VOR and ILS Radio Navigation Receiver Test Using Option 302 for Keysight Signal Sources

Keysight Technologies

VOR and ILS Radio Navigation Receiver Test Using Option 302 for Keysight Signal Sources

Introduction

The Keysight X-series (EXG and MXG) analog and vector signal generators with Option 302, avionics license, are ideal signal generators for VHF Omnidirectional ranging (VOR) and instrument landing system (ILS) radio navigation receiver test solutions from Keysight. This application note describes the signals used by VOR and ILS transmitters and how the X-series signal generators can be used to create the proper signals to provide a test solution for ILS and VOR radio receiver test.

Abstract

This application note describes the Avionics License Option 302 that is available in the Keysight Technologies N5171B EXG, N5172B EXG, N5181B MXG, and N5182B MXG signal generators. These performance signal sources can be used to generate AM, FM, ØM, and pulse modulated signals across 9 kHz to 6 GHz, option dependent. In addition to general purpose modulation format capabilities, these signal generators can be used to generate VOR and ILS radio navigation waveforms. This application note provides a brief overview of the VOR and ILS radio navigation signals, as well as using the Keysight EXG and MXG series signal generators to generate VOR and ILS waveforms.

Overview

Overview of VOR and ILS radio navigation signals

The VOR system in an aircraft provides directional information while in flight by interpreting the specially coded information transmitted by VOR stations located on the ground. VOR stations transmit over the 108 to 118 MHz frequency spectrum [1]. The VOR signal transmitted by the station is modulated using two distinct parts. The first part of the VOR signal is amplitude modulated onto a 9.960 kHz sub-carrier at a rate of 30 Hz [2] and in such a way that its phase is not dependent on the bearing angle between the receiver and VOR station [1]. This signal is referred to as the reference signal. The second signal is also modulated at 30 Hz. The phase of this second signal is adjusted such that the difference in phase relative to the reference signal corresponds to the compass bearing of the receiver to the VOR station [1]. The aircraft’s VOR receiver demodulates the transmitted VOR signal comparing the phase difference between the two transmitted signals. This phase difference can then be used to determine the aircraft’s compass bearing to the VOR station. An aircraft’s position can be determined by observing multiple VOR stations and the compass bearing relative to the VOR stations. Position location utilizing the VOR system can be used to provide confirmation of the location information reported by GPS systems.

Brief overview of the ILS radio navigation signal

ILS radio navigation provides a method of positioning an aircraft vertically and horizontally with respect to a runway during landing. These systems are especially important during situations where visibility to the runway may be impaired due to climate conditions. A variety of signals and systems are utilized as part of the ILS system to provide the three dimensional spatial orientation required. These signals are the localizer and glide slope signals that are used to orient the craft horizontally and vertically, respective to the runway. There are also a set of beacons; the outer marker, middle marker, and inner marker, which provide visual and audible cockpit alerts to the pilot indicating the distance to the runway’s edge.

Localizer and glide slope signals

The localizer signal is used to provide information to the pilot to align the aircraft horizontally with respect to the centerline of the runway. This system operates on carriers from 108 MHz to 118 MHz. The localizer system in an aircraft determines left, right, or on-center orientation of the craft with respect to the runway utilizing two directionally radiated fields that are each amplitude modulated at rates of 90 Hz and 150 Hz, illustrated in Figure 2 [1]. In addition to the two localizer signal amplitude modulations, an audio Voice/Identification signal is placed on top of the localizer carrier. This identification signal uses the baseband frequencies from 350 Hz to 2500 Hz [1]. The antenna arrays are arranged such that the 90 Hz is stronger than the 150 Hz signal on the left side of the runway. Correspondingly, on the right side of the runway the 150 Hz is stronger than the 90 Hz signal. There is a section of equal amplitude located in the center that is approximately 5° wide [1]. The left and right orientation of the aircraft is determined in a relatively straightforward manner by demodulating the received localizer signals and comparing the difference in the recovered amplitudes of the 90 Hz and 150 Hz tones. The difference in depth of modulation (DDM) is defined as the difference in the percentage of modulation depth of the stronger signal less the percentage modulation depth of the weaker signal, divided by 100 [1].