UNIT 12

Read and translate the following text

GPS AND GLONASS GLOBAL NAVIGATION SATELLITE SYSTEMS (GNSS)

GPS and GLONASS integrated use

Specifically, the GLONASS 3-plane versus GPS 6-plane constellation and the differing inclinations offer distinct availability features as a function of latitude, with GLONASS favouring the extreme latitudes while GPS favours the mid latitudes. This effect becomes more pronounced as mask angles increase. A receiver able to operate with both GPS and GLONASS would offer the best of both worlds, with one system making up for the limitations of the other at specific latitudes. The added availability of GLONASS satellites would not only increase the number of visible satellites, which is the case at any location, but at high latitudes would provide satellites at higher elevations than achievable with GPS alone.

In addition, the GLONASS P code is currently not encrypted, available on two frequencies and provides civilian users with accuracy comparable to the GPS Ycode (the encrypted Pcode). Carrier phase measurements using both systems should provide better performance due to the easy access to dual frequency GLONASS measurements.

Dual GPS/GLONASS operation

A faster acquisition time can be achieved in the cold start mode, as more satellites are visible at any given time and location, thereby increasing the probability of acquisition. Better coverage in an obstructed environment is one of the primary advantages of the dual implementation. For applications where it is essential that tracking be continuous even when surrounded by buildings, trees or mountains, the increased constellation will greatly enhance operational capability. The GPS Standard Positioning Service (SPS) horizontal accuracy is 100 metres. This approximately leads to 50 metres and considering an HDOP of 2-0 we obtain a measurement accuracy of 25 metres. For GLONASS, experience has shown a measurement accuracy of the order of 8 metres. The major software issue relating to dual GLONASS/GPS operations has to do with timing. Specifically, in a GPS only or GLONASS only configuration, all measurements include a receiver clock error with respect to either GPS or GLONASS time, whichever system is being used. As this error is common to all measurements, it will only affect the time estimate and not the position or velocity estimates. In the case of dual operation, some measurements will include a receiver to GPS clock error, while others will include a receiver to GLONASS clock error. The relation between the GPS and GLONASS time references has to be known for acceptable position accuracies to be achieved.

GPS time is related to UTC(USNO) while GLONASS uses UTC(SU) (this can also be known as UTC(RF)). The difference between the GLONASS time system and UTC(SU) is about 20 seconds, while UTC(SU) has been offset from UTC by about 7 seconds. The GPS time system is maintained within about 20 ns of UTC. The Russian authorities may start to broadcast the time offset differences between the two systems as a part of the GLONASS navigation message. Following a recommendation from the ComitQ Consultatif pour la Definition de la Seconde, the Russian authorities may also steer the GLONASS Time System closer to UTC.

The dual mode operation is also affected by the different ellipsoids used by the two systems, (GPS and GLONASS) each employing a different geocentric Cartesian co-ordinate frame. GPS uses the WGS 84 frame and GLONASS uses the PZ 90 frame (can be referred to as the Soviet Geocentric Co-ordinate System 1990 (SGS 90)). The major difficulty in evaluating any differences between these two co-ordinate frames has been the lack of detailed knowledge of PZ 90. Reports from The (United States) Institute of Navigation working groups on the Interoperability of GPS and GLONASS suggests that, as an estimate, the difference between the exact co-ordinates of a position on the Earth's surface in both WGS 84 and PZ 90 is <15 m with a mean average of about 5 m. In order that dual mode operation can be effective, the relationship between these two ellipsoids has to be very clearly defined.

Overall accuracy and system integrity

The overall positioning accuracy can be obtained, in a statistical sense, by multiplying the measurement accuracy with the Position Dilution Of Precision (PDOP). With a GLONASS measurement accuracy of 8 metres and a GPS accuracy of 22-5 metres and assuming a PDOP of 3-0, this leads to 24 metres (1o) accuracy for GLONASS and 67-5 metres (1o) for GPS. The accuracy of the dual operation would depend on the combined effect of improved PDOP over either system alone, coupled with the reduced accuracy of GPS measurements versus GLONASS measurements, as well as the residual error in the estimate of the difference between time references.

Integrity is a major obstacle for widespread acceptance of GPS (or GLONASS). In the case of operation with a single system, it can be tested by comparing an over determined solution using the various possible combinations to arrive at a solution. For example, in the case of five satellites, there are five combinations of four satellites. If one satellite is providing 'bad' measurements, one of the five solutions will be good, whereas the other four will be poor, thereby pointing to the bad satellite. But this approach requires calculating many solutions, thereby putting an increased computational burden on the receiver processor.

An alternate approach would be to process all in view. This, with a combination of residual tests, would rapidly point to a bad satellite, without requiring the calculation of multiple solutions. This requires at least five and preferably more satellites in view at any given time. The dual GPS/GLONASS implementation, with its doubled constellation, would provide a much better integrity capability.

Performance and functionality of integrated GPS/GLONASS receivers

The 8 channel GLONASS dual frequency P code receiver and 12 channel GPS/GLONASS single frequency C/A code receiver can be used in a combined mode. GLONASS measurements are single frequency P code carrier smoothed pseudo ranges from four satellites, while the GPS measurements are single frequency C/A code carrier smoothed pseudo ranges from seven satellites. Integrated GPS/GLONASS receivers can also be used in the differential mode for both code and carrier phase measurements.

Except for differential measurements over short baselines, ionospheric delays are responsible for a substantial portion of the residual errors in the measurements. The availability of dual-frequency GLONASS measurements allows the measurement of the ionospheric delay for all GLONASS satellites in view. This information is then used to obtain good estimates of the ionospheric delays for the GPS satellites (for which only single frequency measurements are usually available).

Time transfer in the receivers is implemented according to the standard established by the Bureau International des Poids et Mesures (BIPM) ComitC Consultatif pour la Definition de la Seconde (CCDS). A least squares quadratic fit is applied to 15 second sets of pseudo range measurements taken every second.

Several commercial companies are now making available receivers capable of receiving and processing signals from 14 (or more) navigational satellites from any (satellite) navigational system, e.g.: GLONASS, GPS NAVSTAR or satellites from both systems simultaneously. These instructions will also display the co-ordinates of your location in one of several selected horizontal datums, e.g.: PZ 90, WGS 84, NAD 83 etc.

Answer the following questions

What benefits are in the GPS and GLONASS integrated use?
What are the primary advantages of the dual implementation of GPS/GLONASS?
What factors is the dual mode operation affected by?
How can the overall positioning accuracy be obtained?
What does the accuracy of the dual operation depend on?
What is a major obstacle for widespread acceptance of GPS (or GLONASS)?

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