GNSS Receiver’s Testing: Study of GPS Test Software
Ooi Wei Han
National Space Agency (ANGKASA)
Dr. Mustafa Din Subari
National Space Agency (ANGKASA)
The Global Navigation Satellite System (GNSS) are becoming a global utility used in various applications. Currently, Europe in the middle of the Galileo development, the GPS system started with its modernization, the restoration of the Russian system GLONASS was started last few years, Japan and India are intended to establish regional satellite navigation systems and China started the first satellite on 14 April 2007 for another GNSS called Compass or Beidou.
A wide variety of applications are strongly dependent on GNSS applications including land, air, sea and space navigation, positioning, surveying and mapping, military, personal location and location-based services (LBS), and etc. Depending on the type of GNSS receivers used and the procedures followed, GNSS technology could be used to provide information of the location accuracy from a few centimeters to tens of meters. In the meantime, there have numerous tests and calibration procedures on GNSS receivers and more and more researches are carried out nowadays to ensure the collection data by GNSS receivers providing a better level position accuracy whatever in all conditions and environments. For example, Simulation test, Mobile test, Timing and Frequency test and GPS Integration test are among the GNSS tests which are widely used in most developed country likes United States of America.
One of the capability software in GNSS testing is GPSTest. It is one of the software exists in the market for testing and evaluating GNSS receivers based on the NMEA 0183 standards. This software enables to help the users understanding the quality, stability, performance and accuracy achieved by GNSS navigation receivers. Also, it helps the users to find out the better receivers for their projects indirectly.
This paper is discussed on effectiveness of GPSTest in GNSS navigation receivers (NMEA output). Several type of navigation receivers have been used to test its performance and accuracy achievement. Report of the results which consisted of several parameters was come out and it is proved that GPSTest has strong capability in GNSS receiver’s testing. In future, the National Space Agency (ANGKASA) will continue to further study and hope could be implemented and applied for navigation sectors in Malaysia. The role objective of having this done is help to ensure safe and accurate use of local GNSS receivers.
2.0 Current Establishment of the GNSS Test Centre
In surveying sector, a several tests and calibration on the GNSS receivers are performed to ensure the work results have achieved an optimum on accuracies. It is consists of 3 tests commonly, which are Zero Baseline Test, EDM Baseline Test and GNSS Network Test. A zero baseline test is performed to ensure the correct operation of the receivers, antenna, cabling and software and it’s done before any GNSS survey activities is carried out. Meanwhile, an EDM baseline test is performed to ensure the correct operation of a pair of GNSS receivers that will be used for baseline measurement. This test is carried out at an established EDM baseline test site, by occupying pillars with at least 90% sky visibility. However, for high accuracy surveying works, the GNSS network test is used. It usually carried out on annual basis, or when the receiver’s firmware or post-processing software is upgraded to a new version.
In industry sector, several GNSS test centers are established worldwide nowadays. Varieties of GNSS equipment’s tests are developed such as Environmental and electromagnetic Interference testing; Electronic Warfare and vulnerability testing; Mobile testing; Timing and frequency tests; GNSS integration testing; Flight testing and etc. In addition, certain test centers could design such tests to assess user system’s performance and determine whether its meet specified requirements or not and offer integration support to system designers, as well as troubleshooting expertise when systems are not performing to specification. Services include advising on navigation system architectures and GNSS-based technologies, developing integration studies and test and evaluation of integrated GNSS navigation systems.
The Naval Research Laboratory (NRL) performs absolute calibration of GNSS receivers for time transfer. NRL did a simulation of this calibration method to verify the procedure which focused on the effects of filters, external to receiver, on receiver calibration. It offers receiver timing measurement and calibration to better than one nanosecond in time accuracy. Also, NRL has Space and Ground Receiver Evaluation which it operates thermal vacuum chambers capable of reproducing the satellite environment as well as unique capabilities for specialized testing of GNSS receivers.
For aviation navigation part, certain industries conducts fixed-wing and rotary flight testing of inertial, GNSS and integrated navigation and guidance systems in benign environments as well as under various Electronic Warfare conditions. They could conduct such testing on any of their readily available fixed-wing or rotary aircraft. Other aircraft may be available upon request. For land navigation part, there have mobile test laboratories to provide cost effective performance and susceptibility navigation evaluation in a dynamic environment.
