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PDOP - How to Improve The Reliability Of Your RTK GPS System
Does your auto-steer system drop out of auto-steer for no apparent reason? And then a few minutes later or even an hour later everything seems OK. Whats going on. This article hopes to shed some light on what could be at the source of the problem.
First a primer on how an RTK GPS works. An RTK system is made up of both a rover GPS unit (the Tractor) and a fixed base station or group of base stations as in the case of a CORS network(continuously operating reference stations). A base station has a fixed known position, it has software that compares this known position to the position that is calculated from the data sent to it via the GPS satellites. The difference between the known position and the calculated position is the correction. For each satellite that the base station sees, it calculates a correction. This correction information is then sent to the rover unit, which can then use this information to correct its own position.
To calculate a 3D position both rover unit and base station require a minimum of 5 satellites. Since the base station and rover are separated by some distance it is possible and common for the rover unit to see a different set of satellites then the base station.
But in an RTK system, the base station and rover unit must use the same set of satellites when calculating their position. So they have to find a set of at least 5 satellites that they have in common.
The GPS satellite system is comprised of many satellites. These satellites are in constant motion and throughout the day the satellites that are visible to a given location are constantly changing. In calculating a Lat/Lon, it has been shown from geometry, that the more spread out the satellites are the more accurate the calculated position will be. Since the satellites are in constant motion there are times when more satellites are visible and it is easier to pick a set of satellites that are spread out (Figure 1), and there are times when fewer satellites are visible and a suitable set of satellites are not spread out (Figure 2). In addition, as satellites approach the horizon their radio signal may be blocked by trees or land forms (hills).
 
An algorithm has been developed that quantifies the accuracy of the calculated Lat/Lon from the set of satellites that are currently in view to a particular point. This number is called PDOP (Percent Dilution of Position). The lower the PDOP number the better the accuracy of the calculate Lat/Lon. A PDOP from 1 to 3 will usually indicate that the GPS system should be able to calculate an accurate Lat/Lon. A PDOP from 3 to 5 may not produce a very accurate Lat/Lon. When a PDOP is greater then 5, the Lat/Lon should not be used. So PDOP can be used to determine whether the measurements coming from the GPS system can be trusted. If the PDOP number is low, an auto-steer system can trust the results, if the PDOP number is high, the auto-steer system should stop the rover unit.
All RTK systems, John Deere, Trimble, AutoFarm, Leica, etc are affected by PDOP. The nice thing about PDOP is that it can be calculated in advance, not just minutes, but days and months in advance. And it can be calculated on a minute by minute basis. So it is possible to see which times during the day an auto-steer system should run trouble free and which times during a day when an auto-steer system would have trouble navigating. (See figure 3). When the PDOP is below 3 there should be very few problems. When the PDOP is between 3 and 5, problems near tree lines and other obstructions may become more noticeable. When the PDOP is above 5 an auto-steer system may not function at all.
Figure 3
It is possible to be proactive and minimize problems caused by poor PDOP. Software is available from John Deere, Trimble and other sources that will calculate the PDOP for any day and Lat/Lon combination. This allows a user of RTK GPS to plan ahead so that times of poor PDOP can be avoided or to at least know that when problems do occur that by waiting for some time period the PDOP will improve and the operation can then continue.
For more information on this subject contact your precision ag specialist.
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