A relevant excerpt from the
Calibrated Data Record (CDR) Software Interface SpecificationPage 16:
Shift to time-zero (t=0) at antenna delay
Shifting the data such that “time-zero” (t=0 ns) corresponds to the antenna delay, physically
corresponding to the antenna feed point (Section 3.3.2), is most easily and precisely done in the
Fourier domain. The antenna delay is in ns, and the shift is performed by multiplying the
frequency domain sounding data with the frequency vector in radians.
The position of the Mars ground surface in the Calibrated sounding data will be directly related
to the distance of the antenna above the surface that it is “seeing”. While rover attitude and
surface topography will cause some variability from sounding to sounding, this distance should
remain relatively constant at ~74.4 cm over the mission, which would nominally translate to a
~5.0 ns two-way travel time delay from the start of a calibrated sounding (the first data-sample
of which starts at time = 0 ns in all CDR table records). This value should assist users constrain
the location of the surface return in the CDR sounding data.
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I think that means that the time 0ns (the top of the image) should correspond in all cases to the antenna feed point, 74.4cm above ground level.
Another excerpt could explain the blanking seen in Shallow and Deep mode:
Page 11:
2.6.2 Surface Operation
RIMFAX is designed to operate in different modes in which radar parameters are set to optimize
data collection for different subsurface conditions and depths. The RIMFAX gating makes it
possible to omit the recording of close-range reflections, typically from the antenna and surface,
which would otherwise limit the dynamic range. The removal of these reflections makes it
possible, when desired, to increase the radar’s gain to capture weak subsurface reflections.
Shifting the receiver dynamic range window particularly to each mode effectively increases the
radar’s total dynamic range when soundings from different modes are considered together.
1. Surface Mode
The antenna reflection is captured in the receiver window.
Measures the surface reflection and the very upper subsurface only.
2. Shallow Mode
The antenna reflection is removed from the receiver window.
Measures the surface reflection and the shallow subsurface.
3. Deep Mode
The antenna and surface reflections are removed from the receiver window.
Measures reflections from the upper subsurface (~1 m depth) through the instrumented
range.
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So Shallow and Deep mode have strong close-range reflections removed, to increase dynamic range. This makes me suspect that the first strong reflections seen in Surface mode are from the ground level.