To: Birding-Aus <>
Subject: Re GPS
From: Paul Taylor <>
Date: Thu, 27 Mar 2003 23:13:26 +1100
Paul Foxworthy wrote:
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.

I found this interesting website showing the specifications of a Russian
GPS jammer:
The range stated is up to 200km, so the chance of a jammer in Iraq causing
problems in Australia is precisely zero.  (Curiously, the second picture
shows what appears to be a map of Iraq - there have been recent news
reports that Russian companies have been supplying military equipment to
Iraq in violation of UN sanctions.)

> 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.

I don't believe GPS signals are reflected (the frequencies are too high?),
but they are affected by ionospheric and tropospheric refraction which
also reduces the accuracy (but to a lesser extent than SA.)  GPS signals
can be reflected off buildings, fences, the ground etc. resulting in
multipath errors - the same phenomenon that causes "ghosting" on TV
pictures.  In situations where the signal direct from the satellite
is blocked but a reflected signal is visible, this can affect the
reading since the apparent distance to that satellite is longer than
is actually the case.

   Paul Taylor                                  Veni, vidi, tici -
                           I came, I saw, I ticked.

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