Satellite-Based Augmentation Systems

By on 13 May, 2010

ROBERT LORIMER

America has the wide area augmentation system (WAAS), Europe the European GNSS overlay system (EGNOS), Japan the multifunction satellite augmentation system (MSAS) and India is launching its GPS-aided Geo-augmented navigation (GAGAN) system. They are all satellite-based augmentation systems (SBAS) and are already delivering improved accuracy and integrity for GPS users over much of the northern hemisphere.

SBAS systems are independent of the orbiting navigation satellites used by GPS, Glonass, Galileo and Compass which collectively make up GNSS. The role of an SBAS is to observe these GNSS and provide autonomous data to user equipment about the performance of the navigation satellites they are processing.

An SBAS can achieve this by monitoring GNSS at a number of ground reference stations across the area to be covered (the underlying concept is similar to CORS, which are discussed in the next article). The observations made by the reference stations are processed at master control stations, then correction and integrity data are broadcast via geostationary satellites to user equipment in the satellites’ footprint.

The geostationary satellites are essentially communications satellites (such as Inmarsat or Optus) which have a transponder at the SBAS frequency. The broadcast signal emulates the structure of those from GNSS satellites which makes it easy to incorporate SBAS into end user products.

A typical satellite footprint might be 120 degrees of longitude and 80 of latitude both north and south of the equator. An important point here is that even though SBAS signals reach well into the southern hemisphere, unless there are reference stations monitoring the GNSS on the ground the data from those signals is meaningless (in fact they can actually worsen the performance of a receiver). Consequently many coverage diagrams don’t show the full extent of southern hemisphere footprints for these satellites.

As an example of SBAS performance, a GPS receiver using WAAS typically achieves better than a metre horizontally and 1.5 metre vertically over the contiguous US, Mexico and large parts of Alaska and Canada. Of equal importance is the delivery of integrity, again using WAAS. An error in the GPS system can be detected and transmitted to users in just over six seconds. It is by providing both improved accuracy and integrity that an SBAS such as WAAS raises the theoretical availability of GPS to 99.999 per cent when in the coverage area.

SBAS has been driven by the aviation community and so it is no accident that WAAS meets the requirements for aircraft approaches with vertical guidance which, under the International Civil Aviation Organization (ICAO) Resolution A36-23 of September 2007, is supposed to be available for all aircraft approaches globally by 2016.

However the use of SBAS goes well beyond the aviation community. The improved accuracy delivered by WAAS is credited with boosting adoption of both low-cost GPS/GIS mapping and agricultural GPS guidance products in North America. In Europe the GNSS Supervisory Authority (GSA) has conducted studies into end user applications for EGNOS and is also predicting strong adoption in key markets such as agriculture, where up to 240,000 tractors are expected to be fitted with GNSS by 2012.

The upshot is that most commercial GPS receivers now include at least one channel dedicated to SBAS, indicating the utility of these services is both understood by manufacturers and valued by non-aviation users alike.

So if SBAS is proving to be so popular elsewhere, what plans does Australia have for an equivalent service?

The agencies responsible for the safety of civil aviation and air traffic management in Australia are the Civil Aviation Safety Authority (CASA) and Airservices Australia respectively.

According to CASA’s own analysis, presented at the Naverus Performance Based Navigation Summit held in Seattle in October, without some form of GNSS augmentation Australia will not meet the objective of 100 per cent vertical guidance of aircraft by 2016.

Australia will be able to provide this service to the bulk of the passenger aircraft fleet (about 15 per cent of all aircraft carrying 97 per cent of passengers) by equipping around 200 aerodromes for barometric vertical navigation. However without GNSS augmentation the remaining 85 per cent of aircraft are left out of the picture.

Airservices spent several years developing a Ground Based Regional Augmentation System (GRAS) which would have used existing terrestrial VHF towers to broadcast the signals skyward rather than a geostationary satellite to broadcast the signals earthward.

However this approach was abandoned in September 2008 ‘following a project review which identified some technical limitations, the emergence of cheaper alternative technologies and the prohibitive ultimate cost to regional customers’.

Although some in the aviation industry continue to argue that GRAS was the right approach for Australia, from a non-aviation perspective, its design meant it would never in any case have provided the same utility as an SBAS.

So with the 2016 ICAO deadline looming, both aviation and non-aviation industry observers are asking: What are the alternative scenarios for an Australian SBAS, who would pay for it, and who would use it?

Given that the footprint of SBAS geostationary satellites stretches into the southern hemisphere already, all that is required to provide an SBAS service are suitable reference stations and connection into the control segment of the system.

Latin American countries are already in discussions with the US about extending WAAS into Central and South America. Similarly European agencies are in discussions with both South Africa and Middle East countries on possible future EGNOS service coverage for those regions.

So one plausible scenario is for Australia to approach Japan and co-operate with the existing MSAS system. This would require Australia to build reference and master control stations which would be integrated into the MSAS control segment.

An advantage of this approach is that MSAS is up and proven, so the timeline would be determined principally by how long it would take the two countries to reach agreement.

A second scenario is for Australia to retain more sovereignty and lease SBAS transponders on existing commercial satellites. This is what the US currently does with WAAS. The requirements for reference stations and control segment remain the same as the previous scenario except this time everything is under Australian control.

A third scenario is to launch our own microsatellites to provide the communications part to the SBAS. Once again, the requirements for reference stations and control segment remain largely unchanged. This approach might find favour given the renewed interest in national and international civil space activities by the Australian government, and the formation of the Space Policy Unit in the Department of Innovation, Industry, Science and Research.

Although costs would obviously vary depending on the scenario, realistic estimates are $300 million to put an SBAS over Australia, which interestingly, is about the same as a CORS-based national positioning infrastructure.

The big question is: ‘Who pays?’ If we turn the previous discussion on vertical guidance on its head, then it could be argued we would be spending $300 million dollars to service a mere 3 per cent of air passenger traffic and just over 3000 aircraft (85 per cent of the 3600 in Australia).

That works out at about $100,000 per aircraft – without taking into account operational costs. Notwithstanding Australia’s obligations under the ICAO conventions, on this basis SBAS is certainly a challenging case to make, based on cost recovery from the aviation community alone.

If we broaden the discussion on SBAS to include the wider GNSS community does that help justify the investment?

Unfortunately, we have no published studies showing the economic case for an SBAS for the general GNSS community. There is plenty of anecdotal evidence about the popularity of the North American WAAS which has been operational since 2003. Whether this would translate to an Australian SBAS is up for debate.

What we do know is that at least user equipment is SBAS-ready so there would be no additional cost involved with that.

From the perspective of the general GNSS community the emerging SBAS question means we now have not one but two major pieces of GNSS infrastructure under discussion, the other being the national positioning infrastructure.

One danger is that these two infrastructures are seen by the Treasury as competing programs, whereas in reality they could be positioned as complementary and mutually supportive.

For example, an SBAS could use a subset of the reference station sites put in place for the NPI. Conversely, an SBAS could provide integrity information for automated machines using the national positioning infrastructure.

These are important issues for the future of Australia, but appropriate forums in which they can be debated are few and far between. The new Space Policy Unit may provide one.
 
Robert Lorimer is a managing partner of Position One Consulting.


 
Issue 44; December – January 2009
 


 

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