Differential GPS pushes our capabilities beyond those of GPS. Differential GPS, or DGPS, can yield measurements good to a couple of meters in moving applications and even better in stationary situations. This advanced technology has had a profound effect on the importance of GPS as a resource, becoming a universal measurement system capable of positioning on a very precise schedule. Read more here about the history of GPS and how Differential GPS formed as a result.
The History of GPS
To understand the advantages of Differential GPS, an explanation of GPS is first necessary. The GPS system, designed and implemented by the US Department of Defense, became fully operational with 24 satellites in the mid 1990’s. These satellites orbit the earth in a medium earth orbit, which is approximately 23,000km above the earth’s surface.
Satellites in the GPS system continuously transmit messages containing information on timing, their position in the orbit, and the general health of the system and estimated location of other satellites in the orbit. A GPS receiver works by receiving this message from a minimum of four satellites and performs a trigonometric calculation. Based on the information in the satellites message and transmission time of this message, a GPS receiver is able to calculate its position.
The Evolution of GPS Accuracy
If a GPS receiver was being used in a perfect vacuum, the time taken for the satellite’s radio signals to reach it would correspond exactly with the speed of light – 186,000 miles per second. Unfortunately, because the signals must pass through the Earth’s atmosphere, they are subjected to a number of factors that can slow them down.
The signals are transmitted in the high frequency L-band, which is highly resistant to interference, but charged particles in the ionosphere as well as water vapor in the troposphere can play their part in an unpredictable way. Once on the Earth, the signals may bounce off other objects or landscape features to cause local multipath errors. Although the signal that reaches the GPS unit’s antenna directly will be the most accurate, echoes of the same signal received from buildings, mountains or other objects will blur its accuracy in the same way that they can cause the more familiar ghosting sometimes experienced on televisions.
Other potential sources of error may also exist; these can be caused by the actual satellite not positioned where it should be in its orbit causing errors called ephemeris errors. The GPS receiver may also be less than perfect and cause internal errors of its own. Most significantly, however, are errors caused by a deliberate distortion known as Selective Availability (SA).
The GPS system was originally intended primarily for government use, the Department of Defense wanted it to be available for civilian use, but at a different accuracy. Initially the highest quality signal was reserved for military use, and the signal used for civilian purposes was known as Selective Availability. Selective Availability added an ambiguity to the signal which affected the accuracy of the position determined by a GPS receiver. In 2000 Selective Availability was turned off, significantly improving the accuracy of positions gained by a GPS receiver.
Correcting the Errors
The answer to the problem of GPS inaccuracy lies with Differential GPS. This was devised as a simple system for correcting the errors simply by measuring them. With the exception of multipath errors and the technical shortcomings of individual receivers, the factors affecting the accuracy of GPS positioning will be common throughout an area that may cover thousands of square kilometers. All GPS users in the region will be tracking the same satellites and the GPS signals will be passing through much the same distorting layers of atmosphere.
By sitting a GPS receiver on a very precisely surveyed location, it becomes a simple matter to identify the extent of any errors in the satellite signals. The true location of such a reference receiver, or reference station as it is known, will be surveyed to within a few millimeters using a highly accurate international scientific reference framework. The reference station will compare the time taken for each signal to reach it with the time that it knows the signal should have taken. It is these time differentials that are provided to the mobile receiver – hence the name Differential GPS.
Current DGPS Practices
Traditionally DGPS was only received by a mobile GPS receiver through some form of radio link. The nature of this link varies according to the DGPS service being used and can include terrestrial radio beacons or communications satellites. The corrections themselves are usually transmitted in a format known as the RTCM SC-104 protocol. This is an internationally agreed standard that enables any mobile GPS receiver equipped with the appropriate box to receive, understand and apply the corrections.
The problems associated with terrestrial radio beacons are more pronounced due to the properties of the MF frequency. This is more susceptible to electromagnetic noise and weather conditions such as thunderstorms, atmospheric distortion, multipath, masking and the receiver simply being beyond the range of the radio transmissions. Because of the curvature of the Earth, these may typically only be received up to 200 km away.
Communication satellites are now providing a solution to this problem. Instead of the correction messages being sent from a beacon at ground level, they are uplinked to a satellite in geostationary orbit above the earth.
The satellite then re-transmits the corrections, which may be received by users anywhere within a vast area of the Earth’s surface. Together, they cover the Earth’s entire surface with the exception of the north and south poles, which are masked by the curvature of the planet. These signals are relatively low powered and require a big antenna comparable to a satellite TV receiver.
This is not a problem for large, relatively slow moving users such as ships or offshore oil drilling rigs. They are, however, unsuitable to be used by GPS receivers in the agriculture, mapping, or survey/construction industries.
Since 1995, a new type of communications satellite has become available that transmits telephone, television and other services via powerful spot beams. These are focused on specific geographical areas and are now being used by services such as LandSTAR-DGPS and SkyFix to transmit differential corrections. Because of their power, the signals can be received by modern antennas.