VOR

Technology Overview

VHF omnidirectional radio range (VOR) provides directional information to aircrafts. The ground-based VOR system transmits a special VHF radio signal. It is demodulated by the aircraft’s VOR receiver. The result is the actual course of the aircraft in relation to the VOR station. VOR stations are strategically distributed. If an aircraft receives signals from two or more VOR stations, it can determine its approximate position by triangulation.

The VHF signal transmitted by the VOR station contains the station’s identifier, navigation signals and optional voice. The station’s identifier is morse-coded. The directional information is derived by comparing a 30 Hz reference signal and a 30 Hz variable signal to determine the phase angle between them. The reference signal is frequency modulated on a 9.96 kHz carrier, which is then amplitude modulated onto a VOR carrier in the 108 MHz to 118 MHz range. The variable signal is directly amplitude modulated onto the VOR carrier using an antenna array with the signal rotating at a 30 Hz rate. The phasing of the two signals is set to be in phase at magnetic north. The signals are 90 degrees out of phase at magnetic east, 180 degrees at magnetic south and 270 degrees at magnetic west. The radial azimuth is always equal to the phase angle between the variable signal and the reference signal.

Proper operation of VOR stations must be ensured through regular monitoring of the main parameters including the reference and variable signal modulation indices, the signal level and the bearing accuracy.

Doppler VOR (DVOR) is typically two to three times more accurate than VOR (CVOR). The difference lies in how the signal is generated. A frequency modulated signal is less susceptible to interference than an amplitude modulated signal and therefore provides more accurate directional information. In the future, DVOR will gradually replace CVOR.

Applications

VOR is applied as an analog navigation aid in international air traffic. It is used in both civil fields and military fields. In military air traffic, however, TACAN is somewhat dominant. VOR stations are often combined with DME ground stations, allowing aircrafts to determine their exact position in relation to the station.

Rohde & Schwarz Test Solutions

Air navigation systems such as VOR systems are subject to the highest safety requirements. Rohde & Schwarz offers unique and highly reliable test solutions for both VOR transmitter and receiver testing – from R&D to production as well as during calibration, installation and maintenance.

Excellent accuracy and highest spectral purity make the R&S®SMA100A signal generator ideal for VOR receiver testing. The generator simulates VOR and COM/ID signals of a VOR station in line with ICAO standards.

VOR signal generators can be fully calibrated with the R&S®FSMR measuring receiver, a one-box solution that provides highest linearity. In combination with the R&S®NRP-Z37 power sensor module, minimum power measurement uncertainty down to lowest signal levels is possible.

The R&S®FSU spectrum analyzer and the R&S®FSQ signal analyzer deliver the optimal solution for testing VOR and ILS ground stations in R&D, production and maintenance. The instruments precisely demodulate and analyze VOR/ILS signals, provide highest performance and flexibility, and are easy to operate.

The R&S®EVS300 ILS/VOR analyzer offers the high accuracy, fast measurement speed and low weight required in test applications for ground and flight inspection of VOR and ILS ground infrastructure in the field. The portable analyzer also features FFT, frequency scan and oscilloscope options in a single box for maximum flexibility in the field.

Key Parameters

Standardization

To ensure the accurate operation of radio navigation systems, the International Civil Aviation Organization (ICAO) has standardized the critical parameters for VOR systems in Annex 10, Volume 1. The conformance of VOR stations with these standards is regularly controlled, calibrated and certified by flight inspection organizations.