Most navigation today is a quiet conversation between a receiver in the panel and a constellation of satellites, and the rules for trusting that conversation go by a family of abbreviations: GNSS, RNAV, RNP, RAIM, SBAS, LPV. They fit together more logically than the alphabet soup suggests, and once you see the structure, an RNP approach chart reads like a menu rather than a puzzle.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
From beacons to any path at all
Conventional navigation tied aircraft to the ground: you flew beacon to beacon, because a VOR or NDB could only tell you where you were relative to itself. Area navigation (RNAV) removed that constraint. An area navigation system computes the aircraft's position continuously, from GNSS above all, and steers toward any waypoint, a point defined by coordinates in a database rather than by a transmitter on the ground. Routes, arrivals and approaches become sequences of waypoints, which is why they can run straight, save track miles, and serve runways no beacon ever pointed at.
That flexibility raised a fair question for the people who design procedures: how accurately can a given aircraft actually fly the path? The answer became performance-based navigation (PBN), set out in ICAO Doc 9613: instead of mandating a particular radio, a procedure states the navigation performance required to fly it, and any aircraft demonstrating that performance qualifies.
RNAV versus RNP: the alerting difference
PBN specifications come in two families, and the naming is mercifully literal. The number is the lateral accuracy in nautical miles the aircraft must hold for at least 95 per cent of the flight time: RNAV 10 for oceanic legs, RNAV 5 for continental enroute airspace, RNAV 1 and RNP 1 for SIDs and STARs, RNP APCH for approaches.
The word in front of the number is the real distinction. An RNP specification requires on-board performance monitoring and alerting: the equipment itself must watch the quality of its navigation and warn the crew when the required performance can no longer be assured. An RNAV specification carries no such on-board alerting requirement. That is the entire difference, and it explains where each family is used: where a procedure threads terrain or traffic tightly enough that a silent navigation failure would be dangerous, the aircraft itself must raise the alarm, so the procedure is RNP. The most demanding flavour, RNP AR APCH (authorisation required), curves approaches through terrain with containment measured in fractions of a mile and needs specific aircraft and crew approval.
How GNSS actually fixes you
GNSS is the umbrella term for satellite navigation constellations: the American GPS, Europe's Galileo, Russia's GLONASS and China's BeiDou. Each satellite broadcasts its position and an extremely precise time signal; the receiver measures how long each signal took to arrive, converting time into distance. Ranges from four satellites pin down latitude, longitude, altitude and the receiver's own clock error, which is why four is the magic number for a three-dimensional fix, out of a nominal constellation of 24 or more satellites per system, per the FAA AIM.
A fix is not the same as a fix you can trust, and aviation's obsession is the second one. RAIM, receiver autonomous integrity monitoring, is the receiver checking its own homework: with a fifth satellite it has enough redundancy to detect that one measurement disagrees with the rest, and with a sixth it can work out which satellite is lying and exclude it (fault detection and exclusion, FDE).
There is a shortcut worth knowing, because it is why RAIM is available more often than the raw satellite count suggests. The FAA AIM allows the altitude input from the aircraft's barometric altimeter to stand in for one satellite, a technique called baro-aiding: with baro-aiding a receiver needs only four satellites plus barometric altitude to detect a fault, and five plus barometric altitude to exclude one. So the numbers come in pairs, five or six without baro-aiding, four or five with it, and the pair that applies to you depends on your installation.
When geometry is poor, RAIM may be unavailable regardless, which is why flights relying on unaugmented GPS for an approach check a RAIM prediction for the destination and arrival time, and why GNSS NOTAMs, covered in our NOTAM guide, deserve a look on every briefing.
SBAS, satellite-based augmentation, improves both accuracy and integrity from outside the receiver: a network of ground stations measures GNSS errors and broadcasts corrections and health warnings via geostationary satellites. The American implementation is WAAS, the European one EGNOS. SBAS is what turns satellite guidance into something precise enough to fly an approach with vertical guidance to low minima.
Unlike a VOR needle's angular spread, GPS course guidance is linear and auto-scaling, tightening in stages as you approach the field: 1 NM full-scale either side of course in the terminal area, and 0.3 NM on final approach. The needle sharpening as you near the airport is the system doing its job; being surprised by it is not.
