Vicarious Calibration of an Ocean Salinity Radiometer from Low Earth Orbit

Abstract

Statistical properties of the brightness temperature (T B) measured by a low-earth-orbiting radiometer operating at 1.4 GHz are considered as a means of calibrating and validating the sensor. Mapping of ocean salinity by such an instrument requires that its calibration be extremely stable over time. Whether certain statistical properties of the measurements are stationary (time invariant) enough to be of value as benchmarks to which the calibration can be referenced is considered. The global minimum, maximum, and average T B are considered, together with a vicarious cold T B statistic that makes use of a sharp lower bound on naturally occurring values for T B. Examination of simulated global distributions of the T B measurements suggests several things about the stationarity (or lack thereof) of the statistics in question. Global minima can vary widely due to instrument noise and are not a reliable calibration reference. Global maxima are strongly influenced by a number of environmental factors as well as by instrument noise and are even less stationary than the minima. Global averages are largely insensitive to instrument noise and, in most cases, to environmental conditions as well. The global average T B varies at only the 0.1-K rms level except in cases of anomalously high winds, when it can increase considerably more. The vicarious cold T B is similarly insensitive to instrument effects and most environmental factors. It is not significantly affected by high wind conditions. The stability of the vicarious cold T B is, however, found to be sensitive at the several tenths of a kelvin level to variations in the background cold space brightness. The global average is much less sensitive to this parameter, so using the two approaches together should be mutually beneficial.

Sea Ice and Atmospheric Parameter Retrieval From Satellite Microwave Radiometers: Synergy of AMSR2 and SMOS Compared With the CIMR Candidate Mission

“…The total polarized brightness temperature observable by an instrument in orbit is then according to Ulaby et al () with p the polarization vertical (v) or horizontal (h), SST the sea surface temperature in Kelvin, and, specific to L‐band radiation, is the background cosmic radiation which is given in Ruf () as 6 K.…”

“…with p the polarization vertical (v) or horizontal (h), SST the sea surface temperature in Kelvin, and, specific to L-band radiation, T c is the background cosmic radiation which is given in Ruf (2003) as 6 K. For simulating L-band measurements over sea ice covered areas the same implementation of mixed surface types as for the AMSR-E implementation of the forward model is used. Thus, any pixel is considered to contain three different surface types, open water for which the upward contribution is calculated as detailed above, and first year and multi-year ice respectively for the sea ice cover.…”

Microwave Radiometry at Frequencies From 500 to 1400 MHz: An Emerging Technology for Earth Observations

“…An alternative wide-band receiver implementation for measuring the thickness of layered media is also under investigation through the measurement of the time-domain autocorrelation of received thermal noise [ 74 ]–[ 78 ], although results from such systems in the 500–1400 MHz frequency range have yet to be reported. Finally, we note that current methods for “vicarious” external calibration based on expectations for the long-term behavior of the brightness temperature of Earth’s ocean or rain forest regions [ 92 ]–[ 95 ] will require the creation of new models for these effects at frequencies less than 1400 MHz.…”

Review of recent technical accomplishments of Aquarius - NASA's first global sea surface salinity mission

“…It operates within the 1400-1427 MHz primary exclusive allocation for passive sensing to avoid severe radio-frequency interference (RFI) and still maintain sensitivity to SSS. The radiometer is internally calibrated with a reference load and multiple noise diodes to maintain 0.13Krms/7-day stability [2] and externally calibrated using global averaged expected antenna temperatures or the oceanic vicarious cold-point brightness temperature [3] for longer term time periods. Pre-launch calibration measurements were carried out to measure values and temperature coefficients of front-end losses and noise diode excess noise temperatures after the methods in [4].…”