MRO Sharad Reduced Data Record of Radar Backscatter Power (USRDR and USRDR V2)

USRDR – Reduced Data Record of Radar backscatter power

PDS Data Set ID: MRO-M-SHARAD-5-RADARGRAM-V1.0, MRO-M-SHARAD-5-RADARGRAM-V2.0

For more information about SHARAD U.S. RDR products, see the Radargram Processing Document.

The Shallow Radar (SHARAD) Reduced Data Record of Radar backscatter power (USRDR) is a data product produced by the U.S. SHARAD team. The image product, the radargram, is a backscatter power presented with along-track distance in the horizontal dimension and round-trip time delay along the vertical axis.

The U.S. radargram processing uses a uniform amplitude model for the frequency components of the SHARAD linear frequency-modulated chirp signal. This leads to asymmetric offset of the characteristic transform sidelobes, with greater sidelobe amplitude on the downrange (greater delay) side of any echo. This approach preserves a two-fold oversampling (i.e., 3600 complex samples from 3600 real samples) of the signal after range compression, which allows for flexibility in later interpolation. Ionospheric distortion is compensated using a model for the frequency dependence of the phase errors. The resulting correction term is approximately linear with total electron content (TEC), and thus with the change in delay time for signals from the surface and subsurface. The image-restoring and range corrections are applied where the solar zenith angle (SZA) is less than 100°. A Hann window function is applied to the frequency-domain data prior to the inverse Fourier transform to reduce sidelobe levels in the range-compressed echo.

Two basic parameters define the processing of SHARAD data. The first is the coherence time or aperture length, T_C (in seconds). The frequency resolution (in Hz) of the resulting Doppler spectrum is 1/T_C, independent of the time spacing between the pulses that make up the synthetic aperture. The degree of pre-summing, which reduces the pulse repetition frequency (PRF) from the initial value of 700.28 pulses per second, affects only the Doppler frequency bandwidth (in Hz) of the Doppler spectrum, which is given by 1/PRF. The second parameter specifies the Doppler frequency bandwidth (in Hz), B, of echoes about the center of the Doppler spectrum to be included in the radar backscatter mapping. If this frequency width is less than 1/T_C, the output map will have only one sample for each output pixel location. This is a one-look radar image. If the frequency width is larger than 1/T_C, more than one Doppler resolution cell will be averaged, yielding a multi-look image.

U.S. Radargram Processing Parameters

Parameter
Chirp Envelope	Uniform with frequency
Ionospheric compensation	Empirically derived function
Aperture Length	8.77 seconds
Along-track Posting	128 ppd = 460 m
Delay Resolution	0.0375 μs
Number of Looks	7

Each 32-bit floating-point format radargram file is accompanied by a TIFF image that logarithmically scales the backscattered power over an 8-bit range corresponding to -3 dB to +32 dB with respect to the noise background (i.e., each DN step is about 0.137 dB). The noise-scaling factor was determined from the average behavior over the period between orbits 7500 and 32999. Tracks collected before about orbit 7200 have a slightly higher (about 1.4 dB) background noise, so their TIFF radargram products will appear slightly brighter due to the use of the lower scaling factor. A reduced-quality JPEG version of each TIFF is provided for browsing the archive.

USRDR V2 products are reprocessed SHARAD radargram data added to volume MROSH_2101.

SHARAD USRDR and USRDR V2 products have the following file names:

S_yyyyyyzz_RGRAM.IMG (with detached PDS label)

where:

S = Standard prefix for US-team SHARAD products

yyyyyy = Orbit number, prefixed by zeroes to allow for 6-digit values

zz = Observation number along the orbit track, prefixed by a zero for values less than 10

In ODE, USRDR and USRDR V2 products have the following Product IDs:

S_YYYYYYZZ_RGRAM