SHARAD radargram data time offset

[Andy]

Dear all,

The RDR radargram data of such as SHARAD orbit 25019 (http://ode.rsl.wustl.edu/mars/indexproductpage.aspx?product_idgeo=19642615) has been obtained, but we don't know how to compensate the reative offset of ech echo. The common reference time for all echoes is described by keyword MRO:RADARGRAM_RETURN_INTERVAL    = 316344 in label file (http://ode.rsl.wustl.edu/mars/indexproductpage.aspx?product_idgeo=19642615) , the shift of  the first sample of a given echo from this reference time is reported as RANGE_SHIFT in rdr.fmt file (ftp://pds-geosciences.wustl.edu/mro/mro-m-sharad-4-rdr-v1/MROSH_1002/label/rdr.fmt). How to understand these two parameters and how to use them?

The questions are as follows:

1. What is meaning of  MRO:RADARGRAM_RETURN_INTERVAL= 316344, and what is format of 316344? integer? real?...

2. What is the meaning of the relative shift of the echo with respect to the value of the MRO:RADARGRAM_RETURN_INTERVAL in rdr.fmt?  For example, the reative shift of first echo is described as two bytes 171, 253 by unit8, which is read by unit16 as 64939.  then what is the relationship of the 316344  and 64939? what is the format of these two parameters or different?  whether the way that we read these paramters is wrong or not ? If wrong, we should how to read then and use them? 

3 what is the relationship of these time offset with 667 sampes with 0.075 microseconds sampling interval ? how to use them to compensate the offset and reestablish the true radargram?

 

Thank you vey much.

 

Best Regards,

Andy

[Roberto Orosei]

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