GPS
Erik Rydell, IxD 2, October 2010
History
- GPS which stands for Global Positioning System, became fully operational in 1995. It is owned and controlled by the American Ministry of Defence.
- In the 70’s, the armed forces in America wanted an accurate positioning
system which had to be available day and night, regardless of any weather
condition. In order to investigate in the project, the US government demanded that the system should be available for everyone in the society.
- P -code (Position code) and CA- code (Coarse Acquisition which is information about the present time) were created. P- code is coded so
that only GPS receivers with special decoders could receive and understand the signals. The civilian GPS system became more accurate than
the army thought. They solved this problem by creating Selective Availability (SA) which send out incorrect information every now and then to
other users accept from the US army. When Europe started to design
their own positioning system, Galileo, they stopped using the SA- code.
How it works
GPS is used by both professional users and private persons in different
situations and different weather conditions. The wavelengths is very short
which means that it is quite safe from interferences from other data traffic.
The wavelength is dependent on the frequency, see the illustration. On this wave, two binary signals has been modulated; the carrier wave and a data wave (CA -code). Together,
they create a signal with code that is unique for each satellite.
The GPS system consists of 24 satellites and they rotates around six
axises. The GPS receiver that the user has, receives data from satellites about their routs. The receiver has to have contact with at least
three satellites in order to get an exact position (in 2-D). For a 3-D po-
GPS data code (carrier wave, CA- code and carrier wave
with CA- code). The data stream is sent by 50 bit/s.
24 satellites rotating around six axises.
sition (with height), the receiver has to be connected to four satellites.
In order to estimate the position, the GPS receiver creates a copy of the
code that has been sent out by the satellite and counts the time difference between different defined time- stamps. The time difference is a
measure of how long time it took for the signal to reach the receiver.
The time is then multiplied by the speed of light which ends up with
the distance between the satellite and the GPS receiver and gives the
position to the user. By calculating the distance between the different satellites, one will get the radius to the spheres. The GPS receiver’s
position (the black ´x´) is in the intersection between the three circles.
Data code.
Time shifting.
Alternatives to GPS
For hobby users there are no alternatives to GPS in terms of price, reliability or comprehensiveness. However, there exists other systems too.
* The Russian system Glonass has existed since 1980’s. It is not fully operational and it is expensive.
* The European project Galileo started in the beginning of the twenty-first
century. Due to it has been delayed several times, it has become expensive to run.
References
Handbok i GPS, Praktisk navigering till sjös och på land,
Magnus Lindén, Naturiska förlaget (2002).
Navigatorboken, om navigering med plotter och GPS,
Jonas Ekblad, Bilda förlag (2008).
Other documents
http://www.findmespot.com (see: spot_gps.pdf)
Time difference.