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A study on the utility of combined use of GPS and GLONASS constellations in India
It is very difficult to make general conclusion on the availability of GLONASS satellites as number of satellites in
GLONASS constellation has been found to be gradually depleting
during the course of this study. This may be attributed to the facts
that the satellites were not surviving its full life time and also the
scheduled replenishment was not carried out possibly due to
economic and political reasons. During the period of study the
number of satellites in the constellation varied widely from 16 to 7.
For different constellation status the visibility of GLONASS is shown
in Fig.2. For the best status of 16 satellites, 95% of time 3 or more
satellites were available whereas only 1 or more satellites were
available for worst status of 7 satellites. In other words, GLONASS
constellation was never so good that one may have 3D solution out
of only GLONASS constellation. However, GLONASS alone may be
used for timing application.
 Fig.2: Probability of a certain number or more GLONASS satellites with varying status of the constellation. Geometrical Configuration of Available Satellites It has been noted that sufficient number of GPS satellites or GPS+GLONASS satellites is always available at all the monitoring sites to get the navigation solution. But to achieve the desired accuracy, it is not only necessary to track sufficient number of satellites but also it is equally important that the geometrical configuration of the tracked satellites is also favourable. The effect of geometrical configuration is numerically expressed by the factor Position Dilution of Precision (PDOP). The following subsection presents the detailed analysis of PDOP value of monitored data and also probability of occurrence of different values of PDOP. It is obvious the PDOP is dependent on the position of the receiver and changes with time due to relative motion of the satellites. All modern receivers find out PDOP for all combination of available satellites and select the set of satellites with minimum PDOP for navigation fix. The variation of PDOP for each day of observation has been studied. Percentage of occurrence of PDOP has been found out by just counting the number of samples that corresponds to PDOP values within a particular range.  Fig.3: Probability that PDOP remains within a certain value. From the availability study it is clear that present status of GLONASS constellation does not encourage using these satellites alone for navigation applications. So attention was given to explore the use of GLONASS in combination of GPS. So the impact on PDOP by the presence of GLONASS satellites along with GPS satellites was emphasized. As expected, appreciable improvement in PDOP in the combined mode by the supplement of GLONASS satellites was observed (e.g. PDOP of 2 in GPS only mode has been improved to 1.6 for combined mode) as shown in Fig.3. But it is of interest to note that the improvement of PDOP was hardly effected by depletion of GLONASS satellites. This apparently implies that the improvement of PDOP may be optimally achieved by addition of only one or two more satellite suitably placed. Further the addition of satellites with the improvement of GLONASS constellation status, may not definitely make any further perceptible impact on PDOP. But it should be noted that the addition of GLONASS satellites surely improves the probability of having at least one or two more GLONASS satellites in favourable position and thereby showing more confidence in improvement of PDOP. Study on the Position Accuracy It is necessary to know the precise coordinates of the location of the antenna to study the position accuracy. So the coordinates of all the three monitoring sites were known a-priori fairly accurately. With the help of predetermined position coordinates, the error in the latitude, longitude and height corresponding to each observation may be found out. But to find out the 3D error the following relation have to be used.  where Lt= Nominal value of latitude Dh = Departure of the observed height from the known height in meters DLn= Departure of the observed longitude (in minute) from the known longitude, and DLt= Departure of the observed latitude (in minute) from the known latitude. The above formulation assumes that 1 minute of arc of the earth curvature is equivalent to 1852 meters. In Presence of SA Since May 1, 2000, GPS Selective Availability (SA) has been withdrawn by USA. Observations taken before May 1, 2000 have been studied in a perspective that assumes the presence of SA. SA had been implemented in Block II satellites and had been effective since April 1990 intermittently at various level of accuracy. According to US DoD, the introduction of SA would provide a horizontal accuracy within 100 meters and vertical accuracy within 159 meters for standard positioning service (SPS) with C/A code. The time output accuracy would deteriorate to 350 nanoseconds. One of the primary objectives of this effort was to study the optimal use of GLONASS constellation in combination of GPS to reduce the effect of SA in position accuracy. In one special campaign, the receiver has been made to operate in three different modes (GPS only, GLONASS only and GPS+GLONASS mode respectively) for 10 minutes each sequentially and the operation were repeated for few hours. In each slot the position solutions corresponding to the each sample of observation has been recorded. Errors in latitude and longitude have been shown for different modes of operations. Numbers at the top in each slot indicate the average number of satellites that participated in the observation. In the mixed mode the first number corresponds to GPS and second number is for GLONASS. These numbers are very useful for proper assessment in right perspective. In GPS only mode the wide fluctuation in error is quite significant reflecting the effect of SA. The diminishing effect of SA in GLONASS only mode and GPS+GLONASS mode has been obviated by smooth variation of the errors. One may argue that due to total absence of SA in GLONASS, the GLONASS only would exhibit best performance. Due to insufficient number of GLONASS satellites, the optimum configuration of satellite geometry would not be possible leading to higher PDOP in GLONASS only mode. But in combined mode, good number of GPS satellites supplemented by fewer GLONASS satellites would allow the selection of satellites with better PDOP. Contribution of GLONASS satellites by higher number helps in more efficient countering of the telling effect of SA. |