The Clementine bistatic radar experiment

Science

Volumn 274, Issue 5292, 1996, Pages 1495-1498

  Nozette, S ^a   Lichtenberg, C L ^b   Spudis, P ^c   Bonner, R ^d   Ort, W ^d   Malaret, E ^e   Robinson, M ^f   Shoemaker, E M ^f  

^a AIR FORCE RESEARCH LABORATORY   (United States)

^b NAVAL RESEARCH LABORATORY   (United States)

^c LUNAR AND PLANETARY INSTITUTE   (United States)

^d Protasis Inc   (United States)

^e Applied Coherent Technology   (United States)

^f U S GEOLOGICAL SURVEY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; GEOMETRY; ILLUMINATION; MEASUREMENT; MOON; PRIORITY JOURNAL; SUN; TELECOMMUNICATION;

EXTRATERRESTRIAL ENVIRONMENT; ICE; MOON; RADAR;

CITRUS RETICULATA;

EID: 0029806791     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.274.5292.1495     Document Type: Article

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33. Analysis was conducted using fast Fourier transform (FFT) techniques. The target area is isolated by Dopplo shift, which relates bands of constant frequency to a set of lunar ground locations (the β = 0 track) The ground points were close enough in distence to inelude all of the bands of constant frequency in the adopted area. Repeat responses were filtered out. The analyses to extract radar scattering information from local regions on the surface were performed by sorting the Doppler data according to the parameter of interest. Typical frequency domain transform parameters used were 1 to 4 seconds of noncoherent averaging. 4096 to 16,384 points per FFT, a von Hann time data window without zeropadding, and magnitude-only (power) data stored in double-precision output.
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35. The effective system noise temperature baseline was estimated by comparing measured noise from the zenith (20 K), moon center (235 K), and lunar poles (90 K). Measurements of ambient temperature microwave resistors added further corroborating measurements to the set of calibration data. The noise baseline, when considered against the recorded attenuator values, was used to calibrate the amplitude of the data files into units of signal-to-noise ratio (SNR). Calibration to flatten the frequency response variation arising from receiver filters was performed using noise-only segments of data. The calibrations included: linear gain change, nonlinear gain change, recording channel frequency response, system noise temperature changes, erroneous data bridge-over, transmitter frequency variation, transmitter power variation, and antenna pointing, A small amount of corrupted data is inevitably recorded. The short periods of corrupted data were flagged and suppressed during subsequent analyses. Spacecraft attitude files were corrected for known time-base and pointing systematic errors. One-way light-time propagation delay effects were included. Systematic errors that simultaneously affect the absolute baseline or bias measurement of each polarization channel were estimated to be lsss than ± 2 dB. Systematic errors in the ratio measurements are estimated at ± 0.25 dB. The systematic errors common to both channels are suppressed when considering the ratio. Thermal noise variation is negligible because several hundred to several thousand frequency bins were averaged together, each having a thermal SNR greater than 10. Target speckle variation is believed to be the dominant stochastic error source. The mean value and error bars given in Fig. 3 are derived by reducing the data set standard deviation by the square root of the number of noncoherently averaged samples represented by each point on the plot. The use of noncoherently averaged FFTs and numerous frequency bins reduced this variation to about ± 0.1-0.2 dB standard deviation. Median filtering was used. Due to the time sampling, regional averaging, and spacecraft system characteristics the resolution in β is ±0.2°. Due to Doppler bin migration (±1 bin), phase noise of the spacecraft oscillator (±2 bins), and FFT windowing effects (±2 bins), the Doppler band regions have an estimated rms resolution uncertainty of about ±25 km at -80° latitude, for the 16,384 point FFT data files.
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37. A standard analysis of variance (ANOVA) for unbalanced design was performed on the data from each orbit. ANOVA tests the null hypothesis that the means are the same and only appear different in measurement because of random fluctuations in the data. This analysis tests the statistical significance of the differences among the means. The data represented target return bins corresponding to angles of incidence greater than 82°.
38. The authors thank the NASA/JPL and Deep Space Network individuals who supported and helped carry out these observations, in particular S. Asmar; the Clementine lunar operations team led by T. Sorensen, assisted by R. Campion and T. Tran; P. Rustan of the U.S. Air Force, the Clementine 1 program manager, D. Duston of BMDO, and L. Wood of LLNL: and R. Simpson and G. Pettengill for review and insight. Funding for this work was provided by the Department of Defense, including the Ballistic Missile Defense Organization, the Naval Research Laboratory, the U.S. Air Force Phillips Laboratory Space Experiments Directorate, the Department of Energy, Lawrence Livermore National Laboratory, and NASA. This paper is Lunar and Planetary Institute contribution 899.