Removal of Early Biases

During the early months of the SMAP mission, the SMAP radiometer operated in two modes: high-rate (HR) data collection over and close to land and low-rate (LR) data collection over most of the open ocean. This was because, during the operation of the SMAP radar (until July 2015), it was necessary to reduce the data volume that was downlinked from the satellite.

Starting on 11-Aug-15, data have only been collected in HR mode. There is a bias between the HR and LR antenna temperature (TA) of about 0.25 Kelvin in all polarizations, which is not corrected in the Level-1B TA processing. This bias results in salty salinity biases near the continental shelf during the early months of the SMAP mission, when both HR and LR collection were done.

In the V6.0 release, this bias between HR and LR TA has been manually removed.

Look Direction-dependent Biases

Small SSS biases that depend on the SMAP look direction have been observed up to and including V5. They can be traced back to the reflected galaxy correction. For the V6 release, an adjusted correction for the reflected galaxy was derived by low-pass filtering the observed TA measured minus expected in the galactic right ascension system. The result shown in the figure below is an additional correction in the V6 salinity retrieval algorithm.

In V6, a small adjustment has been made to the reflected galaxy correction, which mitigates these look-angle-dependent biases.

Salty Biases at High Northern Latitudes

Salty biases with seasonal variability are observed at high northern latitudes up to and including V5. Independent of look angle, these biases can be traced back to the model for the physical temperature of the SMAP mesh antenna. In V6, a small adjustment – which is slightly different for vertical and horizontal polarizations – has been made to the physical reflector temperature model.

The effect of this V6 adjustment mitigates salty biases at high northern latitudes.

Sun-glint Flagging

Small sun glint is seen in V5 at higher sun glint angles and low wind speeds. This is likely caused by the reflected sun intruding into the antenna sidelobes. For V6, a geometrical procedure was developed to flag for that. For each look angle, an angle, gmax, is determined. Then, an elliptical shape is cut out in the (glint angle, wind speed) plane around gmax.

Lower Antenna Temperature Threshold for RFI

The V6 Level-1B L1B antenna temperature uses a lower threshold for RadioFrequency Interference (RFI) exclusion based on Piepmeier et al., 2023. (Note: Until the full back-processing of V6.0 is complete, the evaluation Version 5.3 can and should be used instead.)

Data Latency

The timing for V6 final products is as follows:

• Level-2C: 4-day latency.
• Level-3 8-day running average: 7-day latency (after the end of the averaging period).
• Level-3 monthly average: 7-day latency (after the end of the averaging period).

The latency for the NRT (near-real time) Level-2C files is about 5 hours.

References

Piepmeier J. et al., 2023. SMAP L1B SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 6. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center, doi: 10.5067/GWYQTF307Y9Y.