Research On Robust Phase Unwrapping Algorithm In Radio Interferometry

Radio interferometry is an important measurement method in deep space navigation. Compared with traditional radio ranging and velocity measurement, it has the advantages of far distance and high precision of angle. The core of the radio interferometry is to extract the difference of signal's delay, arriving at different stations, so that get the relative angular position between the signal source and the interference baseline. There are two categories of interferometry delay, group delay and phase delay. Phase delay has the advantages of high resolution and low measurement noise.However, there is a key problem in obtaining phase delay, which is phase unwrapping. The accuracy of phase unwrapping has an important influence on phase delay accuracy.In this paper, we carried out two categories of radio interferometry experiment, long-term continuous interferometry and short-time differential interferometry using Connected Element Interferometry of Beijing Aerospace Control Center (BACC-CEI), to verify phase unwrapping algorithms.For the long-term continuous interferometry, we uses three algorithms for interferometric phase unwrapping:1. The classical unwrapping algorithm, based on the difference of the adjacent wrapping phase. If the absolute value of the difference between adjacent phases is greater than a threshold (usually use ?), all of the latter phase will be increased or decreased by 2?. This method is simple and fast.However, it may cause the error transmission in the case of low SNR.2. An algorithm based on prediction. This algorithm unwrap the phase from the points with high SNR to both sides, predicting the next phase value according to the trend. If the absolute value of the wrapping of the phase which is the difference between the predicted value and the measured value, is greater than a certain threshold value, the predicted value will be used as the phase of the next point. Otherwise, the measured value will be used. Its processing speed is slower, but this method can yield more robust and precise results.3. A phase unwrapping algorithm based on segmenting regions containing phase jumps. This algorithm uses a probabilistic condition to divide the wrapping phase into hopping region and non-hopping region.First, the phases in non-hopping area will be increased or decreased by 2?.Then the phase of the hopping region will be fitted. Finally, we use filter to improve the quality of result. The efficiency of this algorithm is relatively low, but it is good to eliminate the error transmission.In short-time differential interferometry, its observed arcs are intermittent in time and discontinuous in phase. The phase difference of adjacent observation arc is much larger than 2?. In view of this situation,this paper presents two methods: The first method is to fit the adjacent observation arc phase to the middle region. According to the result,increase or decrease the phase of the entire observed arc by integer multiple of the 2?. Another way is using precise ephemeris to assist. Method 1 is a general method and method 2 is only applicable to the satellite with precise ephemeris.