Understanding GPS and RNAV Systems

GPS (Global Positioning System) and RNAV (Area Navigation) systems have transformed IFR flying by enabling precise point-to-point navigation without the constraints of ground-based navaids. GPS provides continuous three-dimensional positioning using a constellation of at least 24 satellites, with receivers calculating latitude, longitude, and altitude through time-of-arrival measurements. RNAV, meanwhile, is a method of navigation that allows aircraft to fly any desired flight path within the coverage of ground- or space-based navigation aids — it is the capability, while GPS is one of the sensors that can provide it.

When we talk about GPS and RNAV systems in IFR scenarios, we are usually referring to either a stand-alone GPS navigator (such as a Garmin GTN 750 or an Avidyne IFD540) or an integrated Flight Management System (FMS) that incorporates GPS, inertial sensors, and possibly DME/DME inputs. In modern cockpits, these systems feed data to moving maps, flight directors, and autopilots, reducing workload and increasing situational awareness. However, their effectiveness depends on proper setup, database currency, and pilot proficiency.

The key difference between a basic GPS unit and a full RNAV system like an FMS equipped for RNP (Required Navigation Performance) is the level of integrity monitoring and the ability to fly curved paths (RF legs) without step-down fixes. RNAV systems used in IFR must be certified under TSO-C129 (for GPS), TSO-C145/146 (for GPS/WAAS), or TSO-C166 (for FMS). Understanding these certifications helps pilots know what kinds of approaches and routes they can fly legally.

For a deeper dive into the technical specifications, the FAA’s Aeronautical Information Manual (AIM) Chapter 1 provides an excellent overview of satellite navigation principles and WAAS (Wide Area Augmentation System) which improves GPS accuracy over the continental U.S. to allow vertically guided approaches (LPV) down to 200-foot decision heights.

Pre-Flight Preparation for GPS/RNAV Operations

Effective use of GPS and RNAV in IFR begins long before engine start. Pilots must ensure the navigation database is current — most GPS navigators require a database update every 28 days. Flying an approach with an expired database can lead to missing or incorrect waypoints, which may result in a pilot deviation or worse. Always check the database effective dates and any NOTAMs that may affect satellite availability or RAIM (Receiver Autonomous Integrity Monitoring) prediction.

RAIM Prediction and Outage Planning

RAIM is the system’s ability to detect a faulty satellite and warn the pilot. For any GPS-based IFR flight, you are required to check RAIM availability for the planned route and alternate airports. Many GPS units have a built-in RAIM prediction tool; if not, free online tools from the FAA or third-party vendors like AOPA’s RAIM prediction can be used. If RAIM is predicted to be unavailable for more than five minutes, you must have an alternate means of navigation (such as VOR/DME or an alternate airport with a non-GPS approach) and ensure your alternate is legal.

Route Validation and Waypoint Checks

Even with a current database, waypoints can move, become restricted, or be permanently removed. Always review your route in the navigation system after programming it. Verify that each waypoint is in the correct order, that there are no discontinuities (especially after fixes where a leg type changes), and that the distance and track values look reasonable. Cross-check the route against the IFR low or high enroute chart. Some GPS units allow you to insert a route from the flight plan and then activate Direct-To after a missed approach; be sure you know how to sequence legs manually if the autopilot does not follow the autosequence correctly.

Battery and Backup Power

For portable or panel-mounted GPS navigators, ensure the battery is fully charged if the unit can operate without aircraft power. In the event of an alternator failure, a depleted battery can render your primary IFR navigation source useless. Have a backup plan — know how to identify and switch to VOR or ADF navigation, even if you haven’t flown an NDB approach in months.

In-Flight Use of GPS and RNAV Under IFR

During flight, the GPS/RNAV system becomes your primary source of navigation data. However, the prudent IFR pilot never relies on a single source exclusively. Maintain a continuous cross-check against other instruments: the attitude indicator, heading indicator, altitude, and airspeed. For enroute segments, compare your ground track to the desired bearing and check that the distance to next waypoint is decreasing appropriately.

Monitoring Leg Sequencing and Autopilot Coupling

Many GPS units automatically sequence to the next leg when you cross the active waypoint. However, this assumes the “arm” feature is enabled and the system knows you want to continue. If you are given a vector by ATC and later ask for “direct to a fix on the flight plan,” the system may try to intercept a later leg incorrectly unless you properly activate a Direct-To or rejoin the active flight plan. Always verify the autopilot is following the correct navigation source — especially after a mode change (e.g., from HDG to NAV or GPSS).

Using the OBS (Omni Bearing Selector) Mode

In an approach or when holding, you may need to hold over a fix or fly a certain radial. The OBS mode on many GPS navigators simulates a VOR indicator, allowing you to select a radial and track inbound or outbound. This is handy for executing a published hold or a non-precision approach that requires a course reversal. Remember that in OBS mode, the GPS does not automatically sequence when crossing the fix; you must manually sequence or revert to normal mode when you wish to continue.

