Refining the usefulness and safety of satellite navigation

More accurate GPS flightpaths, curved RNP approaches, lower minimums—all coming soon to an airport near you.

WAAS cover. This map predicts WAAS coverage for LNAV, VNAV and LPV and gives color-coded vertical protection levels in meters. This is basically a diagram of WAAS service volume.


Receiver autonomous integrity monitoring (RAIM) is part of all IFR GPS navigators. You need at least 5 GPS satellites or their equivalent in reception to permit RAIM to do a comparison check on individual satellite integrity and accuracy.

If you are not receiving at least 5 satellites or equivalent, RAIM will deny a navigation signal after a buffer period. Most RAIM systems will note a degraded satellite signal but do not exclude this from the navigation solution—hence the possibility of up to 300 ft lateral and 460 ft vertical error.

If a GPS satellite drops offline, predictive RAIM will estimate (or "predict") whether you will have sufficient satellites for minimum coverage in the enroute, terminal or nonprecision approach phases.


Wide area augmentation system (WAAS) was developed from 1994 onwards, first commissioned in 2003, and fully commissioned in 2008 as a means to increase GPS accuracy to a Cat I ILS minima level.

WAAS works by retransmitting position-corrected GPS data from accurately surveyed ground receivers back to 2 communication satellites which then transmit to an airplane's onboard GPS receiver. WAAS increases accuracy enough to permit approaches with GPS vertical guidance and minima to as low as a 200 ft DA and 1/2 sm visibility.

The WAAS signal is received by your GPS navigator on a separate frequency and from separate communications satellites from the GPS satellites. This separate path of recep­tion makes a WAAS correction signal a satellite equivalent for certain aspects of navigation.

In addition, WAAS enabled navigators have a fault detection and exclusion (FDE) circuit. FDE will detect a faulty satellite and exclude this line of position from your navigation solution in real time. The WAAS corrected signal plus FDE permits greater accuracy and reliability of WAAS enhanced navigation.

Can you hear me?

Approach plate for RNAV (GPS) Rwy 36 at TPF (Peter O'Knight, Tampa FL).

Civil GPS navigators received GPS satellite signals on frequency L1C (1575.42 MHz). Recognizing that this is a potential single point of failure for civil navigation, a 2nd frequency—L2C (1227.60 MHz)—has been allocated.

This 2nd easy-to-track signal will improve accuracy of navigation and act as a redundant signal in case of local interference with the L1C signal. GPS satellites in orbit today are transmitting on both L1C and L2C—however, your GPS navigator must be configured to receive the L2C signal.

A numbers game

To obtain a 3D position solution, at least 4 measurements (GPS satellites or equivalent) are required. To detect a fault (RAIM), at least 5 measurements are required. And to isolate and exclude a fault, at least 6 measurements are required (RAIM–FDE).

More than the minimum 6 measurements are needed if satellite geometry relative to your aircraft is poor, such as when satellites are low on the horizon. Typically there are 6–8 satellites in view in the lower 48 states. IFR navigation in the NAS requires a minimum of 5 satellites or equivalent. "Equivalent" here means a WAAS signal, or a calibrated pressure value for vertical positioning, referred to as "baro-aiding."

GPS approaches

The first published GPS approaches were exact overlays of existing nonprecision VOR or NDB approaches. This allowed FAA's Flight Procedures branch to get GPS in the field quickly and collect data on how well it all worked. Shortly thereafter, Flight Procedures created the stand-alone nonprecision GPS approach.

In this early era, conventional ground-based navaids were required for IFR flight. Today, when an alternate is required, an other-than-GPS approach must be available at the alternate to be legal, unless your navigator is WAAS enabled.

Prior to 1998, the concept of GPS as part of the RNAV family of navaids was not in place—thus, GPS approaches did not reflect this. Some FAA AeroNav format approach plates still use the older format.

The concept of GPS as part of the RNAV family and the concept of RNP to describe navigation performance both came about in 1998 from the Intl Civil Aviation Organization (ICAO). This ICAO concept of RNP and the commissioning of WAAS in 2003 allowed for a family of GPS approaches.

In 2000, the FAA Flight Procedures office revised the format for GPS approach plates and revised the terminology for approach categories to LPV, LNAV and LNAV/ VNAV, and circling.

The first laterally guided nonprecision approach minima were simply termed straight-in or circling, such as "S-15" for a straight-in GPS approach to Runway 15. After the formatting change in 2000, FAA began using the term lateral navigation (LNAV) for straight-in nonprecision laterally guided GPS approaches.

With the implementation of WAAS in 2003, vertical guidance was added, with the lateral and vertical navigation approach (LNAV/VNAV) and the localizer performance with vertical guidance (LPV) approach.

The key difference between LNAV/ VNAV and LPV is that LPV permits GPS-only navigation, while an LNAV /VNAV approach may use approved terminal navigation signals from other sources, such as baro-aiding. Some IFR GPS navigators will display an LNAV+V mode.

This is a nonprecision LNAV approach with advisory-only descent guidance. The LNAV+V must be flown to the appropriate LNAV minima for your aircraft—the vertical guidance simply makes for a smoother descent to MDA.

The first LPV approaches had artificially high visibility limits as a precaution. Once FAA was sure that WAAS enabled vertically guided approaches worked as advertised, these limits were revised to the allowable lower limits.

A further development is the localizer performance (LP) approach. This is a laterally-guided-only approach that requires WAAS. The advantage of the LP approach is that the final segment may be offset from the runway heading while still using the smaller protected airspace footprint of a WAAS enabled navigator and thus lower minima.

The LP approach is best suited for airports in highly congested airspace.


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