Achieving CAT I Service with KASS

Leveraging Galileo and introducing new GNSS measurements to the current network enables Korea Augmentation Satellite System (KASS) availability below the vertical limit. 

THIERRY AUTHIÉ, GUILLAUME COMELLI, SÉBASTIEN TRILLES, THALES ALENIA SPACE, TOULOUSE, FRANCE BYUNGSEOK LEE, MINHYUK SON, KOREA AEROSPACE RESEARCH INSTITUTE, REPUBLIC OF KOREA CHEON SIG SIN, ELECTRONICS AND TELECOMMUNICATION RESEARCH INSTITUTE, REPUBLIC OF KOREA

The Korean Augmentation Satellite System (KASS) is a satellite-based augmentation system (SBAS) developed by the Republic of Korea (South Korea) to augment the functionality of the Global Positioning System (GPS) in the Korean Peninsula and the surrounding regions. KASS was designed to enhance the accuracy and reliability of GPS signals within the country. In aviation, it enables more precise and reliable navigation for aircraft, supporting instrument approach procedures and improving operational efficiency.

A SBAS is a Global Navigation Satellite System (GNSS) augmentation system standardized in the International Convention on Civil Aviation SARPS Annex 10 [1], Volume 1, published and maintained by the International Civil Aviation Organization (ICAO). KASS provides safety-critical services for civil aviation, up to Approach with Vertical Guidance 1 (APV I ) service level, as well as an open service usable by other forms of transportation and possibly other position, navigation and timing (PNT) applications.

The KASS system provides improved GNSS navigation services for suitably equipped users in the agreed service areas of the Republic of Korea by broadcasting an augmentation signal of the GPS Standard Positioning Service (SPS).

To ensure the smooth operation of the system, KASS includes a network of ground receiver stations dedicated to collecting GPS measurements, a set of ground processing stations responsible for monitoring and controlling the satellites, and a set of ground uplink stations managing the transmitted signals toward two Geostationary Earth Orbiting (GEO) satellites. These ground control stations accurately calculate the orbit and clock information of the satellites, as well as the mono frequency L1 ionosphere delay, and continuously update the transmitted signals accordingly.

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One of the system’s key features is its ability to enhance the integrity and reliability of positioning information. KASS incorporates integrity monitoring functions to detect and alert users of any potential errors or anomalies in the GPS signals. This is crucial for safety-critical applications that require precise positioning data, such as aviation and maritime navigation.

The augmentation signal provides corrections of GPS satellites orbits and clocks and integrity bounds of orbit/clock residual errors, as well as corrections and integrity bounds for ionosphere delays. The KASS satellites transmit signals that are compatible with GPS, allowing KASS-capable receivers to seamlessly switch between GPS and KASS signals and to compute a navigation solution with greater accuracy.

The KASS system qualification was achieved by December 15, 2023, and safety of life aeronautical services have been fully operational since December 28, 2023. 

The current network of KASS Reference Stations (KRS) is composed of seven KRS sites all deployed on the Republic of South Korea land masses. Each KRS includes two independent channels based on NovAtel WAAS GIII receivers that provide GPS signal tracking and measurements. This concentrated network allows users to reach APV I service level but not more stringent service levels like the Category 1 (CAT 1) approach.

This article presents an analysis of a system upgrade to reach the CAT I service level based on representative synthetic data scenario. This analysis involves an extension of the KRS network and minor changes in the navigation algorithms.

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KASS Services and Architecture

KASS is designed to provide performance-based navigation (PBN) aeronautical procedures, such as RNAV or RNP approaches, enable aircraft to fly along precise paths during departure, enroute and approach phases. PBN is a modern concept in aviation that uses satellite-based navigation technology and proposes a set of procedures that enhance operating efficiency, reduce flight distances, improve airspace capacity and enhance safety.

