Carrier-Phase Inter-Frequency Biases of GLONASS Receivers
August 25, 2011 By: Lambert WanningerThe present GLONASS navigation system uses frequency division multiplexing (FDMA – Frequency Division Multiple Access) to make the signals from individual satellites distinguishable. This results in the use of several adjacent frequencies for the broadcast signals in the two frequency bands L1 and L2. In both frequency bands each GLONASS satellite transmits its signal on one of 14 frequencies. Antipodal satellites share the same frequency, so that a full satellite constellation of 24 GLONASS satellites can be used. In the coming years new signals will use code division multiplexing like GPS. But for the next decade or so, only GLONASS FDMA-signals will provide continuous dual-frequency coverage.
As a consequence of this FDMA-approach, different hardware biases exist in GLONASS receiving channels even within one frequency band. Already several years ago it was shown that the GLONASS carrier-phase inter-frequency biases are usually a linear function of frequency (Pratt et al. 1998, Wanninger and Wallstab-Freitag 2007, Zinoviev et al. 2009). Furthermore, receiving equipment of the same type usually experiences quite similar inter-frequency biases, and thus they may be removed to a large extent in differential mode. In baselines with receivers from different manufacturers, these GLONASS inter-frequency biases can produce coordinate errors and they can even prevent correct and reliable carrier-phase ambiguity fixing. For Real-Time Kinematic (RTK) and other precise geodetic applications which require GLONASS ambiguity fixing, it is crucial to introduce a priori bias values into the data processing. After ambiguity fixing, baseline individual inter-frequency biases must be estimated from the carrier-phase data.
The ideal data set to determine and study such GLONASS inter-frequency biases would consist of observations from many receivers of various receiver types employed at the same site. Such results for a small number of receivers were published in Wanninger and Wallstab-Freitag (2007). Due to the high costs of collecting such data for a larger number of receivers, it was decided to use already existing data sets in this study (Wanninger 2011). In the archives of continuously operating GNSS reference station networks, data sets from GPS/GLONASS receivers have become quite common in recent years. These data sets have the disadvantage that long inter-station distances have to be accepted and thus some ionospheric and tropospheric refraction effects will influence the results. In order to include a large number of receivers and receiver types in this study, while at the same time keeping the maximum baseline lengths well below 1000 km, only sites across the European continent were selected. Here, a high density of GPS/GLONASS receivers from various manufacturers with publicly available data exists.
Observations of 133 GPS/GLONASS reference stations were selected, comprising 19 receiver types produced by 9 different manufacturers. The data processing was performed in baseline mode. Such baseline results include estimates of the differences of GLONASS inter-frequency biases between pairs of individual receivers for L1, L2, and the ionosphere-free linear combination L0. These differences do not contain any information on the absolute level of the inter-frequency biases. Thus, we can choose freely how to set this absolute level. It was decided to adjust all baseline solutions with the constraint that the mean bias value of 12 selected JPS Legacy receivers equals zero. The reasons for the decision are explained in Wanninger (2011).
The processing results confirmed that the biases can be very well modeled as linear functions of frequency. Selected results are shown in Figs. 1 and 2. The carrier-phase inter-frequency biases shown here refer to delay differences between adjacent frequency channels. The values are given in units of meters.
The comparison of the L1-results and the L2-results (Fig. 1) shows that in general similar biases exist in the two frequency bands. The one distinct exception is not shown in the figures but it is discussed in detail in Wanninger (2011). In general, large bias differences exist between receivers from different manufacturers. 5 manufacturer groups can be distinguished: Trimble, old Ashtech/Javad/JPS/TPS, new Ashtech (Pro Flex 500 CORS), Leica/Novatel, and Septentrio. Please note that there is only a single Septentrio receiver which contributed to the data set. More details on the receiver types are published in Wanninger (2011).
Figure 1. Comparison of GLONASS L1 and L2 inter-frequency biases of 132 individual receivers.
Bias differences between receivers from different manufacturers can reach up to 5 cm (0.2 ns) for adjacent frequencies and thus up to 73 cm (2.4 ns) for the complete L1 or L2 frequency bands. When relating this to the signal wavelengths of about 19 cm or 24 cm or the wavelengths of often used linear combinations e.g. 84 cm or 11 cm, it becomes obvious that reliable ambiguity fixing requires accurate a priori bias corrections.
When forming the ionosphere-free linear combination L0 similar values are obtained as long as the L1/L2 bias differences are small. Fig. 2 shows the L0-results sorted by manufacturers. Here, the sample sizes become more visible and also the variations among receivers of the same type. There is a good agreement within the groups of JPS Legacy, Leica GRX1200 GG PRO, and Trimble NetR5 receivers. Larger variations are noticeable for Javad and TPS receivers. 
Figure 2. GLONASS inter-frequency biases in the ionosphere-free linear combination L0 of 132 individual receivers grouped by manufacturers and receiver types.
The biases typically seem to be stable over time, but it was found for some receiver types that a restart of the receiver may cause small modifications of the biases. No indications were found that the biases are sensitive to temperature changes, e.g. caused by seasonal variations.
Table 1. Proposed a priori corrections of L1 and L2 GLONASS inter-frequency biases for receivers of nine different manufacturers.
The correct and reliable fixing of GLONASS carrier-phase ambiguities requires a priori correction of the inter-frequency biases. Table 1 summarizes the results of this study in the form of such a-priori corrections for 5 groups of manufacturers. They are recommended for use when ambiguity fixing in baselines with receivers from manufacturers of different groups. After successful ambiguity fixing, differences of individual receiver inter-frequency biases must be estimated for each baseline.
References
Pratt M., B. Burke, P. Misra (1998): Single-Epoch Integer Ambiguity Resolution with GPS-GLONASS L1-L2 Data. Proc. ION GPS-98, 389-398
Wanninger, L., S. Wallstab-Freitag (2007): Combined Processing of GPS, GLONASS, and SBAS Code Phase and Carrier Phase Measurements. Proc. ION GNSS 2007, 866-875
Wanninger, L. (2011): Carrier-Phase Inter-Frequency Biases of GLONASS receivers. Journal of Geodesy, in print, DOI 10.1007/s00190-011-0502-y
Zinoviev, A.E., A.V. Veitsel, D.A. Dolgin (2009): Renovated GLONASS: Improved Performances of GNSS Receivers. Proc. ION GNSS 2009, 3271-3277
Prof. Lambert Wanninger
Geodetic Institute
TU Dresden, Germany
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