062 Radio Navigation topic guide
GNSS and RAIM
GNSS positioning is built from three distinct segments: the space segment, the constellation of satellites broadcasting timed signals, the control segment, the ground stations that monitor each satellite's orbit and clock and upload corrections, and the user segment, the receiver in the aircraft that turns those signals into a position. Understanding the segments matters because integrity failures are caught at different points depending on which segment is at fault.
The receiver's own position calculation is where most exam questions live, because the number of satellites required changes depending on what the receiver is being asked to do: simply fix a position, detect that something is wrong, or identify and remove a faulty satellite while continuing to navigate.
Why four satellites, not three
Each satellite range places the aircraft somewhere on a sphere centred on that satellite, and three spheres intersect, in principle, at a single point, so three ranges would be enough if the receiver's clock were as accurate as the satellites' atomic clocks. It is not: an inexpensive receiver clock drifts, and that drift adds the same timing bias to every range it computes. A fourth satellite gives the receiver enough information to solve for that clock bias as an additional unknown alongside the three position coordinates, which is why a usable 3D fix needs four satellites in view, not three.
This is also why GNSS altitude is treated with more caution than latitude and longitude: the satellite geometry that resolves height is generally weaker than the geometry resolving the horizontal position, because an aircraft only ever sees satellites above its horizon, never below it.
RAIM: detection and exclusion
Receiver autonomous integrity monitoring checks the position solution for internal consistency by comparing redundant combinations of the available satellites. Four satellites give a fix but no spare information to check it against, so detecting that one range is inconsistent needs a fifth satellite in view. Excluding the faulty satellite, identifying which one is wrong and continuing to navigate on the remaining good ones without interruption, needs a sixth.
The practical consequence is a RAIM prediction check before any flight that depends on GNSS for a critical phase, particularly an approach: with fewer satellites predicted to be in view, or one predicted to be temporarily unavailable, the aircraft may have a position fix but no integrity protection, which is precisely the gap RAIM exists to close.
What degrades accuracy
Satellite geometry matters as much as satellite count: satellites clustered together in one part of the sky give a poor geometric spread, described as a high dilution of precision, and produce a less accurate fix than the same number of satellites spread widely across the sky. Ionospheric delay varies with solar activity and time of day and bends the signal's effective travel time; multipath, the signal reflecting off the ground or airframe before reaching the antenna, adds further small timing errors.
Augmentation systems exist to correct or bound these errors: aircraft-based augmentation, of which RAIM is the standard example, checks the receiver's own solution; satellite-based augmentation, broadcast over a wide region by geostationary satellites, corrects for ionospheric and orbital errors and tightens the achievable accuracy; ground-based augmentation, local to one airport, supports the most demanding approach guidance by broadcasting corrections generated by reference receivers sited on the airfield itself.
Worked example
Worked example: satellites required for RAIM fault detection
A GNSS receiver has four satellites in view, enough for a 3D position fix. How many satellites in view does it need to additionally detect that one satellite's range is inconsistent with the others, without yet being able to identify which one is faulty?
- AFour
- BFive
- CSix
- DThree
Show the answer and walkthrough
Correct answer: B
- A. Four satellites is exactly enough to compute a position and solve the receiver clock bias; there is no spare information left over to check that solution against, so a fault cannot yet be detected.
- B. Correct. One additional satellite beyond the four needed for the fix gives the receiver enough redundant information to notice that the ranges are not all consistent with a single position, which is fault detection.
- C. Six satellites support fault exclusion, identifying which specific satellite is faulty and continuing to navigate on the rest, a step beyond simply detecting that a fault exists.
- D. Three satellites would only work with a perfect receiver clock. In practice a fourth satellite is required just to solve for clock bias, before any question of fault detection arises.
Step by step
- A basic 3D position fix, including solving for receiver clock bias, needs four satellites.
- Detecting a fault requires redundant information beyond the minimum fix, which needs one satellite more than the fix itself.
- Four plus one gives five, the number needed for fault detection.
- Six would be needed to go further and exclude the specific faulty satellite, which the question does not ask for.
Common mistakes
Confusing the satellite count for a fix with the count for RAIM
Four gives position, five gives fault detection, six gives fault exclusion. Blurring these numbers into one figure produces a confident wrong answer whenever the question specifies which capability is being asked about.
Treating GNSS accuracy as constant regardless of satellite geometry
The same number of satellites can give a good or poor fix depending on how spread out they are across the sky, and ignoring dilution of precision leads to overconfidence in a marginal fix.
Skipping the RAIM prediction check because the receiver currently shows a valid fix
A present fix says nothing about whether integrity monitoring will remain available during a later, more critical phase of the flight such as an approach, which is exactly what the prediction check is for.
Related topic guides
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Last reviewed July 2026