History of Air Navigation – Part II: Transition to Radio Navigation Systems
- Early air navigation shifted from reliance on human skill to radio-based systems in the early twentieth century. Technological developments accelerated during the Second World War, and many of these systems were later adopted for civil aviation use.
- The Telefunken Kompass Sender (1907) used 32 cables forming 16 dipole pairs to transmit an initial omnidirectional signal, followed by a rotating directional signal. Aircraft determined their approach direction by identifying variations in received signal strength.
- Later systems included LF/VLF navigation, inertial navigation, and hyperbolic systems such as LORAN, OMEGA, and DECCA, which used time or phase differences between transmitters to determine position. These systems supported long-range navigation and were later withdrawn as newer navigation systems became available.
The early twentieth century saw a transition of navigation from dependence on human skill to the utilisation of radio systems. Investments made in the development of technology during the Second World War resulted in systems developed to support war efforts being used for civil aviation after the war ended.
Many systems were developed independently for long-range navigation, wherein the pilot of an aircraft could derive their position in the air. The systems developed were used both for nautical navigation and for aeronautical purposes. Some of the systems developed and adopted during this period are described below.
Telefunken Kompass Sender
One of the earliest radio navigation systems was the Telefunken Kompass Sender, developed by the German electronics company Telefunken in 1907. It was used primarily for long-distance navigation by Zeppelins (Fig. 1). The system was taken out of service around 1918.
The system consisted of a series of 32 individual 60m long cables (placed around a central mast at 11.25 deg intervals) supported at the centre by a single mast and reaching the ground at the other end, forming a sort of umbrella (Fig. 2). Pairs of cables were wired to create 16 pairs (formed by opposite cables) 120m long dipole antenna.
First station identity code was transmitted from the central antenna to generate omni-direction radio pattern; and then transmission was made through switched dipoles (16 dipoles) to produce a rotating radiation pattern. An approaching aircraft would first receive the omnidirectional station identity signal, and when the omnidirectional signal expired, a rotating directional signal (2 revolutions per minute) was transmitted.
Antenna pair, currently transmitting, was identified by measuring the lapsed time between the end of the omnidirectional signal and the transmission made by the switched antenna pair. Pilot listened for the rotating signal. As the switched radiation pattern maxima approached the receiver direction, the strength of the signal went on getting stronger.
The strength of the received signal was maximum when the direction of the aircraft approach was perpendicular to the line joining the elements of the dipole. The direction where the signal was strongest was the right approach path. Later, however, it was realised that signal minima (weakest) were easier to identify (when the antenna pair is aligned to the direction of the receiver).
Low Frequency (LF)/Very Low Frequency (VLF) Systems
LFR was the first true system for identifying established aerial flight paths. Low Frequency (LF) and Very Low Frequency (VLF) (30 to 300kHz) systems operate on long/very long wavelengths, which enables them to travel long distances, follow earth’s curvature and penetrate ‘lossy’ media like water and soil.
First widely used radio navigation system was the Low Frequency Radio Range (LFR), developed in the United States in the late 1920s by Ford Motor Co. Using ground based crossed loop stations broadcasting N (dash dot -.) and A (dot dash . -) Morse Code signals (by two antenna systems at right angle), it created a constant strength “beam” for the pilots to follow. If the aircraft veered off course, the pilot would hear a clear Morse Code “A” (dot-dash) or “N” (dash-dot) signal indicating which way to turn to get back to the equi signal path.
The first commercial crossed-loop antenna systems were installed at Ford’s private airfields in Dearborn, Michigan, and Chicago in 1926. In this system, pilots would listen for a continuous humming signal in their headphones, any deviation from the path would be indicated by a stronger ‘N’ or ‘A’ signal as explained.
Inertial Navigation System (INS)
The history of Inertial Navigation began with the 19th-century gyro principle (Foucault), which was advanced by Schuler’s gyrocompass (1908). The application of the Inertial Navigation System (INS) started in World War II, with its first use in missiles.
Initially, INS was not adopted for aeronautical navigation because of its size and weight; the system was later adopted for aviation when the system’s weight was reduced. INS used for navigation is an autonomous system which does not need any external reference. INS consists of linear sensors (accelerometers), rotational sensors (gyroscopes), and a computer that continuously calculates positional information using the principles of deduced reckoning.
