Caiyun Wang

A Matching Method for Establishing Correspondence Between Satellite Radar Altimeter Data and Transponder Data Generated During Calibration

This letter presents a method for matching satellite radar altimeter data and transponder data generated during in situ calibration. The transponder generates a measurement error when it measures the arrival time of the altimeters transmitted signal and embeds the error in both the transponders recorded data and the altimeters recorded data. The second-order finite difference sequence of this error sequence can be extracted from the raw data; thus, the correspondence between two identical but mismatched second-order difference sequences can be uniquely established. The measurement error is utilized, and a data matching method that can uniquely establish the correspondence between the altimeters recorded data sequence and the transponders recorded data sequence is presented. This postprocessing method does not increase the real-time signal processing workload of the transponder. Furthermore, the principles underlying this method can be used for any transponder that can adjust the response signal delay during calibration.

MICAP (Microwave imager combined active and passive): A new instrument for Chinese ocean salinity satellite

Sea surface salinity (SSS) plays an important role in global water cycle. In recent years, satellite based remote sensing has proven to be a promising approach for global SSS observation. A new payload concept, named MICAP (microwave imager combined active and passive), has been introduced in this paper. MICAP is a suit of active/passive instrument package, which includes L/C/K band one-dimensional MIR (microwave interferometric radiometer) and L-band DBF (digital beamforming) scatterometer, sharing a parabolic cylinder reflector. MICAP has been selected to be a candidate payload for future Chinese ocean salinity mission. In this paper, the MICAP instrument concept, specification and preliminary system design will be introduced.

HY-2A Radar Altimeter Ultrastable Oscillator Drift Estimation Using Reconstructive Transponder With Its Validation By Multimission Cross Calibration

This paper presents a method estimating the HY-2A altimeter ultrastable oscillator (USO) drift with a reconstructive transponder. The frequency of the USO of the in-orbit altimeter changes with age, and a bias between the actual frequency and the nominal one exists. The USO bias contributes a portion of the altimeter range drift. The HY-2A altimeter transmits signals at a fixed time interval during calibration, and the actual interval between adjacent altimeter transitions, which is controlled by the USO, is different from the nominal one due to the USO drift. The reconstructive transponder measures the arrival times of the altimeter transmitted signals accurately with the atomic clock. The drift of the USO on board the HY-2A altimeter can be estimated accurately by using the ranges from the reconstructive transponder and the HY-2A altimeter. The USO drifts of around 40 cm/year are presented. Furthermore, the multimission cross calibration provides preliminary validation of the HY-2A altimeter USO drift.

In-orbit calibration of HY-2 altimeter using a signal reconstructive transponder

A reconstructive transponder for the internal path delay and backscattering coefficient calibration of the dual frequency radar altimeter onboard the HY-2 marine dynamic environment satellite is introduced. In comparion to the bent-pipe transoponder, which has been utilized for the in situ calibration of sereval satellite radar altimetry missions, the reconstructive transponder has real-time chirp tracking and response signal delay adjustment functions and provides a more flexible approach for radar altimeter in situ calibraton. The HY-2 calibration processure using reconstructive transponder and corresponding data post-processing are presented. Biases introduced by environment and instruments are corrected, and inner path delay, clock drift introduced delay and point target backscattering coefficient of the HY-2 altimeter can be obtained.

Development of the reconstructive transponder for in-orbit calibration of HY-2 altimeter

HY-2 satellite, the first ocean dynamic satellite of China, was launched in August 2011, carrying a dual frequency (Ku band and C band) altimeter, to measure the global mean sea level [1]. To provide truly sea level measurements, in-orbit calibration is essential. This paper concerns the transponder in-orbit calibration method. A signal reconstructive transponder for HY-2 altimeter in-orbit calibration is presented. Different from the existing bent-pipe ones, this kind of transponder can capture and track altimeter pulses, record them, and reconstruct the chirp pulses, transmit them back to the altimeter. It is carried on a SUV, reduces requirement for a permanent installation site. With the characteristics of capture and record altimeter signals, the HY-2 altimeter inner path delay and clock drift could be derived from the calibration data.

