GNSS (GPS-based navigation)

GNSS / GPS — Fundamentals

GNSS (Global Navigation Satellite System) is the umbrella term for satellite-based navigation. Multiple constellations, jointly standardised in ICAO Annex 10.

Source: ICAO Annex 10 Vol I §3.7 GNSS.

Constellations (core systems)

NAVSTAR/GPS — what is transmitted

NAVSTAR/GPS serves to transmit signals from which time, position and speed can be determined:

• Time via atomic-clock timestamps in satellite signals.
• Position via trilateration.
• Speed via Doppler shift or position change over time.

Operating principle — trilateration

1. Each satellite continuously broadcasts its exact position and time (atomic clock).
2. Receiver measures signal travel time — the distance to a satellite is computed by measurement of the signal run time.
3. Travel time × c (speed of light) gives distance to the satellite.
4. 3 distances give 2D position (3 sphere intersection), 4th distance corrects receiver clock.

Number of satellites — 3D and RAIM

To obtain three-dimensional information, four satellites need to be receivable; additionally, accuracy can be monitored with a fifth satellite to check the computed position (RAIM):

• 4 satellites → 3D position (Lat, Lon, Alt) + clock correction.
• 5th satellite → RAIM (Receiver Autonomous Integrity Monitoring) checks consistency and warns if accuracy is insufficient.
• 6th satellite → RAIM-FDE (Fault Detection and Exclusion) can exclude a bad satellite measurement.

WGS 84 — global datum

WGS 84 (World Geodetic System 1984) is a chart base covering the entire Earth that serves as base for GNSS navigation:

• A reference ellipsoid with defined semi-axes (a = 6378.137 km, b = 6356.752 km).
• Defines the global coordinate system for Lat/Lon and altitude.
• All modern charts (ICAO 1
  since ~2000) are WGS-84 referenced.

Map data / geodetic datum

The term "map data" means a match of geoid and ellipsoid (geodetic datum) as accurate as possible for a certain region:

• Geoid: actual Earth shape including local gravity variations.
• Ellipsoid: mathematically smooth reference body.
• Datum: local fit of the ellipsoid to the geoid in a region (e.g. ED-50 Europe, NAD-83 North America, WGS-84 global).
• → Different datums can offset positions by up to 300 m → charts and GPS must use the same datum (today standard: WGS 84).

Frequencies (GPS Civil)

• L1: 1575.42 MHz — primary civil code.
• L2: 1227.60 MHz — second code (atmospheric correction).
• L5: 1176.45 MHz — new civil code (modernised GPS).

Augmentation systems (SBAS / GBAS)

SBAS

Augments GNSS for better accuracy and integrity:

• WAAS (USA), EGNOS (Europe, EUSPA), MSAS (Japan), GAGAN (India), SDCM (Russia).

GBAS

Local corrections for precision approach at a specific airfield (CAT I/II/III).

Accuracy (2024)

Accuracy under adverse conditions

An accuracy of approx. 70 to 100 metres within adverse influences can be assumed during navigation using GNSS:

• Good conditions: 3-5 m (95 %).
• Adverse (ionosphere, multipath, poor geometry): up to 70-100 m.

Improved accuracy

An increase in accuracy can be achieved with differential GPS and carrier phase analysis:

• DGPS: ground station knows its exact position and broadcasts correction → receiver < 1 m.
• Carrier phase / RTK: uses carrier-wave phase (instead of just code) → cm-level accuracy.

Integrity — warning on impairment

The integrity check of a GNSS receiver is the ability to issue timely warnings to the user that the receiver is no longer available for navigational use:

• RAIM is the standard for this check.
• For IFR GPS approaches it is legally required.

Accuracy technically reducible — US DoD

The accuracy of GNSS can be technically reduced by the US department of defence:

• Historic: until May 2000 Selective Availability (SA) was active — US DoD intentionally degraded civil accuracy by ±100 m.
• Since May 2000: SA switched off by Clinton directive → full civil accuracy.
• Hardware: SA control is still built into modern GPS Block IIR-M and newer hardware but currently inactive. DoD could theoretically reactivate it in conflict zones.

Error sources — GNSS positioning errors

GNSS positioning errors can be caused by clock error and topography, pseudo-range and selected availability as well as disruptions in the ionosphere:

GNSS accuracy influenced by

GNSS positioning accuracy can be influenced by the ionosphere (diffraction, reflection) and weather phenomena in the tropopause (humidity, thunderstorms):

• Ionospheric effects: diffraction and reflection during solar flares.
• Tropopause weather: high humidity and thunderstorms can attenuate the signal.

Limits — signal shadowing

NAVSTAR/GPS signals can be completely shaded by parts of the aircraft:

• Wings, tail, fairings can block signals — especially with low-mounted antennas.
• Consequence: in banks or steep turns the GPS receiver may "lose" individual satellites.
• Solution: install the GPS antenna in the highest, clearest position (typically atop the fuselage).

POOR COVERAGE message

A disruption between receiver and satellite can cause a "POOR COVERAGE" message on the GNSS receiver:

• Meaning: too few satellites or signal quality too low.
• Causes: topography (mountain, building), antenna position (bank), jamming, solar storm.
• Pilot action: do not trust the GPS reading, fall back to conventional navigation.

Searching the sky

The notification "Searching the sky" means the system is executing a procedure after switching on the receiver, when no satellite data is available:

• On first start or after a long pause: GPS searches all satellites → may take minutes.
• On warm restart: seconds to 1 minute.
• During this phase no position is available.

Database currency — AIRAC 28 days

The validity of a database memory card is 28 days according to print — corresponds to the AIRAC cycle (Aeronautical Information Regulation and Control):

• Every 28 days ICAO issues a new AIRAC cycle with updated waypoints, airspaces, approaches.
• Outdated database can lead to wrong waypoints, approaches, airspaces.
• For IFR flight legally mandatory (EU 965/2012, NCO.IDE.A.190).

CDI on GPS — lateral offset

The course deviation indication (CDI) of a GNSS nav device indicates lateral offset to the desired course line:

• Unlike VOR (where CDI shows angular deviation in °), GPS CDI shows a direct distance offset (typically 0.3-5 NM full scale, depending on phase).
• En-route: typically ±5 NM full scale.
• Terminal: ±1 NM.
• Approach: ±0.3 NM.

Selective Availability (SA)

Historic: until May 2000 GPS was intentionally degraded for civil users (US DoD, ±100 m error). Clinton lifted SA by directive — full civil accuracy since. SA control is no longer technically present in modern Block IIR-M and newer hardware.

Position lines

Position lines are helpful in case of loss of orientation:

• A position line is a line on the chart along which the pilot is certain to lie (e.g. a VOR radial, an NDB bearing, a GPS distance-from-waypoint).
• Two position lines intersect → position fix.
• On lost orientation: the pilot flies to the nearest distinctive position line (river, railway, VOR radial) and works from there.