RAIM
Some conditions decrease the accuracy of GPS:

Terrain/aircraft position could reduce the number of satellites within line of sight. 
The very low power of GPS satellite signals creates the potential for signal jamming – intentional or unintentional. 
Fortunately, Receiver Autonomous Integrity Monitoring (RAIM) allows GPS receivers to alert a pilot in such instances. They can even predict such outages. RAIM works by comparing the position solutions from different groupings of at least four satellites. These groupings may contain some of the same satellites, but since at least one must be unique, RAIM requires at least five satellites to be in view. A barometric altimeter may substitute for the fifth satellite. A RAIM-enabled receiver alerts the pilot if there aren’t enough satellites in good positions to make a comparison or when a satellite is transmitting faulty signals. RAIM alerts are triggered at different levels of accuracy for enroute, terminal, and approach environments.

RAIM provides three levels of warnings/alerts:

Enroute – 2 n.m. 
Terminal –1 n.m. 
Approach – .3 n.m.
If a student doesn’t seem interested in how the GPS system works, RAIM is one topic that can demonstrate what happens if the system doesn’t work. Understanding the system helps them appreciate the importance of RAIM availability for ensuring the integrity of GPS data. This would be a good opening to discuss the dangers of shooting an approach when RAIM is not available.

Also, tell students that GPS NOTAMS are issued regarding RAIM availability and other issues. Make sure these NOTAMS are part of any preflight briefing before a flight that depends upon GPS.



Unit Certification
Prior to IFR use, the GPS unit and installation must be certified. TSO-C129 (TSO-C146a for WAAS) specifies performance standards for IFR units, such as:

One-second position updates. 
Fixes stored in a non-corruptible database. 
Pilot selectable CDI sensitivity. 
RAIM integrity alarms.
Make sure students understand that IFR receivers can be approved for different levels of use. Just because it’s IFR-certified doesn’t necessarily mean it will provide approach navigation.

Class A1 – enroute, terminal, and non-precision approaches. 
Class A2 – enroute and terminal operations only.
Installation
AC 20-138A (or AC 20-130A) provides guidance for installation procedures and requirements. 
Discuss how students can verify these conditions for the GPS in the plane(s) they fly. 
Manufacturer confirms TSO. 
Logbook should document installation and performance tests (ground and flight tests are required prior to IFR use). 
FAA-approved Aircraft Manual Supplement must be in aircraft with POH. 
Most IFR installations require a 337. 
Should be an STC in maintenance logs for the installation. 
Substitutions for NAVAIDs
There are pages and pages of guidance in the AIM as to when and how a pilot may substitute GPS navigation for ground-based facilities. Some acceptable uses are to:

Determine the aircraft position over a DME fix. 
Fly a DME arc. 
Navigate to/from an NDB/compass locator. 
Determine the aircraft position over an NDB or compass locator. 
Determine when the aircraft is over a fix defined by an NDB/compass locator bearing crossing a VOR/LOC course. 
Hold at an NDB


WAAS Ready and Legal
Remember, a receiver must meet the following requirements to be used for vertical guidance:

WAAS-enabled equipment required in aircraft 
Manufactured and approved under TSO C145A or C146A. 
Installed per AC 20-138A. 
A supplement must be added to the airplane’s flight manual and be carried in the aircraft.