The KASS system is designed to ensure four safety critical service levels:

• Enroute continental over the Incheon FIR area. Flight segments after arrival at initial cruise altitude until the start of descent to the destination.

• Enroute terminal over the Incheon FIR area for descent from cruise to Initial Approach Fix. 

• NPA over the Incheon FIR area. For non-precision approaches in aviation, this instrument approach and landing uses lateral guidance but not vertical guidance.

• APV I over South Korea landmasses (including Jeju Island) for precision approaches with vertical guidance.

KASS will provide open service over Incheon FIR area. 

Figure 1 shows the KASS service areas.

The KASS system is designed to be a system-of-systems ensuring the following main functions [2-3]:

• Collect GPS data at various locations in the Republic of Korea (and possibly other states in the future) through Korea Receiver Stations (KRS).

• Compute corrections and associated integrity bounds from ranging measurements of GPS satellites in view of KASS, and format messages compliant with the SBAS user interface standardized in ICAO SARPS Annex 10 [1] and the RTCA MOPS 229-D Change 1 [4]. This function is ensured by the Korean Processing Stations (KPS).

• Uplink a signal carrying these messages to navigation payloads on the KASS GEOs with Korean Uplink Stations (KUS).

• Broadcast the signal to users after frequency-conversion to the L1 band.

The KPS is the core component of the KASS system responsible for computing orbit, clock, ionosphere corrections, and alert information below the Navigation Overlay Frame (NOF). It uses data from a set of reference stations, the KRS, to perform these calculations. The KPS consists of two independent elements known as the processing set (PS) and the check set (CS).

The PS is responsible for computing the complete navigation context for the GNSS constellation, including orbits, clocks and the ionosphere model. It then prepares and sends the NOF, which is broadcasted to users. The CS acts as a supervisory entity by applying the NOF to GPS messages, ensuring consistency with an independent set of measurements to maintain integrity and control.

The KPS-PS component plays a critical role in achieving high-performance levels, specifically for the APV I service level. APV I is a type of PBN approach that provides lateral and vertical guidance to aircraft during the approach and landing phase. APV I approaches typically use satellite-based augmentation systems like the Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS) and now the Korean Augmentation Satellite System (KASS). These approaches provide accurate lateral and vertical guidance, allowing pilots to perform precision approaches with reduced reliance on ground-based navigational aids.

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From APV I to CAT I Aviation Service for KASS

CAT I (localizer performance with vertical guidance 200) is another type of PBN approach that offers even higher precision and accuracy than APV I. CAT I approaches also use satellite-based augmentation systems like WAAS or EGNOS. With CAT I, the lateral and vertical guidance is provided by the aircraft’s flight management system, enabling pilots to fly approach paths that closely resemble traditional instrument landing system (ILS) approaches. CAT I approaches can provide the same level of guidance and minimums as Category I ILS approaches, with decision heights as low as 200 feet above the runway.

Both APV I and CAT I approaches provide increased flexibility, safety and efficiency compared to traditional ground-based navigation systems. They allow for greater access to airports in various weather conditions, reduce reliance on infrastructure, and enable more precise and efficient aircraft operations. These approaches have become increasingly popular and are being implemented worldwide to enhance flight safety and optimize airspace use.

Performance of a satellite navigation system can be expressed through Five Criteria: accuracy, integrity, continuity, availability and time-to-alert (TTA). The overall detailed performance specifications are depicted in Table 1.

Accuracy is the difference between the computed value and the actual value of the user position and time. Usually, accuracy is defined as the 95th percentile of the positioning error distribution.

The system TTA is defined as the time starting when an alarm condition occurs to the time the alarm is displayed in the cockpit. Time to detect the alarm condition is included as a component of integrity.

The alert limits are the maximum allowable error in the user position solution before an alarm is to be raised within the specific time to alert. This alert limit is dependent on the flight phase, and each user is responsible for determining its own integrity in regard to this limit for a given operation phase following the information provided by the SBAS SIS.

