urn:esa:psa:context:instrument:gio.gre 1.0 1.11.0.0 Product_Context urn:nasa:pds:context:instrument:gre.gio 2021-02-24 1.0 Changed inst LIDs from u:n:p:c:i:instID.scID to u:n:p:c:i:scID.instID Changed LIDs from urn:nasa:pds: to urn:esa:psa: And per "Guide toPDS4 Context Products" v1.7, changed all lidvid_reference to lid_reference urn:esa:psa:context:instrument_host:spacecraft.gio instrument_to_instrument_host Asmar, S.W., and R.G. Herrera, Radio Science Handbook, JPL D-7938, Volume 4, Jet Propulsion Laboratory, Pasadena, CA, 22 January 1993. reference.ASMAR-HERRERA1993 Edenhofer, P., M.K. Bird, H. Buschert, P.B. Esposito, H. Porsche, and H. Volland, The Giotto Radio- Science Experiment, ESA SP-1077, 1986. reference.EDENHOFERETAL1986A Edenhofer, P., M.K. Bird, J.P. Brenkle, H. Buschert, P.B. Esposito, H. Porsche, and H. Volland, First Results From the Giotto Radio-Science Experiment, Nature, 321, 355-357, 1986. reference.EDENHOFERETAL1986B Edenhofer, P., M.K. Bird, P.B. Esposito, H. Porsche, and H. Volland, Preliminary results of the Giotto Radio Science Experiment, Advanaces in Space Research, 5, 201-209, 1986. reference.EDENHOFERETAL1986C Edenhofer, P., M.K. Bird, H. Buschert, P.B. Esposito, H. Porsche, and H. Volland, Measurement Technique of the Giotto Radio-Science Experiment, Journal of Physics E: Science Instrumentation, 20, 768-777, 1987. reference.EDENHOFERETAL1987A Edenhofer, P., M.K. Bird, J.P. Brenkle, H. Buschart, E.R. Kursinski, N.A. Mottinger, H. Porsche, C.T. Stelzried, and H. Volland, Dust Distribution of Comet P/Halley's Inner Coma Determined from the Giotto Radio-Science Experiment, Astronomy and Astrophysics, 187, 712-718, 1987. reference.EDENHOFERETAL1987B Hall, P.J., The Giotto Radio Science Experiment at Parkes, Proceedings of the Astronomical Society of Australia, 6(3), 298-301, 1986. reference.HALL1986 GIOTTO RADIOSCIENCE EXPERIMENT Atmospheric Sciences not applicable not applicable Instrument Overview =================== The Giotto spacecraft telecommunications subsystem served as one element of a radio science experiment for investigations of comet Halley. The second element was a set of ground antennas and associated electronics, most of which were in Australia. The spacecraft element of the experiment is specified below. Instrument Id : RSS Instrument Host Id : GIO Instrument Name : GIOTTO RADIOSCIENCE EXPERIMENT Instrument Type : RADIO SCIENCE The Giotto Radio Science Experiment (GRE) utilized instrumentation with elements on the spacecraft and on Earth. Much of this was shared equipment, being used for routine telecommunications as well as for Radio Science. The experiment is described in more detail by [EDENHOFERETAL1986A] and [EDENHOFERETAL1987A]. The spacecraft radio system was constructed around a redundant pair of transponders which received at S-band (2.3 GHz, 13 cm wavelength) and transmitted at both S-band and X-band (8.4 GHz, 3.6 cm wavelength) frequencies. The transmitted frequency during the Halley encounter was controlled by an on-board oscillator; at other times it was controlled by a signal transmitted from the ground. Each transponder included a receiver, command detector, exciter, and low-power amplifier. The transponders provided the usual uplink command and downlink data transmission capabilities. Traveling wave tube amplifiers, driven at saturation, amplified the transponder output before the signals were radiated via a high-gain antenna (HGA). The HGA offset reflector had a diameter of 1.46 m and was despun with respect to the spacecraft body. HGA polarization was right circularly polarized for S-band up/downlink and for X-band downlink. S-band beamwidth was about 5 degrees; X-band beamwidth was about 2 degrees. Ground stations included antennas, associated electronics, and operational systems at two complexes in Australia. The prime system for GRE was the NASA Deep Space Network (DSN) station near Canberra; its 64-m antenna is known as DSS 43. Geodetic coordinates for the 64-m antenna are 148 deg, 58 min, 48 sec E longitude and 35 deg, 24 min, 14 sec S latitude. Raw