Frequently Asked Questions


What are GPS Primary Means of Flight outages?

Those are the conditions when the GPS constellation may not be able to support Primary Means flight operations as specified in FAA N8110.60. Although such outages are rare events, aviation GPS users should note that this can happen, particularly if GPS navigation is heavily relied on. During the outage, aviation GPS receivers may not be able to provide positional information with integrity. Integrity requires that at least 5 satellites be in-view so that a cross-check of data can be performed. Note that the GPS constellation is in constant motion with respect to the earth and the outage condition eventually clears itself.

Why is Integrity important?

A GPS receiver needs 4 satellites to produce a position fix. If there is a measurement error, or if a GPS satellite produces faulty data, or if the constellation geometry is poor, the position fix error can be very large, and may be large enough to put the pilot, his aircraft, his cargo, and his passengers at risk. All aviation GPS receivers are required to qualify the position fix and estimate the position fix error. To do that, special software was developed, often referred to as Fault Detection and Exclusion (FDE) algorithms. These FDE algorithms qualify the position fix with an Integrity number. Without integrity, the GPS position fix should not be relied upon in life-and-death applications.

How does Fault Detection and Exclusion (FDE) software work?

All FDE software works by essentially performing a cross-check of the raw GPS data. To do a basic cross-check, 5 GPS satellites need to be processed. Differences in the satellite data are used to determine the error bounds of the navigation solution as well as provide an indication of satellite data integrity. To determine which of the data may be faulty, 6 GPS satellites need to be processed. Once identified, the faulty satellite can be removed from the position fix solution, eliminating the error. Unfortunately, there are instances in time and place where less than 6 GPS are visible. These are called "outages".

Why is baro-altimeter augmentation important?

The baro-altimeter improves overall GPS availability and continuity of operation. All CMC aviation GPS receivers certified to TSO-C129a B1/C1 automatically calibrate the pressure altimeter and use the pressure altimeter during periods of GPS outages, extending FDE coverage that would normally become unavailable during a GPS constellation outage. This permits extended use of GPS for all phases of flight during these adverse conditions. These receivers meet or exceed the requirements in DO-229C concerning the use of pressure altimeters.