11 November 1993 To: Gordon Pettengill From: Dick Simpson Subject: Bistatic Quasi-Specular Observations - 9 Nov 1993 (Note 1 from archivist: References to figures and attachments have been modified to reflect file structure in archive. File 93313FGn.EPS is Figure n in encapsulated PostScript format, file 93313_Xn.ASC is page n of ASCII Attachment X, and file 93313_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). (Note 2 from archivist: References to detected and observed X-band signals are incorrect. Real-time observers were confused by labeling of the SSI displays. Later processing showed only S-band signals present). (Note 3 from archivist: 7 of the 22 tapes eventually received from the DSN were seriously degraded; but good data appears to be present on the others). Three bistatic observing sessions were carried out on 9 November, using the Magellan spacecraft as a continuous-wave microwave signal source and the Madrid 70-meter station of the DSN as the receiver. During an approximately 40-minute segment within each of the orbits 9846-9848, the spacecraft high gain antenna (HGA) was aimed at the (moving) specular point on Venus' surface. The reflected signal was collected at both S- and X-Band and in both right- and left-circular polarizations (RCP and LCP, respectively) by the 70-meter ground antenna. The objective of the experiment was to determine the variation in microwave scattering as a function of the location of the specular point on the surface. The regions probed were on a curved path that started near (27S, 67E), crossed Venus' equator at 67E longitude, extended to (76N, 167E) and ended near (57N, 210E). Angle of incidence (without accounting for refraction effects) was minimum (PHI=75.5 deg) at (41N, 73E) and increased to about PHI=78 deg at both ends of the specular track. Preliminary calculations indicate that refraction will reduce the incidence angle at the surface by about 2.5 in Hestia Rupes, which is near the minimum PHI and about 2 km above the mean 6051 km surface. These orbits were chosen because they provided a range of surface types and because 70-m coverage could be scheduled. The 70-m pass had originally been awarded to the Galileo Project for collection of Galileo solar conjunction radio science data; Richard Woo and Galileo kindly relinquished the pass so this experiment could be carried out. 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 Stanford (e.g., 93313_A1.ASC and 93313_A2.EPS through 93313_A8.EPS). These indicated that the reflected signal would never be more than about 1 kHz from the frequency of the directly propagating signal while the expected echo width would be about 2 kHz,. Given that the receiver passband is about 20 kHz and the variation in the on-board oscillator frequency resulting from thermal changes is about 1.5 kHz, it was decided to use the DSN one-way frequency prediction file for receiver tuning without modification. To maximize strength of the illuminating signal, Magellan telemetry was turned off prior to the antenna maneuver. Two minutes of open loop data collection at the beginning of each experimental sequence were scheduled to observe the directly propagating signal in both amplitude and frequency without telemetry. These provide estimates of the signal amplitude and frequency in the region illuminated on Venus' surface and of the relative phasing between S-RCP and S-LCP systems at the ground station. A primary concern during experiment planning for both these observations and the observations on 6 October was carrier suppression resulting from a modulation spur seen on the X-Band downlink for many months. 20 MHz wide spectrum analyzer plots of the X-Band carrier and spur were scheduled during the two minutes of direct signal with no telemetry. Configurations Spacecraft: Spacecraft configuration is mentioned above. Specifically it included: HGA selected Telemetry OFF Frequency Reference: AUX OSC (one-way) HGA pointed toward the specular point Ground: All three observations were conducted using the 70-m antenna at Madrid (DSS 63). The station was configured as follows 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 System noise temperature measurements were requested for each of the four data channels during pre-cal in the "low noise" configuration. In fact, only "X" and "S" temperatures were reported and it is not clear whether those were obtained in the experimental configuration. A request to repeat the measurements during the post-cal yielded four numbers; but the station had reconfigured to do an uplink at the end of the pass and it is quite possible that these were also not obtained in the "low noise" configuration. Signal Maser Pre-Pass SNT (K) Post-Past SNT (K) ------ ----- ---------------- ----------------- S ? 19.5 X ? 22.4 S-RCP SPD 18.7 S-LCP MOD-3 32.0 X-RCP XRO-1 24.0 X-LCP XRO-2 27.0 Attenuators were to be set during pre-cal for 1 volt rms on each OLR channel. 15 dB additional attenuation was to be added when the carrier was present to prevent saturation of the receivers (particularly X-LCP). There was some confusion on this point. We believe proper levels were finally set during a star cal shortly before the first experimental period. Levels were reset during another star cal between the first and second sessions as part of a station reconfiguration implemented in an attempt to reduce NBOC errors and provide station operators with a wider selection of displays. Nominal, and apparently successful, attenuator settings are shown in the table below for both the strong (carrier or ambient load) and weak (specular scattering) input signal conditions. No spurious signals were seen in the SSI display, which is sometimes an indication of inadequate attenuation. Channel Strong Signal Weak Signal Attenuation (dB) Attenuation (dB) ------- ---------------- ---------------- 1 54 39 2 52 37 3 54 39 4 39 24 We requested that the masers be connected to the ambient load for one minute during the spacecraft turn from Earth-point to Venus so that independent determinations of OLR system temperature on each channel could be made at a later time directly from recorded data. The 15 dB additional attenuation also covered this period. Switching to the ambient load may not be a difficult process, but our intentions apparently did not match the expectations of station operators. The