deep space network enhancement for the galileo mission to jupiter

Acta Astronautica Vol. 17, No. 3, pp. 321-330, 1988 0094-5765/88 $3.00 +0.00 Printed in Great Britain Pergamon Press plc

DEEP SPACE NETWORK ENHANCEMENT FOR THE GALILEO MISSION TO JUPITERt~:

T. K. PENG, J. W. ARMSTRONG, J. C. BREIDENTHAL, F. F. DONIVAN and N. C. HAM Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive,

Pasadena, CA 91109, U.S.A.

(Received 5 February 1987)

Abstract--The Galileo mission to Jupiter has unique scientific objectives never attempted before by a planetary mission. These objectives have presented technical challenges to the NASA Deep Space Network. New technologies and system concepts have been developed to meet these challenges. Major implementations are underway to equip the ground stations in the Network. Significant improvement in performance is expected. The ground-based navigation is expected to achieve an angular precision of 50 nanoradians using very-long-baseline interferometry (VLBI). The frequency stability of ground instrument will be 5 x 10 -~5 for the detection of gravitational waves. The precision of Faraday rotation angle measurement of the spacecraft signal will be better than 2 deg.

1. INTRODUCTION

The Galileo mission to the planet Jupiter, which is likely to be launched in 1990, presents new technical challenges to the network of NASA ground tracking stations known as the Deep Space Network (DSN). Better accuracy is required in ground-based navigation to precisely estimate the probe entry trajectory and to navigate the spacecraft along a course that conserves fuel and maximizes the number of satellite encounters. New reception capability is needed to measure the Faraday rotation angle of spacecraft signals during occultations by Jupiter and the sun in order to map their magnetic fields. A 7.1 GHz uplink frequency capability and an extremely stable two-way Doppler measurement system are needed to enable a search for gravitational waves.

The principal gravitational wave experiment will be conducted at solar oppositions when the spacecraft and the sun are on the opposite sides of the Earth. The probe entry, the Faraday rotation experiment, and the satellite tour will take place when the spacecraft encounters Jupiter.

The Galileo mission will use the DSN 70-m and 34-m parabolic antennas located at Canberra in Australia, Madrid in Spain, and Goldstone in Calif., U.S.A. to track the spacecraft (Fig. 1). In early 1986, the DSN completed the task of supporting the Voy- ager mission encounter with the planet Uranus. Many basic capabilities needed for the Galileo mis-

"['Paper IAF-86-304 presented at the 37th Congress of the International Astronautical Federation, Innsbruck, Austria, 4-11 October 1986.

:~The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Tech- nology, under a contract with the National Aeronautics and Space Administration.

sion were demonstrated during the Voyager-Uranus encounter. These capabilities include telemetry ar- raying using multiple antennas, central control of station equipment, a reliable radio science recording system and a navigation technique using VLBI. In addition, the Network is also enlarging it's 64-m antennas to 70-m during 1986 and 1987. These capabilities form the basis on which the enhancements for Galileo will be made.

In what follows we will outline Galileo's requirements, their rationales, and describe the enhancement of the DSN to support these requirements.

2. PRECISION NAVIGATION USING VLBI

The Galileo mission requires determination of the spacecraft position from ground based observations with a one-sigma angular precision of 50 nanoradians or 0.01 arc sec (40km at jupiter's distance of 7.5 x 108 km). Such precision will be needed to accurately reconstruct the entry trajectory of the probe into the Jovian atmosphere. The observations made both before and after the separation of the probe from the orbiter will provide an accurate trajectory of the orbiter. Knowledge of the separation conditions will then permit determination of the probe trajectory. Such precision will also be needed during the satellite-tour phase of the mission. Ground-based radio metric data, including VLBI, combined with on-board optical navigation data, will be used to maximize the number of satellite encounters within the orbiter fuel constraints.

The DSN plans to use VLBI to meet this require- ment. The VLBI technique[l] uses simultaneous observations of a distant radio source by two antennas separated by hundreds to thousands of kilometers. A unique wavefront emitted by the source will arrive at

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322 T.K. PENG et al.