Gyroscopes measure the angular displacement of the sensor frame with respect to the inertial reference frame. By using the original orientation of the system in the inertial reference frame as the ‘initial condition’ and integrating the angular displacement (measured by the gyroscope), the system’s current orientation is known at all times.
Accelerometers measure the linear acceleration of the moving vehicle (aircraft) along all four directions, and with the acceleration data, the distance travelled can be calculated. Based on the information provided by the accelerometer and the gyroscope, the current position can be calculated.
As with any sensor, there is a threat of inherent drift in the system, resulting in inaccuracies. A number of new technologies, like fibre-optic gyroscopes, have been developed to provide a lighter weight and better accuracy of the system.
Adcock Array
Adcock Array was invented and patented by British Engineer Frank Adcock in 1919. Adcock Array in aviation was crucial for early radio navigation, forming the backbone of the Low Frequency Radio Range (LFR) system in the 1930s to 1940s.
System produced airways using four equidistant vertical antennas (Fig. 4) to define directional signals. Frank Adcock originally used this antenna system as a receiving antenna to find the azimuthal direction a radio signal was coming from, in order to find the location of the radio transmitter, a process called radio direction finding.
Prior to the invention of the Adcock Antenna, the loop antenna was being used to find directional information.
The problem with the loop antenna was that the horizontal portion of the antenna (top and bottom) was contaminating the vertically polarised genuine signal with the horizontally polarised signal received by the horizontal section of the loop.
This antenna system was subsequently used for many of the radio direction finding systems.
Hyperbolic Navigation System
This system uses the time difference measure between pulses received from a pair of radio transmitters to assess the position of a suitably equipped receiver. Concept emerged in the 1930s from Second World War needs, and systems were developed in Britain (1941) for bombers and in the United States (1942) for long-range nautical and aeronautical use. Popular hyperbolic navigation systems used during WWII are described below:
LORAN
Long Range Navigation (LORAN) was developed in the United States during Second World War.
LORAN (Hyperbolic Navigation) provides positional information using time differences in synchronised radio pulses received from a Master Station (A) and several Slave Stations (B and C). A receiver measures the Time Difference (TD) between master/slave signals, placing the aircraft on a hyperbolic line of position (LOP), which is an equi-time-difference path (Fig. 5). A second LOP from another master/slave station intersects the first to pinpoint the location.
The system utilises low-frequency signals (like 100 kHz for LORAN-C) that travel long distances. With two master/slave pairs, positional information is received with an ambiguity of 2; with three such pairs, we can get correct, unique position information.
In India, Loran-C systems operate specifically in a “mini-chain” covering ports on its East and West coast, but this chain does not provide service to aviation. On the West Coast, stations include Veraval, Billimora and Dhrangadhra and for the East Coast, stations are provided at Diamond Harbour, Balasore and Patpur (the Diamond Harbour station was decommissioned in 2006).
This system is similar to the Gee System developed in the United Kingdom. Initially, the LORAN system was used by the military and large commercial users because of the cost involved.
OMEGA
OMEGA was the first global-range radio navigation system, operated by the United States in cooperation with six partner nations. It was a hyperbolic navigation system, enabling ships and aircraft to determine their position by receiving very low frequency (VLF) radio signals in the frequency range 10 to 14 kHz transmitted by a global network of eight fixed terrestrial radio beacons. The system became operational around 1971 and was shut down in 1997 in favour of more advanced global navigation systems being available.
DECCA
DECCA was another hyperbolic radio navigation system which was used for navigation until the 1990s. The system used phase comparison between pairs of low frequency signals between 70 and 129kHz, as opposite to pulse timing systems in LORAN etc. This made it much easier to design receivers using 1940’s electronics, and information was provided on a Cathode Ray Tube, making it much easier for use. This system was developed by Decca in UK during World War II.
The system consisted of a Master Station and three/two Secondary Stations termed Red, Green and Purple. Secondary stations were positioned at the vertices of an equilateral triangle with the master at the centre. Master – Secondary distance was typically 60 – 120 NM.
The systems described above were developed mainly for military use, but were utilised in commercial nautical and aeronautical applications immediately after the war ended. These systems provided positional information to airborne systems directly. In addition, some ground-based systems were developed to provide information to ground-based controllers about the direction from which aircraft transmissions were received. These systems will be described in Part 3 of this series.
Also Read: History of Air Navigation – Part I: The Origins of Navigation from Sea to Sky
