Glossary
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Overview of the GPS
Overview of the GPS
A global positioning system (GPS) was used for military purposes in the beginning but was expanded to the civilian sector in 1983, and is currently being used in various fields. Satellite signals are received through two carrier waves: L1 (1575.42 MHz) and L2 (1227.6 MHz), which are 154 times and 120 times, respectively, the 10.23 MHz of atomic oscillators (two units each of cesium and rubidium)'s basic frequency. The two frequencies go through phase-shift keying (PSK) through irregular codes, C/A code and P code.
| Category | GPS | GLONASS |
|---|---|---|
| Number of satellites | 32 | 27 |
| Cycle | 11 hours 58 minutes | 11 hours 15 minutes |
| Altitude | Approx. 20,200 km | Approx. 19,300 km |
| Data speed | 50bps | 50bps |
| Angle of inclination | 55° | 64.8° |
| Frequency | 1575.42MHz | 1602.5625MHz-1615.5MHz |
| PN code clock | 1.023MHz | 0.511MHz |
| Geodetic datum | WGS-84 | SGS-90 |
Components & Controls of the GPS
Components of a GPS is largely divided into space stations, ground stations, and users. The GPS ground control work is divided among one master control station (MCS) (Colorado Springs Falcon Air Force and Army), which gives overall commands related to the GPS satellite's orbit correction and presatellite operation; five monitoring stations (MS; Diego Garcia, Ascension Island, Kwajalein Hawaii, Colorado Springs), which are responsible for inspecting GPS satellite signals, tracking/predicting the orbit, and monitoring ionospheric/tropospheric delays; and three ground control stations (GCS) (Diego Garcia, Ascension Island, Kwajalein), which are responsible for managing the antennas that can transmit satellite information (clock, correction value, orbit correction value, messages on users).
Principles of the GPS
The GPS positioning method uses the principle of triangulation. The positioning method used in land and cadastral survey involves determining the location of an unknown point by excluding one unknown point and measuring the other two angles and length of the side. However, the GPS positioning method is different compared to the principle of triangulation as it determines the location of an unknown point by measuring the length of two sides. In other words, the typical positioning method uses the angle and length of two sides, whereas the GPS positioning method uses the length of two sides.
For positioning, it is necessary to know the distance between the location of the GPS satellite and the GPS reception period using triangulation based on the positioning method above. As shown in the figure (How to check satellite signals) below, the satellite adds the C/A code on the L1 (1575.42 MHz) frequency and carries it back. The receiver also generates the same code as the satellite signal, compares it with the received satellite code, and measures the time it took for the satellite signal to arrive to the receiver. When the pseudo range between the satellite and the reception period is measured at the speed of the satellite signal (speed of light), the distance between n satellite and the receiver is calculated. Once the distance is calculated by observing four satellites, the location of the receiver can be derived.
GPS Satellite Status
| Total satellites in constellation | 32 SC |
|---|---|
| operational | 31 SC |
| In maintenance | 1 SC |
Positioning Error
Satellite positioning error is divided into range error caused by structural factors and geometric error increase caused by the satellite arrangement.
Range Error Caused by Structural Factors
Range error refers to the error in the measured range between the satellite and the reception period. It is caused by the following factors and is about five to ten m.
- Satellite clock error
- It refers to error caused by the error in the atomic clock installed in the satellite. Fortunately, a satellite clock error can be predicted to some extent; therefore, the master control station minimizes it by making adjustments
- Satellite orbit error
- Satellite orbit error is predicted based on the data acquired by the monitoring station. The satellite is required to broaden its parameter together with the code information. However, when a difference between the predicted orbit and the actual orbit occurs, range error occurs accordingly.
- Transmission delay in the atmosphere
- Since the altitude of the satellite is about 20,000 km, the signal travels through the atmosphere's ionosphere and troposphere after leaving the satellite until it arrives at the receiver. An error occurs due to the transmission delay that occurs during this process. In particular, the transmission delay in the ionosphere increases when electronic activity in the ionosphere is high. The error becomes smaller around midnight when the activity is weak. The gap in the error is quite significant between different days and seasons. The master control station predicts the delays above and broadcasts that with the code information. The receiver makes corrections accordingly when calculating the position to reduce positioning error.
- Error made in the receiver
- A range error also occurs due to electromagnetic noises or electromagnetic multipaths that occur in the receiver. Such range error is combined with geometrical factors caused by the satellite arrangement, and it translates to a positioning error.
Increase in Geometrical Error Due to Satellite Arrangement
Error increases according to the arrangement of satellites used for positioning. When selecting targets of appropriate intervals like finding a position on the ground through map reading, the cocked hat becomes smaller, which results in accurate location. However, if targets that are being used are clustered together, the cocked hat becomes larger, results in inaccurate locations. Likewise, the positioning error becomes small when the satellites are properly arranged. As demonstrated in the follower formula, the GPS receiver uses the observed data to calculate the position dilution of precision (PDOP). The result is then multiplied by the range error to calculate the positioning error. In other words, range error x PDOP = positioning error. Accordingly, most receivers select a satellite arrangement where the PDOP is small to calculate and indicate the positioning. Receivers nowadays have great capabilities; thus, when PDOP is 3, the positioning error or circular error probability (CEP) is approximately 15 m; that is about 15 m flat with error probability range of 50%.
