ATM - Aircraft Guidance

The Very High Frequency Omni Range is a radio beacon par excellence for navigation. The VOR has been and continues to be the most popular primary navigational aid.

In the late 1940s, air navigation was revolutionised when the Civil Aeronautics Administration of the US (now Federal Aviation Administration) introduced a new and precise radio air navigation aid known as very high frequency (VHF) omni range (VOR). By mid-1952, 45,000 nautical Miles (NM) of VOR airways, referred to as Victor airways, supplemented the 70,000NM of federally maintained low frequency airways. The VOR facilitated accurate navigation within 200NM of its location on 360 equally spaced tracks one degree apart, radiating from the beacon.

VOR Navigation

With a VOR, a pilot can simply, accurately and without ambiguity navigate from point A to point B, provided point B is within reception range of the signals transmitted by the VOR beacon. Unlike previous systems, the VOR provides directional information to the aircraft at all points within the reception range of the radio signal.

The concept is similar to a lighthouse which employs two beams of light. One beam sweeps at a uniform rate through 360 degrees of the azimuth and the other when triggered, emits light rays in all directions at the same instant. When the sweeping beam passes north, the lighthouse flashes a white light visible in all directions at the same time. This serves as a reference signal. The second sweeping beam passes the observer, providing one with directional information. With the observed time difference between the omni directional flash and the sweeping beacon, the observer may determine one’s bearing to the lighthouse. For example, consider the sweeping beam to complete one rotation every sixty seconds. When the beam passes north, an omni-directional flash of light provides the observer a cue to start one’s stopwatch. If he were to see the sweeping beam 38 seconds after the white flash, the bearing relative to the lighthouse is determined as bearing from the lighthouse = [Time elapsed/beam sweep period] x 360 = [38/60] x 360 = 228 degrees.

The VOR employs a similar principle. Here, instead of beams of light, radio waves emanate from the transmitter located on the ground. The radio waves are sensed by instruments onboard the aircraft and displayed as simplified and easily comprehensible information to the pilot.

The VOR provides directional information through phase comparison between two signals transmitted from the same facility, one reference and the other, variable. Both signals are 30 Hz sine waves, the difference lying in the phase relationship between the two as a function of direction in azimuth. For example, an aircraft located due east on a bearing of 090 degrees from the VOR, will see the variable signal lagging in phase by 90 degrees with respect to the reference signal. The aircraft, when on a bearing of 175 degrees from the VOR, will receive the variable signal lagging in phase by 175 degrees from the reference signal. Onboard electronic circuitry displays the phase difference to the pilot.

Older VORs, also known as Classical VORs, now few in service, used two sets of antennae—a fixed antenna which transmitted the 30 Hz reference signal in all directions and a sweeping directional antenna which created the 30 Hz variable phase signal. The sweeping antenna rotates at 30 revolutions per second, serving as the backbone of the VOR’s operation.

Frequency Allocation

VORs are generally assigned radio frequencies between 108.0 MHz (Megahertz) and 117.95 MHz, with 50 KHz spacing. This 9.95 MHz bandwidth assigned to VORs falls into the VHF range, above the FM broadcast band. Every VOR is assigned a unique frequency so that two VORs do not overlap their service volumes, that is, the volume of airspace within which the VOR signals may be received.

As is evidenced in the table on Page 28, no two VORs in the vicinity of Bangalore have the same frequency. Rajkot in Gujarat has a VOR operating on 115.4 Mhz, which is the same as the Kancheepuram VOR, but the geographical distance between the two stations makes it impossible for an aircraft to receive transmission from both the VOR beacons simultaneously. Despite commonality in frequency, there is no possibility of signal overlap and erroneous directional information. Similarly, Jaipur VOR operates at 112.9 MHz, same as Coimbatore’s CCB. The pilot can select the desired unique frequency to determine his bearing from the selected VOR station and navigate accurately.

Information in The Carrier

The VHF waves serve only as carriers into which information is injected for transmission. When there are two waves with the same frequency (30 Hz reference and 30 Hz variable), there is a need to selectively code the two in such a way such that the onboard equipment is able to differentiate and compare the two signals.

Reference Phase Signal

The reference signal, which is transmitted omni-directionally, is frequency modulated on a 9,960 Hz sub-carrier. The sub-carrier is modulated with a deviation ratio of 16 +/- 1 (tolerance of 1 unit). This means that the sub-carrier frequency varies with the instantaneous amplitude of the reference 30 Hz, the maximum deviation being (16+/- 1) X 30 = 480 Hz +/- 30 Hz. After modulation, the sub-carrier frequency varies from 9,480 Hz (9,960 Hz – 480 Hz) to 10,440 Hz (9,960 Hz + 480 Hz), the frequency deviation representative of the 30 Hz reference signal phase. Thus, the sub-carrier’s modulation as a function of time is f_SUB = 9,960 Hz + 480sin(t) Hz.

This frequency modulated sub-carrier amplitude modulates the VHF carrier, successfully placing the 30 Hz reference information on the carrier as a distinguishable signal.