4.3 Main RA-2 Level 1b algorithms
Question 4.15 : How is
the time window delay calculated?
The window delay (in time) is the first measurement
of the range performed during the Level
1b RA2 processing. It is referred to the
antenna phase centre, and therefore, the
delays of the Tx and Rx paths, characterised during
testing on ground, are taken into account.
In fact, during the L1b processing, the on board Rx
distance (obtained by combining Rx
coarse and fine components, and
compensating for the PRI ambiguity rank and for the
on board tracking filter rate of change)
is corrected for instrument induced
errors using both ground (Tx and Rx paths) and
flight PTR calibration parameters. Finally, the
retrieved on board time delay
measurements are corrected for the Doppler
effect, before making it available in the L1b products.
These measurements are used at Level 2 to calculate
the final retracked range.
Question 4.16 : How is
the sigma0 scaling factor calculated?
The power scaling factor for sigma0 evaluation is
obtained at Level 1b from the radar
equation, taking into account the
- on board AGC, from the AGC coarse and fine
values, corrected for instrument
errors using both ground (Tx and
Rx paths) and flight PTR calibration parameters,
- antenna gain and overall gain of the receiving chain
- correction of illuminated area for Earth curvature
The scaling factor thus obtained, is used at Level 2
to compute, from the amplitude of the
retracked waveform, the backscattering
coefficient of the echo.
Question 4.17 : How is
the datation calculated?
The UTC datation is the exact time when the on-board
averaged waveform is reflected from the
surface, and hence, when the middle of
the 100 pulses averaged waveform is at the surface.
It's of vital importance to have a precise and
coherent measure of the time, since the
altimeter range is derived from the
measurement of the time between the transmission of
the pulse and its reception.
The datation is derived through correlation of the
RA-2 ICU clock counter OBDH datation
with the satellite on-board reference
binary time which is in turn correlated to a UTC
The following components are also taken into account
in the final L1b expression of the
- the location of the mid point of the averaged waveform,
- estimated time for the first received echo of
the current data block, since
the signal that is actually
triggering the request for the datation is the
pulse transmitted just before
the first returned echo
contained in the current data block
- the propagation time delay