8 October 1993 To: Gordon Pettengill From: Dick Simpson Subject: Bistatic "Spotlight" Observations of Gula Mons Using Magellan (Note from archivist: References to figures and attachments have been modified to reflect file structure in archive. File 93279FGn.EPS is Figure n in encapsulated PostScript format, file 93279_A1.ASC is ASCII Attachment A, and file 93279_Xn.EPS is page n of Attachment X in encapsulated PostScript format. For each EPS file, there is a corresponding PostScript Version 1 file with suffix *.PS). Three bistatic observing sessions were carried out on 6 October, using the Magellan spacecraft as a continuous-wave microwave signal source and stations of the DSN as receivers, spatially separated from the signal source (93279FG1.EPS). During an approximately 8-minute segment within each of three Magellan orbits (9331, 9335, and 9336), the spacecraft high gain antenna (HGA) was pointed toward the summit of Gula Mons on Venus and the scattered signal was collected at both S- and X- Band and in both right- and left-circular polarization (RCP and LCP, respectively) by 70-meter ground antennas. The objective of the experiment was to determine the variation in microwave scattering efficiency as a function of the bistatic angle beta near the backscatter direction (beta is the included angle between ks and ki in 93279FG2.EPS). Gula Mons was selected because of its high backscatter (and relatively low emissivity) based on Magellan monostatic and other observations. These orbits were chosen because they provided a range of bistatic angles which included beta~0. Certain models predict an enhancement in the backscatter direction from symmetry and coherence; the angular width dbeta of the enhancement lobe may be related to the scale over which the scattering takes place. Planning Spacecraft sequences, including HGA pointing, were developed by the Spacecraft Team (SCT) at Martin Marietta's Astronautics Group in Denver. Doppler offsets for the carom path were also calculated by the SCT and independently at JPL and Stanford (e.g., 93279_A1.ASC). These indicated that the reflected signal would be about 20 kHz higher than the directly propagating signal at the time of minimum beta. The differential frequency as a function of time was added to the DSN frequency predictions for normal one-way spacecraft tracking; the modified file was used to drive the Programmable Oscillator Control Assembly (POCA) of the DSN Open Loop Receivers (OLRs). Predicted timing in the final Magellan navigation files differed from timing in the approved sequences by only 3 seconds; no last-minute updates were carried out. To maximize strength of the illuminating signal, Magellan telemetry was turned off prior to the antenna maneuver. Earlier calculations at MIT (and later at Stanford) predicted a "best case" detectability of 5-10 dB, meaning that the reflected signal would likely be indistinguishable from noise in real time. In fact, no scattered signal was observed in real time. A primary concern during experiment planning was carrier suppression resulting from a modulation spur seen on the X-Band downlink for many months. Sal Abbate was present at SPC 10 during the first set of observations and obtained spectra of the X-Band downlink signal prior to the HGA maneuver (93279FG3.EPS and 93279FG4.EPS). If the spur were present, less power would be available in the carrier to illuminate the target and the probability of an echo detection at X-Band would be reduced accordingly. No spur was identified, indicating that the spur had moved more than 10 MHz away from the carrier and/or that its strength was reduced to a negligible level. Spectra obtained prior to the second and third observing sessions (93279FG5.EPS and 93279FG6.EPS) showed a similar signal level but did not extend as far from the carrier in frequency. Preliminary estimates of transmitted power and spacecraft antenna gain provided by Bill Adams of the SCT are (93279_B1.EPS): S-Band 36.22 dBm (4.188+/-0.025 w) 35.9 dBi (HGA gain) X-Band 42.38 dBm (17.298+/-0.100 w) 48.3 dBi (HGA gain) Configurations Spacecraft: Spacecraft configuration is mentioned above. Specifically it included: HGA selected Telemetry OFF Frequency Reference: AUX OSC (one-way) HGA pointed toward Gula Mons Ground: The first observing session was conducted using the 70-m antenna at Goldstone (DSS 14). The second