Systems for safety

The latest navigational guidance aids in the aviation sector that use the GPS technology are expected to replace many of the existing aids.

AVIATION traffic requires a reliable and continuous navigational service for safe and comfortable travel. The sector has seen a gradual development in navigational facilities with traditional technologies making way for the latest in equipment.

The invention of radio navigational aids, which are now indispensable in the aviation sector, grew out of the inadequacy of navigational aids such as smoke, light and magnetic compass in inclement weather conditions. The advantages of radio navigational aids are that they do not get affected by any weather condition and can guide aircraft at great distances as well as at very high altitudes. The precision of guidance is of a very high order. The latest guidance aids use the Global Positioning System (GPS) technology and are expected to replace many of the existing radio navigational aids.

Aviation safety demands the availability and security of an adequate and well-protected radio frequency (RF) spectrum. The useful frequency range of RF is limited. There are many users who want bigger and bigger portions of it for their use. To protect the interests of individual groups, specific slots have been allotted to each. The number of channels that is possible within the given slot is limited by channel spacing.

The rising demand to increase the number of channels is reducing channel spacing, thus giving rise to instances of channel interference. To avoid any complication out of such interference, a system is required to manage and monitor the frequencies. Mobile phones, Wi Fi facility, radio toys, ground-to-air communication and airto-ground communication systems, radar, radio navigational aids, high frequency radio, radio and television and many such things are active at airports. If RF is not managed properly, it could lead to chaos.

The International Civil Aviation Organisation (ICAO), a specialised agency of the United Nations, issues guidelines and mandatory requirements for providing radio navigational aids for use in international air traffic. The guidelines cover technical characteristics and performance, site selection of the facility for optimum usage, performance evaluation of the facility, and so on. It is mandatory to meet these requirements as a protection against unknown transgression of frequency spectrum, hazardous indication in air, and malfunctioning of the airborne receiver. It also ensures standard safety and continuity of operation for civil aviation.

The Radio Construction and Development Unit (RCDU) is entrusted with the responsibility of site finalisation for installation of radio navigational aids such as the VHF omnidirectional range (VOR), the instrument landing system (ILS), the distance measuring equipment (DME), and the non-directional beacon (NDB). The unit also carries out the installation of these facilities. The unit has been functioning under the Airports Authority of India (earlier the Directorate General of Civil Aviation and National Airports Division, or DGCA/NAD) as a central institution for more than 20 years for the installation of almost all types of radio navigational aids.

The Flight Inspection Unit (FIU) is entrusted with the responsibility of carrying out flight testing of facilities to check if these meet the ICAO-specified performance criteria; it issues a test report thereafter. The FIU was established in the 1970s under the DGCA. A manual flight inspection system was used at that time. The system was then upgraded to an advanced flight inspection system from Sierra Inc., United States.

This system had an ultra violet recorder, which used UV-sensitive paper, an RTT (real-time technology) system for sending automatic positional information from the theodolite installed on ground, an onboard signal generator, an oscilloscope and a spectrum analyser. Manual intervention was required in the analysis of recorded signals, and aircraft tracking was done manually.

In 1983, the Sierra system was replaced with a semi-automatic flight inspection system from M/s NORMARC, Norway. The system was built around a computer with a data acquisition system to capture desired signals during flight testing.

In 2004, the Normarc system was replaced by a fully automatic flight inspection system (AFIS) from M/s Aerodata, Germany. It is a fully computerised system and its reference positional system makes extensive use of GPS, which allows flight testing to continue even in bad weather conditions. It also makes extensive use of computer networking.

The reference position system under ideal conditions can give the aircraft a position within 20 cm of its actual position in space. In addition to the GPS, the system uses a radio altimeter, a barometer, aircraft heading, and roll and pitch information in its reference positioning solution. The coordinate system used for the purpose is dependent on the facility under test. For ILS, the threshold coordinate system is used, which is a local system based on ground survey data of the runway and the ILS components.

The FIU is equipped with proper tools and expertise to carry out the ground survey and develop the telecommunications subsystem (TCS) for the ILS under test. The position reference system is complemented by a laser autotracker system.

The laser autotracker, once installed properly at the desired position, can be operated remotely from the flight inspection aircraft. It is fully automatic and can search, lock and track the aircraft without manual intervention. The autotracker data are continuously received by the aircraft through a UHF radio link. After every autotracker operation, its calibration and error is automatically checked. On error, the autotracker operation is inhibited and the flight inspector is informed by the system.

A VHF radio is used to establish a communication link between the flight inspector and the ground team during flight testing. The AFIS is capable of analysing the collected data automatically to generate the results of the test run. It can also indicate if the results are within or outside the ICAO-specified tolerable limits. The flight inspector can observe various parameters of importance in both graphical and alphanumeric form during flight testing.

The AFIS system is also capable of flight testing of area navigation (RNAV) procedures, the satellite-based augmentation system (SBAS), the automatic dependent surveillance broadcast (ADSB) system, and so on. It is going to be upgraded to meet the requirement of flight testing the ground-based augmentation system (GBAS) in the near future.

For continuation of radio navigational aid services, satisfactory practices on ground have to be maintained, in addition to satisfactory flight test results. This is to ensure that the likelihood of a change in the performance of the equipment resulting in false information to air traffic is reduced to the very minimum. Any deterioration in the equipment can also predict the requirement of preventive maintenance and thereby reduce the chances of a sudden breakdown. It increases the facility reliability as well as the confidence level of the user group. Routine checking of equipment performance as per standards applicable is mandatorily carried out by maintenance engineers on ground.

It includes not only checking the performance at the equipment site inside the building, but also field-checking of the radiated signal at specific points on ground. In addition, normal test equipment such as oscilloscopes and multimeters and special equipment are also used. The maintenance personnel are trained in this. Licensing is still a far cry, though. Time and again, this issue has been raised at international fora by various organisations and countries.

Equipment design alone cannot provide safety in air traffic. There are many factors on ground that can affect these services. Hence, there is a need to choose a proper configuration of the equipment commensurate with the level of service to be provided. The extent of monitoring and service by the maintenance team also helps the continuity of service. For certain facilities, it is important to provide adequate standby power supply, which should be able to take over the equipment within a few second in case of mains failure. Facility performance is also affected by ground conditions and objects in the specified critical/sensitive area. For continuity of service for such facilities, these areas need to be protected during operation.