***** File GIOGRE.TXT                                                                                                             
                                                                                                                                  
NOTE: This file was created by scanning the original hardcopy article                                                             
and only the Figure captions are included.  Table 1 was missing                                                                   
from the printed version and is not included.                                                                                     
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
The Giotto Radio- Science                                                                                                         
Experiment                                                                                                                        
                                                                                                                                  
P.  Edenhofer                                                                                                                     
Institut fur Hoch- und Hochstfrequenztechnik, University of Bochum, W.  Germany                                                   
                                                                                                                                  
M.K. Bird                                                                                                                         
Radioastronomisches Institut, University of Bonn, W. Germany                                                                      
                                                                                                                                  
H. Buschert                                                                                                                       
Institut fur Hoch- und Hochstfrequenztechnik, University of Bochum, W.  Germany                                                   
                                                                                                                                  
P.B. Esposito                                                                                                                     
Jet Propulsion Laboratory, Pasadena, USA                                                                                          
                                                                                                                                  
H. Porsche                                                                                                                        
Institut fur Physik der Atmosphare, Deutsche Forschungs- und Versuchsanstalt fur Luft- und Raumfahrt (DFVLR),                     
Oberpfaffenhofen, W.  Germany                                                                                                     
                                                                                                                                  
H. Volland                                                                                                                        
Radioastronomisches Institut, University of Bonn, W. Germany                                                                      
                                                                                                                                  
                                                                                                                                  
Abstract                                                                                                                          
  The scientific objectives of the Giotto Radio-Science Experiment (GRE) are to                                                   
determine both the columnar electron content of Comet Halley's ionosphere and the                                                 
cometary mass fluence from atmospheric drag by using the radio signals from Giotto                                                
during the Halley encounter on 13 - 14 March 1986. The radio-science data (S- and                                                 
X-band Doppler and range measurements) will be collected at NASA's deep-space                                                     
64 m tracking antenna at Tidbinbilla near Canberra, in Australia. In order to separate                                            
the effects of the terrestrial ionosphere and the interplanetary plasma, S-band Doppler                                           
measurements will also be taken at Tidbinbilla along the line-of-sight of Japan's com-                                            
etary probe Sakigake during the Giotto - Halley encounter. The measurements of com-                                               
etary electron content and mass fluence will be inverted to derive the spatial                                                    
distribution of the electron and mass (dust and gas) density within Halley's coma. The                                            
GRE is the only experiment on Giotto capable of measuring the low-energy (~ 10 eV)                                                
electron bulk population of Halley's ionosphere and the total cometary mass flow im-                                              
pacting upon the spacecraft.                                                                                                      
                                                                                                                                  
1. Scientific Objectives                                                                                                          
  The Giotto spacecraft is equipped with redundant radio transponders capable of                                                  
downlink transmission at S-band (2.3 GHz) and X-band (8.4 GHz). It is expected to                                                 
travel through the dayside atmosphere of Comet Halley (Reinhard, 1981), with a                                                    
relative velocity of 69 km/s. The target distance for the cometary flyby will probably                                            
be 500 +/- 100 km. The goal of the GRE (Edenhofer, 1980) is to take measurements                                                  
of the phase (Doppler) shifts of the downlink carrier signals both at S- and X-band                                               
as a function of time during the Halley encounter. The specific scientific objectives                                             
are determination of:                                                                                                             
(a) the columnar electron content of the ionosphere of Comet Halley                                                               
(b) the cometary mass fluence and total mass flow due to atmospheric drag.                                                        
                                                                                                                                  
During encounter, the carriers of the telemetry signals from the Giotto spacecraft will                                           
be subject to: (a) dispersion by cometary plasma influencing the phase and group                                                  
velocities of signal propagation (strongest at S-band); and (b) Doppler frequency shift                                           
induced by the deceleration of the spacecraft due to atmospheric drag (strongest at X-                                            
band). Since both effects will be intermingled in the data collected, joint S/X-band data                                         
analysis and evaluation will be required to determine both the columnar electron con-                                             
tent and the mass fluence. By applying appropriate models of the cometary atmosphere                                              
and using integral inversion techniques, the spatial distributions of electrons as well                                           
as mass (dust and gas) density will be derived for the comet's coma.                                                              
  The GRE investigations will complement the in-situ particle measurements to be                                                  
made onboard Giotto. The GRE determination of the total cometary electron content                                                 
(along the Giotto -Earth line-of-sight) and mass flow (along the spacecraft trajectory)                                           
will provide useful reference values for the on-board plasma analyzer (RPA experi-                                                
ment) and dust detectors (DID, PIA). In view of the sensitivity threshold of the RPA,                                             
only the GRE is capable of measuring the low-energy electrons (~ 10 eV) which con-                                                
stitute the bulk of the charged-particle population within Halley's coma.                                                         
                                                                                                                                  
