Comparative study between SPP and DGPS using RTKLIB...
SATELLITE NAVIGATION AND POSITION TECHNOLOGY 2016 Final Paper Comparative study between SPP and DGPS using RTKLIB Prepared by: BOUCHIBA MOHAMMED Student ID: 2016276190002 E-mail:
[email protected] ABSTRACT GPS positioning techniques may be categorized as being predominantly based on code or carrier measurements. Code techniques are generally simple and produce low accuracies, while carrier techniques are more complex and produce higher accuracies. For both code and carrier measurements, a variety of positioning methods exist. The suitability of each for a specific application is dependent on the desired accuracies, logistical constraints and costs. The paper focuses on a comparison of two different positioning methods provided by free and open source software package called RTKLIB. The RTKLIB supports real-time and post-processed positioning of GNSS, such as single point positioning (SPP), precise point positioning (PPP), differential GPS (DGPS) and real time kinematic (RTK) solutions.
Citation: M. BOUCHIBA (2016), Comparison between SPP and DGPS using RTKLIB, Final Paper of Satellite Navigation and Position Technology, Liesmars, Wuhan University. Components: 1655 words. Key words: GPS, SPP, DGPS, RTKLIB. ________________________________________________________________________________
1. Introduction Positioning with GPS may take the form of single point positioning or relative positioning. In single point positioning coordinates of a receiver at an "unknown" point are sought with respect to the earth's reference frame by using the "known" positions of the GPS satellites being tracked. Single point positioning is also referred to as absolute positioning, and often just as point -1-
positioning. In relative positioning the coordinates of a receiver at an "unknown" point are sought with respect to a receiver at a "known" point. The aim of this paper is to make clear with functionality of RTKLIB using two different positioning modes. In this perspective, for our study case, we will use a sample datasets in post processing mode (RTKPOST) with two different positioning modes: Single Point Positioning SPP and relative positioning DGPS.
2. Single point positioning: The concept of single point positioning is illustrated in Figure 1. Using the broadcast ephemerides, the position of any satellite at any point in time may be computed. In the figure, s1, s2, s3 and s4 represent four different satellites being tracked. The positions of these satellites are referenced to the centre of the earth in the x, y, z coordinate frame. The coordinates for s1 are shown as (xs1, ys1, zs1). The coordinates of r, the unknown point, as referenced to the centre of the earth, are (xr, yr, zr). The observed code,
Prs1, relates the known coordinates of
satellite 1 with the unknown coordinates of the receiver shown in Figure 1 using the equation for a line in three-dimensional space. That is,
(1)
The same equation showing the relation between satellite 1 and the receiver may be formed for all satellites tracked. With at least four satellites all the unknowns (xr, which forms part of the errors) may be computed.
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yr, zr and a clock term
Figure 01: Single point positioning Single gle point positioning is achieved by intersecting the measurements from four or more satellites at a single receiver on the earth's surface. The accuracies achievable using single point positioning are 100 m 2drms horizontally and 156 m 2s vertically (U.S. DoD and DoT, 1986), assuming favourable geometry (e.g. PDOP < 6). These accuracies apply equally to static or kinematic single point positioning. Solutions may be attained almost instantaneously, using an inexpensive hand-held held GPS receiver. The receiver need only collect C/A code measurement measurements to achieve the specified single point positioning accuracy. 3. Differential GPS (DGPS): The concept of relative positioning positioni is illustrated in Figure 2. Instead of determining the position of one point on the earth with respect to the satellites (as done in single point positioning), the position of one point on the earth is determined with respect to another "known" point. The term differential positioning is sometimes used interchangeably with relative positioning. However since differential positioning is more often associated with a specific type of relative positioning which applies corrections measured at a "known" site to measurements surements at an "unknown" site, relative positioning will be the term used herein to describe the general co concept illustrated in Figure 2.
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Figure 02: Relative positionning ( DGPS)
Differential positioning may be conducted with either post-mission or real-time processing. The former is simpler and less expensive, while the latter is complicated by the requirement for a data link. Differential corrections may take the form of measurement corrections or position corrections. Although the former is the more rigorous recommended approach, the concepts behind both correction forms are explained. With either approach, the coordinates of one point which is used as a monitor station, must be "known". With the measurement method, the "true" range, r, between a satellite and monitor station is computed as
(2) Where (x
s
, ys, zs) are "known" satellite coordinates derived from the broadcast ephemerides and
(xrk, yrk, zrk) are "known" receiver coordinates. The errors for the satellite-receiver range are computed by rearranging equation (2), as
(3)
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Where
is the observed satellite-receiver range and
is the "true" satellite-
receiver range. Because the major errors which affect GPS observations (orbital and atmospheric) will affect points near the monitor station by approximately the same magnitude, equation (2) may be used to correct observations made at nearby sites. The errors computed at the monitor site thereby form differential corrections which are applied at the rover site. This is the basic concept of differential positioning using the measurement method.
