ICAO has tasked the ALL Weather Operations Panel (AWOP) with considering the possibility of using satellite based navigation systems as a runway approach aid. Trials have commenced in several countries, most notably in the USA. These satellite systems are generally referred to as GNSS (Global Navigation Satellite System). This refers to any satellite based system. There are two available today, the US GPS and Russian GLONASS systems. As the current developments are almost entirely based on GPS, this is the system discussed in this paper.

The GPS system consists of a constellation of 24 satellites (21 in use and 3 in orbit spares). The system was developed by the US Department of Defence, but it includes the ability to be used by civil operators. The full 24 satellite constellation was completed in the spring of 1993.

Each satellite transmits coded time signals which are compared by the receiver in the aircraft (Fig1). Comparing the signals from a number of satellites enables the position to be calculated. The GPS position will normally be fed into a navigation system such as an FMS. The GPS system could potentially offer a single navigation system for both en-route and approach & landing. As there would be considerably less equipment required on the ground, and at each airport, the overall cost of the system could, potentially , be less than today’s ground based aids. It is generally accepted that to achieve the accuracy and reliability required for Cat I approaches or better, a system called Differential GPS will be required. This uses a ground station at the airport which also receives the satellite signals. As the ground station knows its position very accurately, it can measure any inaccuracy in the satellite signals. The ground station then calculates a correction signal which it transmits to the aircraft on a separate frequency (Fig 2). Differential GPS considerably improves the accuracy of the system, but increases the cost, and removes one of the potential benefits of GPS, that it is independent of ground equipment.

In the USA , the FAA claims that the accuracy for Cat I approaches has been demonstrated in trials. They have overlaid 5000 domestic non-precision approach procedures with GPS procedures.Their intention is to develop and test these procedures so that they can be used for Cat 1 operations by 1998, allowing Cat I ILS to be phased out shortly after then. It is not yet known if it will be possible to achieve the accuracy required for Cat II /III . The FAA has commissioned a study into the use of GPS for Cat II / III operations and this is due to be completed in 1995.

Airlines in the USA have demonstrated the use of FMS in conjunction with Differential GPS for approaches and departures. When used with these systems in the aircraft it is possible to fly the curved approaches and other advanced procedures possible with MLS. It was noted during the development of MLS advanced procedures that they can complicate the ATC situation, reduce the flexibility available to the controller , and increase workload. these considerations will also apply to advanced procedures using GPS.

The major technical challenges with GPS are the integrity and reliability of the system. There are techniques that can be used to resolve these problems. These include the differential ground stations. There is a proposal that other satellites should be used to check the signals from the GPS satellite. The GPS receiver will have to be improved for use on approach with built in checking systems. Considerable improvements are required in this area before the system will be acceptable for precision approaches. The GPS receiver also needs to be much more sophisticated for approaches compared with one used only for en-route navigation in order to convert the position signal into final approach guidance. The differential GPS datalink must also be added. The cost of these items will be a significant factor in the use of GP as an approach aid. Other questions to be resolved cover the vulnerability of the system to jamming, the ownership, control method of payment for the system.

Airlines, particularly in the USA , but also in underdeveloped countries, are supporting the use of GPS for approaches. They see the need to introduce GPS for use over the oceans and over sparsely populated areas. Having bought the GPS equipment for this , they believe that it can be used as a replacement for ILS as well, so avoiding the costs of buying MLS. General Aviation in the US also strongly supports the introduction of GPS as meeting their needs for a straightforward and affordable system both for en-route and as an approach aid.

It is expected that Cat 1 GPS can be certified in the period 1998 to 2000 and this will then be available as the replacement for Cat I ILS. It seems likely, however , that Cat II / III operations will not be achieved for many years, and MLS will be required at least initially for Cat II / III operations. As a result of this, it appears that there will be a proliferation of approach aids in the years to come. It also appears unlikely that there will be a short, clearly defined, transition period between ILS and MLS, but rather a period of evolution, perhaps with regional differences, where GPS, MLS and maybe other systems gradually replace ILS.

To Conclude

It is almost certain that GPS will be used as final approach aid within the next few years. The institutional issues of ownership and payment are being addressed in order to allow ADS operations. The standards and procedures for its use as an approach aid will be developed by ICAO by the AWOP. It is also likely that GPS will only be the first stage in the development of other GNSS systems.

The problems of the proliferation of landing aids is being addressed under the MLS and RNP work study items.

It appears at present that GNSS will be used as a standard approach aid and at present its use does not appear to create any special problems for ATC. SC 1 is happy, in principle, that the introduction of GNSS based approach aids does not require any additional policy.

Until specific issues regarding the use of GNSS for approach are encountered, this subject on the SC1 work programme should be reduced to a monitoring role.