07-05-2014, 11:54 AM
Ground Based Augmentation System (GBAS)
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INTRODUCTION
This paper describes a particular satellite navigation technology known as a ground-based augmentation system (GBAS). Satellite navigation has become a critical component of the emerging worldwide air traffic management (ATM) infrastructure. As congestion grows and rising costs demand ever greater efficiencies, the management of air traffic will rely more and more on the management of airplane trajectories in four dimensions (lateral, vertical, and longitudinal path and time). GBAS promises to become an indispensable tool in the future for the management of airplane trajectories for ATM, particularly near and at airports and landing sites.
The International Civil Aviation Organization (ICAO) committee on Future Air Navigation Systems (FANS) developed a vision for a Global Navigation Satellite System (GNSS) to support aviation navigation needs. The Global Positioning System (GPS) was offered to the world's aviation community by the United States in a letter to the ICAO in October 1994 [1]. ICAO accepted the offer, establishing GPS as an important component of the GNSS. The Russian Federation made a similar offer with respect to use of the Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS). Hence GPS and GLONASS became the core constellations in the system of systems defined by ICAO as the GNSS. However, because of certain limitations (real and perceived) in the performance of GPS and GLONASS, additional system components were added to GNSS to augment performance, including:
1. space-based augmentation systems (SBASs);
2. ground-based augmentation systems (GBASs);
3. airplane-based augmentation systems (ABASs);
4. ground-based regional augmentation systems (GRASs).
The GNSS as defined by ICAO includes the core constellations (GPS and GLONASS) as well as these augmentation systems. Formal standards and recommended practices (SARPS) for GNSS were developed and published in 2000 [2]. These SARPS are intended to ensure interoperability between components of the GNSS and to ensure that equipment based on GNSS operates safely and with consistent performance that meets the operational needs of aviation users.
The augmentation systems listed above were developed to provide improved accuracy, integrity, continuity of service, and availability of navigation to support a wide variety of operational needs. These augmentation systems have been the subject of nearly two decades of research and consequently much has been written about them. This paper focuses on only one of these four augmentation types: GBAS [3], [4].
GBAS is significant for air transport users for a variety of reasons. It is the only augmentation system defined at this time that is expected to be capable of meeting the most stringent operational needs of aviation (e.g., takeoff and landing in very low-visibility conditions). Furthermore, the system is relatively inexpensive, physically compact, and self-contained, so that deployment in response to demand anywhere globally is technically feasible. Although GBAS relies on the core constellations, it does not rely on any other large and expensive infrastructure.
GBAS SYSTEM DESCRIPTION
GBAS System Architecture
GBAS is fundamentally a local differential satellite navigation system [5]. As such, the basic principle is that pseudorange observations made by ground-based receivers are used to develop differential corrections for each satellite. These corrections are provided to the airborne user's receiver via a data link. The airborne receiver then applies these corrections in order to produce a set of corrected pseudoranges that are then the basis of a position solution. The underlying assumption is that, for relatively short baseline separations between the ground-based receivers and the airborne-based receivers, the most significant error sources will be common to both observers and will therefore be eliminated by the differential processing [6].
GBAS Signal in Space
The GBAS signal in space is defined as the combination of the satellite signals from the core satellite constellations and the VDB signal [14]. The current standards allow a GBAS to augment signals from either the GPS or GLONASS constellations.
The GPS signals are direct sequence spread spectrum signals. These signals consist of a binary phase-shift keyed (BPSK) modulated carrier with a pseudorandom binary code at a chipping rate of 1.024 Mchips per second. In addition, a 50 bit per second navigation message is combined with the direct sequence signal. The primary civil GPS signal, designated “L1 Coarse/Acquisition” or “L1 C/A,” is centered nominally at 1575.425 MHz. All satellites transmit at the same nominal frequency, and code-division multiple access is used to share the band. The GPS signal structure is well documented in the GPS Interface Control Document (GPS ICD-200 [8]) and the interested reader is referred there for further details. A good description of the signal structure as well as a general description of entire GPS system can be found in [10] and [11].
A GBAS currently uses only the primary GPS civil signal L1 C/A. GPS also includes signals at another frequency, designated “L2.” A GBAS makes no use of those signals, as they are not fully supported for civil applications and are primarily intended for use by Department of Defense authorized users. Modernization plans for GPS include the addition of a third frequency, designated “L5,” which is intended to support civil applications. The current definition of GBAS does not support the use of these planned signals, but it is anticipated that the standards will be expanded to allow for use of these signals once they are available.
GBAS Performance
GBAS performance is characterized in several ways. One fundamental metric is the “signal in space” (SIS) space performance. The SIS performance is defined in terms accuracy, integrity, continuity, and availability of the service. This SIS performance is referenced to the output of a “fault free” instance of compliant user equipment. For example, integrity is defined at the output of user equipment that is conforming to certain mandatory functional requirements that define how the data from the ground station is combined with measurements made by the airborne equipment.
GBAS Landing System (GLS) Approach Selection
Unlike ILS or the microwave landing system (MLS), GNSS-based approaches are not referenced to a ground-based antenna. Both the desired flight path and the measured or estimated position of the airplane are referenced to an earth fixed coordinate frame. The desired reference path is defined by a set of coordinates supplied via the data broadcast. A mechanism is required that allows the correct set of coordinates defining the approach to be selected. Also, a mechanism to verify that the correct selection has been made is required.
As new operational capabilities are introduced, consistency with existing operations is highly desirable. Consistency reduces the cost of integrating the new functionality, the cost of training pilots, and safety hazards that can occur if inconsistent system interfaces are used. Consistency with existing ILS and MLS operations is also important to enable air traffic management service providers to handle mixed-mode operations at airports. The transition from ILS to GBAS is likely to take decades, and some airports will need to support operations with both systems during the transition.
The data broadcast on a single frequency may contain FAS datablocks for several different runway ends, multiple approaches to a single runway end, or even multiple runway ends at multiple airports. Due to the TDMA frequency sharing, a receiver tuned to a given frequency could see FAS definitions provided by multiple different ground stations for airports separated by large distances. Therefore, unlike ILS, the simple selection of a frequency does not uniquely identify a specific approach.