(ATBD) SMAP Level 1 Radar Data Products - NASA

ithm (possibly simplified) in the radar L1C processor and compute the correction
from the radiometer telemetry data. This variation would add significant development
and testing effort to replicate major parts of the radiometer calibration and
RFI processing algorithms.
The second approach, which we will adopt for L1 radar processing, is to use
estimates of Faraday rotation derived from externally supplied measurements of the
ionosphere total electron content (TEC). These measurements are available from
several sources (including JPL) on a daily basis. They are based on measurements
of GPS signals which also operate at L-band. Since the GPS constellation is located
at much higher altitude than SMAP, it sees a more complete integrated TEC
than SMAP will see. Based on Aquarius experience, a correction factor of 0.75 is
applied to the GPS-based TEC values to account for the shorter atmospheric column
seen by SMAP. A numerical study performed by Oh-Ig Kwoun assesses the
impact expected from Farady rotation on SMAP data using long term models of
the ionsphere, and the expected performance of the Faraday rotation correction derived
from GPS-based measurements of TEC. The GPS-based measurements supply
an estimation of the Faraday rotation angle with an uncertainty (one-sigma) of
about 2 degrees. Propagating the effect of the Faraday rotation angle into the radar
backscatter measurements required a simulation of the planned SMAP orbit, and
a land surface backscatter model also used by the L2 retrieval algorithms. If we
assume no Faraday rotation correction, then 3% of the AM co-pol backscatter measurements,
and 2% of the AM cross-pol measurements will not meet requirements.
All of the PM measurements are expected to meet requirements. The PM data
show less effect from Faraday rotation because the latitude restriction eliminates
the most heavily impacted locations. If a Faraday rotation correction is applied using
the GPS-based measurements of TEC, then all measurements are expected to
meet the Faraday correction residual error budget.
These results were derived using GPS measurements from 2011 which are similar
to levels forecast for early in the SMAP mission. The forecast comes from
climatology based models that follow the 11 year solar cycle. These models also
can provide backup TEC estimates for those occasions when GPS-based measurements
are not available. Such interruptions are exepcted to be rare, and given the
low levels of error expected from Faraday rotation effects, will not have a significant
impact on data quality. As described later, the SMAP radiometer estimate of
Faraday rotation angle will be used to validate the GPS measurement and model
based approach.
The assessment study also showed that applying the Faraday rotation correction
can increase the errors in areas where the Faraday rotation effect is small due to the
uncertainties in the GPS-based measurements. For this reason, the Faraday rotation
correction will only be applied when the estimated rotation angle exceeds its own
uncertainty. A flag in the L1C product will indicate when the correction is applied.
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