SBAS Crashing

April 22, 2010  - By

It’s been a tough couple of weeks for SBAS (Satellite-Based Augmentation System), namely the USA’s WAAS program and India’s GAGAN program. WAAS and GAGAN have taken big hits recently that threaten the integrity of the programs. Both events were totally unexpected and are causing disruptions of GPS correction services.


Let’s Start with WAAS

First of all, consider the following infrastructure graphic describing WAAS.

WAAS Infrastructure (note: GEO satellites positioning not geographically correct in graphic)

At the moment, WAAS uses two geostationary satellites (referred to as GEOs) to broadcast GPS corrections throughout the WAAS service area, which covers the U.S., Mexico, and most of Canada. The user’s GPS receiver must be able to “see” at least one of the WAAS GEOs in order to receive the GPS corrections. Currently, one WAAS GEO (PRN 135) is located at 133°W longitude and one (PRN 138) is located at 107°W longitude. They are positioned, for the most part, to provide “dual coverage” in case one fails as the following graphic illustrates. The solid line represents the visibility above the horizon of PRN 138 (107°W). The dashed line represents the visibility above the horizon of PRN 135 (133°W). In New York, for example, PRN 138 is visible at 30°+ above the horizon while PRN 135 is visible at ~15° above the horizon.

WAAS GEO Footprint Coverage (Dashed = PRN 135, Solid = PRN 138)

The Federal Aviation Administration (FAA) is the WAAS steward. WAAS (and SBAS) was designed for aviation use and paid for by the FAA. The fact that surveying and mapping users benefit from WAAS is a by-product. The FAA owns and controls most of the WAAS infrastructure, such as the 38 WAAS reference stations located throughout the U.S., Canada, and Mexico. About the only thing they don’t own are the WAAS GEO satellites, and this has been the source of most of the problems with WAAS in the past few years.

Lease vs. Buy

It would be prohibitively expensive for the FAA to own GEO satellites that were exclusively used by WAAS. Instead, the agency leases bandwidth from owners of commercial satellites. These are the same commercial satellite owners who lease bandwidth to media (e.g., television) customers. It’s not unlike a utility pole you see along the road with many different wires and devices attached to the pole from different companies who pay to lease space on the pole, except it’s a very expensive pole orbiting in space.

If you’ve been using WAAS for a number of years, you’ll remember back in 2006 there was a hiccup with the WAAS GEOs at that time. The FAA was leasing space on two Inmarsat satellites (AOR-W and POR). They began transitioning to the current WAAS GEOs but before the transition was complete, Inmarsat began moving AOR-W. This was a headache for some WAAS users and really showed the vulnerability of WAAS.

Losing Control

The vulnerability reared its ugly head again last week when one of the commercial satellite operators (Intelsat) that the FAA leases space from announced it had lost contact with its Galaxy 15 (G-15) satellite, which is the GEO that WAAS PRN 135 is broadcast from. Intelsat reported it had lost the ability to send commands to G-15. Without the ability to control the satellite, it will slowly drift out of orbit until it becomes unusable. The FAA estimates this will occur in one to three weeks.


Intelsat’s answer was to bring in an older generation backup satellite (G-12), which was in a backup orbit at 122°W. It arrived at 133°W around April 14. Intelsat said that G-12 has virtually an identical C-band package as the G-15 and they could transfer C-band customers to the G-12. The problem is that there is no L-band package (which WAAS needs) on the G-12, so the FAA was out of luck.

Since Intelsat’s G-12 backup won’t help WAAS, the FAA is looking at other alternatives:

  1. Contract with Inmarsat to bring back POR (178°E). The FAA says that will take 12-18 months. Personally, I don’t think it’s a good solution. It’s too far to the east to help much at all. Its coverage footprint barely covers the western U.S.
  2. Speed up the testing on the new PRN 133 (98°W) and bring it into service more quickly than the original December 2010 schedule. The FAA says it can accelerate testing by one to two months. This is good and I see the benefit, but it still doesn’t help Alaskan users.
  3. The replacement backup satellite being moved to 122°W to backup G-12 may be a solution. It will be a few weeks before it is known what is possible. That would be the best scenario from a coverage footprint standpoint. The question is how long it would take to bring it into service.

On another note, the FAA stated that with the money they are saving with G-15 going out of service, they will be able to accelerate the acquisition of another WAAS GEO. I have no doubt that this has put a new level of fear into the FAA folks, and they have to realize that they can’t be running thin on WAAS GEOs. If you weren’t aware, the future of aviation navigation is based on GPS, WAAS, LAAS, etc. These sorts of hiccups would be an absolute nightmare if the National Airspace System (NAS) was already dependent on GPS.


GAGAN (GPS-Aided Geo Augmentation Navigation) is India’s SBAS. It has been under development for many years and is quite far along in development. It is funded through implementation by the Airport Authority of India with the Indian Space Research Organization. In 2008, GAGAN was broadcasting a test signal from an Inmarsat GEO with reasonable results.

India’s intent was to launch its new GSAT-4 communication satellite with part of its purpose being a GAGAN GEO satellite. GSAT-4 was to be India’s first rocket with an Indian-designed and built cryogenic-fueled third stage. Apparently it is a very difficult technology to master as it reportedly took India 16 years to develop.

Last week, after much anticipation, the rocket with GSAT-4 onboard was brought to the launch pad. Liftoff was reportedly flawless. At 8:25 minutes into flight, the rocket failed and the entire rocket, GSAT-4 and all, ended up splashing into the Bay of Bengal. It’s a crushing blow to India’s GAGAN SBAS program, which has suffered a number of delays.

P.S. Veeraraghavan, director of the Vikram Sarabhai Space Centre in Thiruvananthapuram, said “Our target is to fly a GSLV with our indigenous cryogenic engine within one year. But it will be tough.”

Following is a video report from an India news organization describing the event:











Webinar Tomorrow

If you don’t receive this too late (or you can access the archive if you do miss it), you might want to catch my 60-minute webinar “GPS, GLONASS and SBAS Constellation Updates.” It’s free and full of the latest information. I’ll also be answering a number of questions from people who registered. I hope to see you there!


GITA and ACSM Conferences Next Week

Next week, I’ll be blogging and such from the Geospatial Infrastructure Technology Association (GITA) annual conference and American Congress on Surveying and Mapping (ACSM) annual conference in Phoenix, Arizona. In addition to presenting at both conferences, I’ve got a number of interviews scheduled with interesting people. Follow my blog on the Geospatial Solution’s website Live Event Blog area.


Thanks, and see you next week.

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This article is tagged with , and posted in Opinions, Survey

About the Author:

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributor to GPS World magazine, serving as editor of the monthly Survey Scene newsletter until 2015, and as editor of Geospatial Solutions monthly newsletter for GPS World's sister site Geospatial Solutions, which focuses on GIS and geospatial technologies.

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