What is RAIM and how many satellites does GPS need for RAIM to work?

RAIM (Receiver Autonomous Integrity Monitoring) is a GPS receiver function that checks satellite signals for consistency to ensure the position solution is reliable enough for navigation. Basic RAIM requires at least five satellites in view (or four plus baro-aiding), while Fault Detection and Exclusion (FDE) — which can remove a bad satellite and keep navigating — requires at least six satellites.

When and how do I check RAIM before an instrument approach?

You must predict RAIM availability at your estimated time of arrival (ETA) at the destination, not at departure time, because satellite geometry changes continuously. Use the FAA's free Satellite Availability Prediction Tool (SAPT) online, your avionics' built-in RAIM prediction page, or check GPS NOTAMs. If RAIM is predicted to be unavailable for more than five minutes during the final approach segment, you must use an alternate navigation system or choose a different approach.

Does WAAS eliminate the need for RAIM prediction on GPS approaches?

WAAS-equipped receivers receive integrity assurance from the WAAS ground network instead of performing conventional satellite-geometry RAIM, which allows LPV and LP approach authorizations not available to non-WAAS GPS. However, pilots using WAAS must still check NOTAMs for WAAS outages, especially in Alaska and high-latitude regions where WAAS coverage can be limited, and must always ensure the navigation database is current for IFR GPS approaches.

GPS RAIM Prediction and Receiver Autonomous Integrity Monitoring

Global Positioning System (GPS) navigation has transformed instrument flying, but GPS receivers cannot always guarantee that their computed position is accurate enough to rely on safely. Receiver Autonomous Integrity Monitoring — universally abbreviated RAIM — is the onboard capability that monitors satellite signals and alerts the pilot when the position solution may be unreliable. Understanding RAIM is essential for any pilot who uses GPS for instrument flight, and it is a cornerstone topic on the Airline Transport Pilot (ATP) knowledge exam and oral examinations.

RAIM is not merely a background software function. It is an active safety layer with direct go/no-go implications. When RAIM is unavailable, GPS cannot legally or safely be used as the primary navigation source for an instrument approach, and pilots must have an alternate means of navigation ready. The rules, limitations, and prediction requirements surrounding RAIM are thoroughly addressed in the Aeronautical Information Manual (AIM), Section 1-1-17.

How RAIM Works

A GPS receiver computes its three-dimensional position by measuring the time signals travel from satellites to the receiver. Accurate positioning requires at least four satellites — three for latitude, longitude, and altitude, plus one more to solve for receiver clock error. RAIM, however, demands at least five satellites in view (or four satellites plus a barometric altimeter input, often called baro-aiding) to perform its integrity function. With five signals available, the receiver can compare redundant position solutions and detect when one solution diverges beyond an acceptable threshold.

The receiver continuously computes a Horizontal Protection Level (HPL) — a statistical radius around the computed position within which the true position is guaranteed to lie with very high probability (typically 10-7 per approach). RAIM is available only when the HPL is smaller than the Horizontal Alert Limit (HAL) defined for the phase of flight. If the HPL grows too large — because satellites are poorly spaced, few in number, or producing weak signals — RAIM annunciates a warning and the GPS cannot be used for navigation guidance.

Most modern GPS avionics also implement Fault Detection and Exclusion (FDE), an enhanced form of RAIM that requires six satellites. FDE can not only detect a faulty satellite but actually exclude it from the position solution and continue navigating with the remaining constellation. FDE is particularly important for extended oceanic or remote area operations where no other navigation aid is available for cross-checks.

RAIM Prediction — The Pre-Flight Requirement

Because satellite geometry changes constantly as satellites orbit the Earth and because satellites are periodically taken out of service for maintenance, RAIM availability must be checked before every GPS-based instrument approach. The AIM specifies that pilots must predict RAIM availability at the estimated time of arrival (ETA) at the destination, not at departure time. Satellite geometry at departure may be entirely different from what exists an hour later at the approach destination.

Pilots have several ways to perform the RAIM prediction:

FAA's GPS RAIM Availability Analysis website (SAPT): The FAA provides a free Satellite Availability Prediction Tool accessible online. Pilots enter the airport, date, and ETA window to receive a RAIM availability forecast.
Onboard avionics prediction: Many GPS receivers have a built-in RAIM prediction function accessible through the satellite status or prediction pages. The pilot enters the destination and ETA, and the receiver computes whether RAIM will be available at that location and time.
NOTAMs: The FAA publishes GPS NOTAMs (formerly known as FDC NOTAMs) that warn of satellite outages or periods of GPS unreliability due to testing, satellite maintenance, or constellation problems. These must be checked during preflight planning just like any other NOTAM. The AIM notes that the term used in GPS NOTAMs is often the GPS signal status with predicted outages for specific regions and time windows.
Flight service or ATIS/D-ATIS: In some cases, GPS advisories are broadcast through automated terminal information services.

