Instrument Information |
|
IDENTIFIER | urn:nasa:pds:context:instrument:lidar.clem1::1.0 |
NAME |
LASER RANGEFINDER |
TYPE |
ALTIMETER |
DESCRIPTION |
Instrument Overview =================== Laser Rangefinder (LIDAR): The LIDAR unit shares the telescope of the HiRes camera, splitting the 1064 nm return signal from the NdYag source off to an avalanche photodiode (APD) detector with dichroic filter. The optics are non-imaging, providing an exit pupil through simple relay optics at the APD. The APD electronics include a temperature compensation feature for the APD bias voltage and programmable thresholding of the output signal. The APD current is amplified and inverted to a voltage by a transimpedance amplifier with a gain of 230X, a low frequency cutoff of 3 MHz, and a high frequency cutoff of 23 MHz. The voltage derived from the APD current is amplified, then discriminated for changes (increase) through a 14 MHz discriminator. Voltage changes exceeding the programmed threshold are flagged as returns [NOZETTEETAL1994]. The LIDAR instrument measured the slant range from the spacecraft to the lunar surface at spacecraft altitudes of 640 km or less. These measurements were used to determine the global lunar topography field. Scientific Observations ======================= Throughout the Lunar Mapping phase of the mission, the LIDAR system acquired high resolution profiles of lunar topography. Over those parts of each revolution where radio tracking of the spacecraft was possible, variations in the gravity field of the Moon could be measured. The combination of topography profiles and gravity maps places important constraints on the interior structure of the Moon. The orbital topography data were gridded and used to produce a spherical harmonic topography model (GLTM- 2B). This topographic model represents the first reliable global characterization of surface heights for the Moon [ZUBERETAL1994]. Calibration =========== N/A Operational Considerations ========================== Below 640 km, the instrument had adequate sensitivity to obtain range measurements from both mare and highland surfaces. While the ranging success rate for all measurements was ~20%, many more successful returns were obtained from the smooth mare surfaces than the rougher highlands. However, the distribution of points allowed a qualitative physical description of the shape of the Moon. Detectors ========= The LIDAR uses a avalanche photodiode (APD) detector with a dichroic filter. Electronics =========== The APD electronics includes a temperature compensation feature for the APD bias voltage and programmable thresholding of the output signal. Filters ======= There were no instrument filters. Optics ====== The optics are non-imaging, providing an exit pupil through simple relay optics at the APD. Operational Modes ================= The Clementine LIDAR operated in two distinct modes, 'ON' and 'OFF'. There was also the ability to range at 8Hz, and this was done several times the week before the spacecraft left the Moon. Subsystems ========== N/A Measured Parameters =================== The LIDAR instrument measured the slant range from the spacecraft to the lunar surface at spacecraft altitudes of 640 km or less. Clock cycles were returned which were then converted to range by a multiplicative factor. Basically, the range value was determined by the number of clock cycles between the laser start pulse and the received signal. These cycles were time-tagged, and from the time tag it was possible to reconstruct the latitude and longitude position on the surface by knowing the spacecraft orbit and orientation. The instrument collected data for approximately one-half hour per 5-hour orbit during the 2-month lunar mapping mission [ZUBERETAL1994]. The clock counter has only 14 bits owing to hardware availability limitations. In order to allow returns up to the 640 km maximum range required in the lunar mission, returns are binned four to a clock count, turning the 15 MHz response into a 39.972 meter height bin. Internal memory in the LIDAR unit saves up to six 'returns' per laser firing, with up to four saved in the programmable search range. The programmable search range simply refers to the range of possible detection thresholds, defined as a fixed number of set 10 mV voltage levels which had to be exceeded [NOZETTEETAL1994]. The detection threshold was never formally 'confirmed'. It was selected on the basis of: 1) a link analysis, which is a statistical analysis which looks at how to maximize the probability of getting a valid hit while minimizing the probability of triggering the detector due to solar background or electronic noise; and 2) experience on how the instrument was operating in orbit. Clementine mission scientists or the LLNL engineers manually uploaded the threshold values every day, and the values were dependent on: spacecraft orbital height, surface albedo (maria or highlands), sun angle (sunlit or dark), and instrument gain. During the course of the mission the LIDAR typically ranged at a rate of 1 shot per 1.6 seconds and triggered on about 123,000 shots, corresponding to 19% of the transmitted laser pulses. Typical along-track shot spacings were on the order of 20 km, but this varied considerably [ZUBERETAL1994]. |
MODEL IDENTIFIER | |
NAIF INSTRUMENT IDENTIFIER |
not applicable |
SERIAL NUMBER |
not applicable |
REFERENCES |
Nozette, S., P. Rustan, L.P. Pleasance, D.M. Horan, P. Regeon, E.M.
Shoemaker, P.D. Spudis, C.H. Acton, D.N. Baker, J.E. Blamont, B.J.
Buratti, M.P. Corson, M.E. Davies, T.C. Duxbury, E.M. Eliason, B.M.
Jakosky, J.F. Kordas, I.T. Lewis, C.L. Lichtenberg, P.G. Lucey, E.
Malaret, M.A. Massie, J.H. Resnick, C.J. Rollins, H.S. Park, A.S. McEwen,
R.E. Priest, C.M. Pieters, R.A. Reisse, M.S. Robinson, D.E. Smith, T.C.
Sorenson, R.W. Vorder Breugge, and M.T. Zuber, The Clementine Mission to
the Moon: Scientific Overview, Science, 266, 1835-1839, 1994. Zuber, M., D.E. Smith, F.G. Lemoine, and G. Neumann, The Shape and Internal Structure of the Moon from the Clementine Mission, Science, Vol. 266, pp. 1839-1843, 1994. |