Radar altimeter can map the global geoid with high precision. This paper presents a new concept of mutibeam
delay/Doppler radar altimeter which combines a liner array of nadir receivers and a nadir transmitter. The delay/Doppler
technology is applied in the along track, and the array signal processing technology in the cross track. The key
innovation in this concept is that fan-beams form in both along track and across track, and range correction is applied in
across track as in along track, so that it could provide high precision with wide swath. But the 3D-matrix store and
processing of the raw data from receivers are much more complicated than a conventional radar altimeter and a
delay/Doppler radar altimeter. It could provide height profile with high precision for left scene and right scene,
respectively, and leave small footprints at far range as at near range. It could also lead to uniform cross section in each
The concept of Delay/Doppler Radar Altimeter (DDA) was proposed by R.K Raney to improve the spatial resolution in the along-track direction, for rougher surfaces more irregular than open oceans. DDA also has many new advantages when it is used for open oceans, such as stronger response, being less sensitive to off-nadir angle errors, and being more sensitive to significant wave height (SWH) than Conventional Radar Altimeters (CRA). These new features and advantages will be validated by simulations in this paper.
The design of the China Imaging ALTimeter (CIALT) and the flight experiment of its airborne model are presented in this paper. The system is aimed for providing observation measure for both oceanic applications and continental topographic mapping in the future. The motivation of this project is to develop a three dimensional imager fitted for small satellites with small volume, mass and power consumption. An experimental airborne model of the CIALT has been developed for verifying the design concept. The CIALT integrates three techniques together, i.e. the height measurement and tracking technique of traditional radar altimeter used for ocean applications, the synthetic aperture technique and the interferometric technique. A robust height tracker has been designed for meeting the requirements of both oceanic surfaces and continental surfaces (including surfaces of ice continent). The synthetic aperture technique is used for achieving a higher azimuthal resolution along the cross range direction compared with that of a traditional altimeter. The interferometric technique is used for retrieving the height information corresponding to each image pixel and for boresight angle correction of the antennas, which is crucial for accurate height measurement. The CIALT is different from other proposed imaging altimeters, such as SAR altimeter and scanning altimeter, in which no height tracker is involved. Some key technologies regarding the development of imaging altimeter are addressed, such as the antenna design, the transmitter, the receiver and the robust tracking algorithm.
The IF simulator is an important equipment to test and calibrate spaceborne radar altimeter before launch. In this paper, a new IF simulation algorithm (IFSA) is introduced, based on which an IF simulator is developed successfully. The IF simulator is designed to provide simulated I/Q signals and related interface, so that the tracker of altimeter can be tested and calibrated in close-loop under the simulated circumstance. That means the IF simulator should have the ability to simulate the variety of the distance from the satellite to the sea surface, the significant wave height (SWH), the backscatter coefficient of the sea surface, the sea scattering noise, and the thermal noise of the radar altimeter electronic system. Besides IF simulation, IFSA can be used in RF simulator, and it has been proved to be correct and useful by lots of experiments. The IF simulator includes the monitor unit, the data processing unit, timing unit, D/A unit, buffer unit, and interface unit.
An airborne multi-function microwave remote sensing system has been developed in order to verify the design and performance of future Chinese spaceborne system. Future Chinese spaceborne microwave remote sensing system is for ocean research, atmosphere research and soil moisture content monitoring. Like spaceborne system, the airborne system also includes altimetry, scatterometry and radiometry functions. There are five operate modes: altimetry mode, scatterometry mode, radiometry mode, altimetry and radiometry mode, scatterometry and radiometry mode. The operate mode can be changed by program. There are five channels in radiometry mode. The altimetry mode, the scatterometry mode and the second channel of radiometry mode operate at the same Ku band. In the airborne system, the scatterometry mode uses two pyramidal horn antennae. One is for horizontal polarization. The other is for vertical polarization. The horizontal polarization antenna is also used in the altimetry mode. The radiometry mode uses five pyramidal horn antennae.
Flight experiments have been conducted on southern sea of China. The results verify the design and performance of the airborne multi-function microwave remote sensing system. They also show that the design of future Chinese spaceborne system is practicable. In this paper, the principle of airborne system is briefly introduced. Flight experiments and results are described.