Also, the U.S Coast Guard Navigation Center (NAVCEN) provides quality navigation services that promote safe transportation, support the commerce of the United States, and directly benefit worldwide trade. NAVCEN provides worldwide users with reliable navigation signals, timely operational status, and general navigation information and services. They provides information for all radio navigation system which is staffed 24 hours a days, 7 days a week, providing information on the current status, effective policies, and general information for GPS and DGPS. Using the latest computer and internet technologies, it gathers, processes, and disseminates user reports of anomalies, interference, and other problems related to navigation safety. It disseminates safety broadcasts, Local Notice to Mariners, and the latest Notice Advisory to Navstar Users (NANU).
3.0 The Need of Testing GNSS Navigation Receivers
Accuracy and continuity of the location information in navigation sectors are required based on safety and environmental concerns. Currently, it has standardization in accuracy ranges for all the navigation sectors. Table 1 is showing the accuracy requirements for land, marine and aviation navigation sectors. By overall, it indicated the accuracy requirements are much lower if comparing to other applications such as surveying sector. However, tests have to be implemented on such receivers for ensuring the reliability outputs.
Apart from this, most of the current GNSS tests and calibrations are concentrated on interference of radio frequency (RF) signal to receivers. For example: environmental and electromagnetic interference testing, electronic warfare and vulnerability testing, mobile testing, and etc. has been practiced in the GNSS receivers to ensure it works in well conditions with minimum signal interferences. However, all the tests are rarely performed tests on spatial accuracy of point locations. Point’s position should be proved is reliable and the accuracy range value of every times measurement are not much dissimilar. In addition, most of the GNSS errors preclude the assumption that if a GNSS-derived position of location is accurate at a single point, it must be accurate for all other points.
4.0 Description of GPS Test Software
GPSTest is a useful tool for testing and evaluating GNSS navigation receivers which are in general based on the NMEA 0183 standard. Several parameters of receiver could be calculated such as time to first fix; conduct accuracy analysis with three (3) methods which is Known point, Mathematic mean and Model mean; conduct outliers or jump point’s analysis with different error distances, generate full test report for analysis and etc. The software also enables to do a comparison on performances among different GNSS receivers by logging the data at the same time. GPSTest is also capable to plot the positional errors distributions in Latitude (X), Longitude (Y) and Altitude (Z) direction into graphs for further analysis.
5.0 Test Products
Several tests have been conducted to several types of GNSS navigation receivers by using GPSTest software. All the test NMEA receivers are used the SiRFstar III chipset and the selling prices in between RM 120 – RM 1800. The information of receivers is listed in Table 2:
6.0 Test Results and Discussions
6.1 Test 1 – Receiver Evaluations
The aim of the test is to determine efficacy of navigation receivers. This is done by comparing the time to first fix (TTFF) / cold start time and accuracy are within the range of receiver’s specifications which stated by manufacturer. The PDA GPS phones (Samsung and HTC) have to install software from website to enable the phones log the NMEA data. On the way to make an evaluation, every receiver has to collect 15 minutes or almost 900 points positioning of NMEA data in static mode at the field. This procedure was accomplished in three (3) epoch’s observations at the same location and the collected data was used to process through GPSTest. Result of the test is showed in Table 3.
Comparison on receiver’s performance between manufacturer’s specification and GPSTest (3 epochs).
From the Table 3, it was indicated the test results of Haicom and Navisys receivers were not much different. The cold start time and accuracy achievements were below the range of specification which stated by manufacturer. Meanwhile, test results for the HTC and Samsung phones were showed huge different on TTFF. Cold start time for both PDA GPS phones indicated 288sec and 239sec (almost 4 minutes). However, it took fast time acquisition during warm and hot start’s conditions (1sec). These situations happen due to technology of Assisted GPS (A-GPS) and the chipset itself. By overall, the performances of every one test’s receivers are in well conditions and could be used in the following tests.
6.2 Test 2 – Evaluation Between Similar Receiver’s Types or Brands
This test is aiming to find out the better performances and outputs among the same type or brand of the navigation receivers. Evaluations among the output of receivers Haicom 305 and 306; HTC TyTn II and Samsung SGH-i780 PDA GPS phones have been made. Alike as previous test, each pair of test receivers mentioned have logging 15 minutes NMEA data simultaneously and the procedures were completed it in two (2) epochs at the same location. Results from GPSTest were used to plot graphs for comparison purposes. Figure 1, 2, 3 and 4 are showed accuracy results in graphical view for Haicom 305 and 306. Meanwhile the figure 5, 6, 7 and 8 are showed accuracy results in graphical view for HTC and SGH PDA phones.
