Survey Perspectives – Late April 2008

MSAS: SBAS in the Land of the Rising Sun

Quietly, the Japanese Civil Aviation Bureau (JCAB) has been developing that country’s MTSAT Satellite-Based Augmentation System (MSAS) over the years. MSAS is Japan’s satellite-based augmentation system (SBAS) that is designed to enhance GPS for aviation navigation. It’s similar and compatible with the United States’ Wide Area Augmentation System WAAS, as well as Europe’s European Geostationary Navigation Overlay System (EGNOS). It became operational in September 2007.

So, why am I writing about an aviation navigation system in Japan when this is a survey and construction column? Well, for the same reasons I’ve written about WAAS and EGNOS in the past. The use of SBAS by high-precision, non-aviation users is growing by leaps and bounds. I’m not talking about consumer GPS users. Autonomous GPS is so accurate these days that the average consumer doesn’t feel the benefit of SBAS vs. autonomous GPS. I’m talking about the professional users in mapping and surveying who require GPS equipment that will consistently deliver positioning at the meter-level and also at the centimeter-level. MSAS can help achieve both.

MSAS Coverage Area
MSAS was designed to be compatible with the United States WAAS program. Some of the same contractors, like Raytheon, for example, that helped develop WAAS were also involved in the development of MSAS. A GPS receiver designed to use WAAS, or EGNOS for that matter, can also be used to receive MSAS corrections.

Like WAAS and EGNOS, MSAS is a regional SBAS. It serves the region around Japan in support of aviation navigation. Also like WAAS and EGNOS, MSAS is a powerful tool for non-aviation users who require a highly accurate source of GPS corrections in applications like mapping and surveying.

If you are interested in using MSAS, the first question to answer is whether MSAS “covers” the area you are working in. Following is an MSAS coverage map:

It should be noted that this is the minimum coverage area. Some manufacturers have developed schemes to extrapolate the ionospheric grid when working outside of the published coverage area as well. Using this scheme, the grid map can be expanded by approximately 10 degrees in latitude and longitude.

For example, Thailand is slightly outside of the published MSAS coverage map. A typical GPS receiver designed for SBAS won’t be able to use MSAS corrections in Thailand. However, a GPS receiver designed to operate on the fringes of SBAS coverage areas may well be able to operate in areas like Thailand where there is no central SBAS for that country. Granted, this wouldn’t be useful for aviation users, but for ground users looking for meter-level accuracy, it may work just fine.

Essentially, the official, published coverage area is determined when a GPS receiver is in a position where it can see GPS satellites that are in view of at least two MSAS ground reference stations. This is the same for all SBAS. Following is a map of the MSAS ground infrastructure as presented by a representative from the Japan Aerospace Exploration Agency at the Institute of Navigation conference last September in Forth Worth, Texas. The full presentation can be viewed here.

Another factor in determining ability to use MSAS corrections is the visibility of the geostationary broadcasting satellites (GEOs). Like all SBAS, MSAS corrections are broadcast via geostationary satellites. To use MSAS, a GPS receiver must be able to receive corrections from one of those satellites. There are two satellites broadcasting MSAS corrections located at 140.0 degrees east longitude (PRN 129) and 145.0 degrees east longitude (PRN 137). Following is a map of the “footprint” that is covered by the two MSAS broadcasting satellites. Please not that just because a receiver located within the broadcasting satellite footprint does not mean it will be able to use MSAS corrections; it must also be within or on the fringe of the MSAS grid map discussed above.

Finally, not only does a receiver need to be within the MSAS broadcasting footprint of one of the two broadcasting satellites (GEOs), the signal from the GEOs is line-of-sight. This means that buildings, terrain, trees/vegetation, etc. can block the signal. Some manufacturers have developed various schemes to overcome this, but others have not, so just because a particular receiver is not able to use MSAS corrections, doesn’t mean that MSAS is unusable in that area.

MSAS Accuracy
While the JCAB doesn’t make available test bed results like the FAA does for WAAS, I’ve seen results of data collected using high-performance GPS receivers and MSAS as the correction source. The results are on the same level as WAAS, being well under a meter horizontally. Of course, there are a million caveats when discussing GPS accuracy, so I’ll leave it at that for now, but it does demonstrate the potential accuracy that non-aviation users could realize when using MSAS as a source of GPS corrections.

MSAS for Survey-grade GPS Receivers?
Up until this point in the article, the discussion has been for those interested in sub-meter or meter-level positioning for mapping.

Development efforts by some companies in the last couple of years have resulted in survey-grade GPS receivers with centimeter-level accuracy, benefiting from SBAS signals. One example is the Magellan PM3 RTK. It was introduced last year as one of the first L1-only real-time kinematic (RTK) systems. A key innovation in the PM3 RTK technology is its ability to use SBAS GEOs as a source of ranging measurements. Essentially, the two MSAS GEOs are treated as another constellation of satellites to augment GPS satellite measurements.

According to Magellan, the addition of SBAS makes a significant difference in resolving ambiguities for centimeter-level positioning. Following is a chart from a Magellan white paper.

SBAS for Non-aviation Users Continues to Strengthen
The utility of SBAS in the mapping/surveying/construction industries continues to grow. One indicator of a successful technology is that it spurs the development of other technologies around it. MSAS and SBAS in general have certainly done that.

Japan’s MSAS is the latest SBAS to become operational. Look for future reports on other SBAS, as India develops and introduces its system called GPS and Geo Augmented Navigation (GAGAN), which is referenced on the first graphic above.