Radio Navigation Aids
by Roger Meyer


The provision of radio aids to navigation for Australian civil aviation is the story of invention and innovation on the part of the Department’s engineering and flying operations organisations. With aircraft of ever-improving performance and the demands on air traffic control to process the flights of more and faster aircraft, the challenge was to use the latest available technology to meet these needs, and to anticipate future requirements.

The Bellini-Tosi Medium Frequency direction finding receiver was the first radio aid to navigation used in Australia. Designed for ship-to-shore use in 1911, it was in common use in Europe when introduced here in 1935. These direction finding receivers were installed in Aeradio stations located along the major air routes. The operator could determine the direction of a radio signal transmitted from an aircraft and relay the bearing back to the pilot. Due to the frequency band in which the system operated, the accuracy of the bearings was poor, varying by up to ten degrees at sunrise and sunset. In 1940 a number of HF Cathode Ray DF units were installed and these gave much better performance due to the higher frequency of operation. Direction Finding remained in use as an aid to navigation into the late 1950s.

The USA adopted a different philosophy: namely, that the pilot of an aircraft should have a continuous form of cockpit indication on which he could act without being dependent on the ground station. The system they used was the ‘medium frequency four-course radio range’. One such experimental system was installed at Brighton, near Mascot, in 1936. Each leg of the beacon was aligned with the major air routes from Sydney. The four ‘beams’ were focussed on Canberra, Newcastle and Dubbo, with the fourth reserved for the trans-Tasman route. Again, the operating frequency made the system susceptible to man-made and atmospheric electrical interference.

A typical Lorenz beacon

The most important pre-war navaid was the Lorenz 33 MHz Radio Range, developed in Germany as a ‘blind landing system’. It was used extensively in Europe, and during the war was adopted by Great Britain as the standard beam approach (SBA). In the late 1930s Australian Civil Aviation Board engineers redesigned its antenna system to enable the beacon to be used as an en-route navigation aid. Click here to read more about Lorenz.

Nineteen Lorenz beacons were in use during 1950 at the peak of this system’s use, however it had some shortcomings such as false courses caused by interference between beacons, often across the other side of the country. This led to it being replaced by the VAR system, of which more anon.


During World War II the homing beacon, or ‘homer’, used in conjunction with an airborne direction finder in the aircraft, became our cheapest, most enduring and commonly used navaid of them all - the Non-Directional Beacon (NDB). Aircraft receivers were later automated, becoming the Automatic Direction Finder (ADF). In the 1970s the old valve NDBs were replaced with more powerful, reliable and cheaper transistorised NDBs - shown on charts for a while with the annotation NDB(T).

The SCS51 low ceiling landing aid, LORAN, and the ‘talk-down’ Ground Controlled Approach (GCA) radar were the result of wartime developments in pulse techniques, and these were to form the basis for many post-war radio navigation systems.

Following the war years DCA set about the enormous task of expanding and updating the network of airways navigation and communications facilities along the major air routes of Australia, and adhering where possible to the standards set by the International Civil Aviation Organisation (ICAO). The major contribution to radio navigation aids in the 1950s were Distance Measuring Equipment (DME), the four-course Visual-Aural Radio Range (VAR) and the Instrument Landing System (ILS).

 

A typical VAR installation

 Ground and airborne VAR equipment was purchased from the USA under the Lend-Lease scheme between 1941 and 1945, sufficient to build 30 beacons. Delays in the supply of key components and a shortage of trained technicians delayed the work, but by July 1950 the installation of 30 VARs and 27 associated Airway Marker Beacons was well advanced. The Lorenz beacon was entirely replaced by the VAR network in 1953. Click here to see some photos of VAR installations or to read the VAR and Marker Beacon Operational Notes.


By 1952 thirty beacons had been commissioned, and they continued to give excellent service until superseded by the VOR in the mid-1960s. If it was required to ‘bend’ the course of a VAR to align with an airway the Americans would install two beacons, set at an angle corresponding to the desired course bend. DCA engineers replaced the American antenna system with the Alford Loop, with which they could electronically bend the course by up to 30 degrees, using the one ground beacon. The VAR was a great success, particularly when used in conjunction with a co-sited DME beacon. It was replaced in the mid 1960s by the VHF Omni-Directional Radio Range (VOR), which offered 360 courses to the VAR’s four.

 

A typical DME installation

 The Provisional International Civil Aviation (PICAO) in 1946 invited member States to develop Distance Measuring Equipment to operate in either the 200 MHz or 1,000 MHz frequency band. Because of wartime experience with radar systems operating at 200 MHz, Australia opted to adapt the Rebecca-Eureka secondary radar system for its DME system. In a joint project, the CSIRO developed the technology, AWA assisted in the design and built the airborne and ground equipment, while DCA prepared the technical specification and undertook the operational acceptance of the system. DME was in use between 1955 and 1995, when it was phased out in favour of ‘International’ DME, operating at 1,000 MHz. For the historical record, the Australian DME was in service five years before the American 1,000 MHz system. Click here to read more about the history of the Australian DME system.


The Instrument Landing System (ILS) went into operation at Sydney and Melbourne Airports on 23 October 1956 - just in time for the Olympic Games. ILS reduced the time that airports were closed by poor weather and, being a precision approach system, made it possible for pilots to fly a landing approach through cloud to much lower weather minima than was possible with previous non-precision approach aids (VAR/VOR & NDB). Essendon Airport was often closed for days on end due to foggy conditions, and thus the introduction of ILS was a great boon to airline operators. Not only did it reduce diversions to alternate airports, but it also saved costs in airline operations and added to the convenience of passengers. Fourteen ILS systems were manufactured in Australia by STC, who had to overcome many technical problems. DCA engineers assisted STC in modifying the basic American design to make the system meet the stringent ICAO requirements. The whole ILS system comprises the Localizer (azimuth guidance), Glide Path (angle of descent), Marker Beacons (distance to run and altitude checks) and Locators (procedure commencement and holding fix).

A common theme with all of the radio navigation aids described above has been that the Department’s engineering organisation has always been keen to extract the very best possible performance from its equipment. Aerial systems, and the siting of navigation aids in undulating terrain have always presented a problem. It is no longer acceptable to chop down forests of trees or bulldoze mountaintops in order to improve signal coverage!

Some of the most rewarding developments in this area resulted from DCA sponsorship of research activities at Australian Universities since the mid-60s. Dr R.W. Redlich and later Dr G. Lucas led teams of engineers and scientists, particularly in ILS monitoring and aerial systems. The Redlich Glide path aerial system was successfully tested at Melbourne and Sydney airports, and was demonstrated to overseas ILS experts who attended the 1968 IOAO meeting.

One further Australian-invented radio navigation aid became a household name: Interscan. This story will be dealt with separately.

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