and third sessions used the Canberra antenna (DSS 43). Each station was configured as follows (see 93279_C1.EPS through 93279_C3.EPS) for the experiment: RF: Listen-Only Mode (diplexer out) CONSCAN: OFF DSP: Filter Numbers 6/6/6/6 20 kHz each channel Filter Offset -37500 Hz NBOC Mode 1 4 separate channels Sample Rate 50000 samples/sec/channel IVC Switch PRIME Quantization 8 bits Tape Density 6250 bpi Channel Assignments: A=1 X-RCP B=2 S-RCP C=3 X-LCP D=4 S-LCP Nominal SSI Input B Attenuators were set during pre-cal for 1 volt rms on each OLR channel. This led to saturation on X-RCP prior to the antenna maneuver, but it appeared to be a reasonable setting during the data collection period. Tape drives were set so that automatic switching from tape unit 1 to tape unit 2 would occur after (approximately) 8 minutes of data collection. Timing was such that the first tape would contain scattered signal from Gula Mons; the second tape would contain 8 minutes of noise (baseline) for later comparison with echo spectra. System noise temperature measurements were requested for each of the four data channels both before and after the experiment. Only RCP values were obtained from Goldstone; all four channels were reported by Canberra. Operations Local network changes during the preceding 36 hours made the Multi-Mission Radio Science Support Area at JPL useless for monitoring the Magellan experiments; no connection to the DSN data system lasting more than a few minutes could be maintained. Instead, most of the Radio Science monitoring was done using the NOPE facility within the JPL SFOC "Dark Room" area. Goldstone: The Goldstone configuration was confirmed. Predict set was LF7. Attenuator settings reported via displays were Channel Attenuator (dB) ---------- ------------------ 1 37 2 37 3 43* 4 37 * This reading was 37 dB near the start of recording (13:24:04). It is not clear whether it was changed before or after recording began and, if the latter, when. By 13:28 it was 43 dB and remained at that position for the duration of recording. Post-pass system noise temperatures reported were (93279_D1.EPS) Channel Post-Pass SNT ---------- ------------------- X-RCP (XRO1 maser) 19.9 S-RCP (SP maser) 18.8 Time line for the Goldstone observations is given in 93279_E1.EPS. Signal and noise readings reported in displays are below: S-Band X-Band ------------------- -------------------- UTC Elev AGC PPM SNT AGC PPM SNT COMMENTS (deg) (dBm) (dBm) (deg) (dBm) (dBm) (deg) -------- ------ ------ ------ ----- ------ ------ ------ ------------ 13:05:00 15.517 -141.7 -172.9 15.24 -142.3 ------ ------ TLM ON 13:14:00 ------ -134.0 -172.9 15.24 -142.3 ------ ------ TLM OFF 13:20:00 ------ -163.3 -172.9 15.24 ------ ------ ------ HGA maneuver On Abbate's recommendation the DSP was set to IDLE at 13:32:39. The concern was that the second tape unit, which had been unreliable in previous tests, would fail and bring down the entire data acquisition system. A new tape was loaded on unit 1, and recording of the noise data resumed, possibly as early as 13:37:48 (see 93279_F1.EPS). The Controller's Log indicates that DSP recording was terminated at 13:43:33 (93279_G1.EPS) The carrier was visible in the SSI display at 13:40:40. DSP status information was interrupted at about 13:32 and then terminated at 13:46:25 when controllers reported that it was "knocking other data out of the system." Canberra: The Canberra configuration was confirmed (S-RCP through diplexer, S-LCP listen-only, both X listen-only; CONSCAN OFF). Predict set was KI7. Attenuator settings reported via displays (19:36:44 UT) were Channel Attenuator (dB) ---------- ------------------ 1 38 2 34 Changed to 39 for third session (reported at 21:17:30) 3 34 4 36 Pre- and post-pass system noise temperatures reported were Channel Pre-Pass SNT Post-Pass SNT ---------- ------------------ ------------------- X-RCP 20.4 19.7 S-RCP 19.4 19.1 X-LCP 22.3 22.97 S-LCP 20.0 20.24 Time line for the Canberra observations is given in 93279_H1.EPS. Signal and noise readings reported in displays are below: S-Band X-Band ------------------- -------------------- UTC Elev AGC PPM SNT AGC PPM SNT COMMENTS (deg) (dBm) (dBm) (deg) (dBm) (dBm) (deg) -------- ------ ------ ------ ----- ------ ------ ----- ------------ 19:30:20 ------ -142.8 -153.7 35.95 -131.5 -153.7 37.09 TLM ON 19:32:42 ------ -135.5 -153.7 