data from the DSN tracking system included Doppler measurements along the line of sight to the spacecraft. Raw data from the open loop receiver system included digital samples of baseband output. The second system was the 64-m antenna operated by the Australian CSIRO near Parkes; it served as the primary Giotto ground receiving facility for the European Space Agency in addition to its role as a receiving site for the GRE [HALL1986]. Once the radio signal from the spacecraft had been captured by a ground antenna, it was amplified by cryogenic maser amplifiers. Two measurement options were then available: (1) Open-loop Data Acquisition (2) Closed-loop Data Acquisition Open-Loop Data Acquisition -------------------------- Open-loop data acquisition is performed by filtering and then downconverting the received carrier signal to baseband, where it is sampled for subsequent manipulation in digital form. The open-loop receiver is tuned on the basis of frequency predictions that take into account the best estimate of the carrier frequency transmitted by the spacecraft and Doppler corrections based on relative spacecraft-to-ground motion. The quantized samples can then be recorded on magnetic tape. Closed-Loop Data Acquisition ---------------------------- Closed-loop data acquisition is performed with a phase- locked loop receiver; it is usually employed when the spacecraft is operating in its 'coherent' mode. 'Two-way' Doppler shifts are determined by comparing an estimate of the downlink carrier frequency from the phase-locked loop with a reference from the ground station's frequency and timing subsystem. Since the station frequency reference is also used to generate the uplink carrier, the determination can be as accurate as the fundamental station clock -- typically a hydrogen maser frequency standard and, therefore, more stable than the crystal oscillator on board the spacecraft. The Doppler integration time needed to achieve a particular signal-to-noise ratio controls the time interval between successive measurements. Amplitude of the received signal is estimated by sampling the calibrated automatic gain control (AGC) voltage of the phase-locked loop receiver's coherent AGC loop. 'One-way' closed-loop Doppler shifts are determined in much the same way except that the downlink measurement from the phase locked loop must be compared with an estimate of the spacecraft carrier frequency; in this case the accuracy is limited by the relative performance of the reference oscillators on the ground and the spacecraft. Both frequency and amplitude measurements obtained at the DSN station can then be recorded on magnetic tape and/or transmitted electronically to JPL. Files of data specifically intended for navigation and radio science are derived from the raw measurements by the DSN Radio Metric Data Conditioning Team. Operation of the DSN radio science equipment is described in more detail by [ASMAR&HERRERA1993]. The strength of a spacecraft carrier signal, and thus the quality of the radio science data, depends on its modulation state. During the Halley encounter, only the X-band downlink was activated; it was modulated with science data. The link budget for the GRE prime receiver at Canberra, Australia is shown below [EDENHOFERETAL1986B]: S-band S-band X-band uplink downlink downlink ------ -------- -------- Signal frequency (GHz) 2.117 2.299 8.429 TX power (dBm) 73 36.7 43.2 Ground antenna gain (dB) 60.6 61.7 71.9 Propagation loss (dB) -262.5 -263.2 -274.5 S/C antenna gain (dB) 25.3 26.3 39 RF losses (dB) -2.3 -1.9 -1.6 Signal level RX input (dBm) -105.9 -140.4 -122 System noise temperature (dBK) 27.2 13.7 13.8 Boltzmann constant (dBm/HzK) -198.6 -198.6 -198.6 Received S/No (dBHz) 65.5 44.5 62.8 Modulation loss (dB) 0 -3.6 -8.2 PLL bandwidth (dBHz) 15.4 16.8 16.8 Required C/N (dB) 10 15 15 Margin (dB) 40.1 9.1 22.8 Science Objectives ================== The primary objective of the Giotto Radio Science Experiment was determination of the Halley mass fluence resulting from atmospheric drag. A second objective was determining the