The effect of SA may to some extent be lessened through
averaging over time. The position error has been analysed by
averaging them over different length of time. These analyses were
compared for different modes of operation. It has been observed
that in GPS only mode, error in the time-averaged position data
reduces with the increase of averaging time. No obvious improvement
of position error could be seen with further increase of averaging
time. Similar exercise was done with data recorded in GPS+GLONASS
mode. It has been noted that hardly any improvement in error in
positioning could be observed by averaging. There is no change in
the pattern of position error with the increase of averaging time in
this mode. This indicates that the effect of SA had been reduced
sufficiently in each sample of observation in GPS+GLONASS mode
of operation. So averaging process would not improve further the
accuracy significantly.
Table.2: GPS Error budget with and without SA
| Error Source | With SA | Without SA |
| Selective Availability | 24.0 | 0.0 |
| Ionospheric Delay | 7.0 | 7.0 |
| Tropospheric Delay | 2.0 | 0.2 |
| Clock and Ephemeris Error | 2.3 | 2.3 |
| Receiver Noise | 0.6 | 0.6 |
| Multipath | 1.5 | 1.5 |
| User Equivalent range Error | 25.0 | 7.5 |
| HDOP Error | 1.5 | 1.5 |
| Total stand Alone | 75.0 | 11.5 |
| Horizontal Accuracy | | |
The status of position error has also been analysed in grouping
the error of certain range. For a certain period of observation, say
for one complete day, the number of samples found with an error of
5mts or less, with an error of 10mts or less and so on, is found out
and these numbers are normalised against the total number of samples.
These help in indicating the probability of the error to be within the
respective range. It may be noted that probability of error to be
within 5mts or 10mts is much higher in mixed mode than that in
GPS only mode. On some particular day, the probability of position
error remaining within 5mts or less, was as high as 77% in mixed
mode and whereas it was only 10% in GPS only mode. It is clear that
more than 90% of time the position error remains within 10mts in
mixed mode but in GPS only mode the position error goes as high as
30mts.
In Absence of SA
The intentional degradation of the GPS Standard Positioning Service
(SPS) known as Selective Availability (SA), has been discontinued
from may 1, 2000. Since SA was discontinued, the accuracy of GPS
has been expected to considerably improve. So the position error
has been reviewed with altogether different perspective. GLONASS
promises 60 meters of horizontal accuracy with C/A code. At this
point it quite relevant to look at the comparative error budget for
GPS positioning and timing with SA and without SA as shown in
Table 2
Let us look at the observation made after May 1, 2000. If
one refers to Table 2, significant improvement of position accuracy
is expected after removal of SA. A sharp improvement in accuracy
with scatter reducing from 90mts to 18mts is so evident in these
figures. The considerable improvement in position accuracy for
GPS only mode was anticipated. In view of this it is very important
to properly compare the performance of GPS only mode and
GOS+GLONASS mode. To achieve this objective the receiver has
been operated 10 minutes in GPS only mode and next 10 minute was
operated in mixed mode.
flipping of mode of operation after every 10 minutes of
observation was continued for the whole day. The data thus collected
were separated for the respective mode of operation and compared.
Accuracy was found to be equal or marginally less in the combined
mode in comparison to only GPS mode. It shows that the probability
of a certain position accuracy is same or slightly greater in only GPS
mode than that of the mixed mode of operation.
It is well established that the introduction of SA caused a
considerable deterioration of position accuracy in GPS. Through
this study it has also been amply demonstrated that the accuracy
denied by the GPS-SA may be considerably compensated when the
receiver is operated in GPS+GLONASS mode. This observation not
only proves that the GLONASS also has better accuracy capability
than that of GPS-SA showing the improvement in accuracy in
combined mode but also the coordination of two systems could be
successfully achieved.
After the removal of GPS-SA, the accuracy of the combined
mode and that of only GPS mode have been found to be comparable.
It indirectly confirms that both GPS and GLONASS systems have
similar accuracy limits and interoperability of two systems has been
to a great extent achieved. The slight decrease in accuracy on
combined mode may be attributed to the non-near perfect conversion
matrix from PZ-90 to WGS-84.
Concluding Remarks
The usefulness of combined use of GPS and GLONASS in India has
been substantiated by careful analysis of observations. For the
paucity of space, few samples of the data have been illustrated. But
general conclusions that have been drawn are based on ensemble of
observations made at different places for almost one year.
Acknowledgement
Authors are thankful to MIT and DRDO for supporting this work.
Authors are also thankful to CABS, Bangalore for experimental
supports and co-operations.
Reference
- P.Banerjee and Anindya Bose, “ Study on the Reliability and Availability of GPS Signal in India (SRAGI)”, March 1997, NPL, New Delhi