How does GPS work?
GPS works in five logical steps:
- "Triangulation." By very accurately measuring the distance of a location from three satellites, it is possible to "triangulate" a position anywhere on earth
- An accurate measure of distance. To measure the distance to satellites, the GPS receiver uses the travel time of radio signals.
- Perfect timing. To measure the travel time, GPS requires extremely accurate timing.
- Exact satellite location. To achieve the highest accuracy, the exact location of the GPS satellites in space must be known.
- Corrects errors. Finally, corrections must be made for any delays the signal experiences as it travels through the atmosphere.
Measuring Distance
The distance to a satellite is determined by measuring how long a radio signal takes to reach a location from that satellite. Satellites and receivers generate Pseudo Random Code (PRC), which is a complicated sequence of "on" and "off" pulses. To measure the distance, it is assumed that both the satellite and the receiver are generating the same PRC at exactly the same time. By comparing the arrival of the satellite’s PRC with the receiver’s code, you can tell how much time the satellite’s PRC took to reach the receiver’s location. Multiplying that travel time by the speed of light provides the distance.
Perfect Timing
Measuring the travel time of radio signals is critical. At the speed of light, timing that is off by just a thousandth of a second translates into almost 200 miles of error! Satellites are accurate because they have incredibly precise atomic clocks on board. If receivers needed atomic clocks, they would be too expensive to afford. The solution is to make an extra satellite measurement to remove errors, as a cross-check. The receiver looks for a single correction factor that it can subtract from its timing measurements. That correction brings the receiver's clock back into sync with universal time, and applies that correction to the rest of its measurements to calculate precise positioning.
Exact Satellite Location - Ephemeris
To work correctly triangulation requires not only the distance, but also the exact location of the satellites in space. The Air Force has placed each GPS satellite into a very precise orbit. On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment. The GPS satellites are constantly monitored by the Department of Defense. They use very precise radar to check each satellite's exact altitude, position and speed. The errors they're checking for are called "ephemeris errors" because they affect the satellite's orbit or "ephemeris." These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites. The errors are usually very slight but if you want great accuracy they must be taken into account. The error information is relayed back to the satellite itself, and the satellite then includes this new corrected position information in the timing signals it broadcasts. So a GPS signal is more than just pseudo-random code for timing purposes. It also contains a navigation message with ephemeris information as well.
Correcting Errors
The earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors. The signal may also bounce off local obstructions on the ground before it gets to a receiver, causing additional delays. Some errors can be factored out using mathematics and modeling. Differential GPS can eliminate all error.
Differential GPS
One technique to increase the accuracy of GPS is called "Differential GPS." With differential GPS, you can make measurements to better than an inch. Differential GPS involves the cooperation of two ground-based receivers, one that's stationary and one that makes position measurements. The stationary receiver is used as a reference, and is placed on a point that's been very accurately surveyed. This reference station receives the same GPS signals as the roving receiver but unlike other receivers, it calculates the equation backwards. Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It calculates what the travel time of the GPS signals should be and compares it to what it actually is. The difference is an "error correction" factor.
The Different Kinds of Differential Signals
1. WAAS
The Wide Area Augmentation System (WAAS) was created by the Federal Aviation Administration (FAA) as a free-to-air differential correction service. The WAAS network consists of 25 ground reference stations located in the United States. This network receives GPS signals from all GPS satellites in view. The GPS data is sent to a master control site, where GPS differential corrections and GPS satellite health data are transmitted to geostationary satellites. These satellites broadcast the information to all WAAS-capable GPS receivers, which then decode the WAAS signal to provide real-time differential corrections. WAAS signals are broadcast free to all WAAS-enabled receivers. Currently, WAAS coverage is only available in the United States and parts of Canada.
2. OmniSTAR* VBS, XP or HP
OmniSTAR is a commercial subscription service that provides real-time correction for GPS measurements. OmniSTAR continuously monitors the signals of all GPS satellites at over 90 known locations around the world, a number of which are in North America. The base stations measure the errors from all GPS satellites in view at their locations and compute corrections. OmniSTAR utilizes several commercial geostationary satellites around the world to deliver these corrections to customers.
OmniSTAR VBS (Virtual Base Station) subscriptions are available worldwide within the OmniSTAR coverage area. It provides similar accuracy to WAAS DGPS or to within ten inches.
OmniSTAR XP subscriptions provide GPS accuracy within three to five inches for subscribers worldwide within the OmniSTAR coverage area.
An OmniSTAR HP (High Performance) subscription provides GPS accuracy within two to four inches for subscribers worldwide within the OmniSTAR coverage area.
3. A Real Time Kinematics (RTK) base station
An investment in an RTK base station provides the most accurate correction for your GPS measurements. You can locate the base station in a permanent location, or you can use a mobile unit that is tripod or post-mounted. The base station transmits real-time positional carrier phase data over a radio datalink to the receiver on your tractor or equipment. The receiver is then able to compute its exact position in your field to within one inch accuracy.
Note: Some descriptions on this page have been condensed from information provided courtesy of Trimble Navigation.
The accuracy of any guidance system is dependent on satellite communication, field location, terrain, tractor and implement condition and setup, system installation, and calibration. A field demonstration and trial is the best way to determine actual guidance system accuracy in each situation.
* OmniSTAR is a trademark of OmniSTAR Inc. |