2. Scientific Background                                                                                                          
  2.1  Electron content                                                                                                           
  The GRE is an occultation-type, remote-sensing experiment which relies on the pro-                                              
pagation effects of electromagnetic waves. Radio-science experiments of this type                                                 
have been performed in the past to probe the inner and outer coma of the Sun                                                      
(Edenhofer et al., 1977; Esposito et al., 1980; Volland et al., 1977; Bird, 1982), or                                             
the planetary atmospheres (Fjelbo et al., 1975a: Fjelbo et al., 1975b; Levy et al.,                                               
1981; Vasilyev et al., 1980), using occulting space probes such as Helios, Pioneer,                                               
Mariner, Voyager, Venera, etc. A cometary dual-frequency radio-science experiment                                                 
is also being undertaken in conjunction with the Vega- 1 and -2 missions (Armand et                                               
al., 1982).                                                                                                                       
  As a function of time, the ground-received phase phi(t) (in cycles) of a stable,                                                
monochromatic radio signal of frequency f is proportional to the (downlink) columnar                                              
electron content I(t), i.e. the integrated electron density N_e along the ray path from                                           
the spacecraft to the tracking antenna on Earth. The signal's phase will shift in time                                            
with respect to that of a local oscillator tuned to the undisturbed (i.e. vacuum wave                                             
propagation) Doppler frequency                                                                                                    
                                                                                                                                  
  f' = f(1 -(dR(0)/dt)/c)                                                                                                         
                                                                                                                                  
by an amount                                                                                                                      
                                                                                                                                  
delta phi(t) = -f(rho(t))/c+40.3/(cf) I(t),                                                                                       
                                                                                                                                  
where the quantity dR(0)/dt (known from celestial mechanics and orbit deter-                                                      
mination) is the radial component of the relative velocity between Earth and probe at                                             
times close to encounter (t = 0), and the distance rho(t) is associated with the drag-                                            
induced deceleration delta v(t) of the probe, via                                                                                 
                                                                                                                                  
 rho(t) = integral(delta v(tau) dtau                                                                                              
                                                                                                                                  
(c = velocity of light; MKS units; phase constants omitted). This corresponds to a Doppler frequency shift                        
                                                                                                                                  
  delta f(t) = [-f delta v(t)/c] + [40.3/(cf)*I(t)]                                                                               
                                                                                                                                  
(Bird et al., 1982; Porsche & Edenhofer, 1982: Bird et al., 1983; Bird et al., 1984). During the encounter, the                   
first term of this equation is expected to result in a 'blue shift' (delta v<0), caused by the deceleration of                    
the spacecraft in the cometary atmosphere (frequency increase). The second term                                                   
yields an increase in frequency for cometary plasma entering the line-of-sight (time                                              
derivative I>0; immersion), but reverses its sign for maximum electron content and                                                
then produces a 'red shift' or frequency decrease, respectively (emersion).                                                       
  Since the Giotto radio subsystem features two downlink carrier signals at frequen-                                              
cies f(s) and f(x), two such phase shifts delta phi(s) and delta phi(x) can be measured simultaneously.                           
The plasma and drag effects due to the cometary atmosphere (ionized and neutral part)                                             
can be separated unambiguously by a linear combination of these measured dual-                                                    
frequency phase shifts:                                                                                                           
                                                                                                                                  
  delta psi(s) = delta phi(s) - (f(s)/f(x)) delta phi(x) = 40.3/(cf(s)) [1 -(f(s)/f(x))**2] I(t)                                  
  delta psi(x) = (f(s)/f(x)) delta phi(s) - delta phi(x) = f(x)/c [1 -(f(s)/f(x))**2] rho(t)                                      
                                                                                                                                  
With just one downlink frequency, plasma and drag effects cannot be separated, even                                               
if an uplink is maintained. The only possibility is then a least-squares fit of the record-                                       
ed data to model time profiles of the unknown quantities I(t) and rho(t).                                                         
  To determine the deceleration delta v reliably, the ratio between the Doppler frequency                                         
shift delta f and the nominal downlink frequency f(0) must be less than about 10**- 10, which                                     
is the expected order of magnitude for delta v/c at the time of encounter, e.g. delta psi(x)=                                     
f(xo)(delta v/c - delta f/f(so)) [1 - (f(so)/f(xo))**2]. The quantity delta f(t) represents small frequency perturba-             
tions due to systematic drifts, randomly distributed instabilities, etc.                                                          
  The most important parameter controlling the electron density and therefore the col-                                            
umnar electron content in the cometary ionosphere is the total gas production rate Q.                                             
It depends, for example, on the size of the cometary nucleus and the distance from                                                
the Sun (Reinhard, 1981; Mendis & Ip, 1979). A postulated balance between photo-                                                  
ionization and losses by recombination governs the physics of cometary plasma,                                                    
yielding a model representation of the cometary electron density (Mendis & Houpis,                                                
1982) of the form                                                                                                                 
                                                                                                                                  
  N_c(r) = (A/r) m**-3    (r in meters; A ~ Q**1/2)                                                                               
                                                                                                                                  
For Comet Halley, a reasonable value for the quantity Q is taken to be Q(0)= 1 X 10**29 s**-1                                     
at 1 AU (Ip, 1980). Out to about 10**4 km, the electron density is found to be well represented by                                
A = 3 X 10**15 m**-2. Further out, the radial dependence may be somewhat different because of the                                 
compressing solar-wind plasma. The outer boundary of the cometary ionosphere is estimated to be about                             
r(c) = 5 X 10**5 km. In a first approximation, the electron density distribution is considered to be                              
spherically symmetric.   Thus, in a nucleus-centred coordinate system, the cometary content is denoted by                         
                                                                                                                                  
