GPS and RNAV navigation

Separate the sensor, navigation method, and required performance

GPS is a satellite positioning system. The receiver measures signals from several satellites to solve for position and receiver-clock error. RNAV is a navigation method that permits flight on desired paths within the capability of the relevant ground, space-based, or self-contained systems, or a combination of them. GPS enables RNAV, but not all RNAV uses GPS. A multisensor flight management system (FMS) may also use DME/DME or inertial inputs.

Performance-based navigation (PBN) specifies the navigation performance and functionality needed for an operation. An RNAV navigation specification does not require onboard performance monitoring and alerting; an RNP specification does. A number such as RNAV 1 or RNP 1 generally describes lateral total-system accuracy in nautical miles for at least 95 percent of flight time, together with other specification requirements. It is not permission to fly one mile off the line, a CDI dot value, or a substitute for the published procedure’s restrictions.

Eligibility for one specification does not automatically establish eligibility for every other specification or special feature. Read the aircraft’s documented eligibility and a procedure’s PBN and equipment requirements. For example, a navigator that supports ordinary RNAV (GPS) approaches does not thereby authorize an RNP AR approach or a radius-to-fix turn. Special authorization and specific aircraft capability may be required. This initial-rating lesson introduces the distinction; it is not authorization for those operations.

AIM 1-2-1: RNAV, RNP and navigation specifications · AIM 1-2-2: eligibility, required features and charted requirements · IPH Chapter 6, printed 6-3–6-8: coded paths and equipment limitations

Verify the installed approval for the intended operation

IFR use requires suitable approved equipment and an approved installation, operated within the applicable AFM or approved supplement. A component’s technical standard order (TSO) marking alone is not approval to use that installation for every IFR operation. Current AIM GPS guidance identifies equipment standards including TSO-C129/C196 for basic GPS and TSO-C145/C146 for WAAS, but the aircraft documentation determines the actual authorized capability and limitations.

Before flight, verify which operations the installed system supports: en route, terminal, approaches, particular lines of minima, any needed vertical guidance, and required backup or sensor inputs. Confirm the navigator, display, antenna, source selector and flight-guidance integration, not just the faceplate model. Learn startup checks, messages, source selection, database checks, sequencing, and abnormal procedures using the actual manuals and instructor-led practice.

A handheld or VFR-only GPS is not approved for IFR navigation, an instrument approach, or as a principal instrument-flight reference. It can assist situational awareness within its limitations. An accurate tablet ownship symbol does not supply the approval or integrity of an installed IFR navigator. The aircraft can have an approved navigator while its portable app remains an awareness aid; evaluate each function separately.

AIM 1-1-17: IFR equipment, installation and AFM requirements · AIM 1-1-18: WAAS approval and supported capability · ACS II.B, printed page 7: approved versus non-approved devices

Integrity monitoring answers a different question from accuracy

Accuracy describes how close the computed position is to the actual position. Integrity includes the ability to detect an unacceptable error and alert the user in time for the operation. A very stable position or small displayed estimated error is not, by itself, proof that the integrity requirements are met.

Receiver autonomous integrity monitoring (RAIM) checks consistency among available satellite information for the applicable phase of flight. It needs enough observations and suitable geometry. Basic fault detection normally requires at least five satellites, or four with appropriate barometric aiding. Fault detection and exclusion (FDE), which can isolate a failed satellite and continue using the remaining solution, normally needs six, or five with barometric aiding. A satellite count alone does not assure adequate geometry or acceptable integrity.

RAIM messages can mean insufficient information to monitor integrity, or a detected inconsistency in the navigation solution. In the first case a position may still be computed, but its integrity is not assured. In the second, the receiver has detected a potentially faulty solution. Understand the installed message and its operational response; do not clear it merely to recover a tidy display. If barometric aiding is used, supply the actual required altimeter input according to the manual; GPS-derived altitude is not an appropriate substitute for that input.

For basic GPS equipment, complete the required availability prediction for the planned operation and time using the approved receiver or prediction method. Review NOTAMs and update the prediction when material timing or route changes require it. A prediction plans for availability; it does not protect against every in-flight failure or local interference. If required RAIM is predicted unavailable, use other approved navigation, reroute, delay or cancel as appropriate.

AIM 1-1-17: RAIM, FDE, predictions, barometric aiding and fault messages

WAAS adds monitoring and corrections, with limits

WAAS is the U.S. satellite-based augmentation system. Ground reference stations monitor satellite signals; the network computes correction and integrity information and broadcasts it to appropriately equipped receivers. It improves GPS accuracy, integrity and availability. It can support vertical guidance and tighter lateral guidance when the receiver, installation, selected procedure and available service all support them.

“WAAS installed” is not a promise of LPV service everywhere. The approved supplement identifies the supported operations, and the receiver annunciates the service actually available for a selected procedure. A receiver or procedure may support only a lower level. LP is lateral-only guidance; LPV provides vertical guidance. Do not infer approved vertical guidance from an advisory glidepath or from a line of minima printed on a chart.