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The integrity risk is the probability during the period of operation that an error, whatever the source, might result in a computed position error exceeding a maximum allowed value, called alert limit, and the user not be informed within the specific time to alert.

To display the integrity of the satellite corrections for each GPS satellite, the UDRE Safety Index is used to assess the integrity margin. The UDRE Safety Index is defined as the ratio . The Satellite Residual Error for the worst user location (SREW) was computed as the pseudorange error projection due to the remaining satellite ephemeris and clock errors, after KASS corrections were applied for the worst user location of the relevant service area. The relevant service area corresponds to the intersection of the service area and of the monitored satellite footprint.

Also, to display the integrity of the ionosphere corrections for each IGP, the notion of Grid Ionosphere Vertical Error (GIVE) Safety Index is used. The GIVE Safety Index is defined as the ratio , with the GIVE Error defined as the vertical pseudorange error at the considered IGP location due to the remaining ionospheric delay after applying the GIVE corrections.

Continuity defines the ability of a system to perform its function without interruption during the operation planned by the user (for example landing phase of an aircraft). It is evaluated as the probability that from the moment when the criteria of precision and integrity are completed at the beginning of an operation, they remain so for the duration of the operation.

Availability is the percentage of time when, over a certain geographical area, the criteria of accuracy, integrity and continuity are met.

Finally, the service area is the geographic zone where the SBAS shall provide service availability.

As depicted in Table 1, the major difference between CAT I and APV I is more demanding requirements in terms of vertical alarm limit and TTA, resulting in a reduction from 50m to 35m for the vertical protection level and from 10s to 6s for TTA.

The KASS system is designed according to the same architectural principles as the European EGNOS system, which complies with the CAT I service requirements concerning TTA. Therefore, all the elements justifying the TTA performance developed for the EGNOS system apply to the KASS system without any restrictions. This aspect is thus not considered a difficulty.

The main challenge in achieving the CAT I service level concerns the domain of vertical protection volume. Currently, KASS is compliant to APV I precision approach procedure but is limited for the achievement of CAT I service level due to the impact of ionosphere error estimation. With the low number of available measurements from only seven collocated stations, it results in very high integrity bounds (GIVE), which directly affects protection volumes in the vertical direction.

To overcome the limitation of the available measurement volume using only the contributions from the GNSS system, two steps are being considered.

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The first step is to include new observables by incorporating measurements from the Galileo constellation into the ionosphere estimation algorithms. It should be noted that Galileo measurements are not incorporated into other navigation algorithms, meaning the system does not monitor the Galileo constellation or calculate any orbit or clock corrections for it.

The second step is to add stations outside of Korea, which would provide better observability in both the dynamics of ionosphere activity and orbit estimation. This addition of external stations would enhance the overall measurement coverage and improve the accuracy of the estimation process.

For all these assessments, fault-free synthetic data are used (analyzed September 3-5, 2002), allowing a first level of performance the KASS system may reach for the provision of CAT I approach service. The results are produced with the same set of KPS navigation algorithms.

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CAT I Performances Reached with Introduction of Galileo 

The first extended configuration considers the introduction of Galileo measurements and the current KASS reference station network.

The simulations is realized with:

• Seven reference stations (Yangju, Gwangju, Jeju AP, Jeju TS, Yeongdo, Dodong and Yangyang).

• 27 GPS: for satellites and ionosphere monitoring.

• 24 Galileo: for ionosphere monitoring only.

The different cases presented indicate the availability of CAT I by monitoring the satellites from a minimum elevation angle of 5° with at least one station in the network, then at 10° and 15°.

This experimentation shows CAT I service level can be reached by introducing more observables for the ionosphere monitoring. Indeed, the availability at 99% is reached with the constraint of satellites monitored from 5° elevation by one station. 

First, these new measurements enrich the internal ionosphere model and then contribute to reduce the ionosphere correction errors.

