GPS Constellation Management: Playing Not to Lose

October 22, 2009  - By

In sports, there is a phenomenon that sometimes occurs when a team is leading towards the end of a game. It’s called “playing not to lose”.

For example, there’s five minutes left in a basketball game and a team is leading by ten points. The leading team wants to run the clock down as much as possible and still maintain their lead. There are two basic strategies the leading team can take. One is to continue being aggressive and using the strategy that put them in a winning position to begin with. The other strategy is to try to “play it safe” until the time clock expires. The problem with the latter strategy is that the other team can sense the change in mentality and feed off of it. It’s called “playing not to lose” instead of “playing to win”. I’ve seen it happen over and over again in team sports and in business. Once an organization has achieved a level of success, they lose the edge that brought them their success.

The reason you are seeing high HDOP warnings from the NAVCEN and GPS “brownouts” during the day when RTK (GPS-only) isn’t working is because the GPS satellite constellation is sub-optimal. The current design of the GPS constellation is not focused on “playing to win”, but rather “playing not to lose”.

Even the original GPS Program Manager, Dr. Brad Parkinson, has voiced his concern about GPS brownouts and discussed possible solutions. You can read one of his presentations here. In 2006, noted GNSS consultant John W. Lavrakas published a GPS World article entitled Managing the GPS Constellation for Today’s Needs discussing the disparity between the professional user community needs and GPS constellation management.

Today, there are 28 operational satellites. There were 30, but PRN08 is offline for maintenance and PRN24 was placed in active reserve after an “unusual failure”. The 31st one, PRN01/SVN49, never has been declared operational since its launch last March due to the issues discussed here before. The current GPS ground control infrastructure can only handle 30 or 31 satellites.

Given these limitations, GPS looks grim for the GPS-only RTK user, right?

Not necessarily.

The GPS constellation is optimized for 24 satellites. When there are more than 24 satellites in orbit, like there has been for many years, the extras are not positioned to benefit the users but rather to be in a position to replace satellite failures. They are sometimes referred to as “paired orbits”. Simply put, the active spares are orbiting very near other satellites that are most likely to fail. This does very little for the user community.

The current discussion is not whether to launch more satellites, but rather how to reconfigure the satellites that are in orbit. Launching more satellites is a complicated issue. It’s not just an US Air Force (the GPS stewards) technical issue, but a political one because it’s expensive (~$150M per GPS satellite launch). That leaves the Air Force with the option of adjusting the GPS constellation to benefit the user community. Doing this is not completely void of political implications I’m sure, but certainly not near the risk of launching a new satellite and certainly a better bang for your buck to the user community.

For many years in the GPS scientific community, there have been open discussions in the past of GPS constellations designed for 27 or 30 satellites. The good news is that this is an active discussion within the US Air Force today. It’s quite an important discussion because GPS-only RTK users are increasingly being shut down during the day due to the lack of GPS satellite signals and/or high PDOP. Even a constellation designed for 27 satellites would be a significant gain for GPS-only RTK users.

During my webinar a month ago, I submitted to the audience the following question:

“Do you or your crews experience GPS “brownouts” where you have to wait for the GPS constellation to change before you can continue using your GPS system?”


The following results speak for themselves:


Email me your experiences so I can continue to raise awareness of the impact the current GPS constellation is having on GPS-only RTK users. Tell me about your productivity loses, extra mission planning and other time spent dealing with the GPS “brown outs”.

I’m doing my best to make the Air Force aware of that the current constellation is causing GPS-only RTK users a significant loss in productivity. I have a feeling that the Air Force looks at the millions of consumer GPS users who are happy with their Garmins, TomToms, Magellans, etc. because those folks are able to navigate from Point A to Point B with few difficulties given the current constellation. What the Air Force doesn’t realize are that the GPS demands from the professional user community are much higher. We are the infrastructure people. Without our accurate measurements, the consumer GPS community wouldn’t enjoy the benefits they have.

RTK users need at least six satellites above 12 degrees and a PDOP below 3.0 for a robust solution. Furthermore, we have to deal with obstructions such as trees, buildings and terrain that will take out, on average, a couple of those. I think the Air Force plugs in a five degree elevation mask back in the office, looks at the sat visibility graph and says “hey, what are these guys complaining about?” The reality is that satellite signals low on the horizon don’t work as well because the data is noisier and many times rejected by the receiver. Secondly, we don’t work in parking lots where we have an unobstructed view of the horizon. We have to deal with trees, buildings and terrain that block satellite signals.

And the answer is…

At this point, there is only one solution for RTK users who need better productivity…GLONASS. As much as the Russians have taken a beating in the past for having an unreliable constellation, they hold the key for RTK productivity at this point as the GPS constellation continues to deliver “brownouts” that hamper productivity. It could turn out to be a boon for RTK receiver manufacturers. Although a few include GLONASS as a standard, most RTK receiver manufacturers charge an upgrade fee of several thousand dollars to utilize GLONASS. Even worse for some RTK users, their receiver isn’t upgradeable to utilize GLONASS so they would need to purchase a new receiver(s).

On a final note, I just spoke to a user in the field who was using a GPS/GLONASS RTK receiver. I asked him to recite to me how many GPS and GLONASS satellites he was tracking. I suppose I shouldn’t be surprised, but it did saddened me a bit. He was tracking more GLONASS satellites (6) than GPS satellites (5). Sigh…


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