desired procedure needs to be considered and explained more carefully. Tape drives were set so that automatic toggling between tapes units 1 and 2 would occur after every 8 minutes of data collection. There were no real-time problems that could be attributed to tape errors. Operations The Multi-Mission Radio Science Support Area at JPL was not used for monitoring the Magellan experiments. Partially this was because it was still in use, when we arrived, for Galileo observations from the previous day. There also appeared to be problems in bringing it up for Magellan after the Galileo work was cleared. Finally, no SSI displays were available from the Radio Science system. Instead, most of the real-time monitoring was done using the NOPE facility within the JPL SFOC "Dark Room" area; Angela Chen provided assistance. The NOPE displays are excellent and the hard copy capability provides good documentation of the operation. Time line for the observations is given in 93313_B1.ASC." 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 (perhaps as many as 3 dozen) ATDF MON 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) Track Controller's Log Plots from spectrum analyzer Assessment and Preliminary Conclusions Echoes were observed frequently during the course of each observing session. Although detailed correlation is impossible without access to the digital data, it appeared that echo signal behavior was repeated from one orbit to the next. This was expected because the HGA footprint on the surface is large compared with the orbit-to-orbit displacement of the specular track in longitude. Although the X-Band transmitted power is larger and the X-Band HGA gain is larger, I expected the S-Band signal to be more readily observed because those signals are less affected by atmospheric absorption and refraction at the high incidence angles f employed during this experiment. S-Band should also be less sensitive to uncertainties in the HGA pointing. Between X-RCP and X-LCP, I would have expected stronger echoes in the latter because the Fresnel reflection coefficient is higher for same sense circular polarization at 75 deg < PHI <79 deg. In fact, it was the X-RCP echo that was detected first using the SSI display (93313_C1.EPS). We could only observe one SSI channel at a time, so it is quite possible that the other three channels also showed good echoes at the same time. A sequence of left circular echoes is shown in 93313_D1.EPS; in the 36 sec between the lower and middle spectra, the character of the echo changed considerably. This is consistent with our understanding of Venus' surface from earlier Magellan altimetry and SAR data. An unusually narrow S-Band echo is shown in 93313_E1.EPS, while 93313_F1.EPS shows that variability of the S-Band signal is similar to that observed at X-Band (compare 93313_D1.EPS). Echoes were readily observed on all four channels, in some cases standing well above the noise baseline. These are likely to be the easiest to analyze in the near term because they come from very smooth regions where the intrinsic bandwidth of the scattered signal is less than the bandwidth corresponding to the HGA footprint. 93313_G1.EPS is an enlargement of the spectrum shown in 93313_E1.EPS. Here with ruler and pencil we have estimated the width of the echo at a point 3 dB below its peak to be 493 Hz, which corresponds to an RMS surface slope of about 0.5 deg if we scale from model calculations (93313_A1.ASC). Assuming that spacecraft activity was centered on the point where the incidence angle was minimum and assuming this minimum was at 41N, this narrow spectrum may be associated with Venus coordinates (25N, 69E). This point is within a region bounded by Hestia Rupes on the south, Tellus Regio on the northeast, and Bell Regio on the northwest. In our roughness map compiled from inversions of Magellan altimetry echoes [Tyler et al., JGR, 97, 13115-13139, 1992], this region is smoother than Venus' average with some subareas showing rms slopes as low as 1 deg. Recommendations From outward appearances, the essentials of this experiment appear to have been executed well. Areas of possible improvement include: 1) No instructions for disposition of the spectrum analyzer plots were given to the station. I only realized this omission much later. Plots should be sent to JPL by FAX as soon as they are produced. On the other hand, if the data and calibrations recorded on the OLR tapes are accurate, it may be possible to estimate carrier power directly. 2) Our requirement for system temperature measurements (1) on all four channels, (2) in the experimental configuration, and (3) during both pre- and post-cal were never made completely clear to the station operators. The complete requirement needs to be refined so that there are no misunderstandings. 3) Although there were no complaints from the station, the proliferation of late messages to the station from various sources must have been confusing. The SOE (or its equivalent) needs to be developed earlier so that a complete, understandable script is available to the station operators. This can be facilitated, and coordination of distribution of the results made more effective, by scheduling a conference call a week in advance of any future experiment to include: a representative of the investigators (Simpson), a representative of Multi-Mission Radio Science (Gonzalez), a representative from Mission Planning (Lock), Dave Dooty, and the NOPE (Isaacs). 4) We did not understand the current procedure for switching between ambient load and sky. The operators attempted to perform this activity as they have in the past. At best, only a few of these measurements are likely to be valid. 5) The NOPE displays proved very useful both in monitoring progress of the experiment and in providing hard copy documentation for later study. If possible, it would be advantageous to schedule these for future experiments. 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 LMC Link Monitor and Control MIT Massachusetts Institute of Technology NAV Navigation (Team) NBOC Narrow-Band Occultation Converter 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 Gina Gonzalez 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 Richard Woo MS 238-737 Jet Propulsion Laboratory 4800 Oak Grove Drive Pasadena, CA 91109 Len Tyler Stanford