However, because of the direct impact of the integrity bounds on the calculation of protection volumes, the most visible impact of these new measurements on achieving availability is their contribution to reducing GIVE at each Ionosphere Grid Point. 

Second, we observe a clear and direct dependence between the availability of CAT I service level and the minimum angle of satellite monitoring. This phenomenon is the result of waveform deformation (EWF for Evil Wave Form) processing in navigation algorithms, which requires at least one multi-correlator station to lock onto a satellite.

In all maps the integrity target is held with a good margin:

• The maximum satellite Safety Index is 0.77.

• The maximum Ionospheric Grid Point Safety Index is 0.76.

• Integrity is ensured as long as the Safety Index is below 5.33.

Focusing on the Incheon Airport location [127° E;37° N], Figure 3 shows performance in terms of position error and protection levels achieved for Sept. 4, 2022.

The VPL evolves mainly between 15m and 30m, except for a few minutes where it goes above the alarm limit of 35m. The HPL always remain below 30m. 

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CAT I Performances Reached with Reference Station Extension (Large Network)

The second extended configuration considers the introduction of Galileo measurements and additional reference stations located outside South Korea landmass to build a large station network.

The added KASS reference stations are depicted in Figure 4.
Five new references stations are considered: Ulaanbaatar (Mongolia), New Delhi (India), Perth (Australia), Wellington (New Zealand) and Hawaii.

The simulation is performed with:

• 12 reference stations.

• 27 GPS: for satellites and ionosphere monitoring.

• 24 Galileo: for ionosphere monitoring only.

Figure 5 shows the availability performance results with satellite monitoring performed using measurements at a minimum of 15° elevation. 

• Again, in all maps, the integrity target is held with a good margin.

• The maximum satellite Safety Index is 0.92.

• The maximum Ionospheric Grid Point Safety Index is 1.33.

In this case, CAT I availability performance is improved with 12 stations. The effect is not on the reduction of integrity value (GIVE) regarding the ionosphere, as the additional five stations are too far away to have an impact. However, having remote stations allows for much better satellite monitoring. This is clearly visible in Figure 6. 

Because satellites are monitored much earlier with 12 stations, the effect of EWF monitoring becomes negligible by removing satellites if they are not seen by at least one station at more than 10° or 15°. As soon as a satellite becomes visible in the service area, there is always a remote station that can see the satellite with a good elevation.

However, EWF monitoring has a noticeable effect in the case with seven KRS, where the availability decreases as the minimum angle increases.

Focusing on Incheon Airport location [127° E;37° N], Figure 7 shows performance in position error and protection levels achieved on September 4, 2002.

This approach has also high interest in the frame of geographical extension of KASS service, for instance the APV I coverage is extended. Table 2 shows the comparison between the APV I availability with the seven KRS and 12 KRS networks.

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Again, in all maps the integrity target is held with a good margin:

• The maximum satellite Safety Index is 0.92.

• The maximum Ionospheric Grid Point Safety Index is 1.33.

CAT I Performances Reached with Reference Station Extension (Narrow Network)

The third extended configuration considers the introduction of Galileo measurements and additional reference stations located outside the South Korea landmass to build a narrow station network.

Added KASS reference stations are depicted in Figure 8. Five new reference stations are considered: Shangai, Dalian, Mudanjiang (China), Izumo and Goto Tsubaki (Japan).

Figure 9 shows the availability performance results with a satellite monitoring performed using measurements with a minimum of 15° elevation. 

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Here again, in all maps the integrity target is held with a good margin:

• The maximum satellite Safety Index is 0.87.

• The maximum Ionospheric Grid Point Safety Index is 0.85.

Focusing on the Incheon Airport location [127° E;37° N], Figure 10 shows the performances achieved on Sept. 4, 2002.

