Re: Peruvian 757 crash -- possible caus

Date:         01 Dec 96 04:08:52 
From:         Reid Fairburn <>
References:   1 2
Next article
View raw article
  or MIME structure

At 03:36 AM 11/23/96, you wrote:

>There are problems with GPS - even for the equipment that is fitted to
>transport aircraft.
>There are 'holes' in GPS coverage that move around the world (including the
>US), so you can't guarantee accuracy - although you can predict where the
>holes are.

To plan on using the GPS for navigation, the predictions must be run and
coverage verified before the flight.  Not a problem...since the holes are
fewer and smaller with the number of satellites in orbit these days.  Used
to be a big problem.

>Another problem is that the NAVSTAR (and the GLONASS) failure rate is too
>big, and you don't know when you have big errors and when you don't.

I would like to know exactly what the failure rate is for the GPS chain...
It would seem to be very low as I can recall one unplanned problem out of
three years of operation.  Do you have some numbers on this?  Also, the
commercial systems all use RAIM (Random Autonomous Integrity Monitoring)
which will utilize more than 4  satellites at the same time to verify and
isolate bad NAVSTARs.  If the RAIM function is not cannot
use the GPS for critical navigation tasks. With RAIM active, you know who
is bad and who isn't.

Reid Fairburn
Creative Kingdom, Inc.