35.21 -130.9 -153.7 ----- TLM OFF 19:34:30 ------ -135.5 -153.7 34.85 -131.1 -153.7 ----- TLM OFF 19:36:20 ------ -135.4 -153.7 34.85 -131.2 -153.7 ----- TLM OFF 19:38:00 ------ -157.6 -153.7 33.46 ------ -153.7 ----- HGA maneuver 21:05:00 ------ -142.7 -153.7 32.23 -132.1 -153.7 ----- TLM ON 21:07:20 ------ -135.2 -153.7 32.23 -131.2 -153.7 ----- TLM OFF 21:09:15 ------ -135.3 -153.7 32.23 -131.3 -153.7 ----- TLM OFF 21:10:55 ------ -135.2 -153.3 32.23 -131.3 -153.7 ----- TLM OFF 21:12:40 ------ ------ -153.7 32.23 ------ -153.7 ----- HGA maneuver 21:35:45 ------ -142.6 -153.7 32.23 -131.3 -153.7 ----- Earth point TLM ON In both sessions the second (noise) tape ran to completion; those data appear to be contiguous with the data on the first tape. SSI was configured for input from Channel C throughout. Carrier was visible briefly at about 19:59:13 and returned at 19:59:45 completing the second observing session. Carrier was visible at 21:33:55 and 21:34:20 in the third observing session. Data for Analysis The following data will be needed (or would have been desirable) for analysis of this experiment. In some cases (*) the data type appears to be unavailable for the 6 October observations. ODR Tapes from DSP 13:24:04-13:32:04 13:37:48-13:43:33 (times approximate) 19:42:41-19:58:41 (2 tapes) 21:17:21-21:33:21 (2 tapes) ATDF (preferably from OLR antenna, but any concurrent tracking data would be valuable) MON (preferably from OLR antenna, but any concurrent tracking data would be valuable) SPK File (NAIF/NAV trajectory) BQPC (quaternions) SCLK/SCET Conversion (needed only for time of experiment) Transmit Power Estimate (most easily obtained from SCT, as opposed to recovering from engineering data stream and performance models) *SOE (SOE equivalents with BSR events only are available for 6 October) Track Controller's Log Assessment and Recommendations From outward appearances, the essentials of this experiment appear to have been executed well. Areas of possible improvement include: 1) Obtain better understanding of the operation of the IF spectrum analyzer and the characteristics of the Magellan spur. Was the HP analyzer used properly to search for and characterize the spur (e.g., at Canberra)? Did we miss the spur? 2) Include recording of direct no-telemetry signal. This could have been used independently to estimate the S/X-band radiated power and determine the phasing between RCP and LCP channels on the ground. Recording the direct signal may require changing the attenuator settings during the HGA maneuver. It also has repercussions in scheduling the OLR recording times at SPC 10 in light of the tape drive reliability issues. 3) Provide more detailed and more accurate SOE (or SOE-like material) to the stations prior to AOS; there was some confusion about set- up. 4) Ensure that SNT is obtained on all four channels both pre- and post-pass. 5) Use closed-loop system in parallel with open loop receivers to obtain low rate monitor data for amplitude and timing checks. 6) Resolve apparent discrepancies between predicted levels and AGC, PPM, and SNT values in JPL displays ACRONYMS DSN Deep Space Network DSP DSSC Spectrum Processor DSS Deep Space Station DSSC Deep Space Communications Complex HGA High Gain Antenna IF Intermediate Frequency IVC IF Selection Switch JPL Jet Propulsion Laboratory LCP Left Circular Polarization MIT Massachusetts Institute of Technology OLR Open Loop Receiver POCA Programmable Oscillator Control Assembly RCP Right Circular Polarization SCT Spacecraft Team SPC Signal Processing Center Distribution Bill Adams Astronautics Group Martin Marietta Corporation P.O. Box 179 Denver, CA 80201 Sami Asmar MS 161-260 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Jim Cavender Astronautics Group Martin Marietta Corporation P.O. Box 179 Denver, CA 80201 Dwayne Chong MS 161-241 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Gina Gonzalez MS 230-103 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Tony Horton MS 230-103 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Rob Lock MS 230-260 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Dan Lyons MS 230-260 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 David Okerson SAIC 400 Virginia Ave., S.W. -- Suite 810 Washington, DC 20024 Gordon Pettengill 37-641 Massachusetts Inst. of Technology Cambridge, MA 02139 Len Tyler Stanford