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Sea Surface Height Retrieval
The main purpose of the Jason satellite series instruments is to enable measurements of global sea surface height (SSH). The system consists of two instruments – an active radar instrument that transmits microwaves that are reflected back from the Earth’s surface and a passive instrument, which is a radiometer sensitive to emitted microwave radiation. The radiometer is used to determine path length changes primarily due to the presence of water vapor in the atmosphere. By using this measurement in conjunction with the radar measurement, SSH can be retrieved. Since the goal is to measure SSH changes on the order of a few mm/year, high accuracy and stability are necessary for both measurements. Considering Jason-2 for instance, the global mean SSH accuracy requirement is 3.4 cm (k =1) with a drift of less than 1 mm/year.
A crucial part of ensuring that the requirements are met for SSH is by maintaining accurate water path delay measurements using an on-board calibrator for the microwave radiometer. The on-board calibration system used for the Jason series radiometers is unique for satellite instruments. It consists of several redundant noise diodes and a reference load, which emit microwave radiation that can be coupled into the radiometer. A two point calibration is achieved by switching between receiving the signal from a noise diode and the reference load. These two sources have a known radiometric brightness temperature in each of the channels (18.7, 23.8, and 34.0 GHz). The algorithm uses these values to find the calibration equation, which generally is a linear function with a gain and offset term. By studying long-term trends of noise diodes, some instability of the diodes on-orbit has been observed. This poses a key challenge in maintaining the radiometer’s performance. Brown et al. have proposed a recalibration methodology based on long-term stability monitoring of the radiometer on Jason-1 (JMR) by analyzing its response to hot and cold Earth targets. Despite the success in increasing the agreement between the responses from the Earth targets and calibrators, the source of the instability has not been completely resolved.
Vicarious Validation Techniques
The validation of SSH retrievals is done by a number of vicarious calibration techniques. These include simultaneous nadir observation (SNO) between microwave instruments. Such comparisons include the Advanced Microwave Sounding Unit (AMSU) with Jason-2, TOPEX/Posseiden with Jason–1, and Jason-1 with Jason-2. Jason-1-Jason-2 comparisons are particularly useful, because for a period of time, the satellites were on the same groundtrack and made observations at only a few minutes delayed from each other. SNO studies quantify the biases between the different instruments and have led to important contributions to Jason/AMR stability assessments. Another vicarious technique is to compare Jason measurements with ground measurements at specific Earth sites like Cape Senetosa on the island of Corsica and the Harvest oil platform near Pt. Conception, California. For instance, comparisons of Harvest with Jason-1 and Jason-2 revealed that both have a significant bias in SSH compared to the Harvest and Corsica ground measurements. The effects of the atmospheric corrections used in the satellite SSH retrievals have been found to contribute significantly to the biases found between the retrieved SSH values. Quantifying further these and other sources of observed biases is part of a continuing effort to improve satellite data quality for SSH retrievals. Contributed by Aaron Pearlman