160 140 120 100 80 60 40 20 0 20 40 60 80 100 120 140 160 180 80

SHADING INDICATES CHANGES FROM 1986

Fig. 1. NASA Deep Space Network supporting Galileo mission.

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one antenna some time after the other (Fig. 2). The delay depends on the geometric orientation between the source and the stations. Correlation of the signals from the two antennas provides a method to determine the delay, which in turn yields the angle between the baseline separating the antennas and the direction to the source. Observations using two baselines with large orthogonal components will provide a unique position of the source in the plane-of-the-sky.

To determine the angular position of a spacecraft using the VLBI technique, alternate observations are made of the spacecraft and an extragalactic radio source, or quasar, in the same neighborhood of the

sky (Fig. 3). This method, called Delta VLBI, measures the angular position of the spacecraft relative to the quasar with high accuracy. A set of quasars with accurately known positions along the planned trajectory of the spacecraft provides an extremely stable reference frame against which both long and short arcs of Delta VLBI data can be compared and the spacecraft angular positions can be precisely determined.

The reason for the precision of angular measurement by the Delta VLBI technique is the simultaneous observation by two antennas, and of two sources very close to each other. Error sources corn-

STN 2

TIME

Fig. 2. Relationship between antennas, source direction and interferometric delay, and determination of delay using cross correlation.

Deep space network for Galileo 323

Fig. 3. Delta VLBI observations of the spacecraft and a quasar in the same direction of the sky.

mon to multiple observations, such as uncertainties in the propagation media and unmodeled frequency fluctuations on the spacecraft, are cancelled in the differencing stage of correlation processing. VLBI observations for Galileo will be conducted in a mode called Delta Differential One-way Range (Delta DOR), in which reception at a number of narrow- band (250 kHz) channels at different frequencies will be used to synthesize a frequency band, called bandwidth synthesis, with 40 MHz effective bandwidth. Data acquisition at both 2.3 and 8.4 GHz bands will permit calibration for delay due to charged particles.

3. WIDER VLB1 BANDWIDTH AND FASTER PROCESSING

The Galileo mission was the first deep space mission to require VLBI as one of its navigational data types when the mission was planned. As a result, the DSN must augment the existing VLBI system to meet

the Galileo requirements. The new system (Fig. 4) will have higher angular precision, 50 nanoradians for Galileo compared to approximately 150 nanoradians achieved during the Voyager cruises to Saturn in 1980 and to Uranus in 198512,3]. The major increase in accuracy results from synthesizing a frequency band that spans 40MHz vs 14MHz for Voyager. The new system will also have shorter data throughput time, 12 h from acquisition to generation of Delta VLBI data instead of the present average of 2 days or more.

In observing either the spacecraft or a near-by quasar, correlation of the two signals received at separated ground antennas provides a relative delay at a selected epoch. The delay, ~.S/c in Fig. 2, is the projection of the baseline in the direction of the source. A priori knowledge of the baselines between the three DSN complexes in America, Australia, and Spain yields the position of the source. Differencing the spacecraft and quasar delays gives the instanta- neous angular separation of the two. System parameters have been selected such that 10 min of data from the spacecraft and the quasar each will produce a time delay measurement resolution to I ns, equivalent to a differential range resolution of 30 cm. Compared with the intercontinental baseline on the order of 6000 km, this range accuracy corresponds to an angular resolution of 50 nanoradians.

The new VLBI system will include a digitally controlled signal receiving and data acquisition system and a high-stability phase calibration system. The calibration system uses coherent phase calibration tones driven by the station hydrogen maser frequency standard. It will measure the phase drifts in the receiving system for correction in post- processing. To produce a near real-time system, the DSN has implemented high speed processors at each complex to digitize and record the down-converted

DEEP SPACE COMMUNICATION COMPLEX

~ E R - - ~ LOWNOISE ~ FREQUENCY ~ DATA ~1~ MASER DOWN ACQUISITION AMPLIFIER CONVERTER COMPUTER

70-MET

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JET PROPULSION LAB

FROM ANOTHER COMPLEX

~M~ VLBI CORRELATOR ~DELAY DETERMINATION)

H NAVIGATION I COMPUTER I (ORBIT DETERMINATION

DSN CONTROL CENTER

Fig. 4. Conceptual block diagrams of the DSN delta VLBI system.