Much like the previous cases, the VPL remains around 15 to 25m, except for a spike around 5 p.m. The HPL remains around 10m the entire day. The narrow network extension has a benefit aspect on the navigation error NSE, mainly because of better ionosphere corrections accuracy.

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Conclusion

The experimentation shows the KASS system evolution makes it possible to achieve the CAT I operational approach. The system’s adherence to architectural principles warrant its ability to meet the TTA requirements. Leveraging the Galileo system, the introduction of new GNSS measurements with the unchanged KRS network results reduces ionosphere integrity bounds, further enhancing the legacy system’s overall availability below the vertical alert limit compatible to CAT I. The study has also envisaged the addition of new external reference stations whose results have established a significant impact on satellite monitoring. With these new adjunctions, the KASS system would improve its capabilities to provide effective and accurate positioning services, proposing for safer and more efficient navigation in the CAT I operational approach.

References 

(1) “Standards and Recommended Practices (SARPS) Annex 10 to the Convention on International Civil Aviation,” Volume I, up to Amendment 86, July 2006, ICAO (International Civil Aviation Organization). 

(2) Houllier, Carolle, Authié, Thierry, Comelli, Guillaume, Lee, ByungSeok, Lee, Eunsung, Yun, Youngsun, SIN, Cheon Sig, “KASS: The Future of SBAS in Korea.” Inside GNSS, Junary–February 2023, pp 50-57

(3) Thierry Authié, Mickael Dall’Orso, Sébastien Trilles, Heonho Choi, Heesung Kim, Jae-Eun Lee, Eunsung Lee, Gi-Wook Nam, “Performances Monitoring and Analysis for KASS,” In Proc. of ION GNSS+, pp 958–978, 2017

(4) “Minimum Operational Performance Standards (MOPS) for Global Positioning System/Wide Area Augmentation System Airborne Equipment,” RTCA/DO-229D with Change 1, February 1, 2013.

Authors

Thierry Authié is a specialized engineer in space flight dynamics, precise orbit determination and navigation. He received his MS degree in Applied Mathematics from the INSA, Toulouse (France) in 2004. He currently works on SBAS and navigation algorithm at Thales Alenia Space.

Guillaume Comelli
is a system engineer and system architect at Thales Alenia Space. He received his MS degree in electrical engineering from the INSA, Lyon (France) in 1994. He has been the Technical Manager for the KASS program since 2019.

Sébastien Trilles is an expert in orbitography and integrity algorithms at Thales Alenia Space in Toulouse, France. He holds a PhD in Pure Mathematics from the Paul Sabatier University and an advanced MS in Space Technology from ISAE-Supaero. He heads the Performance and Processing Department where high precise algorithms are designed as orbit determination, clock synchronization, time transfer, reference time generation, integrity and ionosphere modelling algorithms for GNSS systems and augmentation.

ByungSeok Lee received a BS degree in electric and electrical engineering, a MS degree and a PhD in electrical and computer engineering from University of Seoul, Seoul, Korea, in 2002, 2009, 2015, respectively. He has conducted research related to a Global Navigation Satellite System (GNSS) including the Satellite Based Augmentation System (SBAS) in Korea Aerospace Research Institute. He was in charge of the KASS program from November 2020 to February 2024. He is currently responsible for the entire KASS operation and maintenance.

Minhyuk Son received his BS and MS degrees in electrical engineering from Daegu University, South Korea, in 2009, and 2011, respectively. He joined the Korea Aerospace Research Institute in 2011 and is currently in charge of operation safety technology development for KASS.

Cheon Sig SIN received a BS degree in electric and electrical engineering from Hanyang University, and a MS degree from Chungnam University, Korea in 1990, 2000 respectively. He conducts research related to a GNSS Signal Interference Detection and Mitigation Technology and Global Navigation Satellite System (GNSS) including the Satellite Based Augmentation System (SBAS) in Electronics and Telecommunication Research Institute. He has been in charge of the GK-3 SBAS Payload Development program since April 2021.

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