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DGNSS of Korea, South Korea's industrial positioning Leading to the Ministry of Maritime Affairs and Fisheries Satellite Navigation Central Office

Overview of GPS

GPS, which had initially been used for military purposes, began to be used in the private sector in 1983 and is now utilized in various fields. It can be used 24/7 anywhere on the planet in all weather conditions and is resistant to external interruptions and disturbances. The GPS location information is reliable and accurate as it uses the World Geodetic System (WGS-84), and the system is comprised of 24 satellites on 6 orbits. The signals from the satellites are transmitted as carrier waves L1 (1575.42MHz) and L2 (1227.6MHz), which are 154 and 120 times higher than the basic frequency (10.23MHz) of an atomic oscillator (two units of cesium and rubidium each). These two frequencies undergo phase-shift keying (PSK) in irregular codes, C/A code and P code. The navigation information is provided by the standard position system (SPS) and the precise positioning system (PPS).

SPS, used to provide location and time information to civilians, can only use the C/A code of L1 frequency. On the other hand, PPS, which is mainly designed for military use, can determine location, timing and speed and uses P (Y) code of L1 and L2 frequencies. PPS is also used by authorized civilians.

GPS & GLONASS
Category GPS GLONASS
# of satellites 24,4×6 orbits 24,8×3 orbits
Period 11 hour 58 min 11 hour 15 min
Altitude Approx. 20,200Km Approx. 19,300Km
Date transmission speed 50bps 50bps
Inclination angle 55° 64.8°
Frequency 1575.42MHz 1602.5625MHz-1615.5MHz
PN code clock 1.023MHz 0.511MHz
Geodetic reference system WGS-84 SGS-90

GPS Structure and Control

The current GPS consists of three major segments: space segment (SS), a control segment (CS), and a user segment (US). The control segment is composed of the Colorado Springs Palcon Air Army, a master control station (MCS), where commands are given with respect to orbit correction and presatellite operation, 5 monitor stations (MS) in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, in charge of inspecting GPS satellite signals, tracking and prediction of orbits, and observation of ionospheric and tropospheric delays, and 3 ground control stations (GCS) in Diego Garcia, Ascension Island and Kwajalein, managing the ground antennas that transmit information on the satellites (clock, correction value, orbit correction value and messages for users).

Positioning Principles of GPS

GPS uses similar principles as triangulation. Triangulation is used in cadastral surveys and surveying methods used in civil engineering to determine the location of a point by measuring the angles and distances to it from two known points. In GPS, on the other hand, the distances of the two sides are measured in order to determine the location of the third point. In other words, the typical surveying method uses the angles and distances to it from two known points, whereas in GPS, positioning is performed using the distances of two sides.

In order to determine the location, the distance between the GPS satellite location and the GPS receiver should be known. As shown in the figure below [Method of Checking Satellite Signals], C/A code carried by L1 frequency (1575.42MHz) is transmitted and the same code as the satellite signal is generated from the receiver to measure the time it takes for the satellite signal to arrive to the receiver after a comparison with the received satellite code. After determining the pseudo range between the satellite and the receiver at the speed of the satellite signal (speed of light), the range between the satellite i and the receiver is calculated, and the location of the receiver can be determined by calculating the range by observing the four satellites.



Positioning Errors of GPS

The positioning errors of GPS are classified into three types: range errors, geometric errors caused by satellite arrangements, and errors caused by selective availability implemented by the U.S. Department of Defense (DoD).

Range Error Caused by Structural Factors

This is an error in the range measured between the satellites and the receiver. It is caused by the following factors and such error is usually within a range of 5 to 10m.

1. Satellite Clock Error

This is an error resulting from an error in the atomic clock of a satellite. Fortunately, satellite clock error can be predicted and thus this type of error is minimized by adjustments made by the MCS.

2. Satellite Orbit Error

The parameters of the satellite orbit are predicted based on the data acquired by the MS and the satellites are controlled to broadcast the parameters with the code information. However, there are differences between the predicted orbit and the actual orbit, and this leads to range errors.

3. Radio Wave Delays in the Atmosphere

Because satellites orbit at an altitude of approximately 20,000km, signals transmitted from the satellites to the receiver must pass through the ionosphere and the troposphere that make up the atmosphere, which can cause a radio wave delay resulting in an error. In particular, the delay in the ionosphere is longer when the electron activity is high and shorter at around midnight when the electron activity is low. As such, the delay time varies significantly by day and season. The MCS predicts the delay time in advance and broadcasts it with the code information, and the receiver performs the necessary corrections when estimating the position to reduce range errors.

4. Errors in the Receiver

Range errors can also be caused by the electromagnetic noise in the receiver or the multi-paths of the radio wave. Together with the geometric factors related to the satellite arrangements, such range errors will result in positioning errors.

Increase in Geometric Errors According to Satellite Arrangements

Errors may increase depending on the locations of the satellites used for positioning. To take map reading on land as an example, selecting tags in reasonable ranges for map reading can minimize the triangle of error and enhance accuracy, whereas the use of tags in a concentrated area will increase the triangle of error and reduce accuracy. As such, satellites must be arranged in reasonable ranges to minimize range error.

As shown in the following figure, GPS receivers use the observed data to calculate the position dilution of precision (PDOP), which is multiplied to determine the positioning error. In other words, range error x PDOP = positioning error. Thus, most receivers are designed to determine and indicate the location information by selecting satellites with low PDOP. The recent improvement in performance of receivers has allowed reduction in range error to be about 15m CEP (Circular Error Probability) when PDOP is 3, i.e. an error of 15m on a plane with the range of error probability of 50%.

Errors Caused by Selective Availability (SA)

GPS error was increased deliberately based on a policy decision of the U.S. DoD, and this is an error caused by Selective Availability. This is called Selective Availability (SA) because the U.S. DoD arbitrarily decided to reserve the highest quality signal for the authorized users. When SA is turned on, the accuracy of GPS is degraded to 100m 2DRMS. According to the U.S. Navigation Plan, the accuracy should never be degraded to more than 100m 2DRMS, which doesn’t cause any problems when GPS is used for navigation. However, in case of a need to determine the precise location to a cm or mm or to acquire GIS data, relative positioning instead of independent positioning is performed. Note that 2DRMS stands for “Twice the distance root mean square” and 100m 2DRMS means that a range error of 100m on a plane with probability of 95%.

Error After Turning Off SA
Error After Turning Off SA
Causes of error Size (meters)
Satellite orbit error 0.57
Satellite clock error 1.43
Ionospheric delay 7.00
Tropospheric delay 0.25
Receiver noise 0.80
Category Size (meters) Result of the 2001 Field Test
With SA Without SA ( Not guaranteed )
Horizontal error (95%) 100 m 13 m 4 m
Vertical error (95%) 156 m 22 m 6 m