324 T.K. PENG et al.

video-band signal at 500 kilobits per second (kbps) for 20 min. Data from each complex will be transmitted to the correlation processor at the Jet Pro- pulsion Laboratory, Pasadena, at rates of 50 to 200 kbps. The total throughput time will be reduced to 12 h or less.

Development of a reference frame of radio sources along the planned trajectory of the Galileo mission is needed to determine the position of the spacecraft relative to the planets. Source-to-source accuracy of 20 nanoradians is needed. Periodic observation of the same sources will also be needed to maintain current knowledge of the frame. Such measurements will be made using receivers with a 400 MHz bandwidth reception system on the 34-m antennas and a recording system with a data rate of 112 Mbits per s. This wide band system will be incorporated into the DSN in 1987. Determination of the relationship between the radio source reference frame and the planetary reference frame is also underway to complete the information needed for navigation.

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Fig. 6. Anticipated source strength for gravitational radiation for various possible sources as a function of character-

istic time scale of the waves; from[4].

4. G R A V I T A T I O N A L W A V E E X P E R I M E N T

The enhancements in the DSN also improve the sensitivity of the gravitational wave experiment, which has been planned for the Galileo mission[4]. In the gravitational wave experiment, the earth and spacecraft act like free test masses. A gravitational wave of dimensionless strain amplitude, h, incident on the Earth-spacecraft system causes very small[5] perturbations in the two-way Doppler frequency. The fractional frequency fluctuations, Af / fo (where f0 = the nominal frequency of the link), are approximately equal to the wave amplitude, h. Thus, the radio link measures gravitational waves via the relative dimensionless velocity Av/c of the Earth and spacecraft. The expected strain amplitudes depend on the type of source but are in any case very small (see also Fig. 6). Thus, extreme care in stabilizing the ground and spacecraft systems is required to achieve an experiment that has an interesting sensitivity.

~ SPACECRAFT

EARIH / ~e = T (STRAIN AMPLITUDE- h)

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Fig. 5. Response of the Doppler tracking link to gravitational radiation. The incident gravitational wave pro-

duces a "three-pulse" response in the link[6].

Figure 5 illustrates the response of the tracking system to an incident gravitational wave[6]. A gravitational wave, a transverse wave of time-varying gravitational potential traveling at the speed of light, is incident on the Earth-spacecraft system. The wave changes the distance between separated test masses and affects the rates at which clocks keep time. The DSN transmits a radio signal having very high spectral purity, which is coherently transponded by the spacecraft and received at the Earth. The observable is a time series of the difference between the frequencies of the transmitted and received signals. The net effect of a gravitational wave is to produce three copies of the gravitational waveform in this Doppler frequency time series (Fig. 5). Two of the three pulses are separated by the round-trip light time, T, to the spacecraft; the third pulse arrives at an intermediate time which depends on the angle of arrival. The sum of the amplitudes of the three pulses is zero. Thus, gravitational waves with time scales long compared to the two-way light time produce responses in the tracking record that overlap in time and cancel to first order. This places a limit on the period of gravitational waves that can be observed with reasonable sensitivity of about T/2. A short period limit is set by the integration time required to reach good frequency stability; for Galileo we expect this to be about 30 s. Between these two limits, the Doppler system is a broadband detector of gravitational radiation.

