Using GAGAN to improve GPS Positioning Accuracy - Part 2

Professor Chinmaya S Rathore
Indian Institute of Forest Management Bhopal, India

This is Part 2 of a two-part article series on Satellite Based Augmentation System or SBAS. In Part 1, we discussed some background information about SBAS. In this concluding part, we will see how to activate and use GAGAN differential correction messages to improve positional accuracy using two commonly used Garmin GPS receivers. The contents of this article are equally applicable to WAAS users in North America, EGNOS users in Europe and MASS users in Japan as all these SBAS are compatible and interoperable. WAAS capable receivers from other manufacturers should function in a very similar way.

I have used two Garmin GPS receivers for this article. These are Garmin GPSMAP 78s and Garmin eTrex 20 (Figure 1). The Garmin GPSMAP 78s can only view the GPS constellation while the eTrex 20 can view both GPS and GLONASS constellations. Both receivers are WAAS capable.

Figure 1

Figure 2

Frankly, there is nothing much for the user to do. Out at a suitable open location in the field, you start your GPS receiver, let it get a fix with as many GPS satellites that it can use and show you your current position and accuracy of the reading. Figure 2 shows a screenshot of the Garmin GPSMAP 78s showing a typical positional fix using 12 GPS constellation satellites with a positional accuracy of 3m (top right corner). Notice that GPS satellites are numbered between 1-32 as shown on the skyplot as well as below signal bars displaying signal strength from each of these satellites. As the GPSMAP 78s sees only GPS constellation satellites, it would therefore never see a satellite ID greater than 32 - the highest satellite number in the GPS constellation. By default typically, WAAS-capable GPS receivers are set to operate in the normal mode. To start receiving differential corrections from GAGAN, we need to switch on WAAS from the main menu of the Garmin GPSMAP 78s using the screen sequence shown in figure 3. 

Figure 3: Activating WAAS on Garmin GPSMAP 78s

Now that WAAS is activated, there is an immediate change in the screen as seen in figure 2 above. This change is shown in figure 4. One of the channels (which was tracking satellite 12) is freed up and allotted to a new satellite numbered 40 (extreme right bar in figure 4a). Satellite numbers between 33 - 64 are reserved for SBAS satellites (current and future) and one of the two GAGAN satellites GSAT-8 or GSAT-10, indicated by numbers 40 and 41 respectively, shows up on the skyplot and bars Figure 4(a). Notice in Figure 4(a) that the bar for satellite 40 is empty and the number 40 in the skyplot (shown by arrows in figure 4a) is grey or cold. This shows that the GPS receiver is in the process of acquiring the GAGAN satellite. Also notice that at this stage i.e. till the time the WAAS satellite has not been acquired, the accuracy is 3m ( Figure 4a top right corner, encircled).

Figure 4

Figure 4(b) is a screenshot taken a few seconds later. It shows the bar for satellite 40 filled and satellite 40 in the skyplot, turning green from grey. This change indicates that the GAGAN satellite has been now acquired and your GPS receiver is now getting differential correction messages in real time from GAGAN. Notice another change from figure 4(a). All signal bars in figure 4(b) are annotated with 'D'  indicating that the differential correction received from GAGAN (or WAAS if you are in North America or EGNOS in Europe) is being applied to measurements from all these satellites. The result is that your positional accuracy improves from 3m in figure 4(a) (i.e. without GAGAN/WAAS) to 2m (with GAGAN/WAAS) as seen in the upper right corner in figure 4(b). As you now start moving in the field collecting waypoints or tracks, this 2m accuracy should hold solid and steady. It is important to recall from part 1 of this article that the operational specifications for GAGAN are 7.6 meters but you end up getting 2m which is within about 6ft of the actual location! 

Figure 5 shows a similar screenshot after WAAS correction from the Garmin eTrex 20 GPS receiver which can track both GPS and GLONASS constellations. Notice in figure 5(a) that GPS constellation satellite bars numbered 1 to 32 appear in the top row while GLONASS satellites numbered between 65 to 96 appear in the bottom row. The GPS receiver has located GAGAN satellite 41 (GSAT-10) this time and is trying to acquire it. The accuracy is 3m. After a few seconds, the screen changes to what is shown in figure 5(b). GAGAN satellite 41 has now been acquired and all the GPS satellites start receiving differential correction indicated by 'D' in each bar in the top row in figure 5(b). Notice that as the GAGAN SBAS is compatible only with GPS constellation satellites, therefore no differential corrections are applied to GLONASS satellites and the bars in the bottom row do not have a 'D'. The accuracy in figure 5(b) improves to 2m with GAGAN. 

Figure 5

If you are in some part of the world where there is no SBAS coverage but after activating WAAS on your GPS receiver (Figure 3) your receiver locates a SBAS/WAAS satellite, differential corrections will not be applied. The SBAS message from the satellite includes coverage area details and all new WAAS-capable GPS receivers do not apply corrections when the GPS receiver is out of coverage area . If differential corrections were applied in such a case by the receiver, positional accuracy may be much worse that what one would get without deploying WAAS i.e. with normal operation mode. Both the GAGAN satellites (40,41) transmit the same correction messages and which one of these two appears on your receiver depends upon the visibility of GAGAN satellites at your location.

While Garmin GPS receivers have been used in this article, the above discussion is valid for any WAAS-capable GPS receiver from any manufacturer. The reader can consult the user manual of the receiver to find out menu options to activate WAAS/EGNOS.


Supplementary Notes 

[1] The satellite numbers that are visible on GPS receivers as discussed above are also referred to as NMEA IDs (NMEA stands for National Marine Electronics Association). While the above discussion and figures mention NMEA IDs for GAGAN (40,41), if you are in North America and you activate WAAS on your receiver, you should typically see one of WAAS satellites numbers 46,48,51. In Europe i.e. with EGNOS, you should see one of satellite numbers 33, 37, 39 and for MSAS over Japan, one of satellite numbers 42 and 50.