Cross-Check with Traditional Navaids

Even with a perfectly functioning GPS, traditional navaids like VOR and DME remain valuable backups. If you are in radar contact with ATC and your GPS fails, they can provide vectors, but you should have a plan to tune the nearest VOR or approach facility. Many instrument-rated pilots practice “sterile cockpit” procedures: during critical phases (like an approach below 1,000 feet AGL), have one pilot (if flying single-pilot, the autopilot) fly raw data from a second source — even if that source is just a full deflection check on the localizer.

Advanced RNAV Procedures: RNP and LPV Approaches

One of the greatest advantages of modern GPS/RNAV systems is the ability to fly LPV (Localizer Performance with Vertical Guidance) approaches using WAAS. These approaches offer glide path guidance similar to an ILS, but without the need for ground equipment at the airport. They are increasingly common at airports where an ILS is not economically feasible. LPV minima can go as low as 200 feet AGL, with visibility as low as ½ mile, provided the approach design and obstructions permit.

RNP AR Approaches – What You Need to Know

Required Navigation Performance Authorization Required (RNP AR) approaches are even more demanding. They require special aircrew authorization and often specific aircraft equipment (dual FMS, fail-passive autopilot, etc.). RNP AR approaches can include curved RF (Radius-to-Fix) legs that enable terrain avoidance in mountainous areas (e.g., the famous approaches into Aspen, Colorado, or Juneau, Alaska). Most general aviation aircraft are not equipped for RNP AR, but the concept is important to understand because it represents the future of precision approach navigation.

If you plan to fly an RNAV (GPS) approach with vertical guidance, ensure your system is WAAS-capable and that you are using a “current” approach that is in the database. Many RNAV (GPS) approaches are being turned into LPV or LP (Localizer Performance with no vertical guidance) minima as old non-WAAS GPS units are phased out. The FAA’s digital IFR approach plates will indicate the type of guidance available.

Handling GPS and RNAV System Failures

System failures can occur suddenly and with little warning. The most common failure in IFR GPS operations is RAIM loss, where the system can no longer guarantee position integrity. Your GPS unit will typically display a warning message (“RAIM not available” or “Loss of integrity”). Immediately revert to an alternate navigation method: identify your position using cross-bearings from VOR or NDB, or request vectors from ATC. If you are in the middle of an approach, you should execute a missed approach unless you have visual contact with the runway environment.

GPS Signal Interference and Jamming

Intentional GPS jamming is rare but can occur near military exercises or from certain vehicle-mounted jammers. Additionally, solar activity and ionospheric disturbances can degrade GPS signals. Although WAAS corrects for some of these effects, it cannot fix a total loss of signal. The best defense is maintaining proficiency with traditional navigation. Always have a current chart showing VOR frequencies and identify any nearby NDBs, even if you rarely use them.

Battery Failure on Handheld Units

A panel-mounted GPS with aircraft power is robust, but a portable unit used as backup can run out of battery mid-flight if not monitored. Some IFR-certified portable units like the Garmin Aera 660 have external power options, but if the battery dies, you lose not only navigation but also the moving map and terrain awareness. Keep a spare battery or a charged secondary unit in your flight bag. We recommend the AOPA article on using a smartphone as a backup for a practical approach to redundancy.

Communicating with ATC

If you experience a GPS failure while on an IFR flight plan, inform ATC as soon as practical. State what you have lost (e.g., “Loss of GPS-based navigation, request vectors to the next fix.”). They will likely provide vectors to help you re-establish on a preferred navaid or radar-vectored approach. If you lose GPS while on an RNAV (GPS) approach, you must abort the approach unless you can continue using another approach (e.g., localizer or VOR) at the same airport or an alternate.

Conclusion: Building Proficiency with Modern Navigation Tools

Mastering GPS and RNAV systems in IFR scenarios is not just about pressing buttons — it is about understanding the underlying technology, preparing thoroughly before each flight, monitoring continuously during flight, and having a robust backup plan. The best pilots combine modern automation with classical stick-and-rudder skills. They practice flying approaches using only raw data (VOR/LOC) periodically to stay sharp. They update their databases without fail and always run a RAIM prediction before departure.

As the FAA continues to decommission ground-based navaids and expand the GPS-based National Airspace System, the importance of RNAV proficiency will only increase. Investing time in simulator training, reading the latest guidance from the FAA’s AIM Chapter 5 on RNAV, and participating in a local IFR safety seminar will pay dividends. The combination of GPS and RNAV, when used effectively, gives the IFR pilot a level of precision and safety that was unimaginable just a few decades ago.