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.


  

Using GAGAN to improve GPS Positioning Accuracy - Part 1

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


This is a two-part article on how to use GAGAN ( or any other SBAS available in your region) to get better positional accuracy from your GPS receiver for free. Part 1 of this article (this article) provides a quick background on differential positioning and Satellite Based Augmentation System (SBAS) concept while part 2 shows how to activate and use the SBAS service on a typical GPS receiver.

In the previous posts, we have seen that many sources of error influence the accuracy [1] of the positioning solution determined by the GPS receiver. In short, a location being identified by the GPS receiver as a certain latitude and longitude could, in theory, be around 15 meters away from its true position. While the user might not be able to control the sources of error that contribute to this positional inaccuracy, there is a neat trick that can provide the user with much better positional accuracy using the same receiver. It's called differential correction. The basic idea behind differential correction, illustrated in figure 1, is rather simple (its implementation is not!). 

Referring to figure 1, a GPS receiver (called a reference or base station) is installed at a location whose latitude, longitude and altitude are precisely known (1). The GPS receiver after getting a fix from GPS satellites gets a latitude and longitude reading as determined via ranging GPS satellites (2). Because it already knows its position accurately, it can compare the position obtained via the GPS satellites with its known position and find out the quantum of positioning error (3). It can now pass on (4) the error correction parameters to any nearby GPS receiver (also called a rover) which can likewise adjust GPS positions with the error corrections received from the base station (5).

Figure 1: The DGPS concept with real-time correction

While this arrangement is really nice, there are three potential issues that come in way to make it work effectively:

1. You need to establish and operate a base station. This typically requires special equipment which is quite expensive. 
2. The base station can pass correction parameters to a rover in real time if it is equipped with a radio transmitter and the rover with an appropriate receiver. This additional capability make both the base station and rover much more expensive (also see supplementary note 4). 
3. This arrangement can operate within a limited distance (usually a few hundred kilometers) in the vicinity of the base station governed by the premise that both the base station and the rover being locationally proximate experience similar atmospheric conditions, and must therefore, be subject to the same errors.  

Satellite-based Augmentation System (SBAS) provide a really elegant solution to the above DGPS issues making available differential corrections over a large area (continents!) to GPS receivers for free! The DGPS and SBAS functioning is quite similar in concept. The SBAS implements the real time differential correction idea by gathering positioning errors from a network of permanent base stations, computing differential corrections and uploading them to geostationary satellites (also referred to as GEO satellites)  which in turn broadcast these corrections over large areas. A GPS receiver, which is SBAS capable, can receive these correction messages in real time and make the required positional corrections. It is for this reason that the SBAS is sometimes also referred to as Wide Area Differential GPS or WADGPS. Currently, four countries are operating SBAS services while some others have proposed to operationalize such services in the near future.  The USA operates the Wide Area Augmentation Service (WAAS) available over North America, European Geostationary Navigation Overlay Service (EGNOS) is operational over the Europe, the Indian GPS Aided Geo Augmented Navigation (GAGAN) available over the Indian sub-continent region and the Japanese Multi-functional Satellite Augmentation System (MSAS) covering Japan. See this exhibit for a summary of various operational SBAS and their coverage areas.

Figure 2 conceptually summarizes the SBAS concept using GAGAN as an example.

Figure 2: SBAS Concept

It is important to point out that these systems are interoperable which means that the same GPS receivers will be able to receive differential correction messages from all these systems. The primary beneficiary of the SBAS is the aviation sector but all GPS users having WAAS-capable GPS receivers can benefit from SBAS by getting a typical positioning accuracy of less than 3 meters, 95% of the time [2]. It will be also worthwhile to reiterate that the differential corrections from the SBAS mentioned in this article are applied to positional measurements from GPS constellation satellites and not GLONASS (or Biedou). A good overall summary of the SBAS/WAAS concept with an interesting animation is available at the US Federal Aviation Administration website.

WAAS/EGNOS/GAGAN/MSAS broadcast correction messages on same frequencies as GPS (L1 / L5) and as such GPS receivers can read the broadcast differential correction data without any additional equipment requirement as long as the SBAS satellite is visible (line of sight) to the receiver.   

In part 2 of this article, we will see how we can activate and use GAGAN to get better positional accuracy using a popular WAAS-capable GPS receiver.This should help get more accurate positional data from field surveys.

Supplementary Notes 

[1] While accuracy is one of the commonly used and understood navigational parameters in reference to GPS, other parameters also characterize the performance of the GPS system. These parameters (in addition to accuracy) are integrity, continuity and availability.  It is important to point out that in addition to providing better positional accuracy, SBAS also improves GPS integrity (by sending timely alerts when positioning cannot be relied upon) which is crucial for aviation applications particularly for flight safety while landing. The interested reader is referred to this article by Dr. Richard B Langley for a fuller explanation of these terms.  

[2] GAGAN, among other component units, comprises of 15 Indian Reference Stations (INRES) spread across India, two master control centers at Bangalore, three uplink stations and 3 Geostationary Satellites two (GSAT-8 and GSAT-10) transmitting correction messages and one (GSAT-15) an in-orbit spare. Technically, the combined footprint of GSAT-8 and GSAT-10 satellites extends from Africa to Australia filling in the airspace gap between EGNOS and MSAS Satellite-based augmentation systems. GAGAN has an operational accuracy performance requirement of 7.6 meters. For more technical information about GAGAN, the reader is referred to an excellent article titled GAGAN - Redefining Navigation over the Indian Region by Ganeshan et. al., InsideGNSS, January / February 2016, pp. 42-48.

[3] Russia is developing an SBAS called System for Differential Corrections and Monitoring (SDCM) and China has announced the Satellite Navigation Augmentation System (SNAS). South Korea has also announced to develop an SBAS by 2021. Some private operators like OmniSTAR also operate SBAS.

[4] DGPS corrections can also be applied after the GPS data has been collected (i.e. not in real time) using a technique called post-processing. Essentially, position data from the rover is corrected with data from the reference station using post-processing software. It has been reported that positions corrected via post-processing generally result in higher accuracy when compared to real time systems such as SBAS. For more details on DGPS and post-processing, the reader is referred to this white paper by Trimble.

[5] Figure 2 is a highly simplified conceptual description of the SBAS concept created to convey an overall general idea in layperson terms. It must be pointed out that while GPS uses signal travel time from 4 or more GPS satellites to the GPS receiver to compute a position solution, the SBAS concept works backwards by calculating a correction factor in signal travel time (ranging error) using the accurately known position to improve positional accuracy resulting in the kind of effect shown in figure 2.