Jason-1 Microwave Radiometer
Artist's rendering of Jason-1 on-orbit and overview of Jason-1 measurement system. (Images courtesy of the NASA Jet Propulsion Laboratory, TOPEX Project Office)
Jason-1 is a follow-on mission to the highly successful TOPEX/Poseidon Mission scheduled for launch in late 2001. We are involved in the on-orbit calibration, validation, and performance assessment of the Jason Microwave Radiometer (JMR). JMR will measure radiometric brightness temperature (TB) at 18.7, 23.8, and 34.0 GHz in the nadir direction, from which is estimated the excess path delay (PD) through the atmosphere experienced by the radar altimeter signal due to water vapor and suspended cloud liquid water. The Jason-1 project-level error budget allocates 1.0 cm of uncertainty to the PD correction provided by JMR. This is a 20% reduction, relative to the TOPEX Microwave Radiometer (TMR), and suggests that an on-orbit validation program is warranted on the order of that conducted for TMR.
There is one significant change in the instrument design, from TMR to JMR, which the on-orbit validation should take into account. In the case of TMR, absolute calibration was referenced to a warm black body load and a cold sky view of space. These calibration reference points bracket the range of Earth TBs measured over the life of the TOPEX/Poseidon (T/P) mission. In the case of JMR, the cold sky horn has been replaced by a trio of internal noise diodes, which provide a hot reference point above that of JMR's warm black body load. This change presents two particular concerns which will be considered during validation. The calibration points will no longer bracket the Earth TBs. Absolute calibration will involve an extrapolation from, rather than an interpolation between, reference points. One obvious cause for concern is the increased sensitivity, in the case of an extrapolation, to non-linearities in the instrument behavior. This issue will be considered both during pre-launch testing and development of the Flight Algorithm for instrument calibration and during the on orbit validation proposed here. The second concern with the noise diodes involves possible long term aging effects in the space environment. Three noise diodes were used for generic reliability reasons and also in order to correct for independent short term variations in their noise power. Correlated long term drifts due, for example, to radiation exposure, cannot be internally detected and corrected. An external absolute reference, such as is proposed below, is necessary. It should be noted that flight results to date with the reference noise diodes used by the NSCATT radar on ADEOS-1 suggest that there should not be a gross drift problem. However, the absolute accuracy of the NSCATT results is not sufficient to guarantee JMR's 1.0 cm uncertainty requirement over the life of the Jason-1 mission.
The objectives of our proposed work will be:
1) Assembly of an on-orbit ground truth data base for the JMR. The data base will include three independent measurements of wet tropospheric path delay and two independent references for radiometric brightness temperature. The first independent source of path delay measurements will be TMR. Intercomparisons with TMR will be coincident in space but not in time with JMR, due to the phase offsets between their orbits. TMR will also provide an independent measure of the three brightness temperatures. The second source of path delay ground truth will be an upward looking microwave water vapor radiometer (WVR). The TOPEX Project supported fabrication and deployment of such a WVR, at the Harvest Oil Platform. Our proposal will include support for a redeployment at Harvest. The third source of path delay ground truth will be derived from routine national weather service radiosonde profiles of atmospheric temperature, pressure and humidity, at selected ocean-island launch sites lying on or near the Jason-1 ground track. The two additional reference brightness temperatures will be derived from depolarized regions of the tropical rain forest, for high levels of brightness, and calm, clear, dry sub-polar regions of the open ocean, for low levels of brightness.
2) Validation and (if necessary) calibration of JMR Flight Algorithms for the measurement of radiometric brightness temperature and the retrieval of wet tropospheric path delay. The PI is currently supported by the Jason Project to develop and implement JMR Flight Algorithms for instrument calibration and PD retrieval. However, current support does not include on orbit validation. The ground truth data bases produced here will be used during the early, "commissioning", phase of the mission to test the initial accuracy of all pertinent flight software, with particular emphasis on possible biases in instrument calibration or path delay retrieval. As more flight data becomes available, possible scale errors in brightness and path delay will also be tested. Also, TMR intercomparisons will become possible once a significant time record is available.
3) Long term assessment of the instrument and path delay retrieval stability. The ground truth data bases will be updated and archived throughout the mission lifetime. JMR stability will be monitored against these data. Of particular interest in the case of instrument stability are the performance characteristics of the on-board reference noise diodes, against which JMR calibration is absolutely referenced. This approach to radiometer calibration has not been tried before by a flight mission. Of particular interest in the case of long term trends in path delay is the recently discovered apparent increase in global water vapor, as detected by the TMR. Analysis of long term JMR retrievals of path delay will specifically include tests for this phenomena.