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GPS Q&A
By Greg May Q.My GPS data collection project requires differential corrections in real-time, however there are a variety of real-time sources I can choose from. Which one is right for me?
M.O. St. Paul, MN A.Differential correction is the standard method of reducing the errors caused by selective availability (S/A) from GPS signals. This improves the accuracy of the computed GPS position from 100 meters to better than 1m, depending on the quality of your GPS receiver. Differential correction can be performed as either a post-processed operation, or in real-time while the user is receiving GPS positions. Real-time differential is important if the user wishes to use GPS for navigation, or if accurate positions are required immediately without returning to the office for post-processing.
A variety of sources of real-time differential corrections are available for users who require real-time DGPS. These are:
¥ MSK beacons
¥ Satellite differential providers
¥ FM subcarrier broadcasts
¥ Private reference stations
Each of these sources transmits differential correction messages via radio frequencies to a radio receiver linked to the GPS receiver, giving a differentially corrected position in real-time. Factors to consider when choosing a source of real-time corrections are:
¥ availability (range) of service
¥ accuracy
¥ cost MSK Beacons
The main purpose of MSK (minimum shifting key) beacons is to provide differential corrections to shipping around the coast, harbors, and navigable waterways of the world. Because these corrections are being broadcast over several hundred kilometers, land-based GPS applications such as mapping and GIS data collection can use the corrections for their own purposes. Availability
Many countries with shipping industries have MSK beacons installed around their coasts, such as the United States, Canada, Iceland, the United Kingdom, and much of continental Europe and Asia. In the United States beacon signals cover both coasts, the Mississippi and the Great Lakes. Even though signals travel anywhere up to 500 km from the site of the beacon, much the country cannot receive these broadcasts. For those who can however, MSK beacons provide a free method of receiving differential corrections.
There are currently plans to implement beacon-style transmissions from the Ground Wave Emergency Network (GWEN) system of broadcast stations throughout the United States. This will significantly increase coverage over land-locked areas. Accuracy
The accuracy of differential correction from MSK beacons varies depending on the particular beacon being tracked. Normally, submeter to 10m can be achieved. The distance you are from a beacon also affects the accuracy; as you move further away from a beacon errors, due to differences in the ionosphere between your location and that of the beacon, become more pronounced. Cost
MSK beacons are usually installed and maintained by government bodies, thus the signals are free for land-based users. Satellite Differential Providers
Satellite differential providers use geostationary satellites to transmit differential corrections to the surface. Differential corrections from a series of ground-based reference stations are sent to a central control center, which then transmits the corrections up to the satellite. The satellite transmits these corrections to an activated GPS receiver which interpolates a correction value based on its current location. The effect is of having a reference station right at your location, no matter where you are or how fast you are moving. This technique is known as Virtual Reference/Base Station (VRS/VBS) technology. Availability
Because these satellites can see huge areas of the surface at once they supply corrections to a large part of the Earth. Much of the world, including the entire continental United States, is covered. Because of the equatorial location of the satellites however, the signal becomes attenuated and eventually disappears in higher latitudes. For this reason large parts of Canada are not covered by the North American beams. Accuracy
The accuracy provided by satellite-based differential corrections is almost as good as results achieved by post-processing, in the range of submeter to 5m. The control center checks and compares corrections as they are received from the various reference stations to ensure that high quality corrections are broadcast from the satellite. Cost
An annual subscription fee is required to activate the service. FM Subcarrier Broadcasts
The Radio Data System (RDS) is a standard developed in Europe for transmitting data over FM radio frequencies. RDS allows data to be transmitted along with regular programs. Differential providers are taking advantage of the existing worldwide coverage of FM radio stations to transmit differential corrections. Availability
Differential corrections are currently being broadcast from selected radio stations, however, coverage is increasing as this method of DGPS becomes more accepted. Coverage from existing stations is in the order of 70 to 100km from broadcast towers. Accuracy and Cost
With current FM subcarrier broadcasts, accuracy and cost go hand in hand. Fees are charged based on the level of service, with higher levels receiving better quality differential corrections. The accuracy ranges from submeter to 10 meters. Private Reference Stations
If none of the above sources of real-time DGPS meet your needs, you can always set up your own reference station and transmit your own differential corrections. In fact this is the way that all differential correction has been traditionally done, before the introduction of commercial services. Setting up your own reference station provides the flexibility to cover your area of interest but adds additional equipment cost. Depending on where you are you may also need to get permission to use specific frequencies. Availability
Setting up your own reference station means that differential corrections are available when and where you want them. The range of transmission depends on the strength of the signal you are broadcasting. As with all radio sources, obstacles such as mountains or buildings may attenuate the signal. Radio repeaters can be used to extend the range of your broadcasts in many circumstances. Accuracy
Normally you can achieve submeter to 5m accuracy from your own reference station; most radios will not transmit far enough for distance degradation to be a factor. You must also have a reliable known position on which to set up the reference station. For relative accuracy this is not critical, however to achieve absolute accuracy a first-order survey marker is preferred. Cost
This is the most expensive option and certainly the most time consuming in terms of setup time. A second high quality receiver and two radios are required. Depending on how often you need to relocate your station you may require additional field crews for scouting and setup. Conclusion
There are four main sources of real-time differential corrections that are available today. When making a decision on which real-time source to use, you must consider factors of availability of service, accuracy of the corrections, and any subscription fees. In most parts of North America all choices are available; in other parts of the world you may be limited to one or even none. In all cases you can set up your own reference station for local differential broadcasts.
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