Electrical properties of the venus surface from bistatic radar observations

Science

Volumn 272, Issue 5268, 1996, Pages 1628-1631

  Pettengill, Gordon H ^a   Ford, Peter G ^a   Simpson, Richard A ^b  

^a MASSACHUSETTS INSTITUTE OF TECHNOLOGY   (United States)

^b STANFORD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

BISTATIC RADAR EXPERIMENT; ELECTRICAL PROPERTIES; MAGELLAN SPACECRAFT; PLANETARY SURFACE;

VENUS;

EID: 0030470375     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.272.5268.1628     Document Type: Article

References (26)

1. G. H. Pettengill, P. G. Ford, S. Nozette, Science 217, 640 (1982); P. G. Ford and G. H. Pettengill, ibid. 220, 1379 (1983); G. H. Pettengill, P. G. Ford, R. J. Wilt, J. Geophys. Res. 97, 13091 (1992).
2. G. H. Pettengill, P. G. Ford, S. Nozette, Science 217, 640 (1982); P. G. Ford and G. H. Pettengill, ibid. 220, 1379 (1983); G. H. Pettengill, P. G. Ford, R. J. Wilt, J. Geophys. Res. 97, 13091 (1992).
3. G. H. Pettengill, P. G. Ford, S. Nozette, Science 217, 640 (1982); P. G. Ford and G. H. Pettengill, ibid. 220, 1379 (1983); G. H. Pettengill, P. G. Ford, R. J. Wilt, J. Geophys. Res. 97, 13091 (1992).
4. G. H. Pettengill, P. G. Ford, B. D. Chapman, J. Geophys. Res. 93, 14881 (1988); G. L. Tyler et al., Science 252, 265 (1991); G. L. Tyler, R. A. Simpson, M. J. Maurer, E. Holmann, J. Geophys. Res. 97, 13115 (1992).
5. G. H. Pettengill, P. G. Ford, B. D. Chapman, J. Geophys. Res. 93, 14881 (1988); G. L. Tyler et al., Science 252, 265 (1991); G. L. Tyler, R. A. Simpson, M. J. Maurer, E. Holmann, J. Geophys. Res. 97, 13115 (1992).
6. G. H. Pettengill, P. G. Ford, B. D. Chapman, J. Geophys. Res. 93, 14881 (1988); G. L. Tyler et al., Science 252, 265 (1991); G. L. Tyler, R. A. Simpson, M. J. Maurer, E. Holmann, J. Geophys. Res. 97, 13115 (1992).
7. K. A. Tryka and D. O. Muhleman, J. Geophys. Res. 97, 13379 (1992); R. J. Wilt, thesis, Massachusetts Institute of Technology (1992).
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9. R. A. Simpson, IEEE Trans. Geosci. Remote Sensing 31, 465 (1993).
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11. J. A. Stratton, Electromagnetic Theory (McGraw-Hill, New York, 1941).
12. The transmitting waveform at both S and X band wavelengths was continuous (CW), with the echoes recorded simultaneously in a pair of orthogonal, circularly polarized coherent receivers at a digital sampling rate of 50,000 8-bit words/s, a rate that satisfied the Nyquist condition for the 25-kHz filtered bandwidth. The local oscillator of the receiver was adjusted to maintain the approximately 2-kHz-wide echo within the receiving bandwidth at all times. We analyzed the recorded data using a complex fast-Fourier transform and temporal averaging to recover the Stokes parameters (8) for each of 128 195.3-Hz-wide spectral components spanning the 25-kHz receiver bandwidth. Further convolution of these data incoherently in time and frequency improved the signal-to-noise ratio and allowed estimates of the position angle of the received plane of polarization (the polarization angle), as well as of the intensities of the linear and circularly polarized components of echo power, at 1-s intervals over the approximately 24-min duration of each pass. Data were also taken at times when the spacecraft antenna was directed toward Earth in a known orientation, to provide an attitude and intensity reference for receiver phase and amplitude calibration. The primary reference for recovering the absolute polarization angle was based on observations of the spacecraft's signal while its antenna pointed directly at Earth. Faraday rotation during these observations varied from about 3° to 1° (A. Coster, personal communication). Its variation over the observing interval (observed to be less than ±2° from the average over the 6-hour observing interval) has been neglected, except as a contribution to the measurement error; in any event, the variation could not be distinguished from possible drifts in the relative phases of the two DSN receivers. The circularly polarized receiver levels were set to yield equal values for the linearly polarized received signal during periods of direct pointing toward Earth; the absence of significant circular polarization in the echo (except over Maxwell Montes) is taken as evidence that this procedure was valid. We believe that small delay differences in the 15-dB receiver attenuators, present during direct downlink but removed during reception of the echo, may be responsible for a bias of a few degrees of electrical phase deduced from the polarization rotation observed at small angles of incidence. In any event, we have increased all of the "raw" values of polarization angle, as obtained from the primary reference, by an estimated 2.7° of bias, in order to bring the average value of the observed polarization angles for reflection from the "normal" surface at 13.7° incidence to its anticipated theoretical value of -43.5° (this value is quite insensitive to the precise value of the dielectric constant at this low incidence, and these data provide our secondary reference).
13. J. D. Kraus, Radio Astronomy (Cygnus-Quasar, Powell, OH, 1986), pp. 4-14.
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16. Handbook of Chemistry and Physics (Chemical Rubber Publishing, Cleveland, 1973), p. F-145; G. R. Olhoeft, in Physical Properties of Rocks, Y. S. Touloukian, W. R. Judd, R. F. Roy, Eds. (Hemisphere Publishing, New York, 1989), p. 257.
17. W. M. Becker and V. A. Johnson, in Chemistry of Selenium, Tellurium, and Polonium, K. W. Bagnall, Ed. (Elsevier, Amsterdam, 1966), chap. 3.
18. Handbook of Chemistry and Physics (Chemical Rubber Publishing, Cleveland, 1973), p. D-142.
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20. R. A. Brackett, B. A. Fegley Jr., R. E. Arvidson, ibid. 100, 1553 (1995).
21. The "skin depth" represents the distance, d, into the conducting surface at which the penetrating electromagnetic wave is reduced to 1/e of its external value; it is given by (6) d = √2/ωσ
22. J. S. Kargel and J. S. Lewis, Icarus 105, 6 (1993).
23. B. Hapke, ibid. 88, 407 (1990).
24. J. A. Wood, in Venus II, S. Bougher, D. Hunten, R. Phillips, Eds. (Univ. of Arizona Press, Tucson, in press).
25. M. K. Shepard, R. E. Arvidson, R. A. Brackett, B. A. Fegley Jr., Geophys. Res. Lett. 21, 469 (1994).
26. We thank the staff of the NASA DSN, as well as the dedicated engineers of the Jet Propulsion Laboratory (JPL) and Lockheed-Martin Aerospace, for their painstaking support in implementing this complex experiment. This work was supported under NASA grants NAGW-3787, NAGW-4351, and NAGW-3702 and JPL contracts 959490 and 957089.