Objectives The frequency division multiple access technology of GLONASS causes IFB (inter-frequency bias) when the receiver receives signals from different satellites, which makes it difficult to fix the ambiguity of GLONASS. It is an important factor that makes the orbit accuracy of GLONASS inferior to that of GPS(global positioning system). At present, only CODE (centre for orbit determination in Europe) among the IGS (international GNSS service) analysis centers provides GLONASS ambiguity-fixed orbits. To improve the orbit accuracy of GLONASS, it is necessary to develop a method to fix ambiguity in GLONASS all baseline and all constellation orbit determination.
Methods Based on a GLONASS ambiguity fixing method for long baseline, GLONASS ultra-rapid orbit determination is propsed. Firstly, the float ionosphere-free ambiguity with a wavelength of about 5.3 cm is solved. Then the Inter-frequency phase bias (IFPB) rate will be determined by interval search method to correct the IFPB. After the IFPB is eliminated, the double difference ionosphere-free ambiguity can be fixed and then the ambiguity-fixed ultra-rapid orbit is obtained.
Results and Conclusions The effect of this method in the GLONASS ultra-rapid orbit determination is verified and analyzed by using globally distributed stations. The results show that the IFPB rates are relatively stable within one month. The fixing-rate of GLONASS satellite ambiguity after correcting IFPB decreases with the increase of baseline length. The highest fixing-rate is 95%, the lowest is 88%, and the average is 94%. After the ambiguity is fixed, compared with the CODE product, the accuracy of the ultra-rapid orbit is increased by 26.8% and 18% for the calculation part and the 3‒6 hour forecast part respectively, the accuracy of the inner precision is increased by 20% and 13% for the calculation part and the 3‒6 hour forecast part respectively, which indicates that this method can effectively fix the GLONASS ambiguity and improve the accuracy of ultra-rapid orbit.
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