  I(c) =  Integral(from S(g) to (S(c)) N_c(r) ds = A ln [s(c) + r(c)/(s(g) + r(g))]                                               
                                                                                                                                  
where s(g)**2 = r(g)**2 - p**2; s(c)**2 = r(c)**2 - p**2, and the quantities r(g), r(c), and p                                    
represent the distances to Giotto. to the outer boundary of Halley's ionosphere, and to the                                       
Earth-Giotto (impact parameter) ray path, respectively (Edenhofer et al., 1980; Bird                                              
et al., 1982; Bird et al., 1983). For a flyby distance of 500 km, Figure 1 shows the                                              
temporal variation of I(c) (time profile) to be expected during encounter (S-band). The                                           
maximum columnar electron content I(comax) = 3 X 10**16 m**-2 will be obtained about 3.5 s                                        
after encounter, where this value may increase to some 7 X 10**16 m**-2, depending on                                             
whether or not there are locally enhanced regions from internal sources of additional                                             
ionization (e.g. jet-stream configurations) within the cometary coma intersected by the                                           
ray path.                                                                                                                         
                                                                                                                                  
Figure 1. (a) Columnar electron content of Comet Halley during Giotto                                                             
encounter for a flyby distance of 500 km at S-band                                                                                
(1 hexem=10**16 m**-2). Models of cometary electron density                                                                       
distribution vary with r**-1 (photo-ionization only) and r**-2,                                                                   
respectively.  (b) Electron content I(c) (---) and rate of change                                                                 
dI(c)/dt (---) versus time, corresponding to variations in phase                                                                  
and Doppler frequency shift. A noise level of +/-3x10**14 m**-2 s**-1                                                             
(+/-15 mHz) is indicated, as derived from Giotto Doppler residuals                                                                
collected at the NASA DSS 42 ground station on 31 July 1985                                                                       
                                                                                                                                  
 The taking into account of such kinds of phenomena can result in significantly dif-                                              
ferent profiles for I(c). For a gas production rate Q = 4 Q0, the maximum electron con-                                           
tent will be raised by a factor of two. Computer simulations indicate, however, that                                              
I(comax) may vary only about +20% and -9%, if the distance changes to 100 and                                                     
1000 km, respectively. For the mission baseline (X-band downlink only during                                                      
encounter), the electron content would be reduced by a factor of (2.3/8.4)**2 = 1:13,                                             
corresponding to a loss in signal strength for I(c) of about 11 dB. Obviously, neither                                            
optimistic variations of parameters for encounter geometry nor for cometary plasma                                                
physics can balance such large a loss in signal strength (see Sections 3 and 4 for                                                
intrinsic noise levels of GRE measurements).                                                                                      
  Figure 2 shows some details of the geometry of encounter and the resulting com-                                                 
etary electron content. The temporal behaviour of the electron content during en-                                                 
counter is expected to be specifically different from the steady-state time signature of                                          
those portions of the total electron content measured that are due to the terrestrial                                             
ionosphere and interplanetary plasma.                                                                                             
                                                                                                                                  
Figure 2. Schematic of the geometry of Giotto's Halley encounter on                                                               
13/14 March 1986. The cometary electron content represents the                                                                    
integrated electron density of Halley's ionosphere along the ray path                                                             
C - A, the cometary mass fluence being accumulated from atmospheric drag                                                          
along the trajectory C - B                                                                                                        
                                                                                                                                  
2.2 Cometary mass fluence: superposition of plasma and mass effects                                                               
  As Giotto travels through the atmosphere of Comet Halley, dust and gas particles                                                
will hit its bumper shield. Cometary dust and gas particles typically consist of grain                                            
materials such as magnetite, graphite, quartz, water ice (grain radii roughly                                                     
10**-7- l0**-2 m; mass spectrum 10**-17- 10**-3 kg) and sublimation products of nuclear                                           
constituents in terms of molecules such as H2O, CO2, NH3 (e.g. mean thermal speed                                                 
400 m/s, outflow speed at surface of nucleus 180 m/s) (Newburn, 1981; Divine,                                                     
19,81; Hubner & Keady, 1982; Hellmich & Keller, 1981; Schwehm & Kneissel,                                                         
1981). The impacts of these particles build up an atmospheric drag effect, causing a                                              
deceleration delta v of the spacecraft (Edenhofer et al., 1980; Porsche & Edenhofer, 1982;                                        
Bird et al., 1983). This deceleration can be estimated, from momentum and energy-                                                 
transfer relationships, to be                                                                                                     
                                                                                                                                  
