The satellite-based augmentation system (SBAS) transmits correction data through satellites, and navigation receivers utilize this correction data to improve the accuracy of the Global Navigation Satellite System (GNSS). In order to implement the SBAS, ground equipment that generates correction signals is required. If a ground verification system is developed, it can be utilized to verify the performance and function of the system on the ground before the full construction of the augmentation system, and check the transmission and reception status of test messages. The ground verification system includes a signal generator. A module that performs signal quality test must be installed in the system to check the quality status of the signal generated by the signal generator. In this study, a signal quality test algorithm required for the ground verification system of the satellite-based augmentation system was designed. The test was performed using the SBAS signal to confirm the possibility of module development. The developed module can be applied to a ground verification system and used to verify the generated signal.
Global Navigation Satellite Systems (GNSS) provide position and time information to users using
navigation satellites. GNSS have been used in military and transportation applications that require
position information. Their applications have gradually expanded. Currently, GNSS have become
essential components in a wide range of technologies such as Internet of Things and smart devices[
CEUR
ceur-ws.org
information signals under the same conditions as other navigation satellites. Signal control accounts for
the fact that the signal passes through the atmosphere twice during uplink and downlink transmission,
requiring adjustment of code and carrier frequencies. When developing a SBAS, it is advisable to
develop a ground verification system in the early stages to perform signal generation and validation
on the ground, and subsequently expand to include satellite transmission and reception checks [
The ground verification system verifies the functionality and performance of relay equipment for the
augmentation system to be mounted on satellites, and validates algorithms of the ground station for the
augmentation system. Figure 1 shows the functional concept diagram of the ground verification system.
As shown in Figure 1, the ground verification system includes a Signal Generation Subsystem (SGS)
that generates augmentation signals and a Radio Frequency Subsystem (RFS). The SGS is responsible
for generating test correction messages and controlling signal timing according to GPS time, while the
RFS performs the function of connecting the geosynchronous equatorial orbit (GEO) satellite with the
ground verification system, transmitting the generated signals to the satellite. The SGS for the ground
verification system must have the capability to generate test correction messages and to generate and
receive L1/L5 signals, which serve as augmentation signals. As shown in Figure 2, which illustrates the
system architecture proposed in [
The SGS uses received measurement data to inspect GPS time synchronization ofset errors, carrier
frequency stability, and code/carrier coherency for verifying the quality of L1/L5 signals [
Inspection techniques for GPS time synchronization ofset error, carrier frequency stability, short-term
code/carrier coherency, and long-term code/carrier coherency were designed to test compliance with
the requirements presented in Section 2. Each inspection technique uses pseudorange measurements,
carrier phase measurements, and Doppler measurements of the augmentation satellite measured by
satellite navigation receivers. The implemented signal quality test simulation processes user-defined
inputs including measurement data, simulation time parameters, and signal quality requirements.
The simulation generates output consisting of signal inspection results, visual graphs, and status
indicators to determine whether tested signals meet the specified requirements. The test for GPS
time synchronization ofset error was performed using the precision of time diference between GPS
and augmentation satellite signals. In satellite navigation systems, time is reflected in pseudorange
measurements since distance between satellites and receivers is measured based on time. Therefore,
the time diference was calculated by computing the diference between the expected pseudorange and
the measured pseudorange of the augmentation satellite. Equation 1 shows the delay time calculation,
which divides the diference in pseudoranges by the speed of light [
3 ( ) < 100 ns The standard deviation of the delay time was calculated and used in Equation 2. Based on the requirements in Section 2, the threshold value was set to 100 ns, and the standard deviation multiplied by 3 must remain under this value. When this threshold is met, the requirement of the GPS time synchronization ofset error is satisfied, and the signal quality is determined to be good.
If a receiver provides pseudorange standard deviation in its output data, the standard deviation output
by the receiver can be directly used for the GPS time synchronization ofset test. The carrier frequency
stability test examines Doppler frequency values against the reference carrier frequency. For testing
frequency stability between 1 and 10 seconds, Allan deviation was calculated and compared with the
requirement to determine whether signal quality meets the requirements. Equations 3 show the Allan
Variance calculation for L1 and L5 frequency stability tests [
1 = Allan Variance ( 5 = Allan Variance ( √ < 5 × 10 −11 ,1 ,1 ,5 ,5 ) ) The short-term code carrier coherency test used code measurements and carrier measurements to check whether the code and carrier generated over a short period of 1 to 10 seconds maintain coherency without divergence. The code frequency and carrier frequency were divided by their respective reference frequencies to create dimensionless values, and coherency was tested using the diference between these values. Equations 4 show how to calculate code and carrier frequencies. In Equations 4, is the code frequency, 0 is the base frequency of the code, Δ is the change in code frequency, is the speed (1) (2) (3) of light, and ̇ is the time rate of change of pseudorange. Equations 4, is the carrier frequency, 0 is the base frequency of the carrier, Δ is the change in carrier frequency, and ̇ is the time rate of change of carrier measurements.
As shown in Equation 5, the code and carrier frequencies are normalized by dividing by the base frequency of the code and the base frequency of the carrier, respectively. Their diference was compared to the threshold value (5 × 10−11) to confirm acceptable signal quality.
