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GPS Satellite Coverage - Scotland designated a Polar Region!

At the March Equator City workshop in Glasgow various tests were carried out using GPS. The results of these tests were disappointing in three main respects:-

Following the workshop further research has been carried out into satellite paths. Figure 1 shows these paths in the sky over Bristol over a 24hr period from the 7th to the 8th April.

azi_el_2.gif

Fig. 1 - Satellite Paths over Bristol

The green tracks are the satellite paths

The blue lines show the limits of visibility of the antenna

The red text indicates a satellite in view

The grey text indicates a satellite not in view

n.b. this figure was produced using VisualGPS downloaded from Apollocom.

The circular area (or 'bald patch') is an area of sky where the satellites are not present - this is the 'polar region' as defined by the GPS system designers. In this plot it appears to start somewhere just north of Bristol (it's actually at latitude 55deg North).

The Department of Geography, University of Texas, has a comprehensive (but sadly not up-to-date) GPS website which includes Figure 2, shown below, confirming that the GPS satellite paths do not extend directly above Scotland (or further north).

svs27.gif

Analysis

The lack of direct coverage over Glasgow explains the poor accuracy and loss of fix. The apparent functioning in E-W streets was probably due to reflections from buildings on the north side of the street, and hence the readings were likely to be of dubious quality (though the receiver may have indicated otherwise).

Urban Canyon illustrations:

gps_sing.gif gps_bris.gif gps_sing.gif
Singapore 1deg 17min N.
Bristol 51deg 26min N.
Glasgow 55deg 52min N.

Alternatives

There are two other global satellite positioning systems under development - GLONASS and Galileo.

GLONASS is the Russian GPS system which has a similar satellite configuration to the U.S. Navstar GPS, and has been designed to provide improved coverage in 'polar regions'. Nine of the 24 satellites have been launched, but two are already defunct. Receivers for GLONASS are available from Russia but appear to be bench mounted. They're probably not much use anyway with only seven operational satellites.

Galileo is the European GPS system currently in the news - it looks as if it's going ahead but won't be operational until 2008. One of it's stated aims is to provide coverage for Northern Europe which is better than GPS.

Recommendations

Clearly if a GPS fix is required in the British Isles, a view of the southern sky is imperative. Attempting to get a fix with an adjacent building (or wood) to the south of the user will result in failure, or a false reading.

This, combined with the poor dGPS coverage in Western Scotland, will make position sensing in Glasgow City Centre using GPS especially difficult, if not impossible. If we are to continue in the streets of Glasgow alternative positioning systems will need to be seriously considered. The most likely candidates are vision in conjunction with the city model; predictive techniques using accelerometers/gyroscopes; or a combination of systems.

Cliff Randell
April 2002

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This page last updated April 26, 2002
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