The sources that might produce waves in this very-low frequency (VLF) band and the magnitude of the expected signals are shown in Fig. 6. The VLF band covers periods roughly 30 to 5000 s. In this range, there are three classes of sources: burst, periodic, and stochastic background. Bursts are expected to arise from violent non-spherically symmetric motion of supermassive objects, for example explosive events in galactic nuclei. Periodic waves should be


produced by massive objects in orbit around one another, for example binary block holes. Stochastic signals could arise from, for example, a primordial background of gravitational radiation. Figure 6 illustrates the range of expected signal strengths for reasonable parameters of the sources. The lines in Fig. 6 show the maximum plausible signal strength for gravitational wave backgrounds (the energy density of the waves would be sufficient to gravitationally close the universe), bursts (100% conversion of mass to gravitational waves), and black hole binaries (the orbit shrinks as the system loses energy to gravitational radiation; peak amplitude occurs just before coalescence of the black holes).

As can be seen, the expected signal amplitudes are small. These weak signals must be detected in the presence of various noises[7]. Although the spectral signature of the 3-pulse response of the Doppler system to gravitational waves can be exploited to improve detection sensitivity, it is desirable to mini- mize the level of the noise sources over which we have control. The most important noise sources are: plasma scintillation noise, tropospheric scintillation noise, ground and spacecraft instrumental instability, and unmodeled accelerations of the spacecraft. The DSN upgrades for the Galileo project address the plasma and instrumental noise problems and will allow order-of-magnitude sensitivity improvements over gravity wave experiments previously conducted in the VLF region. With a sensitivity reaching parts 10 -iS, the Galileo Doppler system should be able to detect gravitational waves emitted from bursts of quasars and the black hole binaries shown in Fig. 6.

5. ERROR SOURCES IN GRAVITATIONAL WAVE EXPERIMENT

5.1. Plasma scintillation

As the radio wave propagates between the ground station and the spacecraft, its phase (hence frequency) is randomly perturbed by fluctuations in the index of refraction along the ray path. The wave must pass through two ionized media: the Earth's ionosphere and the solar wind. Phase fluctuations ("scintillations") caused by these media are the leading noise source for the previous experiments at 2.3 GHz. However, these scintillations depend strongly on radio wavelength: Af / fo scales as radio wavelength squared. Galileo is the first deep space probe to have the capability of receiving a 7.1 GHz uplink and coherently translating it to downlink. The DSN is installing the capability of transmitting a high quality 7.1 GHz signal to Galileo. The result for the gravitational wave experiment is to reduce the plasma scintillation noise by a factor of (3/11) 2 or 0.074. This thirteen-fold improvement brings the level of the plasma noise to about 3 × 10 -15.

5.2. Tropospheric scintillation

Irregularities in the Earth's neutral atmosphere

also perturb the phase of the radio wave. At micro- wave frequencies, the fluctuations are dominated by irregularities in the water vapor distribution. These irregularities can be measured by water vapor radiometers and the gravitational wave data can be appropriately corrected for this noise source. It is hoped that in the Galileo-era the DSN will have implemented water vapor radiometers for this purpose.

5.3. Instrumental stability ,~

To effectively use this improvement in the measurement of plasma noise, care must be taken that the instrumental noise is better than or comparable to the other noise sources. The DSN is improving the stability of the entire ground system to a level of 5 x 10 -15 or better through the following:

Frequency reference distribution. There has been very careful attention to the distribution of reference signals throughout the ground station to ensure that the stability of the hydrogen maser time base is maintained. The stability of the hydrogen maser has been demonstrated at a level better than I x 10 -~5. The critical distribution lines that deliver the reference frequency to the high frequency equipment have been stabilized to cause no significant degradation.

Exciter-transmitter phase correction loop. The phase of the transmitter power amplifier output signal is compared against the exciter reference signal. The resultant phase error information is fed back to the exciter phase correction loop, to compensate for this instability. Stability better than 2 × 10 ~s has been achieved on a prototype 7.1 GHz exciter- transmitter set.

Calibration tones. At the antenna feed input, calibration tones of high spectral purity are injected. These signals follow the same path as the received signal through the first few intermediate frequency conversions. Thus any instability introduced by the receiver is common to the received signal and the calibration tones. By measuring the phase of the calibration tones, corrections for any instability of the receiver system can be made. (Low frequency parts of the receiver where the signal and calibration tone paths are not identical are placed in controlled temperature to achieve very high stability.) The stability of the calibration tones have been measured at approximately 1 × 10 -15 level.

