GPS navigation units

To: "Birding Services Brisbane" <>, "Birding-aus" <>
Subject: GPS navigation units
From: Paul Foxworthy <>
Date: Wed, 26 Mar 2003 21:40:43 +1100
At 02:17 PM 26/03/2003 +1000, Birding Services Brisbane wrote:

I had not used two Magellan GPS units for a few months until last week. I have had great difficulty getting them to give readings here at Nundah in Brisbane. They will usually find one or two satellites but not lock in to a position. Both have new batteries and have been recalibrated. There are no obvious obstacles in the way - trees, buildings etc. A friend at St Lucia is also having problems.
Are others having problems too?   Is George Dubya to blame?

According to this story on Forbes

the US military has no intention of reducing the accuracy of civilian GPS by restarting Selective Availability (SA) which was turned off in the year 2000. What they can and quite possibly are doing is using jamming to distort GPS signals over a small area. That shouldn't affect us in Australia.

According to a discussion at

on Slashdot ( - "News For Nerds, Stuff That Matters"), there are many areas in the world that are having problems with GPS, not due to jamming, rather because one satellite went dead and hasn't been replaced. The current plan is to spread out the sats in that orbital ring (the B plane) to help fill up the gap but that will result in more outages in more places for short times compared to the current 1/2 hour outages seen directly in the flight path. The <>NavCen ( are recommending that you change your mask angle to 5 degrees if it is set higher (many people use 15 degrees).

You can see the problems on <>this map ( . The black areas are where GPS isn't going to give a 3d position and the red areas are where it wont get a 4d (3d+time) fix. The dark blue will have issues if any part of the sky is blocked.

Sorry to say that the Pacific coast of northern NSW and southern Queensland is dark blue... Maybe check your mask angle and see if setting it to 5 degrees makes a difference?


Paul Foxworthy

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