Long before GPS, pilots and mariners navigated using ground-based radio beacons and onboard receivers that measured bearing, direction, and distance from fixed transmitter locations. Many of these systems are decades old, yet they remain active and legally required today, serving as a critical backup layer in case satellite navigation is jammed, spoofed, or simply unavailable. This guide covers the four pillars of legacy radio navigation still in daily use: VOR, ILS, DME, and direction finding.
VOR: VHF Omnidirectional Range
VOR is the backbone of conventional aviation en-route navigation, allowing a pilot to determine their bearing, or radial, from a known ground station.
Frequency range: 108.0 to 117.95 MHz, using 50 kHz channel spacing within the VHF band. This range is shared with ILS localizer transmitters, with VOR stations typically assigned the even-tenths channels and ILS localizers assigned the odd-tenths channels within the lower portion of the band.
A VOR ground station transmits two signals simultaneously: a reference phase signal broadcast uniformly in all directions, and a variable phase signal whose phase shifts depending on the direction it is received from, effectively rotating 30 times per second like a lighthouse beam. An aircraft’s VOR receiver compares the phase difference between these two signals to calculate its exact magnetic bearing, or radial, from the station. Each VOR station also transmits a continuous Morse code identifier at 1,020 Hz so pilots can audibly confirm they are tuned to the correct station rather than a nearby one on an adjacent frequency.
Modern VOR stations increasingly use Doppler VOR (DVOR) technology, which produces a more accurate and interference-resistant signal than the original conventional VOR design, though both types occupy the same frequency range and operate on the same underlying phase-comparison principle from the aircraft’s perspective.
ILS: Instrument Landing System
ILS provides precision lateral and vertical guidance during the final approach to a runway, allowing aircraft to land safely in low visibility conditions that would make a visual approach impossible.
Localizer: 108.1 to 111.95 MHz, using the odd-tenths channels within the VOR/ILS shared band, provides lateral guidance, telling the pilot whether the aircraft is left or right of the extended runway centerline. The localizer transmits two overlapping signal lobes modulated at 90 Hz and 150 Hz; the aircraft’s receiver compares the depth of modulation of each tone to determine position relative to centerline, with equal modulation depth indicating the aircraft is precisely on course.
Glide slope: 329.15 to 335.0 MHz provides vertical guidance using the same 90 Hz and 150 Hz modulation-depth comparison principle as the localizer, but radiated in a pattern that defines a descent path, typically three degrees, toward the runway threshold. Glide slope and localizer frequencies are paired, so selecting a localizer frequency on an aircraft’s navigation radio automatically tunes the corresponding glide slope frequency without separate pilot input.
Marker beacons: 75 MHz mark specific distances from the runway threshold along the approach path. All marker beacons share the same carrier frequency and are distinguished by their modulation tone and a narrow, fan-shaped upward radiation pattern that the aircraft only receives briefly while passing directly overhead. The outer marker uses a 400 Hz tone, the middle marker uses 1,300 Hz, and the inner marker, used at fewer airports, uses 3,000 Hz. Many modern installations have replaced marker beacons with DME-based distance readouts, since DME provides continuous distance information rather than a single momentary crossing point.
DME: Distance Measuring Equipment
DME answers a question VOR alone cannot: exactly how far is the aircraft from the station.
Frequency range: 960 to 1,215 MHz, in the L-band, well above the VHF frequencies used by VOR and ILS localizers. DME operates on a paired-channel system, meaning each of the 126 available DME channels corresponds automatically to a specific VOR or ILS frequency, so selecting a VOR or localizer frequency on the cockpit radio simultaneously tunes the correct paired DME channel without any additional pilot action.
DME works through an interrogation and reply process rather than passive reception. The aircraft transmits a pulse pair on its interrogation frequency, the ground transponder receives it and replies on a different frequency after a fixed, known internal delay, and the aircraft’s DME equipment measures the total round-trip time, subtracts the known transponder delay, and calculates straight-line slant range distance to the station. Because this measures direct line-of-sight distance rather than ground distance, DME readings are slightly longer than the true ground distance when an aircraft is at altitude close to a station, an effect pilots learn to account for during approaches.
Marine and Aviation Direction Finding
Before satellite navigation, both ships and aircraft relied on radio direction finding using low and medium frequency ground-based beacons, a technology that predates VOR and remains conceptually important even where the original infrastructure has been decommissioned.
Marine radio beacons historically operated in the 285 to 325 kHz range, in the medium frequency band, with each beacon transmitting a distinctive Morse code identifier. Ships used a rotatable loop antenna, the radio direction finder, to determine the bearing of minimum or maximum signal strength relative to the ship’s heading, then plotted bearings from two or more beacons to fix their position through triangulation. Many maritime nations, including the United States Coast Guard, decommissioned most dedicated marine radiobeacon networks in the late 1990s and early 2000s as GPS became reliable and widespread, though some countries retain limited beacon networks for emergency backup purposes.
Aviation non-directional beacons (NDB) operate in the 190 to 435 kHz range, in the low and medium frequency bands, functioning on the same basic principle as marine radio beacons. Aircraft use an Automatic Direction Finder (ADF) receiver, which automatically points an onboard indicator needle toward the beacon’s bearing rather than requiring the pilot to physically rotate a loop antenna as ship navigators once did. NDBs were once the primary means of instrument navigation worldwide and remain in limited service today, particularly at smaller or remote airports that lack VOR or modern satellite-based approach procedures, though many countries have decommissioned significant portions of their NDB networks as GPS approaches have become standard.
An important related modern use of this same medium frequency range is differential GPS correction broadcasting. Several countries have repurposed parts of the old marine beacon band, generally within 285 to 325 kHz, to broadcast GPS correction data to ships and other users equipped with differential GPS receivers, improving satellite positioning accuracy to within a few meters rather than relying on the legacy beacons for direct bearing-taking.
Why These Systems Persist Alongside GPS
GPS and other satellite navigation systems are vulnerable to jamming, spoofing, solar interference, and outright service outages, vulnerabilities that ground-based VOR, ILS, DME, and beacon infrastructure simply do not share, since they operate independently of any satellite constellation and are far harder to disrupt across a wide area. Aviation regulators in particular have resisted fully retiring VOR and ILS infrastructure despite the rise of GPS-based approach procedures, recognizing that a parallel, independently operating navigation layer provides essential resilience against any single point of failure affecting satellite positioning.
This is also why many countries maintain what is sometimes called a Minimum Operational Network of VOR stations even as GPS-based area navigation has become the primary method most pilots use day to day, ensuring that if satellite navigation became unavailable across a wide region, aircraft could still navigate safely using conventional radio aids that have remained essentially unchanged in principle since the mid-twentieth century.
Quick Reference
| System | Frequency Range | Function |
|---|---|---|
| VOR | 108.0–117.95 MHz | Bearing/radial from station |
| ILS Localizer | 108.1–111.95 MHz | Lateral runway guidance |
| ILS Glide Slope | 329.15–335.0 MHz | Vertical approach guidance |
| Marker Beacons | 75 MHz | Distance-to-runway markers |
| DME | 960–1,215 MHz | Slant-range distance to station |
| Marine Radio Beacons | 285–325 kHz | Ship bearing/position fixing (legacy) |
| Aviation NDB | 190–435 kHz | Bearing to beacon via ADF |
