(ATBD) SMAP Level 1 Radar Data Products - NASA

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9.1 Antenna Gain Pattern
Knowledge of the antenna gain pattern will be important to both active and passive
calibration. The SMAP antenna pattern will be computed using an accurate antenna
model. During early development of ground software, a sinc 2 pattern characteristic
of a uniformly illuminated aperture will be used for simulation and testing. The
beamwidth will be set to the nominal half-power beamwidth of 1.9 (TBC) degrees.
9.2 Receiver Gain and Noise Temperature
The radar receiver will produce its own internal noise which arises from thermal
fluctuations in the RF components. Most of the noise is produced in the frontend
where signals receive the highest amplification. The noise counts (V n ) can be
described in terms of an effective receiver noise temperature (T r ) and an overall
receiver gain (G)
V n = G(T r + T a ), (38)
where T a is normally the antenna temperature due to thermal emission from the
observed scene. Variations of G will occur as the physical temperature of various
system components vary. Matched load measurements are collected by SMAP
once per antenna revolution. Cold space measurements will be taken monthly for
radiometer calibration. Radar processing can take advantage of these measurements
to determine the receiver gain G when both matched load and cold space measurements
are taken together. The matched load measurement by itself can be monitored
to give some indication of gain and/or noise temperature drift.
The product of transmit power P t and receiver gain G are measured every PRI
by a loopback measurement which is reported with the low-resolution data in the
telemetry stream. These data will be averaged over a time span specified by a processing
configuration parameter. The averaging time will be selected to minimize
the statistical variance of the loopback measurement while still tracking variations
of transmit power and receiver gain. These variations are expected due to component
temperature variations that will occur as the spacecraft orbits the Earth. The
loopback measurement is then used to divide out the short term gain variability in
the affected data.
9.3 Statistical Measurement Uncertainty and Noise Subtraction
The radar equation relates the received signal power P s to the radar backscattering
cross-section (σ 0 ). The radar instrument, however, measures a combination of the
received signal power and internal noise power
P sn = P s + P n (39)