Performance Evaluation Of Rtk Gps Without Sa Effect
Jenn-Taur Lee, Wen-Feng Chen
Department of Surveying and Mapping Engineering
Chung Cheng Institute of Technology
National Defense University
Tahsi, Taoyuan
Taiwan 33509
Tel: 886-3-380-0364
Fax: 886-3-390-7810
Email:jtlee@mail.ccit.edu.tw
Keywords: Global Positioning System, Selective Availability, Real Time Kinematic GPS
Abstract
The policy of Selective Availability (SA) was endorsed in order to artificially widen the gap between the civilian and military positioning services. SA is an intentional degradation of the accuracy of global positioning system (GPS) horizontal positioning to 100m and height determination to 150-170m at the 95% confidence level, for standard positioning service (SPS) users. SA has been implemented since March 25, 1990, and consists of two different components.
Firstly, the so-called epsilon (
e ) component consists of the truncation of the orbital information transmitted within the navigation message so that the WGS84 coordinates of the satellites cannot be computed correctly. Secondly, the so-called delta (
d ) component which is achieved by dithering the satellite clock output frequency. The varying error in the satellite clock's fundamental frequency has the direct impact on both the pseudo-ranges and carrier phase observations. The variations in range have amplitudes of as much as 50m, with periods of several minutes. Fortunately, SA was discontinued on May 2, 2000 that should make GPS more responsive to civil and commercial users.
The objective of this study is to investigate the performance of real time kinematic (RTK) GPS without SA effect, since RTK GPS is widely taken in engineering and geographical information system (GIS) uses. The experiment was accomplished in an existed network that consists of 31 control points, which was surveyed with RTK GPS in addition static GPS, differential GPS, and single point positioning before and after SA effect turned off. EDM and leveling surveying was also done at the same network. Furthermore, the statistical analyses of horizontal and vertical vectors, baseline length, and height difference were completed and will be presented in the article.
1. Introduction
GPS is a dual-use system, providing highly accurate positioning and timing data for both military and civilian users. There are more than 4 million GPS users world wide, and the market for GPS applications is expected to double in the next three years, from 8 billion to over 16 billion. Some of these applications include: air, road, rail, and marine navigation, precision agriculture and mining, oil exploration, environmental research and management, telecommunication, electronic data transfer, construction, recreation and emergency response. GPS has always been the dominant standard satellite navigation system thanks to the U.S. policy of making both the signal and the receiver design specification available to the public completely free of charge. The U.S. previously employed a technique called Selective Availability (SA) to globally degrade the civilian GPS signal. New technologies demonstrated by the military enable the U.S. to degrade the GPS signal on a regional basis. GPS users worldwide would not be affected by regional, security-motivated, GPS degradations, and businesses reliant on GPS could continue to operate at peak efficiency [Leick, 1996]. The technology that makes this extraordinary technology possible grows directly from past research investments in basic physics, mathematics, and engineering supported from U.S. Federal agencies over a period of decades.
GPS works because of super reliable atomic clocks-no mechanical device could come close. These clocks resulted from gifted physics and creative engineering that managed to package devices which once filled large physics laboratories into a compact, reliable, space-worthy device. The improved, non-degraded signal will increase civilian accuracy by an order of magnitude, and have immediate implications in areas such as car navigation, emergency services, recreation, and others. In addition to more accurate position information, the accuracy of the time data broadcast by GPS will improve to within 40 billionths of a second. Such precision may encourage adoption of GPS as a preferred means of acquiring Universal Coordinated Time (UTC) and for synchronizing everything from electrical power grids and cellular phone towers to telecommunications networks and the Internet [Teunissen, 1998]. For example, with higher precision timing, a company can stream more data through a fiber optic cable by tightening the space between data packages. Using GPS to accomplish this is far less costly than maintaining private atomic clock equipment.
2. Error Sources Of GPS
The accuracy with which a GPS receiver can determine position and velocity and synchronize to GPS time is dependent on a number of factors. GPS error sources are allocated into three categories: the space segment, control segment, and user segment. The error components that comprise these segment, as well as an estimate of the one-sigma value of each component is given in Table 1. The most dominant error source by far is Selective Availability (SA) that produces a one-sigma error of 32.3 meters. These error sources are for the civilian coarse/acquisition (C/A) code. The effective accuracy of the pseudorange value is termed the user-equivalent range error (UERE). The system UERE is the root sum square (RSS) of the individual error components. As shown in Table 1, the one-sigma UERE is reduced from 33.3 m with SA to only 8 m when SA is removed. Further reduction in the UERE can be obtained with the proposed 2
nd and 3
rd civilian frequencies to correct for ionospheric delay. A UERE of 6.6 m was examined to account for the ionospheric correction, as well as a UERE of 3.8 m that includes additional benefits of GPS modernization [Kaplan, 1996].
A more conservative user error budget also was examined to account for larger receiver noise and ionospheric errors. The troposphere and multipath errors also were modified to agree with the supplemental GPS Minimum Operational Performance Standards (MOPS). This error budget, shown in Table 2, results in a one-sigma pseudorange error of 12.5 m without SA. The root sum square of the receiver noise, multipath, and interchannel bias equals 5 meters which is consistent with the value in the current Wide Area Augmentation System (WAAS) MOPS. A one-sigma pseudorange error of 12.5 meters for GPS without SA was adopted by RTCA SC-159 Working Group 2 for inclusion in the WAAS MOPS. As shown in Equations (1) and (2), the magnitude of the horizontal protection level is directly related to the standard deviation of the pseudorange error. This reduction of horizontal protection level (HPL) naturally will translate into a significant improvement in availability. The horizontal protection level is determined by HPL = SF
max bias (1)
Where bias =
s Öl
(2)
SF is the scale factor,
l is the noncentrality parameter of the noncentral chi-square density function,
s and is the standard deviation of the satellite pseudorange error.
Table 1 Estimated GPS C/A Code Pseudorange Error Budget
| Segment Source |
Error Source |
GPS 1s Error(m) with SA |
GPS 1s Error(m) without SA |
| Space |
Satellite Clock Stability
Satellite Perturbations
Selective Availability
Other (thermal radiation,etc.) |
3.0
1.0
32.3
0.5 |
3.0
1.0
-
0.5 |
| Control |
Ephemeris Prediction Error
Other (thruster performance,
etc.) |
4.2
0.9 |
4.2
0.9 |
| User |
Ionospheric Delay
Tropospheric Delay
Receiver Noise and Resolution
Multipath
Other (interchannel bias, etc.) |
5.0
1.5
1.5
2.5
0.5 |
5.0
1.5
1.5
2.5
0.5 |
| System UERE |
Total (RSS) |
33.3 |
8.0 |
Table 2 Conservative C/A Code User Pseudorange Error Budget
| Error Source |
GPS 1s Error (m) without SA |
Ionospheric Delay
Tropospheric Delay
Receiver Noise and Resolution
Multipath
Other (interchannel bias, etc.) |
10.0
2.0
4.8
1.2
0.5 |
| System UERE (RSS) |
12.5 |
Space and control segment errors remain the same (without SA).
