Long-term drift calibration of the altimeter radiometer data is essential for removing any systematic radiometer drift behavior as well as for monitoring the continuity of Jason-1 and TOPEX/POSEIDON data. We are using the independent estimates of wet tropospheric path delay from VLBI (very long baseline interferometry) and GPS (global positioning system) to monitor the drift of the wet delay measured by the TOPEX altimeter microwave radiometers and will continue this effort for the Jason-1 radiometer.
One of the contributions to the measured altimetric delay is the wet path delay caused by tropospheric water vapor in the altimetric signal path. The wet path delay is the additional time that it takes for the signal to pass through the water vapor. If this contribution is not subtracted from the measured altimetric delay, this additional time will introduce error to the measured sea surface height. A downward-looking water vapor radiometer onboard the TOPEX/POSEIDON (T/P) altimeter satellite measures microwave radiation at several different frequencies, 18 GHz, 21 GHz, and 37 GHz.
Over the lifetime of the T/P mission, the radiometer performance has degraded with age so that radiance measurement error has increased with time. This has caused an error of about -1 mm/yr in the wet path delay derived from the radiances. Several studies [for example Ruf, 2000 or Keihm et al., 2000] have indicated that the measurements made by the T/P radiometer have drifted since the launch in 1992. This error translates directly into our estimates of the rate of change of sea surface height. This level of error is significant because global climate change analysis is sensitive to variations of the global mean sea level of 1 to 2 mm/year. Historical records of tide gauge measurements indicate that the global average sea level is rising by 1.8 mm/yr [Douglas, 1991]. To detect a signal of this size requires that one carefully calibrate all the systematic errors in the altimeter data.
The objective of our investigation is to monitor the long term drift of altimeter radiometers. Specifically, we will be monitoring the Jason-1 microwave radiometer (JMR) and
To determine the long term TMR drift, we use wet zenith tropospheric delay estimates made during altimeter overpasses at a globally- distributed set of GPS and VLBI calibration sites. Currently, we are analyzing data from about 25 coastal and island sites. A TOPEX orbit track typically lies within 20-100 km for most of these sites. For future monitoring of the JMR in the upcoming Jason-1 mission, additional sites will be used when they become operational. The map in figure 1 shows our current calibration sites as well as new possible sites that have recently begun operation.
Douglas B.C., 1991: Global sea level rise. J. Geophys. Res., 96(C4), 6981-6992.
Keihm S.J., V. Zlotnicki, C.S. Ruf, 2000: TOPEX microwave radiometer performance evaluation, 1992-1998. IEEE Trans. Geosci. and Remote Sensing, 38, 1379-1386.
Ruf C.S., 2000: Detection of calibration drifts in spaceborne microwave radiometers using a vicarious cold reference. IEEE Trans. Geosci. and Remote Sensing, 38, 44-52.
Calibration of the TOPEX and Jason-1 altimeter microwave radiometers using VLBI and GPS derived tropospheric delays