Signal recording. For Galileo, the voltage of the received signal will be recorded on magnetic tape for Doppler extraction in software. This allows synthesis of very narrow phase-tracking loop bandwidths and flexibility in the post-processing.

Antenna structure. The new 34-m High Efficiency antennas have improved structural design which effects minimum phase change due to sagging or wind load. Structural stability better than 2 x 10 -15 is expected under normal wind conditions on computer simulation.

326 T.K . PENG et al.

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Fig. 7. Conceptual block diagram of system to be used in the Galileo gravitational wave experiment. Main system elements are a hydrogen maser frequency standard, a high-stability frequency distribution system, a 7.1 GHz exciter and transmitter (with phase correction loop), a calibration tone generator and estimator, a stabilized receiver, and a recording system. The frequency and time distribution system drives all major system elements. The actual implementation is shown in Fig. 12 (except that gravitational wave experiment

will use 34-m antennas and only one polarization).

5.4. Unmodeled accelerations

Before the launch of Galileo, it is difficult to estimate the magnitude of the unmodeled spacecraft accelerations. These will be evaluated in flight during an early opposition when the plasma scintillation noise is at a minimum.

Figure 7 shows a block diagram of the system to be used in the Galileo era. Figure 8 illustrates the fractional frequency stability versus integration time for a major instrumental noise contributor. These data show the level of performance expected of the ground system in the Galileo era.

The objective for the Galileo experiments, however, is the reverse. Investigators from Stanford University plan to study the magnetic field in Jupiter's ionosphere using Faraday rotation (see Fig. 10), while a team of ESA and NASA investigators will study the magnetic field in the solar corona. The dual- frequency Doppler and ranging capabilities of Galileo will be used to model the electron density for eqn (1)[12], allowing measurements of Q to give the component of magnetic field parallel to the ray path. Another simultaneous measurement by Galileo, the Scintillation measurement, will independently give the direction (but not the magnitude) of the magnetic

6. FARADAY ROTATION EXPERIMENTS

The term "Faraday rotation" refers to a change in the plane of polarization of an electromagnetic wave as it propagates through a magnetized plasma (see Fig. 9). At the frequencies and plasma densities encountered in the solar system, the rotation obeys the equation

D = 5 N B cos 0 dl (1) aypath

where dl is an element of distance in the direction of propagation, N is the electron density, B cos 0 is the component of the magnetic field along the ray, f i s the frequency, and k is a constant[8].

Previous studies have assumed the magnetic field as a known quantity and inferred columnar electron content (the integral of electron density along the raypath) from Faraday rotation, as has been done for the Earth's ionosphere[9] and the solar corona[10,1 l].

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Fig. 8. Measured Allan variance versus integration time for the exciter-transmitter system at the DSN's research and development station. Similar performance is expected for the receiver elements with stabilization of local oscillators

and real-time phase calibration.


d~ Je I Fig. 9. An electromagnetic wave with polarization E0 changes to a new polarization E6, after transversing a magnetic plasma. This phenomenon is known as Faraday rotation. The rotation is proportional to the

product of N and the component of the magnetic field B parallel to the direction of propagation.

field normal to the raypath[13]. All of the Faraday rotation experiments will require extensive modeling of the total magnetic field, since only one component is measured and it tends to be averaged over space by the integral in eqn (1).

The Galileo measurements are important because the magnetic field can be sensed at altitudes between 1.0 and 1.1 Jupiter radii, a previously inaccessible region. The Pioneer and Voyager measurements have all been done above 1.6 Jupiter radii[14]. Predicting the magnetic field at lower altitudes is difficult because non-dipole components increase rapidly near the planet, yet they are weak farther out. Mea- surements of such non-dipole fields will help set boundary conditions on the rest of the magneto-

sphere, and improve the understanding of the mech- anism of field generation within Jupiter.