Experimental performance analysis of the DBF technique on MICAP scatterometer demonstartor

In this paper, we present the experimental results of a DBF (digital beam forming) scatterometer demonstrator performance on the purpose of development and verification of innovative DBF concepts and advanced operational modes for the future space borne MICAP (microwave imager combined active and passive), Chinas ocean salinity mission. The hardware configuration of the demonstrator is introduced. The ground-based experiments, verifying the DBF function on receiving and transmitting separately are carried out on the open field using the demonstrator. Analysis of the experimental results verifies the validity of the DBF technique on the receiving and transmitting. At last, the problems caused by the unbalance of the amplitude and phase between channels are discussed. The function verification experiment is the very first step towards the future space borne MICAP, and further researches are in progress.

Development of the Reconstructive Transponder for In-Orbit Calibration of HY-2A Altimeter

Chinas first ocean dynamic satellite HY-2A was launched in August 2011 which carries a dual-frequency (Ku- and C-band) altimeter to measure the global mean sea level. In-orbit calibration is essential for reliably measuring sea-surface height (SSH). This paper concerns a novel approach for HY-2A altimeter in-orbit calibration using a reconstructive transponder, which is different from the existing bent-pipe transponder. The reconstructive transponder captures, tracks, and records the altimeters pulses, then reconstructs the pulses, and transmits them back to the altimeter. It is carried on a SUV, reduces requirement for a permanent installation site, and is much more flexible and efficient. There is an internal calibration loop to enable the stability of the transponder electronics delay and the system gain. The transponder electronics delay and system gain are obtained through an overall measurement. Another benefit of the signal reconstructive transponder is to accurately estimate the altimeter clock frequency drift. HY-2A altimeter in-orbit calibration campaign started from March 2012, until now more than 70 experiments have been carried out at several sites. On March 31, 2013, HY-2A altimeter side-A failed owing to malfunction of its ultrastable oscillator (USO). And this abnormality is monitored by the transponder. From the successful experiments, HY-2A altimeter side-A and side-B instrument bias are obtained with 1- and 4-cm precisions separately. The USO frequency drifts of the HY-2A altimeter are monitored all through the calibration campaign.

Echo Signal Quality Analysis During HY-2A Radar Altimeter Calibration Campaign Using Reconstructive Transponder

A reconstructive transponder has been utilized for the in-orbit calibration campaign of the HY-2A radar altimeter since March, 2012. The precision of final calibration result is influenced by echo signals quality in the HY-2A altimeters range window. Echo signal dwell time is analyzed considering its influence on signal quality and its fluctuation. Comparing with the length of the HY-2A altimeters range window, the radial component of orbit prediction error is large. The tracking mechanism of the reconstructive transponder utilizes some incoming signal samples without sending echo signals to establish stable tracking. Large radial orbit error and tracking mechanism of the reconstructive transponder are two main factors that limit the precision of the final range calibration result. Finally, approaches for increasing echo signal dwell time are proposed.

Echo signal quality analysis during HY-2A radar altimeter calibration campaign using reconstructive transponder

A reconstructive transponder has been utilized for the in-orbit calibration campaign of the HY-2A radar altimeter since March 2012. The precision of final calibration result is influenced by echo signals quality in the HY-2A altimeters range window. As an indicator of the signals quality, echo signal dwell time is analyzed considering its influence on signal quality and its uncertainty. In HY-2A altimeter calibration, the echo signal dwell time is determined by the radial orbit prediction uncertainty and the real-time signal processing mechanism of the reconstructive transponder. The real-time signal processing mechanism of the reconstructive transponder utilizes some incoming signal samples without sending echo signals before transmitting. Comparing with the length of the HY-2A altimeters range window, the radial orbit prediction uncertainty is large. Large radial orbit prediction uncertainty and signal processing mechanism of the reconstructive transponder are two main factors that limit the echo signal dwell time in HY-2A altimeter calibration. Finally, approaches for increasing echo signal dwell time are briefly proposed.