HOW YOUR GPS WORKS

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TWO OTHER ELEMENTS OF THE GPS SYSTEM

In addition to the receiver we must have a set of satellites in the sky and a method of updating the data in each satellite. There are full time land based sites that monitor the various satellites, which are often referred to as Space Vehicles, SV's. These land based sites check the health of the SV's, check how close they are to their optimum orbits, check the clock accuracy, and send adjustments as needed. The land based sites are located a precisely known positions so that they can verify the operation of the satellites.

The satellites are travelling around the world 11,000 nautical miles high in carefully controlled orbits at a speed that means they will make a complete orbit twice a day. Each orbit takes 11 hours and 58 minutes, so like the stars they will seem to drift 4 minutes a day. The complete constellation consists of a minimum of 21 SV's and 3 working spares. Currently there are 27 total satellites in the sky and it is possible that there could be as many as 31 or 32. There are 6 orbits with multiple satellites in each orbit as depicted in the drawing at the top of the page. Each orbit is inclined 55 degree from the equator and thus there are no orbits that go directly over the poles, but certainly a great many orbits can be seen from the poles or anywhere else on the earth. The goal of the system is to always provide at least 4 satellites somewhere in the visible sky. In practice there are usually many more than this, sometimes as many as 12.

Each satellite contains a supply of fuel and small servo engines so that it can be moved in orbit to correct for positioning errors. With update control from the ground units it can maintain an essentially circular orbit around the earth. It also contains a receiver to get update information, a transmitter to send information to the gps receiver, an antenna array to magnify the weak transmitter signal, several atomic clocks to accurately know the time, control hardware, and photoelectric cells to power everything.

MORE DETAIL ON CALCULATING A RECEIVER POSITION

The steps involved in calculating a position are:

1. Sync with an available satellite and download the navigation information. (See the obtaining a fix section for more details.)
2. Convert the messages to internal format for calculation. These include clock information, ionosphere data, and ephemeris (orbit) data.
3. Calculate the exact satellite position. This will include both the elevation and azimuth data so we can apply troposphere modeling corrections that are dependent on how far above the horizon the satellite is.
4. Calculate the pseudorange data and then correct for ionosphere and other modeling errors. (Note that consumer units may not compensate for ionosphere or tropospheric errors.)
5. Repeat these steps for each available satellite. On a Garmin we will initially attempt to find 3 SV's starting directly overhead and compute a 2D fix using the previous fix altitude (or data input by the user).
6. Correct the SV position for earth's rotation based on the time it takes for the signal traversal using the pseudo range data. (If the internal clock is close this can be done once, otherwise it will have to be repeated after the receiver position is computed.)
7. Correct using differential data if available. (This may have to be done after the initial position is computed as part of the refinement step if the internal clock isn't accurate.) If the differential station is near the gps receiver it will be able to skip the corrections for modeling errors since this is part of the correction data available. Using dgps corrections leads to accuracy considerably beyond the capability of a standard receiver.
8. Calculate the initial receiver position as described in the prior section.
9. Convert the data based on whatever datum and grid system you have chosen and display the answer on the position page. Altitude is also corrected for geoid height prior to display.
10. Add in the leap seconds and time offset from UTC time to the computed time data and convert it for display.
11. Refine the position based on additional satellites and the correct time to obtain a 3D fix and subsequently improve the fix based on choosing SV's with a better DOP, applying an overdetermined solution, etc.

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