Point-source systems, Loran, ILS, urban and indoor positioning, WLAN, UWB, mobile phone, and signals of opportunity.
Before GNSS, aircraft and ships navigated using terrestrial radio beacons. Many of these systems are still operational and remain important as GNSS backups, for indoor positioning, and for environments where satellite signals are unavailable.
The simplest terrestrial systems are point-source beacons. A transmitter at a known position broadcasts a signal, and the receiver measures the range, bearing, or both to determine its position relative to that beacon.
VOR (VHF Omnidirectional Range): A ground beacon that broadcasts a signal allowing an aircraft to measure its magnetic bearing from the beacon. VOR operates at 108–118 MHz and has a typical accuracy of ~1°. At 100 nm range, this corresponds to ~1.8 km cross-track error.
DME (Distance Measuring Equipment): An airborne interrogator transmits a pulse pair, and a ground transponder replies after a fixed delay. The round-trip time gives the slant range:
DME accuracy is about ±0.2 nm (370 m). Combined VOR/DME fixes give the aircraft both bearing and range from the beacon, enabling a position fix. With multiple DME transponders, a position fix from range intersections alone is more accurate than using VOR bearings.
Loran (Long Range Navigation) operates at 100 kHz in the low-frequency band. At these wavelengths, signals propagate as ground waves that follow the Earth's curvature, achieving ranges of 1,000–1,700 km over land and 1,700–2,400 km over sea.
Transmitters are organized in chains, each with a master and 2–5 secondary stations. All stations transmit at the same frequency using TDMA (time-division multiple access), with each station taking turns.
Signal structure: Each transmission is a group of eight 500-μs pulses, spaced 1 ms apart. The master adds a ninth identification pulse. Each chain has a unique Group Repetition Interval (GRI) of 50–100 ms, serving as the chain identifier.
Time Differences (TDs): Traditional receivers measure the difference between arrival times from two stations in the same chain. Each TD defines a hyperbolic line of position (LOP) — a curve of constant time difference. Two TDs give a position fix at the intersection of two hyperbolas.
ELoran: A modernized Loran-C that synchronizes all transmitters to UTC, enabling pseudo-range measurements from individual stations (like GNSS) instead of requiring paired TDs from the same chain. ELoran adds a data channel for differential corrections.
ELoran corrected pseudo-range measurements are formed from the measured time of arrival:
where tst is the time of transmission, c is the effective propagation speed over saltwater, and ΔρASF is the Additional Secondary Factor (ASF) — a correction for slower propagation over land. The ASF is the Loran equivalent of GNSS ionospheric/tropospheric corrections but varies much less with time.
At least three pseudo-range measurements solve for latitude, longitude, and receiver clock offset, using a least-squares or Kalman filter solution analogous to the GNSS navigation processor.
Error sources:
| Error Source | Typical Magnitude | Mitigation |
|---|---|---|
| ASF modeling errors | 50–300 m | Improved databases, differential Loran |
| Sky-wave interference | Variable | Track early part of pulse (ground wave only) |
| Receiver noise | 10–50 m | Longer integration, better processing |
| Cross-chain interference | Variable | GRI selection, interference subtraction |
Differential Loran: A reference station at a known location measures its Loran range errors and broadcasts corrections via the Loran Data Channel or Eurofix. This achieves ~10 m accuracy at harbors and airports — adequate for harbor approach and non-precision aircraft approach.
The Instrument Landing System (ILS) guides aircraft along a precise approach path to the runway, operating in conditions of poor visibility. It provides two angular measurements:
Localizer: Provides left/right guidance relative to the runway centerline. Two overlapping beams at 108–112 MHz are modulated at 90 Hz and 150 Hz. The difference in depth of modulation (DDM) between these signals is zero on the centerline and proportional to the angular deviation from it, typically covering ±2.5°.
Glideslope: Provides vertical guidance along a descent path, typically at 3°. Two beams at 329–335 MHz use the same DDM principle. The DDM is zero on the correct glidepath.
Marker beacons: Transmit vertically at 75 MHz to indicate distance along the approach path (outer, middle, and inner markers). DME is increasingly used as a substitute.
