GENERAL INFORMATION ON GNSS AUGMENTATION SYSTEMS 1. INTRODUCTION Navigation technologies with precision approach and landing systems, for civilian and military purposes, enable aircrafts to perform their operations under adverse weather and terrain conditions. Hence, approach and landing systems and navigation aids (ILS, PAR, TACAN, NDB, VOR, etc.) provide reliable solutions at various operational levels as of today. However, installation and maintenance costs for the current systems and their related sub-units have been dramatically increased due to the factors such as gradual decrease on manufacturer support, the high reliability and availability requirements for aerospace applications.. In addition, many existing systems have geographical constraints which pose criticality for the operational efficiency, operational capability and safety. For these reasons, it is anticipated that the existing systems will soon be replaced with more efficient and costeffective solutions. Navigation technologies with approach and landing systems based on global navigation satellite systems (GNSS) stand as a prominent alternative to the existing systems in terms of usability in all flight phases, providing approaches to airfields which lack navigation aid infrastructure and supporting ease of airfield installation and maintenance. However, due to various types of error sources which affect GNSS based operations, key requirements such as accuracy, integrity, availability and continuity for safety critical aviation applications cannot be fulfilled by stand alone GNSS usage. At this point, augmentation systems for satellite navigation, namely the satellite based augmentation system (SBAS) and the ground based augmentation system (GBAS), present promising, state of the art and cost effective solutions, meeting the performance requirements for different phases of flight from take off to landing by aiding aircraft navigation subsystems to minimize the amount of error from stand alone GNSS calculations. 2. GNSS FOR AVIATION In today s aviation, conventional ground based (ILS, PAR, TACAN, NDB, VOR, etc.) or inertial (INS/GPS) systems have substantially been used as the primary navigation and landing aid for various types of aircrafts and operations. Although the conventional systems are highly accurate and some systems, ILS and PAR, can also support precision approach and landing capability up to CAT-3, there are still some drawbacks related with these systems which prompt the users to search for better alternatives. The main drawbacks of conventional systems are both requirement driven such as efficient use of airspace, manpower and training, frequent maintenance for continuous operation and cost driven such as high installation, maintenance and calibration costs. At this point, GNSS soluitons stand as an attractive alternative to the conventional navigation aids.
Recent developments in GNSS technology has created a potential to overcome the drawbacks of conventional systems. Indeed, GNSS technology is used as an additional positioning and navigation aid to conventional systems in nearly all civilian and military aircrafts. GNSS technology features increased service capacity, high availability and accuracy levels with recent transition to multi frequency capability, which enable GNSS technologiy to be used for all flight phases from en route to landing. In addition to technical capabilities, GNSS technology enables several operational advantages for aviation, such as low maintenance requirements per aircraft, more efficient airspace capacity and usage with lowered seperations, reduction in fuel consumption and optimal noise abatement. Also, GNSS based (GLS) approach procedures can be performed regardless of location and coverage area of legacy ground based navigation aids. Hence, it becomes possible for aircraft to use instrument approach and landing procesdures for airports without any navigation aid infrastructure. Together with all technical and operational benefits above, GNSS should still fulfill the stringent requirements on the performance parameters for aviation in order to be used as the primary navigation aid. These performance parameters that GNSS systems should meet are described for each flight phase and published by international aviation authorities as signal in space performance parameters. These performance parameters are mainly integrity, accuracy, continuity and availability. Among all these parameters; integrity, which is the ability of the system to warn the pilot from inaccurate position calculations in a timely manner, is the absolute must for safety critical applications, as in aviation. In order to use GNSS technology as the primary navigation aid, GNSS shall meet the stringent requirements on integrity as well as other performance parameters, under various error and fault conditions. These error conditions such as multipath, ionospheric and tropospheric disturbances, receiver noise, satellite failures, etc. may result in various situations that may risk integrity, accuracy, continuity and availability of GNSS. For en-route and terminal phases of flight, more tolerant performance limits are applicable, and standalone GNSS system is mostly sufficient to meet the requirements. However; for approach and landing phases of flight and especially the conditions are IFR, standalone GNSS usage becomes insufficient to meet the requirements for performing a safe landing. Especially for integrity, calculated GNSS position should be augmented by another trusted system for enhancing the GNSS based navigation system performance parameters. 3. GNSS AUGMENTATION SYSTEMS 3.1. Overview of GNSS Augmentation The basic operation principle of satellite based navigation systems is calculation of users s position from the GNSS signal. The information transmitted by GNSS satellites includes high accuracy clock and ephemeris data as well as several other parameters including satellite health, orbit, tropospheric and ionospheric information. This information is collected by GNSS receivers and used to calculate position, time and other necessary information for user. Although position and time information obtained through the GNSS data are quite accurate for most users other than aviation, there are indeed several error sources that affect the calculations. Some of the major error sources that affect GNSS can be given as satellite orbit and atomic clock errors, signal propagation due to ionosphere
