Monday, July 28, 2014

Dead Reckoning

As usual I have flight-planned for one task and the onboard satellite communications link to base brings us instructions to do something else. We're to fly direct to a particular airport, and get a maintenance extension from an approved maintenance organization there. We can do that. I punch it in on the GPS, O Mighty Box that guides us safely from place to place.

The GPS in this particular airplane is not that mighty. It's a nice colour moving map, WAAS capable IFR installation, but it has a habit of losing GPS position for exactly two minutes at a time, about twice a day.  We don't fly GPS approaches in this airplane. The database isn't up-to-date and it has a great ADF and VOR.

It's almost nice to have the one or twice a day reminder to keep my navigation skills honed.  DEAD RECKONING says the screen, continuing to show the little airplane symbol, assuming that we're continuing on the same track and speed.  People say that it used to be deduced reckoning, reduced to ded. reckoning, and then converted to dead. I don't know if that's a reverse etymology or not, but some people get quite stuffy about spelling it DED. Garmin does not agree. And two minutes later, almost to the second, it knows where it is again. It isn't a satellite configuration issue. It hasn't lost power. I haven't banked excessively, blocking the GPS antenna. it just does it. We track the issue for a while, trying to troubleshoot but never find its periodic outages corresponding to time of day, time in operation, position, anything.

Eventually when the plane was down for maintenance long enough for this to happen, we pulled the entire unit and sent it in for repair. Like all electronic devices these days "repair" consisted of us receiving a reconditioned model of the same type. It hasn't lost GPS position since, and we bought a database subscription for it, so now I trust it with my life. Or at least with my acute embarrassment.


Sarah said...

I have flown with the same model ( from misty recollections of old posts ). And yes, I've seen those ominous red letters. DEAD RECKONING. Strangely, the much cheaper portable on the yoke kept working fine, but the installed unit lost the signal. It hasn't happened in a long time - we had the antenna checked. So I guess I trust it, but not as much as I trust the one and only engine.

Flying in the north, do you find GPS coverage lacking, signal drops? I think I've heard some far north operators use the Russian sat. system, GLONASS

Traveller said...
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Traveller said...

Not sure of the trigger, but the time to acquire the "bootstrap" data from the satellite is 2 minutes.

Sounds like the old unit was occasionally dumping the data, or marking it invalid, and then reacquiring the satellites.

If you get far enough North, you have all low angle satellites. This makes the derived altitude less accurate. I haven't been far enough north recently to check, bu t I tended to have more satellites visible in the Far North than closer to the equator.