GPS: substitute or alternate?

In these scenarios, we often speak of “substituting” GPS for ground-based navaids, and we understand the general sense of that word. But like many terms in aviation, “substitute for” has a specific meaning, and when operating IFR, it’s important to understand how that definition relates to its counterpart phrase, “alternate means of navigation.”

Current guidance about this topic is mainly in Aeronautical Information Manual, paragraph 1−2−3: Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes, and its counterpart, AC 90-108. AIM 1-2-3 explains how you can use a “suitable RNAV system” (which in general aviation means an IFR-approved GPS) in various situations. You might, for example, prefer to follow GPS guidance when flying a VOR-based airway. That’s using GPS as an “alternate means.” But if you rely on GPS when ground-based navaids are out of service or if you don’t have equipment like DME or ADF in your panel, you’re “substituting” GPS for those navaids. The practical distinction between “alternate means” and “substitution” is best understood through specific examples.

Suppose you want to fly a VOR-based airway, such as V2, which roughly parallels I-90 from Seattle to La Crosse, Wisconsin. If all the VORs enroute are working and you have a VOR receiver in the panel (that you’ve checked per FAR 91.171), you can use an IFR-approved GPS as an alternate means of navigation to stay in your lane the entire way. As long as the GPS signal is valid, you don’t even need to monitor the VORs. However, if ground stations that define segments of the airways are out of service, using GPS to fly those legs is an example of substituting RNAV equipment for a navaid, and your preflight briefing may include a notam requiring you to use GPS along those portions of the route.

The key distinction between “alternate means” and “substitute for” becomes more important when you fly IFR procedures based on navaids. For example, notes in AIM 1-2-3 explain that: “Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.” That means you can’t fly a VOR or NDB approach using only GPS for guidance, as when the navaid listed in the procedure title is out of service or if you don’t have the appropriate receiver in your panel.

But the following note explains that: “Use of a suitable RNAV system as [an alternate] means to navigate on the final approach segment…based on a VOR, TACAN or NDB signal, is allowable.” However, the AIM continues, “The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.”

Sometimes, the FAA requires you to substitute GPS for a navaid. For example, the SCAPO SEVEN departure at Hillsboro, Oregon (HIO), requires only a VOR receiver. But when the VOR it’s based on isn’t working, a notam declares that the DP is “NA except for acft equipped with suitable RNAV system with GPS. UBG VOR out of service.” You can fly that procedure with GPS by creating and then intercepting the leg from UBG to SCAPO. (See “Resolving a Database Dilemma,” August 2024 AOPA Pilot.)

One important exception applies to using GPS as either a substitute or alternate means of navigation. Neither option is approved when flying a final approach segment defined by a localizer. From at least the final approach fix to the missed approach point of an ILS or LOC approach, you must display and use “green needles” to track the funnel-like localizer course, using GPS only to identify fixes along final and as a substitute for DME or ADF if required.

Bruce Williams is a CFI. Find him at youtube.com/@BruceAirFlying and bruceair.wordpress.com