The enroute figure is the one to be careful with, because it is not a single number and a great deal of published material prints it as though it were. For the basic, unaugmented receivers the FAA AIM describes, certified to TSO-C129 or TSO-C196, enroute full-scale deflection is plus or minus 5 NM. SBAS-capable receivers, certified to TSO-C145 or TSO-C146, scale more tightly than that enroute, and the figure often quoted for them is 2 NM, but it is set by the equipment rather than by a figure the AIM publishes for all of them. The practical rule is therefore not to carry a remembered number into the cockpit at all: read the enroute scaling from your own receiver's documentation and its annunciations, and treat the terminal and approach values as the ones that behave predictably across the board.
Reading the minima lines on an RNP approach
An RNP approach chart, titled RNAV (GPS) on US charts and RNP on ICAO-convention charts, usually stacks several minima lines for the same ground track, and the line you may use depends on equipment:
- LNAV: lateral guidance only, flown as a non-precision approach to an MDA.
- LP: SBAS-sharpened lateral guidance, still without vertical, also to an MDA; published where terrain or obstacles rule out vertical guidance.
- LNAV/VNAV: lateral plus approved vertical guidance, from baro-VNAV or SBAS, flown to a DA.
- LPV: localizer performance with vertical guidance, the SBAS flagship. The guidance becomes angular, converging like an ILS beam, and the DA can be as low as 200 ft at suitably surveyed runways, comparable to a Category I ILS.
The discipline is to brief the line your equipment currently supports: a receiver that loses SBAS integrity on final may downgrade the service, and the difference between a DA and an MDA changes how the whole final segment is flown.
The database is part of the system
Every waypoint in an RNP procedure lives in the aircraft's navigation database, revised on the 28-day AIRAC cycle described in our AIP and AIRAC guide. The procedure must be loaded from the database by name, then compared against the chart: waypoint sequence, tracks, distances, altitudes. Building an approach by hand from raw coordinates is prohibited in this world for a simple reason: the containment that RNP promises assumes the coded procedure, complete with its turn geometry, is exactly what the designer published. An out-of-date database is the quiet failure mode here, the same staleness problem that afflicts every other kind of aeronautical data.
A worked example
You brief an RNP approach with LNAV, LNAV/VNAV and LPV minima. Your aircraft is SBAS-equipped, the database is in cycle, and the GNSS NOTAMs are clean, so you plan the LPV line with its DA of 250 ft. Loading the procedure by name, you check the waypoints against the chart and they agree. On the arrival the CDI tightens from enroute to terminal scaling on schedule. On final the receiver annunciates LPV, the guidance turns angular, and you fly the glidepath down to the DA, runway in sight, land. Had the SBAS service degraded on the way in, the plan was already briefed: revert to the LNAV line, treat it as a non-precision approach to the MDA, and fly the stabilised, continuous-descent profile rather than diving and driving.
Common pitfalls
- Flying a procedure the database does not hold, or holding an expired database. The containment assumes the coded procedure; currency is not bureaucracy.
- Skipping the RAIM or augmentation check. Unaugmented GPS approaches stand on RAIM availability at your arrival time.
- Briefing minima your equipment cannot support today. LPV needs SBAS integrity now, not just a capable box.
- Being surprised by CDI auto-scaling. A centred needle on final represents a far tighter tolerance than the same needle enroute.
- Quoting a single enroute CDI full-scale figure. It depends on the receiver class: plus or minus 5 NM for the TSO-C129 and TSO-C196 boxes the FAA AIM covers, tighter for SBAS-class receivers. Take it from your own equipment.
- Assuming RNAV and RNP are interchangeable. Only RNP guarantees the equipment will tell you when performance is lost.
In Pilot EFB
Pilot EFB is a study and planning companion, not a navigation system: it does not compute GNSS positions, provide course guidance, or substitute for your aircraft's approved navigators and current database. Where it helps is in understanding and preparation: this guide sits alongside the instrument approach chart, non-precision approach and AIRAC explainers in Learn, and the app keeps your own briefing notes and records organised and readable offline. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and fly from your certified equipment and official sources.