Introduction to GPS
GPS is a radio-based navigation system capable of providing an exact three-dimensional position (latitude, longitude, and altitude) anywhere on the earth, 24 hours a day, in any weather condition. GPS consists of three components: space, control, and user segments. The space segment is a constellation of 24 active satellites—as well as a few spares—orbiting the earth at a height of approximately 12,600 miles in six evenly distributed orbital planes. The control segment consists of five tracking stations spread out around the earth that monitor the satellites’ orbits and send precise orbital data and clock corrections back to the satellites. The user segment is made up of GPS receivers and the user community. GPS was originally developed by the U.S. Department of Defense (DoD) for military use. However, GPS has also proven to be a very useful tool for civilian use and is available to anyone with a GPS receiver.
Each GPS satellite transmits signals on two frequencies: L1 (1575.42 MHz) and L2 (1227.60 MHz). The L1 frequency contains the civilian Coarse Acquisition (C/A) Code as well as the military Precise (P) Code. The L2 frequency contains only the P code. The P code is encrypted by the military—using a technique known as anti-spoofing—and is only available to authorized personnel. The encrypted P code is referred to as the Y Code. Civilian GPS receivers use the C/A Code on the L1 frequency to compute positions—although high-end survey grade civilian receivers use the L1 and L2 frequencies’ carrier waves directly. Military GPS receivers use the P (Y) Code on both L1 and L2 frequencies to compute positions.
New GPS satellites also transmit on the L5 (1176.45 MHz) frequency. However it will be some time before there are sufficient GPS satellites transmitting the L5 frequency and before there are readily available GPS receivers that are capable of receiving the L5 frequency.
GPS receivers monitor these signals from multiple satellites—at least three for a two-dimensional position and at least four for a three-dimensional position—and through a process called trilateration, they compute a position. This position is accurate from about 10 to15 meters—now that selective availability, an intentional degradation of the satellite signals, has been turned off—down to a centimeter or less, depending on equipment and conditions.
Although GPS receivers give you exact positions—for example, 34° 28' 18.8765"N, 122° 15' 34.0832"W, 302.56 meters elevation—it is important to understand that there is some amount of uncertainty, or error, inherent in these positions. A number of factors contribute to this error including satellite clock drift, atmospheric conditions, measurement noise, and multipath. In addition, due to the satellite geometry, vertical accuracy (elevation) is generally one and a half to three times worse than horizontal accuracy. You should consider each GPS position as a box, and you are somewhere within that box. The size of that box depends on the overall accuracy of your GPS receiver.
The accuracy of GPS receivers can be improved by using a technique known as differential correction, or differential GPS (DGPS), to reduce some of the error. DGPS involves using a stationary GPS receiver, called a base station, at a known location—an accurately surveyed point—to calculate corrections for each satellite it is tracking. The corrections can be calculated by comparing the known location of the base station to the GPS location determined by using the satellites. These corrections are then applied to the satellite data received by your GPS receiver, resulting in positions that are accurate from about five meters down to less than one meter for a civilian C/A Code receiver, depending on the receiver.
There are two approaches to DGPS: postprocessing, in which the corrections are stored and then applied to the field GPS data back at the office after the data collection is complete, and real-time, in which the corrections are broadcasted from the base station to the field GPS receiver as soon as they are calculated. Real-time DGPS allows the corrections to be applied almost instantly so that you can begin to work immediately with the more accurate GPS positions . In addition, accurate in-field GPS navigation requires real-time DGPS. Many modern GPS receivers have builtin real-time DGPS capabilities or support add-on real-time DGPS components. There are various sources of real-time DGPS signals, including Coast Guard beacons; Satellite Based Augmentation Systems (SBAS) such as the Wide Area Augmentation System (WAAS), a U.S. Federal Aviation Authority (FAA) system of equipment and software that supplements GPS accuracy, availability, and integrity; FM-based services; and satellite-based services. The U.S. Coast Guard beacons and WAAS services are free of charge. Other services may require a subscription fee. Since real-time DGPS calculations are handled internally by the GPS receivers themselves, ArcPad supports this form of DGPS. Postprocessing requires additional file formats, proprietary protocols, and additional software and therefore is not supported directly by ArcPad.
Another way to improve the accuracy of GPS positions is by averaging multiple fixes at the same location over time. For example, instead of taking a single GPS position at a particular location, you can stand in the same position for 30 seconds and average all the GPS positions you receive during that time to produce one final position. An averaged position tends to be more accurate than one single position.
Measures of accuracy
There are several indicators of the potential accuracy of particular GPS positions. Dilution of Precision (DOP) is probably the most common indicator and is output by most, if not all, modern GPS receivers. DOP indicates the quality of the geometry of the GPS satellite constellation at a particular time. A higher DOP indicates poor satellite geometry and a potentially less accurate position than a lower DOP. There are several expressions of DOP—for example, horizontal DOP (HDOP) and time DOP (TDOP)—but position PDOP (PDOP) is the most commonly used. A PDOP value of six or less is generally acceptable. By only capturing GPS positions with a low DOP, you tend to capture more accurate positions.