A similar argument applies to the sun. Direct measurements of the sun's magnetic field have all been made outside 0.3 AU[15] and indirect optical observations have been limited to no farther out than 10 solar radii[16]. The gap between 10 solar radii and 0.3 AU, or 70 solar radii, is an active region with several types of transient phenomena[17], but the details of these phenomena remain unclear. Mea- surements of the magnetic field with Faraday rotation will improve our understanding.

The accuracy needed to make progress in understanding Jupiter's magnetic field depends on the existing uncertainty near the planet. A contour map

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Fig. 10. The Galileo orbiter will transmit a linearly polarized 2.3 GHz radio wave through Jupiter's ionosphere toward the Earth, where the polarization will be measured at NASA's 70-m Deep Space Stations. The combination of the free electrons and the planet's magnetic field will rotate the polarization (Faraday rotation). The Orbiter will rotate at about 3 rpm, necessitating constant measurement of its orientation. Faraday rotation can then be calculated from the difference between transmitted and received

polarization. Similar experiments will be performed on the solar corona.

328 T.K. PEYG et al.

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Fig. 11. Predicted differences in degrees of Faraday rotation between two existing Jovian magnetic field models are shown as a contour plot. The calculation was made as a function of the latitude and longitude at the top of the ionosphere. Galileo will observe the Faraday rotation at two points on the map (one on entry and one on exit) for each of up to 10 occulations, and at altitudes below the one shown. (Courtesy

D. P. Hinson, Stanford University Center for Radar Astronomy.)

(Fig. 11) of the difference in ionospheric Faraday rotation predicted by two existing models of the field shows differences of 10 to 40 deg[18]. Interpreting these differences is difficult because the models have been extrapolated into a region where measurements have not yet been made, but it is reasonable to view the differences as a lower bound to the expected size of the differences that the Galileo mission will observe. Measurement accuracy significantly better than this lower bound will be needed to support modeling of the field. The DSN ground system is designed to achieve an accuracy of 2 deg or better.

Much lower measurement accuracy is tolerable for the solar corona since the total effect exceeds 180 deg when the signal comes within 3 to 5 solar radii[12, p. 108]. With a 2-deg accuracy, it should be possible to detect Faraday rotation out to 20 solar radii, and sometimes even out to about 30 solar radii. It is important to have continuous observation near the sun, because the transient phenomena produce changes of typically 90 deg per h[10, Fig. 5-3]. If such changes are not traced, it is likely that Faraday rotation will change by a turn or more, rendering the measurements ambiguous and obscuring information about the absolute magnetic field.

Measuring polarization

Several candidates were considered for measuring the polarization of Galileo's signal at the ground station. One method using two perpendicular dipole antennas was discarded because the spacecraft will rotate the polarization at 3 rpm. The signal would have disappeared from one antenna or the other twice per revolution. We also considered several

mechanical and electrical ways of rotating the antennas to follow Galileo's rotation. However, these methods were too expensive or would have introduced single failure points which could affect the reliability of command and telemetry.

The method chosen takes advantage of the fact that a linearly polarized wave can be obtained by adding two circularly polarized waves rotating in opposite directions. The linear wave can therefore be received by two separate receivers with antennas sensitive to circular waves, which give a constant amplitude even if the spacecraft rotates. Enough information is still present to measure orientation, but the physical angle is converted to a difference in time-of-arrival between the circular waves. If one wave is delayed relative to the other by a fraction of a cycle, then the equivalent linear wave rotates by the same fraction of a half-turn. Thus, it is only necessary to measure the relative time delay between the circular waves to determine the linear orientation.

The particular receivers chosen are the so-called open-loop type, which convert the frequency of the spacecraft signal nearly to dc using predicted tuning. The resulting voltage is sampled, digitized, and recorded on magnetic tape. An investigator later processes the recorded voltages to estimate the phase difference between the two signals. This approach allows a great deal of sophistication in final detection of the signal. In particular, noise can be essentially eliminated by filtering to a very narrow bandwidth (10 to 100 Hz) centered on the carrier, even when the carrier is changing frequency rapidly.