and troposphere, receiver clock errors, receiver noise and resolution, multipath effect and ephemeris prediction errors. These error sources and related range errors are given in Table 1. Although positioning accuracy obtained by the stand alone usage of GNSS are quite high and mostly enough for a wide range of applications, higher accuracy is required for applications which have strict safety and integrity requirements. Table 1: GNSS Error Sources, Range Errors and Effect with DGNSS GNSS Error Sources GNSS Range Error (meter) DGNSS Effect (meter) Satellite Clock 3 0 Ephemeris 5 20 0-1 GNSS Receiver 2 2 Ionospheric Disturbance 15 20 2-3 Tropospheric Disturbance 3 4 1 Multipath 2 2 For applications such as aviation, the amount of error can be minimized and accuracy can be increased for meeting the performance requirements by augmenting GNSS calculations. The technology behind GNSS augmentation is differential GNSS or DGNSS. In DGNSS, GNSS receivers with exactly known locations are used as references to calculate real time GNSS errors and these errors are then eliminated from measurements. Knowing the exact location of the reference receiver, corrections for the GNSS satellite positions can be computed and transmitted to users. Specific software is developed for the reference receiver to track all satellites in view and to calculate the specific corrections for each satellite. If these corrections are to be used in real time, a data transfer means between the reference and user aircraft is needed. In order for the user to apply these corrections from DGNSS system, the user must be in the same vicinity of DGNSS reference receivers since it will only then allow both the reference receiver and the user receiver to experience common errors for a specific satellite signal. In addition to this, user receivers should use the same set or a subset of satellite signals with the reference receiver so that the calculated corrections can be valid for the user. There are two main augmentation systems that rely on DGNSS technology; Satellite Based Augmentation System (SBAS) and Ground Based Augmentation System (GBAS). 3.2. Satellite Based Augmentation Systems (SBAS) Satellite based augmentation systems calculate range and integrity information for the GNSS satellites by a ground system and transmit these data to users within the coverage area through a GEO satellite. The main functions of an SBAS system can be given as data collection from GNSS sources, ionospheric correction and satellite orbit determination, range and integrity calculations, independent data verification and SBAS message generation and broadcast. SBAS system architecture can be divided into three segments; namely the Space Segment, the Ground Segment and Users. In an SBAS system, the reference stations deployed over a wide area collect GNSS information such as the satellite data, tropospheric data, calibration information, etc. These
information are used to perform operations such as pseudorange, ionospheric and tropospheric calculations estimated from signal delay. Central control station which continuously receives data from all reference stations uses these information and to constitute the necessary range correction message for the users. Correction message basically includes wide area GNSS based position correction data for each and every GNSS and GEO satellite, ionospheric delay information with correction, integrity parameters for all satellites, warnings for GNSS sources if necessary, position information for GEO satellites, SBAS network time and UTC offset parameters. Error boundaries for ionospheric, ephemeris and clock corrections are also calculated in the central control station. All of this information is also checked and verified in terms of integrity by central control station before the message is transferred to uplink to be broadcast to users. After the SBAS messages are formed by central control station, they are sent to satellite uplink station where the messages with best quality are selected, transformed into GNSSlike signals with time synchronization and PNR code integration for uploading to GEO satellites and made ready for transmission by combining the formatted messages with the signals in the transmission band. Also, an operation and maintenance station usually monitors and controls SBAS ground segment elements, monitors and logs SBAS mission and performance data and supplies this local integrity data to air traffic controllers. Communication inside an SBAS ground segment is carried out through the terrestrial wide area communication network. This network enhances data transmission between all SBAS elements, enables multicast capability for certain data while monitoring the performance of whole transmission network. Communication network needs to be designed for redundancy and security for SBAS availability, continuity and integrity. When SBAS messages are broadcasted to users within the coverage area, the user aircraft with necessary SBAS compatible avionics, receives and processes the message data. SBAS avionics of year 2015 generally process single frequency SBAS correction and integrity information to perform position calculations which are used for different phases of flight from en-route to landing. SBAS enables up to LPV200 precision-like approach and landing procedures by providing improvements for the accuracy, integrity, availability and safety parameters. Compared from approach and landing system perspective, some of the advantages obtained by SBAS to legacy systems are wide coverage area, low cost for maintenance, suitability for all phases of flight, flexible approach and landing routes, efficiency and cost effectiveness and less susceptibility to environmental conditions. The areas other than the aviation, where SBAS are widely used include some special military applications, agriculture, maritime, railways, land transport and construction applications, geodetic studies and timing standards. WAAS (USA), EGNOS (AB), GAGAN (India) and MSAS (Japan) are the SBAS systems available for use in aviation community as of today. SDCM (Russia), SNAS (China) and ASAS (Africa) are other systems which are still being developed. The coverage areas of the above mentioned systems are shown in Figure 1.
