Understanding GPS

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The US GPS system is a collection of 24 space vehicles placed in an approximately 12 hour orbit at +/- 55 degrees lattitude around the earth. By knowing exactly where these satalites are with regard to the earth, comunication with a GPS device can be trilaterated to calculate it's postition as well. GPS delivers to us 4 peices of data. Longitude, Latitude, Altitude, and Time.

Earth is a rock

The Earth is not spherical. Neither is it an ellipsoid, although we use an ellipsoid to approximated it. The Earth is actually a geoid, or shaped like a rock.

The System

The standard for this is constantly in a state of flux, but at this moment, the system is defined by the WGS-84 standard.

[WGS84 Implementation Manual]


"A plane (as in a flat surface) at 55 degrees to the equatorial plane. That orbital plane will have at least 4 SVs. Repeat this every 60 degress for a total of 6 orbital planes" abadfish

Europe has been working on developing their own Galileo GPS system, mainly in the interest on not relying on a US system that can be locked out as a negotiating tactic. Russia uses it's own GLONASS system.


WGS-84 Implimentation Manual, p.70


The earth surface can be approximated as an Oblate Sphereoid or Oblate Ellipsoid. The GPS system uses a definition contained in WGS-84. This model has an equitorial radius of 20925646 feet and a polar radius of 20855487 feet.

F = (x/a)^2 + (y/b)^2 + (z/c)^2 where a = b = 20925646 and c = 20855487.

WGS-84 Implimentation Manual, p.81

Hardware Suppliers

Trimble - Highly accurate GPS. Survey grade.

External Links

Wikipedia - GPS
Wikipedia Category - GPS

Trimble - Understanding GPS