Re: GPS Vs COMPASS

In article <eC8Ge.17129$Hd4.3787@xxxxxxxxxxxxxxxxxxxx>, Steve Ray <nochace@xxxxxxx> writes 
Much snipping made

After very extensivley using my GPS in all kinds of weather conditions and
keeping my Silva 4 in the map bag I've seen many times when my 16 channel
GPS cannot sync with enough satellites to get a good enough fix, and
sometimes locating me off cliffs.  This glitches have normally been put
down
to *very* harsh weather conditions such as pea-soup's.


In practical terms weather does not affect GPS signals - it wouldn't be
much use as a military tool if it did.


I disagree with this comment.  I've worked with Location Systems since the
late 80's including mililary location systems. Weather conditions are very
good at dispersing all kinds of RF signals, thunder storms and VERY dense
cloud for starters. Very dense local cloud cover containing a large amount
of water content can easily divert or impede a signal therefore giving a
smaller QoS and phase changes in signals.

The effect is dependant upon the frequency of the signal. Ground based systems have a limited frequency band if they are to be of use for a greater distance than line of sight. They also have to contend with other ground based radio systems and avoid interfering or being interfered with.

Space based signals do not have the distance restriction for line of sight frequencies so have a far greater range of frequencies to choose from.

Some links say "all weather" etc.
http://www.af.mil/factsheets/factsheet.asp?fsID=119
Bullet point 3

http://www.usace.army.mil/usace-docs/eng-manuals/em1110-1-1003/c-2.pdf
Para 2

http://gpsinformation.net/gpsclouds.htm

http://www.garmin.com/aboutGPS/
1st para 3rd sentence

http://www.thalesnavigation.com/en/products/aboutgps/howworks.asp
Fifth para.

Water vapour and droplets do not pose a problem to the signals. A thin continuous film will begin to degrade the signals and a few mm of continuous water is enough to stop them.

 Thunder storms have the ability
to cause signal phase changes for land based location systems. These need to
be nearer to the transmitter than the reciever first and a not used any more
(US Omega System used by US military, retired some years ago, using VLF)


If you are having trouble receiving a good mix of satellites (so that a
good position is reported) the chances are the sky is significantly
obscured by terrain or your body. It is also a possibility that your GPS
receiver is not very healthy :-( what model is it?


The GPS I use is an entry level Magellen 16 channel Explorist 100.

I think that is a 14 channel receiver - allowing 2 for WAAS/EGNOS and the usual 12 for the GPS sats.

 To
comment on the obscuring the last time I had exceptional bad fixes was on
top of Ben Nevis (May 1 this year) very little to obscur the signal I was on
the summit ridge.  My location given by the GPS was some 30 m from my known
(mapped and nav'ed) location. The GPS was only able to get good signals from
2 GeoSats due to the water content in the cloud I was stood in

A GPS receiver cannot compute a position from the signals from only two satellites without making assumptions as to the altitude and the time (sub microsecond). Any error in those assumptions will lead to position error. For a good 3d position solution four satellites spread out in the sky are required.

The GPS satellites are not geo-stationary but the EGNOS test signal satellites are. Perhaps it was reporting the use of two of those in which case they could have been contributing to the error. In the current test mode the signals sometimes make the position less accurate rather than more accurate. I do not know if you can switch off WAAS / EGNOS use with that unit but if you can't you may well experience a reduction of accuracy at times.

Unless the receiver had just been switched on or was under wet material or was significantly obscured by your body / rocks / wall / backpack it sounds very much like it is faulty.

If the receiver had not been switched on for a while (the time is dependant upon the software but likely to be between one and four hours) and is next used a considerable distance (hundreds of miles) from the last position or with a difference in altitude of a few hundred metres it may suffer some confusion with the initial position solution. This may take up to a couple of minutes of good signals to recover from due to the fact that there is some moving averaging going on in the position solution to smooth out short term errors.


--

Dominic Sexton
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