What is the purpose of RAIM in a GPS unit?

RAIM (Receiver Autonomous Integrity Monitoring) is a technology that is used in GPS receivers to assess the integrity of the GPS signals that are being received at any given time. It is particularly applicable to receivers intended for safety-critical applications and in aviation or marine navigation.

How long is a RAIM check valid for?

72 hours
Whilst a RAIM check may be performed in the aircraft, the result is only valid for +/- 15 minutes. However, use of the following link to the Eurocontrol RAIM prediction website will provide a RAIM prediction that will be valid for multiple locations for 72 hours from the current day at 00:00 hrs UTC.

Is RAIM required for IFR?

IFR GPS units must automatically perform a RAIM check before beginning an approach. However, performing a RAIM check prior to leaving the ground will better enable pilots to plan ahead and is recommended specifically for pilots without baro-aiding (see below).

Does RAIM have FDE?

Traditional RAIM uses fault detection (FD) only, however newer GPS receivers incorporate fault detection and exclusion (FDE) which enables them to continue to operate in the presence of a GPS failure.

Can you fly without RAIM?

If there is no RAIM available during part of your flight, you can’t rely on GPS during that part. The GPS may still work fine, but there is no way to check its integrity. A single fault in a GPS satellite or, more likely, a corruption in the downlinked GPS satellite almanac will cause the position fix to be off.

How does predictive RAIM work?

RAIM uses redundant signals to produce several GPS position fixes and compare them, and a statistical function determines whether or not a fault can be associated with any of the signals. If the number of GPS satellites is 23 or fewer, RAIM availability must be checked using approved ground-based prediction software.

How many satellites are required for RAIM?

five
RAIM needs 24 or more satellites to operate, although normally only five are needed for position estimation. Older GPS units will simply report a GPS fail, but newer ones are able to recover.

Do I need WAAS?

An IFR approved WAAS GPS is required for vertical approach guidance. That could be simple advisory vertical guidance or LPV approaches that guide you to within 200 feet of the ground more reliably than a typical ILS approach. WAAS GPS gives you more options with planning alternate airports.

How is RAIM calculated?

Viewing the RAIM page On the Flight Plan menu, click RAIM Prediction. Receiver Autonomous Integrity Monitoring (RAIM ) assesses the integrity of the Global Positioning System (GPS) signals. This system predicts outages for a specified geographical area.