Figure 1: Satellite Based Augmentation Systems 3.3. Ground Based Augmentation Systems (GBAS) Ground Based Augmentation Systems, construct corrections and integrity information for the GNSS data through a ground station that is installed on each airport. Corrections and integratity messages as well as the final approach segment route are broadcasted to the users within the 25 nm coverage area of the V/UHF broadcast units. The existing GBAS systems of 2015 depend mostly on GPS L1 C/A signal for civil applications. Considering the developments in GNSS systems and with the introduction of the new signals and satellite constellations, GBAS systems are expected to utilize multi frequency and multi constellation GNSS in medium term. There are three operational segments of a GBAS system; namely the Space Segment, the Ground Segment and the Aircraft Segment as shown in Figure 2. Figure 2: Illustrative Ground Based Augmentation System Architecture
GBAS ground segment contains at least four reference GNSS receivers in order to ensure the integrity and continuity requirements. Reference stations gather GNSS signals, monitor their quality, and perform pseudorange calculations for available satellites. The processing unit of GBAS ground segment is responsible for calculation of the pseudorange corrections and the preparation of navigation link message. Through several different algorithms, performance and integrity monitoring for GNSS signal sources and for the GBAS system itself are executed on ground segment to warn users for any risk that may result due to GBAS system and/or GNSS signals. Final Approach Segment (FAS) data for each runway end is defined as a part of the GBAS message. The FAS data is sent to users in order to support users with specific route of any single approach. After the construction of GBAS messages, which include the differential corrections, integrity information and FAS data, they are transmitted to users within the coverage over the V/UHF data link. Military applications require electronic protection capabilities and may use different communication possibilities. Aircrafts must be within the coverage of data transmission and equipped with GBAS compatible avionics to receive GBAS messages transmitted by ground segment. The basic functions of a GBAS avionic are; to receive and decode signals and correction messages from GBAS ground segment and GNSS satellites, to continuously monitor GBAS and GNSS system availability, to compute aircraft position and integrity information, and to apply the uplinked corrections according to the approach and landing route. It is possible to classify GBAS as a precision approach and landing system which is capable of supporting Cat-1,2,3 and more, whereas an SBAS system is only capable of supporting LPV200 Cat-I like approaches. SBAS is not capable of supplying the integrity requirements for Cat-2 and Cat-3 approaches due to its system architecture. On the other hand because of its wide coverage area SBAS is not only an approach and landing aid, but can also be used for en-route operations. For GBAS, due to its limited service area to vicinity of each installed airport, it can be used only for the operations within that service area. Advantages of GBAS compared to legacy approach and landing systems come forward as; usability by all types of aircraft, support for many different approach procedures on the fly, suitability for usage by more than one airport if the coverage area conditions are satisfied, flexible touchdown points and approach and landing routes, flexible air traffic control and effective airport traffic management, low costs for installation, maintenance and training, etc. Worldwide locations of GBAS installations as of 2015 are shown in Figure 3. Figure 3: Ground Based Augmentation System Locations (flygls.net)