= 0 + Δ = 0 − 0 ̇ = 0 + Δ = 0 − ̇
| Δ − 0 Δ 0
We applied the algorithms designed in Chapter 3 to test signals received from an actual satellite navigation augmentation system and signals generated by the SGS of the ground verification system to perform quality tests. Figure 3 shows the data collection environment used for the test. Two types of signals were collected: signals from the Korean Augmentation Satellite System (KASS) and signals generated by the SGS of the ground verification system. The KASS began test services in 2023 and has been providing full service since 2024. The KASS provides satellite-based augmentation services in the Korean Peninsula area. Both KASS signals and SGS signals were synthesized and simultaneously received by the satellite navigation receiver. The test data was collected for 10 hours. The collected data went through a decoding process to extract the measurements needed for testing. These extracted measurements were used as inputs for each test algorithm for analysis. Figure 4 shows the results of the GPS time synchronization ofset error test using the L1 signal. The blue and red solid lines represent the GPS time synchronization ofset error and the threshold derived from the requirements, respectively. Figure 4(a) shows the test results using KASS data, with an average value over 10 hours of 7.81 ns. Figure 4(b) shows the test results using SGS data, with an average value of 7.13 ns. Both KASS and SGS signal data maintained GPS time synchronization ofset errors below the threshold value of 100 ns over 10 hours, satisfying the requirements and demonstrating good signal quality. KASS data includes error components during transmission from the augmentation satellite, while SGS data is input directly from the signal generator to the receiver, making SGS data more stable than KASS data. The GPS time synchronization ofset error test results using the L5 signal also showed good signal quality, similar to the L1 signal test results. The average values were calculated as 8.51 ns for KASS data and 6.51 ns for SGS data. As with the L1 signal, SGS data was more stable than KASS data. Furthermore, the test results confirmed that the L5 signal was more stable compared to the L1 signal. lines represent the Allan deviation for the carrier frequency stability and the threshold, respectively. Test results using KASS data and SGS data are shown in Figure 5(a) and Figure 5(b). Analysis using Allan variance showed that both data sets met the threshold requirements. The test results confirmed that SGS data had more stable frequency compared to KASS data. Figure 6 shows the results of the short-term code carrier coherency test. The blue, black, and red solid lines represent the Allan deviations of L1 signals, the Allan deviations of L5 signals, and the threshold, respectively. Figure 6(a) and Figure 6(b) show the test results for KASS data and SGS data. Examining the Allan deviation, the L1 signal in both data sets exceeded the threshold value of 5 × 10−11 between 1 and 10 seconds, failing to meet the requirements. For the L5 signal, KASS data exceeded the threshold from 1 to 6 seconds, and SGS data exceeded the threshold from 1 to 3 seconds, failing to meet the requirements during these periods. After these periods, both data sets were below the threshold, meeting the requirements. Although SGS signals do not pass through the atmosphere and satellite, and both the GPS time synchronization ofset error test and the carrier frequency stability test confirmed their stability, SGS signals still failed to meet the requirements in the short-term code carrier coherency test. The failure to meet short-term code carrier coherency requirements despite stable signal conditions confirms that requirements are very stringent. The short-term code carrier coherency requirement is one of the conditions necessary for using augmentation signals as additional navigation satellites, and even if the requirement is not satisfied, there is no problem with the function of transmitting augmentation information. However, additional research and analysis are needed to meet the requirements for using augmentation signals as pseudorange sources for additional navigation satellites. Figure 7 shows the results of the long-term code carrier coherency test. The blue, black, and red solid lines represent the Allan deviations of L1 signals, the Allan deviations of L5 signals, and the threshold, respectively. Figure 7(a) and Figure 7(b) show the test results for KASS data and SGS. Both data sets met the requirements with test results less than one cycle. The Allan deviation analysis from 1 to 100 seconds revealed that SGS data exhibited smaller values than KASS data, demonstrating superior signal quality. Similarly, the L5 signal showed smaller values compared to the L1 signal, indicating better signal quality. The test results show that the requirements for the long-term code carrier coherency test are more relaxed compared to the short-term code carrier coherency requirements. KASS data and SGS data have already been confirmed to have acceptable signal quality. Experimental results demonstrated that both data sets met most requirements when the designed signal quality test algorithms were applied. These experimental outcomes validated the applicability of the designed algorithms to signal quality testing of SBAS. The designed signal quality test algorithms efectively represented the characteristics of L1/L5 signals from both the currently used KASS and the SGS, enabling efective monitoring of signal quality status.
SBAS provide correction information across wide areas, making them highly efective for improving navigation performance for numerous satellite navigation users. The development of SBAS represents a necessary field for continuously enhancing satellite navigation system performance. Developing a ground verification system as a preliminary step in the SBAS development process holds significant importance. The ground verification system must include functionality to inspect the signal quality of generated augmentation signals. This study designed signal quality test algorithms for ground verification systems. Signals from the Korean Augmentation Satellite System (KASS), currently in operational service, and signals from a signal generator were tested using the implemented algorithms to validate system application feasibility. The designed signal quality test algorithms demonstrate high utility potential for testing augmentation signals in future ground verification system implementations. Future work should expand beyond current augmentation signal requirements to develop additional inspection methods in terms of signal characteristics, measurement techniques, and data analysis. These enhancements would enable comprehensive signal quality assessment of ground verification systems from multiple technical perspectives.
This paper was supported by research funding from the Aviation Safety Technology Development Project of the Ministry of Land, Infrastructure and Transport (2021GEOS-C164591-01), and we thank you for the support.
Or (by using the activity taxonomy in ceur-ws.org/genai-tax.html): During the preparation of this work, the authors used Claude-3.7 in order to: Grammar and spelling check. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the publication’s content.