GPS & GNSS Aviation Navigation
Satellite navigation fundamentals for aviators — GNSS systems, RAIM integrity monitoring, SBAS augmentation, approach categories from LNAV to LPV, and IFR certification requirements.
GNSS Fundamentals
GNSS (Global Navigation Satellite System) is the umbrella term covering all satellite-based navigation systems. Aviation primarily uses GPS but modern avionics increasingly use multi-constellation receivers for improved availability and integrity.
| System | Operator | Satellites | Status |
|---|---|---|---|
| GPS | USA (USAF/Space Force) | 31+ operational | Fully operational; aviation primary |
| GLONASS | Russia | 24+ | Fully operational; used in multi-constellation avionics |
| Galileo | European Union | 28+ | Fully operational; increasingly used in aviation |
| BeiDou | China | 45+ | Global constellation; growing aviation use |
- Aviation GPS uses the L1 frequency (1575.42 MHz); modern IFR avionics add L5 (1176.45 MHz) for dual-frequency operation
- Minimum 4 satellites required for a 3D position fix (3 for position + 1 to resolve time ambiguity)
- Standalone GPS accuracy: ±15 m horizontal (95th percentile); SBAS-augmented: ±1–3 m
- Dilution of Precision (DOP): geometry factor — lower DOP = better accuracy; PDOP <6 preferred for IFR
RAIM — Receiver Autonomous Integrity Monitoring
RAIM is a software algorithm within the GPS receiver that uses redundant satellite signals to verify the integrity of the position solution. It is essential for IFR GPS use because GPS has no inherent signal authentication — a faulty or spoofed satellite could provide misleading information without RAIM.
Satellite Requirements for RAIM
| Satellites Available | Capability | Notes |
|---|---|---|
| 4 | Position fix only — no RAIM | Cannot detect faulty satellite |
| 5 | RAIM — fault detection (FD) | Can detect a faulty satellite but cannot exclude it |
| 6+ | FDE — Fault Detection & Exclusion | Can identify and exclude faulty satellite; most robust |
RAIM failure = GPS unusable for IFR. If RAIM is unavailable or fails during an approach, the pilot must execute the missed approach immediately (or revert to an alternative procedure if still en route). A RAIM alert does not mean GPS is wrong — it means GPS cannot verify that it is correct.
RAIM Prediction
Before conducting an IFR GPS approach, pilots must confirm RAIM availability at the destination at the expected time of arrival (ETA). RAIM availability can change due to satellite outages notified via NOTAMs.
- Check RAIM prediction for ETA ±30 minutes
- Use preflight planning software, FMS prediction function, or official RAIM prediction services
- GPS satellites removed for maintenance are published as NOTAMs — always check before flight
- Aircraft with SBAS (WAAS/EGNOS) receivers automatically get enhanced integrity; RAIM prediction is less critical but still recommended
SBAS — Satellite-Based Augmentation Systems
SBAS systems use a network of precisely surveyed ground reference stations to detect GPS signal errors (including satellite clock, orbit, and ionospheric errors), then broadcast correction messages to aircraft via geostationary satellites.
WAAS
USA / North AmericaWide Area Augmentation System. Covers CONUS, most of Canada and northern Mexico. The most mature SBAS; enables LPV approaches to 200 ft DA.
EGNOS
EuropeEuropean Geostationary Navigation Overlay Service. Enables LPV approaches across Europe. Also used in parts of North Africa and the Middle East.
MSAS
JapanMTSAT Satellite-Based Augmentation System. Provides SBAS coverage over Japan and surrounding airspace.
GAGAN
IndiaGPS Aided Geo Augmented Navigation. Operational over India; enables GPS approaches at Indian airports.
SBAS accuracy: SBAS corrects the main GPS error sources, achieving ±1–3 m horizontal accuracy (95th percentile). This enables approach performance equivalent to a Cat I ILS at airports without ILS ground equipment — a major benefit for smaller airfields.
GNSS Approach Categories
GPS/GNSS approaches are published as RNAV (GPS) on approach charts. Multiple lines of minima appear on the same chart, each requiring different equipment capability.
| Approach Type | Full Name | Required Equipment | Typical DA/MDA | Notes |
|---|---|---|---|---|
| LNAV | Lateral Navigation | IFR GPS + RAIM | 300–500 ft MDA | Non-precision; lateral guidance only; baro-altimeter for vertical |
| LP | Localiser Performance | SBAS (WAAS/EGNOS) | ~300 ft MDA | Lateral guidance more precise than LNAV; no vertical guidance |
| LNAV/VNAV | Lateral + Vertical Navigation | Baro-VNAV or SBAS | 300–400 ft DA | Vertical guidance; approach with vertical guidance (APV) |
| LPV | Localiser Performance with Vertical Guidance | SBAS required | 200 ft DA | Precision-equivalent; replaces ILS at many airfields |
| RNP AR | Required Navigation Performance — Authorisation Required | Advanced FMS + SBAS + approval | As low as 100 ft DA | Curved/RF legs; airline and special ops only; operator approval required |
LPV row highlighted — highest performance available from standard SBAS equipment. DA = Decision Altitude; MDA = Minimum Descent Altitude.
IFR GPS Certification Requirements
Not all GPS receivers can be used for IFR navigation. Equipment must meet specific airworthiness standards:
| TSO | Applies To | Notes |
|---|---|---|
| TSO-C129 | Non-SBAS IFR GPS | Older standard; LNAV capable; requires RAIM |
| TSO-C145 | SBAS GPS sensor (airborne) | SBAS-capable sensor unit; enables LPV |
| TSO-C146 | SBAS stand-alone GPS | Complete standalone SBAS IFR unit; enables LPV |
| TSO-C196 | Airborne supplemental GPS | GPS as supplemental means of navigation |
IFR GPS Checklist Requirements
- Current navigation database: AIRAC cycle must be valid (28-day cycles); expired database = GPS cannot be used for IFR approaches
- RAIM check: required before any GPS approach (prediction + actual RAIM status in-flight)
- GPS NOTAMs: check for GPS satellite outages and test/interference NOTAMs
- GPS can substitute for NDB, VOR, DME, and DME/DME when IFR-certified and database current
- Chart title note: older "GPS" approach titles are being renamed to "RNAV (GPS)" — same procedure, updated nomenclature
Database currency is a legal requirement. Under most regulatory frameworks (FAA, EASA), using an expired navigation database for an IFR approach is not permitted. The database must be current for the day of the flight. If expired, GPS may still be used for situational awareness only.