The global positioning system we depend on for our farms is owned and operated by the U.S. Department of Defense and paid for by American taxpayers.
Of the many satellites that orbit the earth in six different orbits, at least 24 function at one time.
The arrangement of the constellation of satellites ensures that no place on earth should see less than four satellites at any given moment. These orbiting GPS satellites travel fast and may orbit the earth as often as twice a day.
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A GPS position is calculated from these satellites with triangulation by timing the signals sent from the satellites to the GPS receiver.
GPS satellites transmit two low frequency radio signals called L1 and L2. Civilians use L1, which work by line of sight so they travel through clouds, glass and plastic but not solid obstacles such as trees or buildings.
Each satellite can calculate how far you are from it because it knows how fast its signal travels. Although we call the process triangulation, we actually need at least four satellites to accurately calculate a position. This is called autonomous GPS and is 15 to 50 metres accurate, which is not nearly enough for agriculture.
We require a differential signal from a differential satellite to calculate an accuracy that suits our needs. This is usually referred to as DGPS. Differential satellites follow the earth’s rotation and therefore stay in the same spot over the equator.
These differential satellites communicate with base stations on earth that know their position and can greatly increase our accuracy.
The lines will still drift occasionally because differential still uses satellites in an orbit and the correction takes time to process.
WAAS is a differential signal that’s free to use, paid for once again by American taxpayers, and uses two satellites labelled PRN 135 and PRN 138. PRN 135 is almost directly south of the Manitoba-Saskatchewan border while PRN 138 is more westerly. Your location will determine which WAAS satellite you should be using. Both satellites were launched in 2007.
Private companies also operate differential correction satellites.
Omnistar is the most common with three accuracy levels:
- VBS at an accuracy of about 12 inches.
- XP has an accuracy of three to five inches.
- HP has an accuracy of two to four inches, the same as John Deere’s SF2.
These signals require a yearly subscription fee and use L1 and L2 signals from GPS satellites. A dual frequency receiver is needed to access them.
Real Time Kinematic (RTK) is an alternative to the satellite differential signals. Operators buy their own base station that can be permanently mounted or mobile on a tripod. The accuracy of RTK should be less than one inch but sometimes varies.
The base station still uses GPS satellites but does a real-time correction in the field without the differential signal. Some producers choose to mount their RTK base station on a high tower to minimize blockage from trees or buildings.
The range of an RTK system depends on the company, ranging from six to 16 kilometres. By placing the RTK base station in exactly the same location every time, operators can achieve repeatability and use the same lines with no drift.
Many factors influence the GPS signal you receive.
- Atmosphere can delay the signals, which changes the distance and can offset the position. The GPS has software to help correct this.
- Multipath occurs when a signal bounces off a building and lengthens the time it takes to get to the receiver. This is like the ghosting effect on television when using an antenna.
- Sometimes there are satellite clock errors or a slight change in a satellite’s orbit. These are usually downloaded by the GPS during start up and should be corrected in the software.
- Dilution of precision (DOP) is the measure of distance between satellites. In a perfect scenario, you want lots of satellites but you also want them to be spaced out for better coverage. DOP provides a number that tells you how well the satellites are spread. Depending on the scale the company uses, most will have perfect spacing, signified as a DOP of one.
There are different types of DOP. The two most common for agriculture are HDOP, which is horizontal, and PDOP, which is positional.
If your accuracy was about six inches all day and your DOP is one, then six multiplied by one equals six inches.
However, if your DOP number jumps to four, then your six inch accuracy multiplied by four equals 24 inches. DOP jumps when the satellites are all bunched together in one part of the sky.
If your GPS flashes “high DOP,” there is nothing to do but wait. It should take five to 10 minutes for the satellites to spread out. Depending on your position on the earth, this will probably happen about twice a day. Free programs are available on-line that will predict when these “high DOP” times will occur, based on your location.
Pamela Haegeman grew up on a small farm at Swan Lake, Man. She has an agricultue diploma from Olds College, a GIS diploma from Assiniboine College and a geomatics degree from Brandon University. She is the GPS/GIS specialist for Mazur Group in Brandon and the Manitoba Zero Till Research Farm. She can be reached at < ahref="mailto:phaegeman@yahoo.com">phaegeman@yahoo.com.