ILS accuracy categories:
| Category | Decision Height | Visibility |
|---|---|---|
| CAT I | 60 m (200 ft) | 550 m |
| CAT II | 30 m (100 ft) | 300 m |
| CAT III | 0–15 m | 0–175 m |
CAT III enables landing in essentially zero visibility, but requires redundant ILS equipment and strict integrity monitoring.
GNSS signals are severely degraded or unavailable indoors and in deep urban canyons. Several terrestrial radio technologies fill this gap:
Mobile phone positioning: Three main techniques are used:
• Cell ID: The serving cell identifies the user's approximate location (100 m in urban areas, several km in rural areas).
• Time of Arrival (TOA): Measuring the signal travel time from multiple base stations gives range circles whose intersection determines position. Requires time synchronization between base stations.
• Time Difference of Arrival (TDOA): Removes the need for phone clock synchronization by using the difference in arrival times, producing hyperbolic lines of position (similar to Loran).
Signals of opportunity: Any radio signal with a known transmitter position can potentially provide positioning. TV broadcasts, AM/FM radio, and digital broadcast signals have been used for ranging. The advantage is that these signals are much stronger than GNSS (often 50+ dB stronger) and penetrate buildings well.
GNSS repeaters (pseudolites): Ground-based transmitters that rebroadcast GNSS-like signals indoors. They provide the familiar GNSS positioning framework but introduce near-far interference problems because the range variation between transmitters is much larger than for satellites.
WLAN (Wi-Fi) positioning exploits the dense deployment of wireless access points in urban and indoor environments. Two main approaches:
• Ranging: Measuring the round-trip time of Wi-Fi signals gives range estimates, but with limited accuracy (5–15 m) due to multipath and the relatively narrow bandwidth of Wi-Fi signals.
• Fingerprinting: The received signal strength (RSS) pattern from multiple access points is compared to a pre-recorded database of RSS values at known locations. This achieves 2–5 m accuracy indoors but requires an extensive survey to build the database, and performance degrades as the environment changes.
UWB (Ultra-Wideband) positioning: UWB transmits very short pulses (sub-nanosecond) with a bandwidth of >500 MHz. The key advantages:
| Property | UWB | Wi-Fi |
|---|---|---|
| Bandwidth | >500 MHz | 20–40 MHz |
| Range resolution | <30 cm | 5–15 m |
| Multipath resilience | Excellent (resolves individual reflections) | Poor |
| Range | 10–50 m | 30–100 m |
| Accuracy | 10–30 cm | 2–15 m |
UWB's wide bandwidth allows it to resolve individual multipath components, making it much less vulnerable to multipath than narrowband systems. This is the same principle behind using wider-bandwidth GNSS signals for multipath mitigation.
Short-range beacons: Bluetooth, RFID, and infrared beacons provide proximity-based positioning with accuracy from 1–10 m. They work well as waypoint markers but have limited precision for continuous tracking.
Relative navigation determines the position of one vehicle or person relative to another, without requiring absolute position knowledge. This is useful for:
• Formation flying (aircraft maintaining spacing)
• Convoy operations (vehicles following a leader)
• Docking and rendezvous (ship to dock, spacecraft to station)
A cooperative transponder on the target responds to an interrogation signal, enabling the interrogator to measure range by round-trip timing and bearing by directional antenna. Many common errors cancel in the relative measurement, just as in differential GNSS.
Tracking systems: In tracking, a fixed station determines the position of a mobile target. The target may carry a cooperative transponder (active tracking) or simply reflect a radar signal (passive tracking). Tracking stations can measure:
• Range (from round-trip time or signal strength)
• Bearing (from directional antenna or antenna array)
• Range rate (from Doppler shift)
Multiple tracking stations enable position determination by intersection of range circles or bearing lines. A single station with range and bearing can track a target, but geometry limits accuracy at long range.
Sonar transponders: Underwater equivalents of radio beacons. An acoustic transponder at a known position on the seabed responds to an interrogation signal from a submarine or AUV. Multiple transponders enable a position fix. Long-baseline (LBL) systems use widely spaced transponders for accuracy of 1–10 m. Short-baseline (SBL) and ultra-short-baseline (USBL) systems use a single array of hydrophones on the vessel.
This simulation shows how a position fix is determined from range measurements to known transmitter positions. Click to place transmitters, then observe the range circles and their intersection.