  delta v =  m(0)*v(0)*(1 + q**1/2 cos(phi)) / M                                                                                  
                                                                                                                                  
where m(0) and v(0) are the mass and the velocity of the impacting (recoiling) particles,                                         
M is the mass of the spacecraft (512 kg during encounter) and phi is the angle of                                                 
intersection between the velocity vectors v and - v(0). Here inelastic energy losses delta Q                                      
(e.g. heat generation and ionization processes) are taken to be negligible, according                                             
to delta Q << m(0)v(0)/2. Since the quantity delta v can be measured from the observables of                                      
phase (and group) delay shift (e.g. delta psi(x)) via rho(t), the cometary mass fluence is finally                                
derivable as the mass flow integrated along the trajectory of the spacecraft. Multipli-                                           
cation by the corresponding spacecraft area (2.64 m**2) then yields the total impacting                                           
cometary mass.                                                                                                                    
   The mass-fluence determination is complicated by the fact that the ratio q of the                                              
mass of the recoiling particles to that of the impacting particles is also a function of                                          
the energy of the impacting particles (Hirao, 1982). High-energy gas molecules in the                                             
cometary coma, for instance, may generate multiple secondary ions. At the encounter                                               
velocity of v(0)=69 km/s, energy levels higher than 1 keV apply for molecules of                                                  
atomic weight as low as 42. As far as dust particles are concerned, the production rate                                           
for emission of secondary ions is expected to be even higher than in the case of gas                                              
particles. In laboratory experiments, q-values as high as 40 have been observed for                                               
velocities much lower than v(0) (Porsche & Edenhofer, 1982).                                                                      
  Following refined calculations of the expected dust-particle fluence based on data                                              
from the last apparition of Comet Halley in 1910 (Newburn, 1981), and on the com-                                                 
putations by Hubner & Keady (1982) for the cometary plasma mass which will also                                                   
impact on the bumper shield of the spacecraft, the total integrated mass flow due to                                              
Halley's atmosphere hitting Giotto during the encounter is estimated to be of the order                                           
of m(c)=9 X 10**-5 kg. Allowing for an uncertainty of 1<q<50, the integrated                                                      
spacecraft velocity decrement is expected to be in the range 0.02<delta v<0.1 m/s                                                 
(Porsche & Edenhofer, 1982). Complementary calculations anticipate an even higher                                                 
dust-particle flow resulting in a factor of two for the total mass fluence (Divine, 1981;                                         
Hellmich & Keller, 1981; Schwehm & Kneissel, 1981).                                                                               
  For a flyby distance of 500 km, Figure 3 shows the drag-induced time variation in                                               
delta v to be expected during encounter. Taking into account the uncertainty limits for q,                                        
the deceleration of the spacecraft corresponds to a Doppler frequency shift                                                       
                                                                                                                                  
  delta f/f=delta v/c)*cos alpha,                                                                                                 
                                                                                                                                  
i.e. at X-band limits of 0.35 <delta f(x) <2 Hz are to be expected (alpha = 44deg being                                           
the angle between the relative velocity vector of the spacecraft with respect to Comet                                            
Halley and the position vector of the spacecraft with respect to Earth). A typical noise                                          
level of the order of 10 mHz can be achieved for two-way Doppler measurements,                                                    
caused by the intrinsic frequency instability of the spacecraft transponder and the                                               
tracking station. The noncometary plasma-induced frequency shift is estimated to be                                               
as low as delta f(x)<0.11 Hz. Neglecting cometary drag effects, the Doppler frequency                                             
shift delta f(xo) due to the relative velocity between the spacecraft and the Earth will be                                       
238.1 kHz.                                                                                                                        
  It is important for the Giotto radio-science drag experiment to know the asymptotic                                             
behaviour of delta v(t) before and after encounter (approximately within t=0 +/- 5 min).                                          
The best way to accomplish this is to collect pre- (and in case of spacecraft survival                                            
post-) encounter range measurements (~ group time delay) using the S-band uplink                                                  
and X-band downlink.  While for Doppler frequency measurements the observables                                                    
are proportional to the time derivative d rho(t)/dt ~ delta v(t) of the drag-induced range, range                                 
observables allow direct measurement of accumulated values rho ~ integral(v(tau) d tau), e.g. bet-                                
ween limits l200 <rho <6200 m for a one-day interval around encounter. This is well                                               
above the noise level of about 10 m that is typical for deep-space ranging.                                                       
  For Doppler measurements at either S- or X-band, the observables involve a super-                                               
position of the cometary effects of electron content and mass fluence. The best way                                               
to separate both effects is to collect dual-frequency downlink measurements according                                             
to delta psi(s) and delta psi(x) as outlined above. An alternative, but ambiguous and less efficient                              
way of separating plasma and mass effects is to use the different time signatures of                                              
both phenomena: the electron content varies like a spike-function, and is mainly                                                  
associated with measurements collected immediately before and after encounter                                                     
(within about +/- 3 min or so), whereas the mass fluence changes like a step-function,                                            
with asymptotes essentially based on measurements (including range) taken sufficient-                                             
ly long before and after the encounter (within about +/- 2 h or so at minimum).                                                   
                                                                                                                                  
2.3 Experimental sensitivities and calibration requirements                                                                       
  During the Giotto-Halley encounter in March 1986, solar-minimum conditions                                                      
will exist. Nevertheless, the electron content of Halley's ionosphere is expected to be                                           
about one order of magnitude smaller than typical values of the electron content for                                              
the terrestrial ionosphere, and about two orders of magnitude smaller than the electron                                           
content of the interplanetary space for a total ray path length of 0.97 AU and a solar                                            
elongation angle of 54deg (Edenhofer et al., l980; Bird et al., 1982: Bird et al., 1983).                                         
Taking, for example, the results of the Helios Radio-Science Experiment, as perform-                                              
ed during the solar-cycle minimum in 1976, a good estimate of the electron content                                                
of interplanetary plasma for the Halley encounter geometry turns out to be                                                        
2.4 X 10**18 m**-2 (Edenhofer et al., 1977; Esposito et al., 1980). To separate the con-                                          
tribution from the cometary plasma, an astute calibration scheme must be used. The                                                
radio-science measurements, for instance, for the Jovian dayside ionosphere (Fjelbo                                               
et al., 1975a), the dayside ionosphere of planet Venus (Fjelbo et al., I975b), and the                                            
plasma torus of Io (Levy et al., 1981) could only yield accurate altitude profiles of                                             
electron density after extensive corrections for the actual conditions of the Earth's                                             
ionosphere, as recorded from geostationary radio beacons, had been applied. The                                                   
interplanetary electron content was eliminated by simply subtracting the slowly vary-                                             
ing component of phase shift from the total phase residuals measured during                                                       
occultation.                                                                                                                      
  Though the electron content of the terrestrial ionosphere will certainly be smaller                                             
for the GRE than that of the interplanetary plasma, the influence of the Earth's                                                  
ionosphere on the total electron content to be measured is something of a problem,                                                
since its scale of time variations might be comparable with the spike-like behaviour                                              
of cometary electron content during the encounter. This expectation is also based on                                              
the fact that the GRE radio measurements during the Giotto-Halley encounter are                                                   
scheduled to take place from Australia at local morning time, i.e. within the perturbed                                           
                                                                                                                                  