Review applicable GPS and WAAS NOTAMs, including site-specific information. A WAAS service warning may affect vertical service while lateral navigation remains available, or it may affect both. A change of service requires reassessment under the installed procedures and the published operation. The later approach lessons teach the decisions around minima and missed approaches; this lesson does not authorize continuing a descent after loss of required guidance.

Basic unaugmented GPS IFR use requires another approved, operational means suitable for the route, with active monitoring when RAIM is lost. Approved WAAS installations can support operations without a separate ground-based navigation installation under the applicable Part 91 guidance. That legal capability does not remove the need for an outage plan. If WAAS is lost, a receiver may revert to GPS-only operation; verify the resulting capability, integrity, annunciations and applicable equipment requirements before relying on it.

AIM 1-1-17: basic GPS alternate navigation and monitoring requirements · AIM 1-1-18: WAAS service, approval, NOTAMs and reversion

Validate the data and the operation, not only a date

The database supplies the geographic fixes and coded paths the navigator uses. Check issuance and expiration for the intended time of use, geographic coverage, applicable provider advisories, and procedure restrictions. A current database can still contain a known error; a chart or NOTAM can change the usable operation during a database cycle. Also verify the chart itself is current and downloaded for use without a network connection.

For GPS approaches, retrieve the procedure by name from the current appropriate navigation database, using the correct airport, procedure version, runway and transition. Do not construct an approach by hand-entering coordinates or a chain of familiar waypoint names. Procedures contain path and sequencing information as well as coordinates; loading an approach depiction alone does not establish approach approval.

Compare the loaded sequence with the published chart: names, order, general geographic relationship, transitions, turns and relevant vertical restrictions. If the quick comparison suggests a discrepancy, resolve it using the required detailed verification and applicable guidance before using the procedure. Do not casually edit a published coded procedure to make a screen match your expectation. Some manual interventions are legitimately required by the operation and installation; practice them from the actual instructions.

Database-expiration rules depend on the equipment and operation. The current AIM gives limited verification provisions for some en route/terminal uses; that is not blanket permission for an expired database on an approach or for every PBN specification. Use the approved supplement and applicable operational guidance. A practical preflight decision is to load current data and resolve expiration, missing-procedure or provider-warning issues on the ground rather than improvise in IMC.

AIM 1-1-17: database validity, procedure retrieval and chart comparison · AIM 1-2-3: database-cycle and amended-procedure restrictions · IPH Chapter 6, printed 6-13–6-17: database capabilities, discrepancies and currency

Loaded, active, and sequenced are different states

A route can be stored without driving guidance. The active leg identifies the path presently followed; activating another leg or Direct To can change that path. A clearance to intercept a specified inbound course is not necessarily satisfied by Direct To the waypoint. Check the active from/to fixes, desired track, distance and map geometry against the clearance before following the needle.

Published legs may specify a heading to an altitude, a course to an intercept, a track between fixes, an arc, or another terminator. Some require pilot intervention or a mode change. Read the chart and installed-system instructions; a magenta line cannot explain every altitude, heading, or clearance condition. Discontinuities, vector legs, OBS selection, and suspend states require deliberate management.

A fly-by waypoint permits turn anticipation to join the next leg. A fly-over waypoint requires crossing the fix before starting that leg’s turn. The navigator’s coded sequence and charted procedure govern; do not apply one rule to all waypoints. A route amendment can also remove or change a necessary transition, so recheck the resulting path after an entry.

AIM 1-1-17: waypoints, chart verification and receiver training · IPH Chapter 6, printed 6-4–6-8: fly-by/fly-over and path terminators

Monitor the path and the airplane when coupling guidance

The FMS or navigator computes a path; the flight director provides commands; the autopilot, when correctly selected and coupled, controls the airplane within its approved capability. Each function has its own status. Verify the displayed navigation source, lateral and vertical armed/active modes, altitude selections and relevant limits. NAV armed is not proof of capture, and lateral capture is not proof of vertical-path capture.

After every meaningful entry or mode selection, observe whether the airplane does what you intended. Compare actual track and position with the cleared path and actual altitude/airspeed with the flight targets. If automation surprises you, maintain control and use a simpler appropriate mode or manual control according to the installed procedures while resolving the issue. Do not let repeated reprogramming consume the scan.

Example: ATC clears an intercept of a 270° course to a waypoint. The pilot presses Direct To, which creates a 245° path from the current position. The autopilot captures that valid 245° path. The system may be functioning perfectly while the aircraft follows the wrong clearance. The correction begins with identifying the intended path, selecting it appropriately, verifying modes, and coordinating with ATC when needed.

Prepare likely selections on the ground, practice unfamiliar avionics with an instructor, and break in-flight entries into short segments between control checks. If capacity is exceeded, request time, a vector, or a different clearance suitable for the aircraft and circumstances. Coupling navigation never transfers the pilot’s responsibility to the automation.

