Letters to the Editor: The Cost of Reliability

December 1, 2011  - By

Thanks to Richard Langley for the constellation update in November GPS World, from ION-GNSS. I’m a GPS constellation junkie, and if there was a history of each GPS space vehicle on orbit, I’d read them all. I love hearing the operational tidbits, about a IIF having problems with its cesium clock, or a reaction wheel failing, or how many spare SVs are on hand, and if SVs are slated to be disposed of, and so on. I’ve never been able to find a good centralized source of that type of information, as it seems to be something that just kind of leaks out into the industry press, from uncited sources. I’d been waiting for an update to The Almanac but it’s a moving target, so I understand why you don’t rush to update it every time a new SV is launched, or an SV’s clock changes. Especially with the increase in GNSS launches.

So thanks for those new updates, and passing them along as they happen.

A second thing, just kind of my musing of the state of the GNSS constellations, and how the U.S. GPS system is so much different than the others: The cost of reliability.

With continued launches by Russia, the GLONASS system has, for all practical purposes, reached a fully operational status with 27 satellites set healthy, being commissioned or in flight tests. They are definitely putting far more SVs into orbit faster than the GPS program ever has. Over the years, they’ve put up so many satellites that they have three times as many disposed satellites (90) as they have operational (27) satellites.
Compass has launched 13 satellites; at least eight are known to be usable.

In the GPS constellation, there are still more SVs active on orbit than have been disposed of, in the entire history of the GPS program. Think about that for a minute.

30 active satellites on orbit, and in the entire 40-year history of the program, only 29 have been disposed of. This is a testament to both the forward-thinking design of the GPS system by its many architects, contractors, and builders of the SVs and their payloads. And of course the Air Force that manages the constellation. The GPS system sets the standard for all other GNSS systems. It is not only the most accurate and dependable GNSS system in the world, it is also the most obsolete, in terms of age of spacecraft on orbit.

The user segment enjoys reliability, at the expense of new features. Because the Block II and II-A satellites exceeded their design life, and now the last of the II-R satellites are reaching their design life, we don’t have all of the signals we could have from a more modernized constellation. Non-professionals like myself don’t have an operational L2C, for ionospheric correction in consumer-level devices. (Waiting on OCX.) We don’t have operational L5 (Waiting on OCX, again.) And what about all of those inter-satellite links for ranging that the IIR, IIR-M, and the IIF (and IIA as well?) satellites have? Are those waiting on OCX too?

Originally, the IIF satellites were supposed to number 51. Then it was reduced to 33. Then 15. Now 12. 12 isn’t even enough to replace the entire remaining IIA fleet, while maintaining the current level of active SVs. Of course, it doesn’t make any sense to launch lots of IIF birds when GPS III is out there on the horizon, only three short years — we hope — away.

If the II, IIA, IIR, and IIR-M GPS spacecraft would have had lifetimes similar to GLONASS satellites, the whole constellation would have either fallen into disrepair, or, more likely, been upgraded to IIF satellites a decade ago. And we’d have all of the modern signals that we could ever hope to need. Civilians have the same signals that we’ve had since the beginning of the GPS program. We could have had new signals years ago. but the old birds keep on flying, and so far, we only have two IIF satellites in orbit.

— Jerry Pasker
Monticello, Iowa

Occupy GPS

It occurred to me recently that maybe all these people all over the country are protesting the fact that 1 percent of the world’s GNSS receivers control 99 percent of the attention.

While 99 percent of receivers actually outperform that select 1 percent in most metrics — time to fix, accuracy in cities, power consumption, sensitivity, dynamic range, jam immunity, and so on — because they live and work in cell-phones and tablets, they are poorly compensated and don’t always get the respect of their better-dressed cousins.

— A Reader

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