Written on 04/04/14 by Paul Oldham

Why you need an alternative to your GPS

An event happened last Wednesday which should give all of us who use a GPS pause for thought. You may or may not know that the GPS constellation of satellites, which was put aloft by the Americans, is only one of four global navigation satellite systems (GNSS) in the world1.

GPS constellation The others are the EU's Galileo (still in the early stages of roll out with only four of the final thirty GNSS satellite constellation in orbit), the Chinese BeiDou (currently only operational over China using three satellites but again planning a global thirty five GNSS satellite constellation) and the Russian's GLONASS.

GLONASS is already fully active, just like the American's GPS, with a twenty four GNSS satellite constellation giving global coverage, and on Wednesday it stopped working properly rendered the system completely unusable for eleven hours.

"So why should this give me pause for thought?" you may be asking yourself. Well, for the technically minded it appeared that what happened was that bad ephemerides data were uploaded to the satellites and this is what every GNSS relies on to ensure the satellites accurately report their position and hence allow your GNSS receiver to accurately report your location by listening to the data coming from the satellites, which includes the ephemerides data2. This is significant because essentially every GNSS uses the same system, not just GLONASS. So America's GPS system could have the same sort of outage, as could Galileo when it becomes a player.

Hopefully we've got your attention now. It's not as simple as carrying a back up GPS (and these days most of us have, in our phone). You might suddenly find yourself without a reliable fix of your location as every GPS you have is affected. So your GPS might lie as to where you are by a lot or, in some ways worse, by only a little.

We love our GPS3, but we use it mostly to record where we've been not to work out where we are and where we're going. For that we still rely on a map and compass. It's nice to have a GPS handy to be able to double check our position but we wouldn't want to rely on that alone and this incident has reminded us why.

  1. There are two more, the Indian IRNSS and the Japanese QZSS but they only have regional coverage.
  2. GPS World has a full report of you want the details.
  3. We mainly use a Garmin Foretrex 301 for our walks.


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