electron content of the ionosphere of the comet; this part of the experiment was badly degraded when Giotto project management chose to operate only an X-band downlink during the encounter. Mass fluence was determined by measuring the change in spacecraft radial velocity (with respect to receiving stations on the Earth) during the encounter. Velocity changes were inferred from Doppler shifts in the received signal. Doppler shifts not in accord with the expected trajectory could be interpreted as resulting from drag on the spacecraft by dust and gas. Passage of the radio wave through a plasma can also cause Doppler shifts on the received signal. The plasma effects can be readily separated from other effects if measurements are made at two well-separated radio frequencies. The inability to make measurements at S-band meant that only indirect measurements and modeling could be used to infer the density of charged particles in the Halley environment. Calibration Descriptions ======================== Several calibration measurements were carried out; most were directed toward estimating the propagation effects of charged particles along the ray path. Earth Ionosphere Plasma Content ------------------------------- VHF Faraday rotation measurements were made at Canberra using the ETS-2 geostationary satellite. These measurements would provide a measure of the plasma content in the Earth's ionosphere over Canberra. Other Earth Atmospheric Effects ------------------------------- Acquisition of data at two Earth sites (Canberra and Parkes) ensured that local anomalies in propagation conditions over a single antenna (such as changes in the ionosphere) could be calibrated and removed. The results from Canberra and Parkes were very similar [EDENHOFERETAL1987B]. Ray Path Reference ------------------ The Sakigake spacecraft was tracked at S-band by the DSS 42 antenna -- a 34-m antenna that is also part of the Canberra DSN complex. Since Sakigake was in the same part of the sky as Giotto, differences in the properties of the received signals would depend primarily on the propagation conditions in the immediate environment of Halley during the Giotto encounter. Vega Spacecraft Measurements ---------------------------- Vega-1 and Vega-2 used dual-frequency radio methods to obtain estimates of peak electron densities during their encounters with Halley (2500 electrons per cubic centimeter at 8890 km closest approach distance and 1500 el/cc at 8030 km, respectively). Extrapolation of these values to the Giotto flyby conditions gave numbers consistent with simulation [EDENHOFERETAL1986C]. Asymptotic Spacecraft Trajectory Calibration -------------------------------------------- Two-way Doppler and ranging data were acquired both before and after the Halley encounter. These showed that the net change in radial velocity was 23.05 +/- 0.05 cm/sec during the encounter [EDENHOFERETAL1987B]. On-Board Oscillator Drift ------------------------- Measured linear drift of the on-board Giotto oscillator was about +1 Hz per minute [EDENHOFERETAL1987B]. Operational Considerations ========================== Giotto Project management chose to operate only the X-band spacecraft transmitter during the Halley encounter. This made extraction of the electron content in the comet environment extremely difficult, if not impossible [EDENHOFERETAL1987B]. Instrument Section / Operating Mode Descriptions ================================================ The instrument sections and modes for operation during the Halley encounter period are shown below: Section Options Mode ---------------------------------------- Ground Equipment: Antenna 64-m N/A Receiver X-band Open-Loop Closed-Loop Spacecraft Equipment: Antenna High-Gain N/A Transponder Non-Coherent Transmitter X-band N/A The closed-loop and open-loop receivers were operated at their maximum sampling rates (10 Hz and 50 kHz, respectively). During the pre- and post-encounter periods the system was operated in the two-way coherent mode to obtain both Doppler and ranging data. Details on those operations are not available.