Preparation of Papers in Two-Column Format for the Proceedings of the 2004 Sarnoff Symposium

Ch Mahesh received his B.E(ECE) degree from Andhra University, Vishakhapatnam ,AP in 1999 and M.Tech(Systems and Signal processing) JNT University, Hyderabad ,AP ,in 2003. He joined AAI in 2006 as JET(Elex) and presently working as Manager(CNS) at Hyderabad International Airport Limited (HIAL),Hyderabad in NAV AID unit since 2011.

GBAS: A future Precision Landing System in Low Visibility

Abstract-In terms of both Airspace and navigation , a major change is envisaged from the navigation requirements to days fixed air routes due to land based navigation system. This paper discuss the limitations of present landing system in use and need of future precision landing based on Global Navigation Satellite System(GNSS) landing. The Ground based augmentation system(GBAS) emerged as a new technology for precision landing in low visibility condition. Further, it greatly improves the airport capacity by the use of multiple runways with single GBAS system within coverage area.I. Introduction

Although the first use of radio navigation has begun in 1904, the practical use did not take place until 1930s with Dr Ernst Kramer who is often called father of instrument landing system at standard Electric Lorenz of ITI subsidiary company. At that time the instrument landing system and navigations were called Bordfunkgeraete means On Board Radio Devices in Germany. After the use of the four-course dot/dash signals at 480MHZ in 1942 at Great Britain, a landing system was developed in the USA in 1943-44 for military purposes [1]. Over last 80 years of successful operation, the landing system is under transition stage due to its potential limitations and cost effective.

A.The Instrument Landing System (ILS)The ILS, a ground based system, provides aircraft with a lateral and vertical guidance relative to an approach path to the runway. In early days of ILS development, the pilot has to take visual decision before approaching to land. Usually, this is known as Decision Height where the pilot visually acquire the runway or must initiate go around. With the development of autopilot and flight display designed, along with the improved ground system accuracy and continuity service made lower decision height. Based on these capabilities the first auto landing in 1964 at United Kingdoms Brodfort Airport and first blind landing in 1972 was performed.

The ICAO introduced series of categories to indicate the minimum approach height which would be achieved with each systems performance. These categories are shown in table 1.CategoryDecision Height(DH)RVR

CAT I 200ft/60m 550m

CAT II 100ft/30m 350m

CAT III

A

B

C< 100ft/30m down or no DH

< 15 m or no DH

No min>200m

200>RVR 50m

No min

Table 1 ILS DH and RVR Each category is associated with a runway visual range

Even though, CATII/III systems provide significant level of safety and efficiency if airport access in poor visibility condition, they required increased separation between arriving aircraft to avoid radio interference.

BILS Limitations

The Instrument Landing System(ILS) has served as the standard precision approach and landing ,it has undergone a number of improvements to increase its performance and reliability. However, in relation to future aviation requirements, the ILS has a number of basic limitations [2]. 1) site sensitivity and high installation costs, 2) single approach path, 3) multi path interference, and 4)channel limitations - 40 channels only.COptions for Change

Early 1970s, Microwave Landing System (MLS) emerged as an alternative to the ILS. The MLS overcome several limitations by using a completely different Radio Frequency but unfortunately failed due to lack of civil airspace user commitment and may be an alternative is in progress. The emerging GPS is the primarily focused and the interest of MLS has utterly neglected.II. gnss landing system-GBASIn the early 1990s, another alternative to ILS was emerged as positioning and landing system based on the Global Navigation Satellite System (GNSS). The ICAO laid international standard for landing system based on local correction of GNSS data to a level that would support an instrument approach. The ICAO SARPS define the characteristics of the GBAS service that can be provided by service provider. GBAS offers greater flexibility by supporting GNSS Landing System operations on multiple runways, with a single system. The aviation world is switching to GBAS in line with ICAO strategy of satellite navigation for all phases of flight[3]. The GBAS provides radiated signal in space that can be used by suitable equipped aircraft as the basis of GNSS landing system. The GBAS consists three major elements:

a) A global satellite constellation

b) The Ground station and

c) Avionics.

The global satellite constellation supports worldwide navigation position fixing, a ground station facility at each equipped airport provides a local navigation correction, and onboard avionics will process and provide guidance as shown in fig 1.