If RAIM prediction indicates the service will be unavailable for more than five minutes during the final approach segment, the flight must use an alternate navigation system for that approach, or the pilot must select an alternate destination served by a non-GPS approach. Simply monitoring the GPS in flight and hoping RAIM remains available is not an acceptable substitute for pre-flight prediction.

Phases of Flight and Alert Limits

The required satellite geometry — and therefore the RAIM threshold — varies with the precision demanded by each phase of flight. The more critical the phase, the tighter the Horizontal Alert Limit and the more geometry RAIM needs.

Oceanic/Remote: HAL = 2 nautical miles
En route: HAL = 2 nautical miles
Terminal (within 30 NM of destination): HAL = 1 nautical mile
Non-precision approach (LNAV): HAL = 0.3 nautical miles
Approach with vertical guidance (LPV/LNAV+V): WAAS provides its own integrity monitoring (SBAS), generally superseding conventional RAIM for equipped aircraft

The tightest demands occur at the final approach fix (FAF) and inbound on an LNAV approach. When the GPS transitions to approach mode (typically within 2 NM of the FAF), the receiver automatically scales CDI sensitivity to ±0.3 NM and applies the approach-phase alert limit. If satellite geometry is marginal, RAIM may be available in terminal mode but fail to meet the approach threshold — the receiver will annunciate accordingly.

WAAS and SBAS — The Modern Upgrade

The Wide Area Augmentation System (WAAS) is a Space-Based Augmentation System (SBAS) that broadcasts correction signals from geostationary satellites and provides its own integrity information. WAAS-enabled GPS receivers receive integrity assurance from the WAAS ground network rather than computing it solely from raw satellite geometry. This means WAAS effectively replaces legacy RAIM for appropriately equipped aircraft and enables approaches to LPV (Localizer Performance with Vertical guidance) and LP minima, which are not available to non-WAAS receivers.

However, WAAS availability is also subject to prediction when signal coverage is marginal (particularly in Alaska and at extreme northern latitudes). Even WAAS users should check NOTAMs for WAAS outages. Non-WAAS GPS receivers must always perform conventional RAIM prediction for any approach.

Why RAIM Matters Operationally

RAIM failures do occur in the real world, most often during periods of GPS constellation maintenance when the FAA takes satellites offline, during solar events that degrade satellite signals, or in areas where terrain or structures block satellite reception. A RAIM failure during an approach is an immediate missed approach situation — the pilot must treat the loss of GPS integrity the same as a loss of the primary navigation signal. Continuing an approach without RAIM would be analogous to continuing an ILS approach with the glide slope and localizer flags in view.

For ATP and commercial operations, operators must ensure that navigation databases are current (expired databases make GPS approaches illegal in IFR operations), and company operations specifications (OpSpecs) often impose additional requirements for RAIM prediction on GPS-based routes and approaches. Under 14 CFR Part 91.185 and Part 121/135 regulations, maintaining navigation integrity is a non-negotiable operational requirement.

Key Numbers and Rules

Minimum 5 satellites required for basic RAIM (fault detection only)
Minimum 6 satellites required for Fault Detection and Exclusion (FDE)
RAIM prediction must cover the ETA ± 15 minutes window at destination
If RAIM unavailable for more than 5 minutes on final approach, select alternate means of navigation
Approach mode CDI scaling: ±0.3 NM, active approximately 2 NM before FAF
WAAS provides SBAS-based integrity — effectively replaces RAIM for WAAS-equipped receivers
GPS NOTAMs must be reviewed during all GPS IFR preflight planning

Memory Aid

Use "5 to DETECT, 6 to EJECT" to recall satellite requirements: five satellites let the receiver detect a fault (basic RAIM), while six satellites allow the receiver to eject the bad satellite and continue navigating (FDE/fault exclusion).

Common Test Traps

Confusing the satellite count: Many students say four satellites are sufficient for RAIM. Four satellites give you a 3D position fix, but you need five for fault detection. The exam exploits this distinction frequently.
Ignoring ETA timing: RAIM must be checked for your estimated time of arrival, not departure. Departure-time satellite geometry is irrelevant to approach-time RAIM availability.
Assuming WAAS eliminates all preflight checks: WAAS replaces conventional RAIM for integrity, but WAAS coverage and outages still require NOTAM checks, especially in Alaska and polar regions.
Forgetting the 5-minute rule: If predicted RAIM unavailability is brief, pilots sometimes plan to "wait it out." Any gap exceeding five minutes during the final approach segment requires selection of an alternate navigation method — not patience.
Expired database confusion: RAIM may be available, but if the navigation database is expired, GPS approaches are not authorized for IFR use. Database currency is a separate requirement from RAIM.