Figure 3. Cometary mass fluence in terms of drag-induced deceleration                                                             
delta v of the Giotto spacecraft as a function of time during the Halley                                                          
encounter (flyby distance 500 km)                                                                                                 
                                                                                                                                  
transition interval of increasing electron content in between the steady-state values                                             
around midnight and noon. In a cooperative effort with NASA/JPL, two procedures                                                   
have been initiated for the GRE to correct for the terrestrial ionosphere: VHF Faraday                                            
rotation measurements will be taken at NASA's deep-space station DSS 43 in Tidbin-                                                
dilla/Canberra, Australia. These measurements will use radio beacons of the                                                       
geostationary satellites ATS 1 or ETS 2 at 136 MHz to derive the time variations in                                               
the electron content of the Earth's ionosphere (and magnetosphere) during the en-                                                 
counter (Royden et al., 1984). Residual errors in the order of 10**15- 10**16  m**-2 will re-                                     
main when the line-of-sight of DSS 43 towards ATS 1/ETS 2 is transformed into that                                                
valid towards Giotto. At the same time dual-frequency time-delay measurements will                                                
be made at DSS 43 using L-band signals (1.2 and 1.6 GHz) transmitted by the Navstar                                               
satellites of the Global Positioning System (GPS). In 1986/87, 18 of these satellites                                             
will be operational, each of them with a 12 h siderial period to calibrate the                                                    
ionospheric electron content for navigational applications also. Generally, at least four                                         
satellites will be visible from each tracking station, so that line-of-sight transformation                                       
errors are expected to decrease considerably compared with the procedure of Faraday                                               
rotation calibration.                                                                                                             
  In addition to the satellite-borne calibration, the University of Armidale, Australia,                                          
will support the GRE by means of ground-based ionosonde measurements to monitor                                                   
the temporal evolution of a local electron density profile around Halley encounter with                                           
respect to the electron content of vertical incidence, at least up to the layer of max-                                           
imum ionization.                                                                                                                  
  If a calibration is to be performed for columnar densities of the order of 10**16 m**-2,                                        
or even smaller, by including also the interplanetary plasma, then corrections are best                                           
applied that make use of the ground-based radio observations of the International                                                 
Halley Watch (IHW) (Irvine et al., 1982), in addition to a deep-space multispacecraft                                             
calibration as first proposed by Fjeldbo et al. (1975a). The concept of this calibration                                          
procedure is to exploit closely spaced radio sources that have nearly identical ray paths                                         
through the terrestrial ionosphere and interplanetary space. When one of these sources                                            
is being occulted by an additional plasma region (e.g. cometary coma or tail), a sub-                                             
traction process using both radio links is applied to isolate the specific plasma con-                                            
tribution of interest. By means of this two-sources calibration technique, Vasilyev et                                            
al. (I980) were able to detect the nightside ionosphere of Venus with an electron con-                                            
tent as low as about 4 X 10**15 m**-2.                                                                                            
  During the Giotto flyby, five other spacecraft will be in the vicinity of Comet                                                 
Halley: the Japanese spacecraft Sakigake and Suisei, the American ICE spacecraft                                                  
(formerly ISEE-3) and the two Soviet spacecraft Vegas-1 and 2 (Bird et al., 1984).                                                
Figure 4 shows the orbital geometry for this satellite configuration and the various                                              
lines-of-sight involved. On the Vega mission, a dual-frequency radio-science experi-                                              
ment will be performed using a two-spacecraft configuration occulted by Halley's                                                  
ionosphere during a flyby about one week earlier than that of Giotto (Armand et al.,                                              
1982). For the GRE, a mission-intercalibration concept has been set up in cooperation                                             
with NASA/JPL and Japan's Institute of Space and Astronautical Science (ISAS). A                                                  
feasibility study has shown that the Sakigake spacecraft (Hirao, 1982) is the most                                                
suitable in terms of orbital position (full coverage of the ray path to Giotto by the ray                                         
path to Sakigake) and on-board radio subsystem (efficiency of S-band signal-to-noise                                              
ratio) to serve as a calibration radio source for Giotto in order to eliminate all the non-                                       
cometary plasma effects. At the same time as the Giotto downlink radio signals are                                                
received by DSS 43 for Doppler measurements during the encounter pass, the                                                        
Sakigake downlink radio signal will also provide Doppler-measurements at the nearby                                               
34 m DSS 42 antenna and, in addition, at the 64 m antenna at Japan's Udusa ground                                                 
station. A cross-correlation of the Doppler data from DSS 42 and Udusa will allow                                                 
potential plasma disturbances to be distinguished that may travel within the Earth's                                              
ionosphere in interference with the cometary plasma-spike of Halley's Doppler data.                                               
For the benefit of correlative data analyses, cooperative efforts are also being under-                                           
taken to collect simultaneous X-band Doppler (phase) measurements at CSIRO's                                                      
Radio Astronomy Observatory in Parkes (64 m antenna dish about 400 km from Tid-                                                   
binbilla, Australia).                                                                                                             
                                                                                                                                  