Fig 1. Basic GBAS ComponentsA.How GBAS Works

GBAS is designed to support precision approach operation at airports within coverage area. It provides desired flight path information for approaches, landings, and other maneuvers within the terminal area, as well as determining ranging sources errors using multiple ground reference receivers. The GBAS ground facility monitors GPS and/or GLONASS signals at an aerodrome and broadcasts locally relevant integrity messages, pseudo range corrections and approach data via a VHF data broadcast (VDB) to aircraft within the nominal range of 37 km (20 NM) in the approach area (when supporting Category I operations) and within the range depending upon intended operations (when providing positioning service). When an Satellite Based Augmentation System(SBAS) service is available, GBAS can also provide corrections for the SBAS GEO ranging signal. The onboard avionics at present containing multimode receivers (MMRs) that support ILS and basic GPS operation with slight modification of these MMRs will receive and process the GBAS data via VDB data link. The MMRs apply the local correction data received from the GBAS to each satellite that the aircraft and GBAS share common. These corrections are more effective when the aircraft relatively closely to GBAS station. A GBAS is designed to support one or more of three categories of precision approach as shown in figure 1. A single GBAS ground installation may provide guidance for up to 49 precision approaches within its VDB coverage, serving several runways and possibly more than one aerodrome.

B.Operation Using the GBAS

A typical GBAS ground station provides approach and landing service to all runways at the GBAS equipped airport. Each runway approach direction requires the definition of a final approach segment (FAS) to establish the desired reference path from an approach, landing, and rollout. Each approach is given a unique identifier for particular FAS, glide slope and missed approach combination. The required FAS data for all approaches supported by GBAS facility through VDB data link. The onboard MMR process the pseudo range corrections and FAS data to produce an ILS like deviation indication from the final approach path. These deviation indications are displayed on the primary flight display for landing guidance. The GBAS is intended to support multiple levels of services to an unlimited number of aircrafts within its coverage area. ICAO defined two levels of service. 1) Performance Type 1(PT1) or Precision approach, 2) GBAS positioning service (GBAS PS). The PT1 service supports ILS look like deviation for an instrument approach. The GBAS PS provides very accurate position, velocity and time measurements within the terminal area. The improved accuracy will benefit other future uses of GNSS positioning. Such as ADS-B and surface movement guidance and control systems. C GBAS performance characteristics Before introducing any new navigation system one should evaluate the four necessary parameters. Those are:

1) Integrity, 2) Continuity, 3) Availability and 4) Accuracy. Integrity is a measure of the trust which can be placed in the correctness of the information supplied by the total system. Integrity includes the ability of the system to alert the user when the system should not be used for the intended operation (or phase of flight). Continuity is the capability of the system to perform its function without unscheduled interruptions during the intended operation. This is expressed as a probability. The availability of GNSS is complicated by the movement of satellites relative to a coverage area and by the potentially long time it takes to restore a satellite in the event of a failure. GNSS position accuracy is the difference between the estimated and actual aircraft position. Ground-based systems such as VHF Omni directional radio range (VOR) and instrument landing system (ILS) have relatively repeatable error characteristics. Therefore their performance can be measured for a short period of time (e.g. during flight inspection) and it is assumed that the system accuracy does not change after the measurement. GNSS errors however can change over a period of hours due to satellite geometry changes, the effects of the ionosphere and augmentation system design.III. BENEFITS and limitations of GBASWhile GNSS offers significant benefits, the technology has its limitations and brings with it a number of institutional issues.

A. Benefits of GBASThe GBAS service can offer significantly better performance than ILS. It has the following capabilities with respect to the user perspective:

Provide approach and takeoff guidance service multiple runways through single GBAS facility

Optimize runway use by reducing the critical protection areas

Provide more flexible taxiway or hold line placement choices

Provide more efficient aircraft separation or spacing standards

The other significant benefits from the service providers perspective are:

Reduce installation, maintenance costs because single system can support all runways

The operational constraint of interlocked condition of ILS can be avoided to serve other runway

The GBAS serves all runway ends with a single VHF frequency , the limited navigation frequency spectrum is used more efficiently

The siting condition of GBAS ground station is not critical as like as ILS

B. Limitations of GBAS

The limitations or challenges of GBAS are: A challenge for GNSS is the achievement of a high availability of service. Interference with GNSS signals directly affects availability. The safety of GNSS navigation depends on the accuracy of navigation databases. Therefore one has to ensure data integrity when developing new procedures.

IV. conclusions

The present ILS under pressure due to its potential drawbacks. Especially, in low visibility and its limitation effects the capacity of airport. GNSS based navigation system i.e. GBAS, has the potential to provide a greater level of efficiency and to be more cost effective. While ILS performance and limitations are well known the need to maintain ILS as a backup until GBAS is fully demonstrate for safe and secure navigation.References

[1]Leon Himmel ., First-Hand: Development of Instrument Landing System Glide Path, IEEE Global History Network.

[2]Ch Mahesh., Dr K Ravindra., Prof V Kamakshiprasad.," The Role of CNS in Aviation: Present and future-A comparative study" Journal of Information, Knowledge and Research in Electronics and communication Engineering., ISSN 0975-6779,Nov 11 to Oct 12., Vol-2.,Issue-01,Page 163-174.[3]ICAO Doc 9849.," Global Navigation satellite System(GNSS) Manual" first adition,1995.

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