16.2 GPS, RAIM, and Radar Services

ACS PA.VI.B.K2 is satellite navigation: equipment, regulations, databases, and limitations. PA.VI.B.K3 is radar assistance to VFR aircraft. AIM 1-1-17 and AIM 4-1 are the source language. Transponder modes were Chapter 4 and 12; this section is what you do with the magenta line and with Approach.

GPS for VFR — what it is allowed to be

AIM 1-1-17 is blunt: do not solely rely on GPS for VFR navigation. There is no design standard of accuracy or integrity for a VFR-only receiver. Integrate GPS with pilotage, dead reckoning, and a VOR when you have one. Minimize head-down time. A suction-cup antenna on the side window will lose satellites in a turn; a handheld with no RAIM will not warn you that the little airplane is now a guess.

RAIM (receiver autonomous integrity monitoring) is the box checking whether the satellite geometry can support the position it is drawing. Without RAIM, AIM says the pilot has no assurance of GPS position integrity. Fault detection needs five satellites, or four satellites plus baro-aiding. Baro-aiding uses a barometric altimeter setting you entered, not GPS altitude — GPS vertical error would invalidate the check. Fault detection and exclusion (FDE) needs six satellites, or five plus baro, so the receiver can throw out a bad bird and keep navigating. Two RAIM messages matter on PAR: not enough satellites to monitor integrity (the fix may still look fine), and integrity failed / inconsistency detected (do not trust the fix). Handhelds and many VFR panel mounts have no RAIM alert at all.

WAAS is the U.S. satellite-based augmentation system. A TSO-C145/C146 receiver that is receiving WAAS gets integrity from the GEO satellites and generally does not need a separate RAIM prediction when WAAS coverage exists along the route. That is an IFR-planning distinction. On the private-pilot written, WAAS means better integrity and vertical guidance on approaches you are not yet rated to fly; it does not mean the moving map is a clearance or a see-and-avoid system.

Database currency (AIM 1-1-17): databases must be updated for IFR. They should be updated for all other operations. There is no requirement that a VFR database be current. The next sentence in the AIM is the one that fails people who stop at “not required”: do not use a moving map with an outdated database in and around critical airspace. Verify waypoints against a current sectional, TAC, or Chart Supplement. VFR waypoints begin with VP, are not for IFR filing, and the five-letter name is not for ATC readback unless the point is collocated with a pronounceable visual checkpoint.

If GPS fails or the RAIM flag appears, go back to the methods in Chapter 15: last heading, last time, next landmark, a VOR radial. The VOR MON (AIM 1-1-3) is the FAA’s reversionary network if GNSS is unavailable — nearly continuous VOR at 5,000 feet AGL over CONUS outside the western mountains, with a MON airport (ILS or VOR approach) within 100 NM. Part 91 WAAS-only airplanes are not required to carry a VOR; prudent planning still asks what you will fly if the satellites lie.

Radar services — flight following is not a clearance

VFR radar traffic information service is what pilots call flight following. It is workload permitting. You remain VFR, you remain responsible for terrain, weather, airspace authorization, and 91.113 see-and-avoid. What you may receive:

• Traffic advisories — clock position, distance, direction of flight, and type if known. “Traffic, 2 o’clock, 3 miles, opposite direction, altitude indicates 4,500” is information. You still have to find the traffic or report it not in sight.
• Safety alerts — issued when the controller sees a situation that places you in unsafe proximity to terrain, obstructions, or other aircraft. This is not optional trivia; answer and maneuver.
• Vectors — a heading to help you, or to sequence you, when you request it or when the controller offers. A VFR vector is not an IFR clearance and does not authorize Class B entry by itself.
• Limited weather or airport information as workload allows.

How to request it. Listen first. Then: “City Approach, Skyhawk 12345, 15 south of Hometown at 5,500, VFR to Destination, request flight following.” Have the destination identifier and type of aircraft ready. They will usually give a discrete squawk and may ask you to ident. “Radar contact” means they have identified you and radar service has begun. It does not mean they see every target, it does not mean you are in controlled airspace you were not already in, and it does not close a VFR flight plan.

“Radar service terminated, squawk VFR” (or “remain this code and contact Tower”) means the advisories just stopped. Resume your own traffic scan, go back to 1200 if told, and do not keep flying as if a controller is still guarding your altitude. Changing frequencies without a handoff, flying into a radar shadow, or the controller getting too busy are all normal reasons service ends. If you still want following, ask the next facility.

Class C and TRSA have their own basic-radar-service packages (AIM 4-1-18 and 3-2-4). In those areas you get sequencing and traffic advisories as part of participating; that is not the same as en-route flight following you requested 80 miles out.

ADS-B traffic is a picture, not a right-of-way

ADS-B Out (91.225) broadcasts your WAAS position. ADS-B In can display traffic (TIS-B / ADS-B) and weather (FIS-B) on a panel or tablet. Limitations PAR cares about:

• Not every airplane is Out-equipped, and not every target is uplinked. A 1946 Cub in the pattern may be invisible on the iPad.
• TIS-B coverage depends on ground radios and on you being a client (usually Out-equipped in rule airspace).
• Latency, ghost targets, and targets that disappear in a turn all happen.
• Cockpit traffic displays do not replace see-and-avoid. AIM and ACS treat them as an aid. 91.113 right-of-way is still settled by the windshield, not by whose symbol was brighter.

PAR scenario: You asked City Approach for flight following and heard “radar contact.” Ten minutes later the GPS pops a RAIM message and the magenta line freezes. Fly the last heading, find the next checkpoint on the sectional, tune the VOR you planned as a backup, and tell Approach you have lost GPS — they can still give a heading if they have you on radar. Do not descend into a valley to “get the satellites back” through a Bravo shelf. When they say “radar service terminated, squawk VFR,” the traffic picture on the tablet is still not a substitute for looking outside.