Figure 4. Ecliptic-plane view of the interplanetary spacecraft around                                                             
Comet Halley and their signal paths to Earth during the Giotto                                                                    
encounter. The integrated electron density along the line-of-sight to                                                             
the Japanese Sakigake (Planet-A) spacecraft will be measured to                                                                   
calibrate noncometary plasma effects for the Giotto Radio-Science                                                                 
Experiment                                                                                                                        
                                                                                                                                  
3  Radio Subsystem of the Giotto Spacecraft                                                                                       
  A block diagram of the characteristic features of the Giotto radio subsystem is                                                 
shown in Figure 5 (a detailed description can be found in the Experiment Interface                                                
Document/EID, Part A). Those elements of the radio subsystem most important to the                                                
GRE are the transponder and the antennas. During the main phase of the mission, a                                                 
high-gain antenna (HGA) capability wi11 be used for all telecommunication purposes                                                
such as telecommand, telemetry (housekeeping and scientific data), and radiometric                                                
operations. In association with the tracking station(s), ground-based radiometric                                                 
operations will allow three different types of data to be received: Doppler                                                       
measurements either in incoherent/openloop mode (one-way without uplink) or in                                                    
coherent/closed-loop mode (two-way with an uplink); ranging measurements using a                                                  
binary-coded phase modulation of the carrier; and DRVID (differenced range versus                                                 
integrated Doppler) which combines the dispersive propagation properties of phase                                                 
and group time delay. Radiometric data are essential for orbit determination and                                                  
spacecraft navigation and, especially during the encounter, for the GRE.                                                          
  The HGA (antenna gain 26/37 dB at S/X-band) radiates a transmitted power of                                                     
25 W (44 dBm) at X- and 6 W (38 dBm) at S-band downlink frequencies. In addi-                                                     
tion, there are two low-gain antennas (LGAs) for the near-Earth-phase operations.                                                 
Two redundant transponders are used, each consisting of an S-band receiver and an                                                 
S- and X-band transmitter. The radio-frequency distribution unit (RFDU), which                                                    
connects either of the transponders with any of the three antennas (HGA or LGA),                                                  
channels the S-band uplink and the downlink signals through a switching network with                                              
two redundant diplexers. The X-band exciters in either transponder can be coupled to                                              
either of the two travelling-wave-tube amplifiers (TWTAs) by means of a TWT inter-                                                
face unit (TWT-IF). The output signal is directed to the X-band feed of the HGA                                                   
through the TWT-IF. During the closest-approach phase of the Halley encounter the                                                 
two TWTAs will be operated in hot redundant mode (heaters turned on), one TWTA                                                    
being operational and the other in standby mode. In the event of any operational                                                  
anomaly, a changeover in the power subsystem will depower the failed TWTA, rotate                                                 
the switch, and activate the standby TWTA.                                                                                        
  As there is no X-band signal at the exciter output when the S-band high-power                                                   
amplifier is turned on, simultaneous S- and X-band operation in one and the same                                                  
transponder, whether in coherent or incoherent mode, is not possible. As the nominal                                              
configuration for the GRE encounter measurements requires coherent transmission of                                                
both S- and X-band downlink signals (maintained by an S-band uplink), the voltage-                                                
controlled crystal oscillator (VCO) of the chosen receiver must drive both transmitters                                           
from different transponders, one providing the S-band and the other the X-band                                                    
output.                                                                                                                           
                                                                                                                                  
Figure 5. Block diagram of the radio subsystem of the Giotto spacecraft.                                                          
Transponders 1 and 2 are connected to the S-band RFDU (from left/---)                                                             
and to the X-channel TWT-IF (from right/---). The HGA operates in RHC                                                             
polarization, serving the S-band up/downlinks via HGA/S (beamwidth ~ 5deg)                                                        
and the X-band downlink via HGA/X (2deg). The HGA offset-reflector                                                                
(diameter 1.46 m) is mechanically despun with respect to the spinning                                                             
spacecraft (15 rpm) and inclined towards Earth (angle of 44deg between                                                            
RF boresight and spin axis)                                                                                                       
                                                                                                                                  
