To: "Duncan Fraser" <>, <>
Subject: Re GPS
From: Paul Foxworthy <>
Date: Thu, 27 Mar 2003 15:40:19 +1100
At 02:12 PM 27/03/2003 +1100, Duncan Fraser wrote:

There have been a few letters about this, so thought I would amplify on my
post. On the Country Hour on ABC Radio yesterday, the spokesman for a firm
in Australia that supplies GPS technology to broad acre farmers, confirmed
that there was interference to the system in Australia due to the Americans
jamming a number of satellites at certain times of the day. It is not all
the time, but it has been causing problems with farmers who rely on the
system for spraying, etc. with GPS guided tractors. DF.

The story is on the ABC site at

I didn't hear the original, but the report on the Web has a carefully worded "He says there *may* be 'selective denial' or jamming", emphasis mine.

Selective availability is not jamming. SA was the deliberate "rounding off" of messages from the GPS satellites so civilian GPS was not as precise as military GPS. SA was turned off in 2000.

Turning SA back on would be bad for business worldwide, not just for farmers in Australia. The Forbes article I mentioned yesterday quoted American officials saying it is most unlikely to happen.

Jamming is broadcasting radio signals at the same frequency as those from the satellites from another transmitter to confuse GPS receivers. If it's happening, surely those transmitters will be in or near Iraq. I doubt very much that they would affect us here. Does anyone know if GPS frequencies are likely to be reflected by the ionosphere? My guess is no, because the signal needs to get from a satellite to earth.


Paul Foxworthy

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