Instrument Flying Handbook Menu >Navigation
Navigation (RNAV)>GPS Components
GPS consists of three distinct functional elements:
space, control, and user.
The space element consists of 24 Navstar satellites.
This group of satellites is called a constellation. The satellites
are in six orbital planes (with four in each plane) at about
11,000 miles above the Earth. At least five satellites are in
view at all times. The GPS constellation broadcasts a pseudo-random
code timing signal and data message that the aircraft equipment
processes to obtain satellite position and status data. By knowing
the precise location of each satellite and precisely matching
timing with the atomic clocks on the satellites, the aircraft
receiver/processor can accurately measure the time each signal
takes to arrive at the receiver and, therefore, determine aircraft
The control element consists of a network of
ground-based GPS monitoring and control stations that ensure
the accuracy of satellite positions and their clocks. In its
present form, it has five monitoring stations, three ground
antennas, and a master control station.
The user element consists of antennas and receiver/processors
on board the aircraft that provide positioning, velocity, and
precise timing to the user. GPS equipment used while operating
under IFR must meet the standards set forth in Technical Standard
Order (TSO) C-129 (or equivalent); meet the airworthiness installation
requirements; be “approved” for that type of IFR
operation; and be operated in accordance with the applicable
POH/AFM or flight manual supplement.
An updatable GPS database that supports the
appropriate operations (e.g., en route, terminal, and instrument
approaches), is required when operating under IFR. The aircraft
GPS navigation database contains waypoints from the geographic
areas where GPS navigation has been approved for IFR operations.
The pilot selects the desired waypoints from the database and
may add user-defined waypoints for the flight.
Equipment approved in accordance with TSO C-115a,
visual flight rules (VFR), and hand-held GPS systems do not
meet the requirements of TSO C-129 and are not authorized for
IFR navigation, instrument approaches, or as a principal instrument
flight reference. During IFR operations, these units (TSO C-115a)
may only be considered as an aid to situational awareness.