  There are several modulation characteristics possible for the radio subsystem. The                                              
S-band uplink carrier may be either unmodulated, phase-modulated with ranging                                                     
signals, or phase-modulated with telecommand signals. The S-band downlink carrier                                                 
may be unmodulated (corresponding to Doppler measurements only), phase-                                                           
modulated with ranging signals, or further phase-modulated with a coded housekeep-                                                
ing telemetry signal. The X-band carrier may be unmodulated (Doppler only), phase-                                                
modulated with ranging signals, further phase-modulated with a coded housekeeping                                                 
telemetry signal, or with a coded science telemetry signal. For X-band telemetry, a                                               
maximum bit rate of up to 92.160 kbit/s is available.                                                                             
  Giotto is not expected to survive the encounter with Comet Halley because of                                                    
particle impacts resulting in the malfunctioning of vital subsystems such as power sup-                                           
plies or causing loss of the radio link due to disturbance of the alignment of the HGA                                            
axis. In the latter case, spacecraft recovery for post-encounter science data may be                                              
possible, if the broader antenna beamwidth of the S-band downlink can be utilized.                                                
Nevertheless, a specific feature of the Giotto mission is real-time transmission to Earth                                         
of all science data using the highest possible bit rate during the 4 h data take.                                                 
  Calibration tests on the intrinsic phase and ranging delay of the transponder have                                              
been carried out on the ground. These tests were performed for various RF input                                                   
power levels, from -90 to - 130 dBm, and temperature conditions, from + l0 to                                                     
+40degC. For the incoherent mode, the intrinsic phase delay turned out not to vary in                                             
standard deviation by more than 0.6 ns, corresponding to a columnar electron content                                              
of about 2.4 X 10**16 m**-2 at S-band (warm up time of 4 h). The reproducibility after                                            
stabilization within 0.5 K was found to be of the order of less than +/-0.5 ns. The                                               
temperature dependence was measured to be about +/-0.3 ns/K.                                                                      
                                                                                                                                  
4.  Giotto Radio-Science Ground Support                                                                                           
Station DSS 43 of NASA's Deep-Space Network (DSN) at Tidbinbilla/Canberra                                                         
will serve as the main telemetry backup antenna to the prime Giotto ground station                                                
at Parkes (Renzetti & Berman, 1983). Based on an interagency agreement, both                                                      
Australian stations will receive Giotto telemetry data simultaneously during the Halley                                           
encounter. DSS 43, with its special radio-science capabilities, will be the prime station                                         
for reception and preprocessing of the GRE radiometric data. The ESA station at Car-                                              
narvon (15 m parabolic dish antenna) will also be engaged in Giotto communications.                                               
A radio uplink may be provided either by Carnarvon (transmitted power 1.2 kW or                                                   
61 dBm) or by DSS 43(20 kW up to as high as 100 kW, or 73 to 80 dBm). Table 1 (not                                                
included) summarizes some characteristic features of the various radio links involved. A                                          
block diagram of the DSN system components that are particularly important for the GRE                                            
is shown in Figure 6.                                                                                                             
                                                                                                                                  
Figure 6. Block diagram of the interfaces for the radio-science                                                                   
subsystem at the GRE prime station (DSS 43). Four-channel reception                                                               
with dual-frequency and dual-polarization capability is effected by                                                               
three parallel receiver blocks (left). Generation and pre-processing                                                              
of the radiometric data is controlled by the metric data assembly (top)                                                           
                                                                                                                                  
  A Signal Processing Centre (SPC) in direct contact with the Network Operation                                                   
Control Centre (NOCC) at JPL/Pasadena will control the Australian DSN complex.                                                    
The NOCC can display real-time radio-science data at DSS 43 or 42. As many as three                                               
receiver blocks are ready to handle the Doppler ranging and DRVID measurements                                                    
for the GRE, either in closed Planetary Ranging Assembly (PRA) or open-loop con-                                                  
figuration. The phase residuals are delivered by two separate Doppler extractors for                                              
S- and X-band. DSS 43 is capable of receiving S- and X-band radiometric data                                                      
simultaneously. The open-loop receiver is not servo-adjusted to the received downlink                                             
frequency, but rather preset to a computer-programmable frequency by radio-science                                                
predictions using an optimum bandwidth control as provided by a Spectral Signal                                                   
Indicator (SSI). The SSI displays the received signal spectrum in real time.                                                      
  The Precision Power Monitor (PPM) of the receiver exciter subsystem is used to                                                  
measure the system noise temperature during cometary occultation by comparing the                                                 
total power in a given channel with a known calibration noise source. The DSN radio-                                              
science subsystem, consisting of an Occultation Data Assembly (ODA) and two                                                       
independent digital recording assemblies, processes the input signals and records them                                            
on tape for immediate delivery to the GRE via JPL/ESOC. Real-time displays of open-                                               
and closed-loop data, as well as the status and configuration of all data-receiving and                                           
recording systems (including station pre- and post-track calibration), will be provided                                           
to a radio-science monitoring station at JPL.                                                                                     
  The phase residual is the actually recorded parameter of the downlink carrier signal                                            
and is given in cycles. The accuracy of the measurement is about 0.01 cycles at                                                   
S-band, which is smaller than the specified phase noise of the downlink signals due                                               
to transponder instabilities (5deg rms at S-band, l4deg rms at X-band). Following a stan-                                         
dard calibration procedure, pre- and post-track calibration data will be taken at                                                 
DSS 43 at the beginning and end of the encounter pass of 13/14 March 1986 to correct                                              
for the intrinsic time delay effects of the radio system on ground (Renzetti & Berman.                                            
1983). These measurements exclude all atmospheric and free-space propagation ef-                                                  
fects by using a station-bound radio-link transmitter - subreflector of the Cassegrainian                                         
antenna subsystem - receiver to calibrate a mean average of time delay, expected                                                  
to be in the order of 3 ms, and a linear drift of about 2 ns/h (valid for a full-length                                           
pass of 8 h), which leaves a standard deviation of approx. 1 to 2 ns. The accuracy of the                                         
combined spacecraft-ground Doppler and ranging system is expected to correspond                                                   
to a columnar electron content of some 10**16 m**-2, while the dual-frequency differen-                                           
tial Doppler accuracy should be in the range of +2 X 10**14 m**-2. These accuracies will                                          
benefit from the GRE request to the Giotto project to provide the encounter uplink by                                             
utilizing the 100 kW transmitter capability of DSS 43 (power increase of about 19 dB                                              
compared with Carnarvon station). Such an uplink considerably enhances the phase                                                  
stability of two-way Doppler data (coherent mode) and also increases the reliability                                              
and safety of the radio telemetry link during encounter, particularly if any spacecraft                                           
emergency should arise. The highest possible sampling rate capability, of 10 samples                                              
per second, will be used to record the phase measurements. The plasma contribution                                                
to the phase- and group-delay dispersion will be identified by dual-frequency differen-                                           
cing (see Section 2.1).                                                                                                           
                                                                                                                                  
