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Roberto Orosei

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Everything posted by Roberto Orosei

  1. Dear Andy, 1. As described in the SHARAD RDR SIS, MRO:RADARGRAM_RETURN_INTERVAL is the round trip time of an electromagnetic pulse from the center of Mars to the first sample of an echo in the data product. This time delay is expressed in terms of number of echo samples. Time distance between echo samples in RDR data products is 0.075 microseconds. Thus, MRO:RADARGRAM_RETURN_INTERVAL is an integer number of 0.075 microseconds quanta. MRO:RADARGRAM_RETURN_INTERVAL = 316344 means that the first sample of the first echo is located 316344 * 7.5e-8 * c / 2 = 3556.4 km away from the center of Mars. This information is provided to allow you to align the radargram to another reference distance if you wish. 2. A SHARAD echo in RDR files is 667 samples long, corresponding to a vertical distance of 667 * 7.5e-8 * c / 2 = 7498.6 m. This implies that if the distance between the highest and the lowest points of the Martian surface along the radargram is greater than 7498.6 m, it will not be possible to keep all echoes both starting at the same time distance from the center of Mars and fitting within the 667 samples length. The RANGE_SHIFT parameter has been introduced to solve this problem. As described in the SHARAD RDR SIS, RANGE_SHIFT is the relative shift of the echo with respect to the value of the MRO:RADARGRAM_RETURN_INTERVAL reported in the data product label. This time shift is expressed in terms of number of echo samples. Time distance between echo samples in RDR data products is, as said, 0.075 microseconds. RANGE_SHIFT is a 2-byte signed integer in little-endian byte ordering, and its absolute value should be of the order of a few hundred, as it should represent a vertical shift of a few km at most. 3. To compensate the offset and reestablish the true radargram, you need to shift echoes up and down relative to each other by an amount corresponding to RANGE_SHIFT. Positive values of RANGE_SHIFT should shift the echo up, that is away from Mars center. Please let me know if this works. Best Regards, Roberto
  2. Hi Jun. the effective spacecraft orientation in space has no effect on the real position of the radar footprint. This is because the antenna of SHARAD has very little directivity, and in effect irradiates in all directions. The real position of the radar footprint is thus the portion of the surface which is closest to the spacecraft, which could or could not be the same as SUB_SC_PLANETOCENTRIC_LATITUDE and SUB_SC_EAST_LONGITUDE. In general, the flatter (i.e. not sloping) and the smoother (i.e. not rough) the surface, the closer the radar footprint is to the computed spacecraft ground track. For sloping terrain (for examples, the flanks of Olympus Mons) the only way to know I can think of is to take the MOLA planetary radii dataset (not topography) and to compute the distance of all points in the vicinity of the ground track (up to a few tens of km, to be sure). The points which result the closest to the spacecraft are those from which the first echo recorded in SHARAD data originates. You can ths consider such area as the footprint of the radar. Hope this helps. Roberto
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