AIM 1-1-17: equipment familiarity and correct navigator use · IPH Chapter 6, printed 6-5–6-8: path limitations and monitoring · ACS V.A, printed page 12

Use the EFB as a prepared reference and awareness resource

An electronic flight bag (EFB) can display charts, checklists, aircraft documents and calculations. FAA AC 91-78A provides Part 91 guidance for using EFB information in place of paper references. That substitution does not allow an EFB to replace required navigation, communication or surveillance equipment. A portable app’s ownship display remains subject to its source, latency, approval and integrity limits.

Before flight, verify current applicable information, offline availability, sufficient charge or suitable power, readable brightness, and secure placement that does not obstruct instruments, controls or visibility. Understand what happens after overheating, loss of power, app failure, a failed position input, or loss of internet. A second device is useful only if the chosen backup remains accessible through the failure you are planning for; two tablets sharing one external GPS or one charging problem may not provide independent protection.

Use a practical backup appropriate to the flight, such as accessible paper or a separately usable device with the needed downloaded information. Test retrieval of charts and documents before workload increases. Cross-check calculation inputs and outputs rather than accepting them because they appear digital. During a demanding intercept, keep aircraft control ahead of a long search or route entry. Defer the entry or request time when needed.

The current AC describes an acceptable means of EFB use, not a new general requirement to carry paper or a device approval for IFR navigation. Aircraft modifications and installed mounting or data connections have their own approval considerations. Apply the actual installation and operating guidance.

AC 91-78A, §§9–11, printed pages 3–4: Part 91 EFB use and assessments · AIM 1-1-17: portable/VFR GPS limitations · ACS II.B.K3/R4, printed page 7: EFB use

Plan for disrupted or misleading GPS

Jamming can prevent reception of usable signals. Spoofing can create misleading position or time information. An anomaly may cause alerts, a large position jump, unexpected track or groundspeed, a clock change, or less obvious erroneous information. WAAS and multiple GPS-fed displays do not make the aircraft immune. Maps, EFB ownship, ADS-B position, terrain systems, wind calculations and automation may share the affected input.

Before flight, check route-relevant GPS/WAAS NOTAMs and available conventional facilities, plan fuel and suitable landing options, and review manufacturer guidance. A RAIM prediction is not a forecast of local jamming. A second GPS receiver can improve hardware redundancy but still share exposure to the same interference; know the whole backup path.

In flight, maintain attitude and performance control, assess the message and dependent systems, and compare usable independent position information such as VOR/DME when available. Use the aircraft’s approved procedures for changing or removing suspect guidance. Do not keep an autopilot coupled to a path you have reason to distrust. Coordinate a suitable conventional route, vectors or landing option with ATC as needed, considering terrain, fuel and actual equipment capability.

The current AIM calls for prompt ATC notification of GPS anomalies, with a specific exception: when flying through known NOTAMed testing areas, do not report the known jamming/spoofing unless ATC assistance is needed. Actual in-flight navigation-equipment malfunctions during IFR in controlled airspace also fall under §91.187. Explain affected equipment, impaired capability and assistance needed as applicable. After flight, document the anomaly and use the FAA reporting channel according to current guidance.

AIM 1-2-4: jamming/spoofing effects, mitigations and reporting exception · AIM 1-1-3: VOR MON contingency resources · 14 CFR §91.187: equipment malfunction reporting

Brief the whole navigation chain

Study scenario: an approved WAAS navigator is installed, current data and charts are available, and the planned arrival includes a GPS-dependent segment. An applicable NOTAM identifies a possible GPS test area during the flight. You also have an EFB and an operable conventional navigation installation, with a suitable conventional route and approach available subject to the actual weather, fuel and clearance.

1. Before departure: verify supported operation and required service, NOTAM timing/area, database and route sequence. Identify the conventional option and fuel needed, and ensure its facilities and aircraft equipment are usable.
2. During a route amendment: load the cleared path, compare the resulting legs and restrictions, then activate and verify the correct source, sequencing and modes. Keep controlling the airplane between entries.
3. If integrity is lost: stop using guidance for an operation requiring the unavailable integrity. Apply installed reversion procedures, assess independent evidence and capability, and request a suitable change or assistance.
4. Debrief: identify which displays shared GPS and which resources actually preserved control, position knowledge and a landing option.

Repeat the exercise assuming the EFB also loses its position symbol. The downloaded chart may remain a useful reference while ownship is unusable. Repeat again with no conventional installation: legal WAAS capability does not manufacture a backup; the preflight go/no-go decision must account for the actual outage options.

AIM 1-1-17: operation and integrity requirements · AIM 1-1-18: WAAS capabilities and NOTAMs · AIM 1-2-4: outage planning and response · AC 91-78A, §§9–11: reference use and failure assessment

FLIGHT SCENARIO

What would change your plan?