ION GNSS 2012
Session E6: Precise Point Positioning 2

Title: Improving Fixed-ambiguity Precise Point Positioning (PPP) Convergence Time and Accuracy by using GLONASS
Author(s): A. Jokinen, S. Feng, W. Schuster, W. Ochieng, Imperial College London, UK; C. Hide, T. Moore, C. Hill, University of Nottingham, UK; C. Milner, ENAC, France
Date/Time: Friday, September 21, 2012, 3:20 p.m.
Room: 209/210 (NCC)

Resolution of GPS carrier-phase ambiguities when carrying out Precise Point Positioning (PPP) has been a major research challenge in recent years. There are two main fixed-ambiguity PPP methods: Fractional Cycle Bias (FCB) estimation and Integer-Recovery Clock (IRC). Both methods should in theory provide similar level of performance.

The main challenge when using these existing fixed-ambiguity PPP methods is long time period (up to 60 minutes) required to obtain the ambiguity fixed PPP solution. For a wide range of applications e.g. land surveying, this long convergence period is not acceptable.

It has been shown that using GLONASS with GPS could facilitate accurate float PPP solution with a lower convergence time compared to GPS alone. Therefore, it is interesting to explore if using GPS/GLONASS float solution could improve fixed-ambiguity PPP. To date, not much work has been done related to the effect of using both GPS and GLONASS when performing fixed-ambiguity PPP. In this paper, GPS ambiguities are only attempted to be fixed to integers and GLONASS ambiguities are kept as float values (i.e. the impact of GLONASS float solutions on ambiguity fixed GPS PPP).

In this paper, Between-Satellite-Difference (BSD) GPS measurements and un-difference GLONASS measurements are used for precise positioning. The BSD operation is to remove receiver clock error and receiver FCB from GPS measurements. The L1/L2 ionosphere-free measurement combination is employed to remove the first-order ionospheric error. GPS ambiguity resolution is carried out in two phases. In the first phase, the Melbourne-Wubbena combination is employed to estimate wide-lane ambiguities and the wide-lane ambiguities are fixed based on a validation test. In the second phase, narrow-lane ambiguities can be calculated based on the ionosphere-free float ambiguities and the fixed wide-lane ambiguities. The LAMBDA method is used in the attempt to resolve narrow-lane ambiguities if there are at least four float narrow-lane ambiguities available.

The used satellite clock corrections are provided by The Space Geodesy team (GS) of the French Space Agency (CNES). The clock corrections are calculated by using a phase clock network solution. Therefore, it is possible to resolve GPS narrow-lane integer ambiguities without employing separate narrow-lane FCB corrections.

In order to reduce the time required for the initial ambiguity resolution, the minimum constellation method, which is based on testing ambiguity fixing for all possible subsets of four or more satellites, is used in this paper when attempting to fix ambiguities.

Ambiguity validation is carried out by using the ratio test with variable threshold computed from the required confidence level. If there are more than one ambiguity candidate vectors which is accepted by the Ratio test, the vector with the largest ratio is selected. Furthermore, integrity monitoring of the solution is carried out using the state of the art Carrier-phase Autonomous Integrity Monitoring (CRAIM).

Data from 12 International GNSS Service (IGS) stations, which is recorded in ten different time periods (four hours each), is used to test positioning algorithms. The test results are analyzed in the terms of time required to obtain an initial ambiguity resolution and the corresponding positioning accuracy. In addition to this, integrity monitoring algorithms are tested.

The test results show that the most important benefit of employing both GPS and GLONASS is to make initial ambiguity fixed position solutions more accurate. For example, in the some tested scenarios, the 3D error of the position was larger than 10cm (smaller than 13cm) when using GPS alone. In contrast, this issue did not appear when employing both GPS and GLONASS. In addition, using GLONASS with GPS can also shorten the time required to resolve the ambiguities in most of test scenarios. The initial ambiguity resolution could be achieved in about 1000s in some cases.

In conclusion, the better performance of fixed ambiguity PPP by using both GPS and GLONASS has the potential to make it more suitable to practical applications such as land surveying.

Previous Abstract Return to Session E6 Next Abstract