Figure 1: Graphs showing the comparison between Horizontal (X, Y) Positioning Error among Haicom s/n 305 and 306 – Epoch 1
Figure 2: Graphs showing the comparison between Altitude (Z) Positioning Error among Haicom s/n 305 and 306 – Epoch 1
Figure 3: Graphs showing the comparison between Horizontal (X, Y) Positioning Error among Haicom s/n 305 and 306 – Epoch 2
Figure 4: Graphs showing the comparison between Altitude (Z) Positioning Error among Haicom s/n 305 and 306 – Epoch 2
Both of the navigation receivers are manufactured by same factory from Taiwan; however the accuracy achievement were showed different. In general, the graphs were obviously indicated the performances of Haicom s/n 305 are better than Haicom s/n 306 as well as horizontal (X, Y) and altitude (Z)’s positioning errors. The accuracy range of 305 was showed much greater compared to 306.
Figure 5: Graphs showing the comparison between Horizontal (X, Y) Positioning Error among HTC and SGH PDA Phones–Epoch 1
Figure 6: Graphs showing the comparison between Altitude (Z) Positioning Error among HTC and SGH PDA Phones–Epoch 1
Figure 7: Graphs showing the comparison between Horizontal (X, Y) Positioning Error among HTC and SGH PDA Phones– Epoch 2
Figure 8: Graphs showing the comparison between Altitude (Z) Positioning Error among HTC and SGH PDA Phones–Epoch 2
Accuracy achievements by both of the PDA phones are showing different, even though there are using the similar chipset which is Qualcomm SiRFstar III. From the figure 6 to 9, the graphs were clearly showed that the positioning errors of HTC PDA phone are not as good as SGH PDA phone. The accuracy range of SGH was much smaller compared to HTC. Meanwhile, altitude error of both devices were uncertainty due to the Standard Positioning System (SGS) GPS’s capability.
6.3 Test 3 – Evaluation between Dissimilar Receiver’s Types or Brands
This test was conducted to evaluate the GPSTest capability in comparing the performance of dissimilar receiver’s type. Two of the better performance’s receivers in Test 2 (Haicom 305 and Samsung SGH-i780) and Navisys 555 receiver have been selected for this Test 3. All the receivers mentioned were done the 15 minutes field observations simultaneously and processed by using the GPSTest software to gain the report. Figure 9 and 10 were showed the comparison between the positioning errors (X, Y and Z) based on collected NMEA data.
Figure 9: Graphs showing the comparison between Horizontal (X, Y) Positioning Error among receiver 555, 305 and SGH Phones
Figure 10: Graphs showing the comparison between Altitude (Z) Positioning Error among receiver 555, 305 and SGH Phones
In overall, the graphs showed the accuracy achievement by Haicom 305 was better than Navisys 555. That is because capability of more satellites tracking by Haicom 305 (20 channel) compared to Navisys 555 (12 channel).
Meanwhile, SGH PDA phone was achieved high accuracy among the test receivers. One of the reasons could be put in consideration was the SGH PDA phone itself is using the Assisted GPS (A-GPS) technology instead of standalone positioning which are used by the other receivers Haicom and Navisys. And this caused the phone receivers obtained much higher quality output in this test.
As conclusion, GNSS has been widely used in various applications in Malaysia since the past few years. Currently, the GNSS navigation receivers are turn into cheaper in selling price and therefore generate a large volume of users yearly. However, the performance and accuracy achieved by receivers have to be concerned seriously to ensure the safety used of local users.
Based on the several test results in this study, GPSTest is proved its capability for ensuring the higher quality GNSS navigation receivers likes Samsung i780 GPS PDA phone. From that, GPSTest has to encourage being practice for entire navigation sectors and beneficial to the users in Malaysia.
Based on the experience of this research, various improvements and extensions to the study are still possible. The following actions are recommended for future studies either to improve the results or to further extend this research.
In order to put up the GPSTest’s results much reliable, continuously tests on several NMEA receivers have to be taken. Also, spending much time in each field observation to enhance the quality of data collected.
Apart from this, test on road track’s accuracy may consider as a good way to gauge the accuracy of the navigation receivers. Through the additional of local electronic maps into GPSTest, it can be tested by driving around the high rise buildings or noisy areas with multiple navigation receivers. Results with graphical view will show up the differences achievement of road track’s accuracy by each receiver.
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