When using separate receivers, however, unknown biases and phase drift between them would translate


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Fig. 12. The polarization of the Galileo 2.3 GHz signal will be determined using two receivers connected to opposite circularly polarized receptors of the 70-m antennas. In each receiver, the signal will be amplified in cryogenic masers, and will be shifted in frequency first to about 300 MHz, and then to between 0 and 10 kHz. The output of each receiver is sampled, digitized, and recorded on magnetic tape, which can then be processed to estimate the orientation of the original wave. A locally-generated wave of definite polarization will pass through all but the last stage of the receiver, enabling calibration of nearly all of

the receiver errors.

directly into measurement errors. To overcome the errors, a separate calibration tone of known polarization angle will be applied at the input of the antenna. This signal will be used to correct bias and phase drift in the front end low-noise amplifiers and transmission lines of the receiving system, which are subject to the most severe motion and temperature change. Phase drift in the low-frequency portion of the receivers, which are located in the control room separated from the antenna, can be minimized by temperature control. Recent measurements have shown that the uncalibrated drift will contribute no more than 0.5 deg of error. Figure 12 gives a conceptual block diagram for the observation system.

A major difficulty with any ground-based Faraday rotation measurement is the Faraday rotation introduced by the Earth's ionosphere before the signal reaches the ground. The magnitude of this effect at 2.3 GHz is usually less than 1 deg at night and 4 to 7deg during the day[10]. If not corrected, the resulting errors would mask effects of Jupiter at the desired 2 degree or better level of accuracy. Essen- tially all of the error is due to the variability of the electron content; the magnetic field is accurately known. Thus, it is necessary to measure the electron content of the Earth's ionosphere during the experiments.

Currently the DSN measures the earth ionosphere using signals from satellites ATS-3 and ETS-2. Un- fortunately these satellites are old and will probably not be available during the Galileo era. Also, spatial

variation of the ionosphere introduces significant errors when measurements along the direction to a satellite are mapped to other directions. To overcome these problems, a changeover to the NAVSTAR Global Positioning System (GPS) Satellites is planned. The new system will use dual-frequency signals at approximately 1 GHz from GPS satellites to measure the electron content in directions sur- rounding the Galileo Orbiter[19]. The measurements will then be mapped to Faraday rotation along the Galileo line-of-sight using a model of the Earth's magnetic field. Tests on a prototype system show that it can measure electron columnar density to better than 5 x 1016 electrons per square meter[20], corre- sponding to Faraday rotation of 0.3 to 1 deg de- pending on geometry[21]. Planned improvements in mapping technique and the availability of more GPS satellites should improve the ionosphere calibration further.

7. CONCLUSIONS

The Galileo mission has unique scientific objectives never attempted before by a planetary mission. These objectives have presented technical challenges to the DSN. As presented above, new technologies and system concepts have been developed to meet these challenges. Major implementations are underway to equip the ground stations in Australia, Spain, and the United States so that they can be ready in time for engineering demonstration and flight support.

330 T. K. PEr~G et al.

As presented above, the precision of navigation is expected to improve from 150 nanoradians, achieved presently, to 50 nanoradians with an improved VLBI system. The two-way Doppler stability for the search of gravitational waves is expected to achieve 5 × 10 -~5, an improvement by a factor of 10, with the implementation of a 7.1 GHz uplink and a very stable set of instrumentation on a new antenna. A new scheme of detecting and tracking linearly polarized 2.3 G H z signals from a rotating spacecraft is expected to provide a 2-deg accuracy and a high level of reliability in detecting Faraday rotation angles. These performances will be demonstrated with the spacecraft before the actual events which depend on these capabilities critically.

These new technologies and systems represent a significant step forward in the technical capability of the Network. The new capabilities will not only support the needs of the Galileo mission but also present new space science and mission design options to future planetary missions.

Acknowledgement--The work described in this paper repre- sents the outcome of a collective effort of hundreds of people associated with the DSN whose contributions are gratefully acknowledged.

REFERENCES

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deep space network enhancement for the galileo mission to jupiter

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