  The encounter scenario for the GRE foresees operations starting about 24 h                                                      
before the time t(0) of closest approach (EID, part B). They will start with                                                      
radiometric measurements (ranging included) after final targeting selection and final                                             
spacecraft manoeuvring. At about t(0)-6 h, the measurements proper will start with                                                
an X-band downlink (telemetry/radiometric data). Simultaneous dual-frequency S/X-                                                 
band radiometric data will be collected starting at t(0)- 15 min. If the spacecraft sur-                                          
vives, post-encounter ranging and Doppler measurements will be taken, particularly                                                
to support the cometary mass-fluence analysis.                                                                                    
  Figure 7 shows how the capabilities of the various ground stations affect the quality                                           
of Doppler measurements to be expected for the GRE. S-band measurements and                                                       
preliminary analyses made so far indicate that the standard deviation of Doppler                                                  
residuals from ESA's Carnarvon station amounts to  +/-0.7 Doppler counts,                                                         
whereas from NASA's DSS 42 station the corresponding standard deviation is as low                                                 
as +/-0.06 Doppler counts (1 Doppler count or cycle at S-band is equivalent to                                                    
130.4 mm in free-space wavelength). The Doppler noise level at DSS 42 is thus                                                     
approximately 11 dB lower than at Carnarvon, the difference in antenna diameter                                                   
(34 m rather than 15 m) contributing about 7 dB in terms of antenna gain.                                                         
In addition, in coherent mode the transmitter uplink capability of DSS 42 can be                                                  
increased to 20 kW, compared with a maximum of 1.2 kW for Carnarvon (correspon-                                                   
ding to another 9 dB in noise reduction). Figure 7 shows also how significantly the                                               
rate of change in cometary electron content as simulated for the Halley encounter ex-                                             
ceeds the noise level at DSS 42. Since the 64 m antenna of DSS 43 will be used for                                                
the encounter pass, we may expect the noise level to be reduced by at least another                                               
5 dB (for S- or X-band). Such radio-science ground-support considerably improves                                                  
the chances of unambiguously resolving the cometary plasma effects, including the                                                 
bulk population of low-energy electrons, from Halley 's ionosphere. A minimum noise                                               
level (and maximum radio-link safety) can be achieved by making use of the 100 kW                                                 
uplink capability of DSS 43 to provide an additional 10 dB gain (19 dB) in link budget                                            
with respect to the nominal transmitter power of DSS 42 (Carnarvon). This means that                                              
a total reduction in noise level of about 30 dB is achievable, if in coherent mode the                                            
uplink is provided by the DSS 43 rather than by Carnarvon. However, Giotto rehear-                                                
sals using DSS 43 are scheduled to take place no earlier than February 1986.                                                      
                                                                                                                                  
Figure 7. Representative subset of residuals for high-order polynomial                                                            
fits of raw Giotto S-band Doppler data (coherent) from the early                                                                  
cruise-phase after Earth acquisition, collected: (1) at Carnarvon on                                                              
8 July and (2) at DSS 42 on 31 July 1985. The rate of change of cometary                                                          
electron content to be expected during the Halley encounter is shown                                                              
                                                                                                                                  
5.  Summary                                                                                                                       
  The Giotto Radio-Science Experiment will perform Doppler and ranging                                                            
measurements during the Halley encounter on 13/l4 March 1986, in order to deter-                                                  
mine the cometary electron content and mass fluence. Using the spacecraft's radio                                                 
subsystem, these measurements will be collected at NASA's Deep Space Station near                                                 
Canberra. The experiment will involve scientific analyses to derive the spatial                                                   
distribution of electron and mass (dust and gas) density within the ionized and neutral                                           
atmosphere of Comet Halley.                                                                                                       
  Noncometary plasma effects due to both the terrestrial ionosphere and interplanetary                                            
space will be calibrated via measurements using a ground-based ionosonde and geosta-                                              
tionary satellites (VHF Faraday rotation/L-band dispersion) and via Doppler                                                       
measurements from the Japanese cometary spacecraft Sakigake with a line-of-sight                                                  
close to that of Giotto. Two-way Doppler measurements taken at a 34-m station during                                              
Giotto's near-Earth phase show that, for S-band Doppler measurements (coherent                                                    
mode), a noise level as low as 0.06 Doppler is achievable, corresponding to a sensi-                                              
tivity level of +/-4.5 X 10**14 m**-2 in terms of electron content, where a typical cometary                                      
electron content is 3 X 10**16 m**-2. The scientific objective of measuring Halley's elec-                                        
tron content can only be achieved if there are dual-frequency S/X-band downlinks dur-                                             
ing cometary encounter.                                                                                                           
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
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