Study On Hy-2 Altimeter System Delay In-Orbit Absolute Calibration Using Reconstructive Transponder

Transponder, as an in-orbit radar altimeter range calibration approach, has been proposed in the 1980 s. After years of development, bent pipe transponder has been an operational approach for in-orbit radar altimeter calibrations, and several space radar altimetry missions have utilized this approach. In the 1990 s, a reconstructive transponder based on rebuilding echo signal was proposed. However, none in-orbit radar altimetry mission has utilized the reconstructive transponder as an absolute calibration approach yet. China’s first marine dynamic environment satellite HY-2 was successfully launched on August 16, 2011. A nadir-looking pulse-limited radar altimeter is one of HY-2’s main payloads, and its missions are measuring sea surface height(SSH), significant wave height(SWH), wind speed, etc. Range absolute calibration is necessary to eliminate range bias in the HY-2 altimeter’s data. HY-2 experimental range calibration using the reconstructive transponder has begun in 2012. This dissertation, based on the calibration campaign, conducts the study on in-orbit radar altimeter system delay absolute calibration using reconstructive transponder. The main works are as follows(The rest of the abstract utilizes ‘transponder’ to call the reconstructive transponder that is used in HY-2 altimeter range calibration work).1, Altimeter and transponder data matching approach and altimeter signal compensation approach based on signal correlation. The transponder records signal during calibration. Real-time signal processing of the transponder is non-ideal. This non-ideal characteristic introduces significant fluctuation in echo signal from the altimeter, and enlarges the uncertainty of the final calibration result. The fluctuation signals in the altimeter’s range are also recorded by the transponder. This dissertation proposed an altimeter-transponder signal matching approach based on signal correlation. After the correspondence between the altimeter data and the transponder data is obtained, the raw signals from the altimeter can be compensated to reduce the fluctuation. Furthermore, the correspondence is important for USO frequency bias estimation.2, Altimeter USO frequency bias estimating approach using transponder. The altimeter measures the range by time measurement. Time measurement is achieved by counting the timing pulses with fixed period. The value of nominal clock period is known. However, the frequency of the oscillator changes slowly with age and a bias is introduced in the range measurement. The time reference of the transponder comes from the atomic clock. The frequency accuracy of the atomic clock is significantly higher than the USO on the altimeter, and the atomic clock can be calibrated regularly. Therefore, the atomic clock taken as a reference clock. This dissertation proposed an altimeter USO frequency bias estimation approach. This approach is based on a comparison of altimeter observation and transponder observation. The biases of HY-2 altimeter USO frequency are presented. Furthermore, dual-mission crossover calibration approach is utilized to verify our results, and a significant reduction of HY-2 altimeter SSH measurement drift is obtained.3, Altimeter internal signal path delay estimation using reconstructive transponder. In theory, the altimeter provides range between earth surface and the reference point of the antenna. However, the signal has to travel through the altimeter’s internal path to measure the range. Altimeter’s internal signal path delay can be obtained from comparison of the ranges from the altimeter and reference ranges. The reconstructive transponder and POD data provide the reference ranges. This dissertation described the approach for HY-2 altimeter signal path delay calibration, and presented pertinent results. The altimeter’s system delay is the sum of the internal signal path delay and the bias from the USO frequency bias.4, Altimeter time tag bias estimating approach using transponder. The altimeter produces the range and corresponding altimeter time tag, at the same time, the precision orbit determination(POD) equipment produces orbit height and corresponding POD time tag. Calculating SSH needs the range and corresponding orbit height, and the correspondence is from the altimeter time tag and the POD time tag. If a bias between the altimeter time tag and the POD time tag exits, then the SSH bias exits. This dissertation estimates the HY-2 altimeter time tag bias. The spatial relationship between the altimeter’s range and POD’s reference range is utilized to estimate the time tag bias of the altimeter’ data.5, Feasibility study of C band signal transmission between reconstructive transponder and altimeter. HY-2 radar altimeter operates at Ku and C band in order to eliminate range delay from the ionosphere. So far, all transponders for in-orbit dual band radar altimeter calibration operate at Ku band. The reconstructive transponder is a Ku-C dual band transponder. This dissertation verifies the feasibility of C band signal transmission between reconstructive transponder and altimeter.