4. Description and applications of the software RTKLIB The RTKLIB is an open source program package for standard and precise positioning with GNSS (global navigation satellite system). It consists of a portable program library and several APs (application programs) utilizing the library. The RTKLIB open source programming package provides many helpful tools and libraries for satellite navigation (RTKLIB: Overview, 2013). The libraries are written in ANSI C and will compile and run in both Windows and UNIX. The provided tools only have Windows support. RTKLIB has many features and support tools, but only a few were needed for this project. The main feature of RTKLIB is support for real time kinematics navigation. The tools provided allow for the use of RTK without needing to understand the theory behind it. The RTKLIB tools support TCP communication which allows for easy integration. The GUI tools allow for easy setup and quick diagnostics and testing. Furthermore, the RTKLIB comes 10 with support for a variety of GPS receivers found on the market. With the RTKLIB programming package, it is easy to get accurate GPS position using RTK. Application Programs (APs) RTKNAVI: Real-time positioning; RTKPOST: Post-processing baseline analysis; RTKPLOT: Plot raw observation data and solutions; RTKCONV: RINEX converter for raw receiver log….
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Figure 3 : interface RTKLIB 5. Comparative study between SPP & DGPS using RTKLIB Software: Through a comparative study between the above described modes, we will use RTKLIB software and a sample datasets (interval of 30 seconds) to prove that GPS relative positioning (DGPS) provides a higher accuracy than that of autonomous positioning (single point). To begin it is necessary to use RTKPOST and set-up the processing settings. Click “Options”. NB: Only the indicated parameters are changed and the remaining ones are set as default.
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A) Single Point Positioning: Under the Settings1 page, change the following parameters:
“Single” for positioning mode.
Set the elevation mask at 10°.
Choose “GPS”.
For the output, select X/Y/Z – ECEF as solution format:
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Click “OK” and load the RINEX files for the receiver from sample data folder:
After loading RINEX files, click on “Execute” to perform the calculation. Once the calculation process is over, click on “View” to show the statistic output file which indicate the Standard Deviation in X, Y and Z and other navigation information as shown in the following figure:
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Click on “Plot” to show the time series in E-W, N-S and U-D directions:
B) DGPS mode: Under the Settings1 page, select the following parameters:
“DGPS/DGNSS” for positioning mode.
Set the elevation mask at 10°.
Choose “GPS”.
For the output, select X/Y/Z – ECEF as solution format:
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For “Positions”, select Average Single Position as a position of base station:
Click “OK” and load the RINEX files for the rover and base station, respectively, from sample data folder:
After loading RINEX files, press “Execute” to perform the calculations.
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Once the calculation process is over, click on “View” to show the statistic output file which indicate the Standard Deviation in X, Y and Z and other navigation information as shown in the figure below:
Click on “Plot” to show the time series in E-W, N-S and U-D directions:
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6. Conclusion and discussion: In our study, we made a comparison test of the accuracy achieved in post-processing of satellite observations by Single Point Positioning and DGPS methods performed by RTKLIB. Performed tests showed that the coordinate in all directions (E-W, N-S and U-D) are considerably improved using DGPS mode. Also, we can find that Standard deviation (Sd) is dropped form meter level in single point mode to centimeter level in DGPS mode as shown in the table below (example: Obs start point):
SPP DGPS
Sd X (m)
Sd Y (m)
Sd Z (m)
2.8186 0.6563
1.5781 0.3923
3.4548 0.7743
Results obtained in our study, proved the advantage of using relative rather than single point positioning is that much higher accuracies are achieved because most GPS observation errors are common to the known and unknown site and are reduced in data processing. Finally, through this simple GNSS data handling, we found out that RTKLIB is free and useful software which can be used for GNSS data manipulation in our future work to obtain the needed results.
Acknowledgments We deeply express thanks to M. Jianghui Geng for the support and data provided to carry out this work, so we are very grateful for his efforts made in the courses in order to provide us his knowledge in GNSS, and we thank him for his sacred time for us in the group for answer our questions and even to share the files. We appreciate him and we wish to work together in the near future.
References [01] GPS positioning Guide; 1995; Third printing; Minister of Supply and Services Canada. [02] B. Wiśniewski, K. Bruniecki & M. Moszyński. 2013. Evaluation of RTKLIB's Positioning Accuracy Using low-cost GNSS Receiver and ASG-EUPOS, DOI: 10.12716/1001.07.01.10. [03] Rock, S., Lin, P., Changsheng, C., Jianjun, Zhu. 2014. Single Point Positioning Using GPS, GLONASS and BeiDou Satellites. Positioning, 5, 107-114. [04] http://www.esri.com/news/arcuser/0103/differential1of2.html. [05] http://www.rtklib.com. -12-
