AIM 

4/20/23 

self-correct. Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi. 

Pilots are encouraged to follow the manufacturer’s or other appropriate procedures to correct possible heading 

misalignment before take off is commenced. 

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16.  Doppler Radar 

Doppler Radar is a semiautomatic self

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contained dead reckoning navigation system (radar sensor plus 

computer) which is not continuously dependent on information derived from ground based or external aids. The 

system employs radar signals to detect and measure ground speed and drift angle, using the aircraft compass 

system as its directional reference. Doppler is less accurate than INS, however, and the use of an external 

reference is required for periodic updates if acceptable position accuracy is to be achieved on long range flights. 

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17.  Global Positioning System (GPS) 

a.  System Overview 

1. 

System Description. The Global Positioning System is a space-based radio navigation system used to 

determine precise position anywhere in the world. The 24 satellite constellation is designed to ensure at least five 

satellites are always visible to a user worldwide. A minimum of four satellites is necessary for receivers to 

establish an accurate three

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dimensional position. The receiver uses data from satellites above the mask angle 

(the lowest angle above the horizon at which a receiver can use a satellite). The Department of Defense (DoD) 

is responsible for operating the GPS satellite constellation and monitors the GPS satellites to ensure proper 

operation. Each satellite’s orbital parameters (ephemeris data) are sent to each satellite for broadcast as part of 

the data message embedded in the GPS signal. The GPS coordinate system is the Cartesian earth

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centered, 

earth

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fixed coordinates as specified in the World Geodetic System 1984 (WGS

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84). 

2. 

System Availability and Reliability. 

(a) 

The status of GPS satellites is broadcast as part of the data message transmitted by the GPS satellites. 

GPS status information is also available by means of the U.S. Coast Guard navigation information service: (703) 

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5907, Internet: http://www.navcen.uscg.gov/. Additionally, satellite status is available through the Notice 

to Air Missions (NOTAM) system. 

(b) 

GNSS operational status depends on the type of equipment being used. For GPS

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only equipment 

TSO

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C129 or TSO-C196(), the operational status of non

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precision approach capability for flight planning 

purposes is provided through a prediction program that is embedded in the receiver or provided separately. 

3. 

Receiver Autonomous Integrity Monitoring (RAIM). RAIM is the capability of a GPS receiver to 

perform integrity monitoring on itself by ensuring available satellite signals meet the integrity requirements for 

a given phase of flight. Without RAIM, the pilot has no assurance of the GPS position integrity. RAIM provides 

immediate feedback to the pilot. This fault detection is critical for performance-based navigation (PBN)(see 

paragraph 1

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1, Performance

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Based Navigation (PBN) and Area Navigation (RNAV), for an introduction to 

PBN), because delays of up to two hours can occur before an erroneous satellite transmission is detected and 

corrected by the satellite control segment. 

(a) 

In order for RAIM to determine if a satellite is providing corrupted information, at least one satellite, 

in addition to those required for navigation, must be in view for the receiver to perform the RAIM function. 

RAIM requires a minimum of 5 satellites, or 4 satellites and barometric altimeter input (baro

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aiding), to detect 

an integrity anomaly. Baro

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aiding is a method of augmenting the GPS integrity solution by using a non-satellite 

input source in lieu of the fifth satellite. Some GPS receivers also have a RAIM capability, called fault detection 

and exclusion (FDE), that excludes a failed satellite from the position solution; GPS receivers capable of FDE 

require 6 satellites or 5 satellites with baro

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aiding. This allows the GPS receiver to isolate the corrupt satellite 

signal, remove it from the position solution, and still provide an integrity-assured position. To ensure that 

baro

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aiding is available, enter the current altimeter setting into the receiver as described in the operating manual. 

Do not use the GPS derived altitude due to the large GPS vertical errors that will make the integrity monitoring 

function invalid. 

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Navigation Aids