AS 18 - AHRS Doppler Flashcards
Review Questions
What happens when the GPS is down to 3 satellites?
2 satellites?
GPS fails?
Lat/Long still valid
Altitude info no longer valid
Lat/Long not valid
GPS not usable, revert to degraded ops
GPS position becomes erratic and varies randomly with large errors
NTS time still operates
What is the purpose of the Standby Compass? Where is it located?
Provides a back-up heading reference independent of AHRS
Located on windshield center post.
What are the AHRS components?
Remote compass transmitter - flux valve
Attitude heading reference unit - AHRU
AHRS control panel
What is the Remote Compass Transmitter and how is it situated on the aircraft?
A sensing element (flux valve) pendulously suspended on a universal joint within the bowl
Damping fluid is added to prevent excessive swinging
Why are the Remote Compasses located on each wing tip?
To achieve the greatest distance from magnetic disturbances from the AC
Describe the construction of the Flux Valve
One primary exciter coil. three secondary pick-off coils, surrounded by three collector horns
(resembles a three spoke wheel)
What does the flux valve do?
Detects the magnitude and direction of the earth’s magnetic field (H component)
Converts this to electrical information
The info is then used to align the Directional Gyro in the AHRU.
The magnitude of the induced current in the pickoff coils in the flux valve depends on what?
Varies according to the number of magnetic lines cutting them
Where is the AHRU located?
Under the floor, aft the air-stair door
What data can be input into the AHRU?
TAS, from the appropriate ADC via the ADC data bus
Magnetic heading information, from the flux valve via the AHRS Control Panel
What systems receive output data from the AHRS?
Flight instruments Flight guidance computers Air data computers Advisory displays FMS
Which flight instruments receive output data from the AHRS?
EFIS
ADIs & HSIs
Pilot/ Co-pilot RMI
True or false
The RMI heading is simulating an AHRS provided mag hdg.
True
Define Doppler
Frequency shift observed due to the relative motion of the source and/or receiver
True or false
The motion of the observer produces a real shift in frequency while the motion of the source produces only an apparent shift in frequency.
False, vice versa
When is the doppler shift at its maximum? When is it at zero?
Maximum when the angle between transmitter and receiver = 0 degrees
Zero when the angle = 90 degrees
What are the four factors that determine the amount of doppler shift?
Frequency of transmitted signal
Relative velocity between tx and receiver
Speed of propagation of the tx signal
Angle between direction of movement and line of propagation from TX to RX
Describe the 4 Beam Janus
3 or 4 beams on an aircraft are used to compare the different front and rear ground speed and drift readings during climb or descent.
How does the 4 Beam Janus cancel out errors?
The front and rear beams will have opposite errors during climb or descent and cancel each other out.
How does the 4 Beam Janus calculate ground speed?
It compares the average shift of the forward and backward beams
Why is it not necessary to precisely control the transmitter frequency for the 4 Beam Janus? What does this allow?
The system uses the average Tx frequency.
Allows for more tolerance and less expense of the Tx
What are the main components of the Doppler?
Receiver / Transmitter and Antenna
Signal Data Converter (SDC)
Emulated Control Display Unit (CDU)
Air train System Computer (ATSC)
Describe the function of the Tx/ Rx & antenna of the Doppler system
It is a fixed mounted system.
The planar array antenna receiver mixes the reflected RF signal and feeds the output to the SDC for further processing
What are the functions of the SDC?
Determines the Doppler Shift of received signals
Processes the signals and computes velocity
Converts velocity data into drift angle and ground speed
Conducts signal to noise checks
Performs system validity checks and sends the status signal to the ATSC
What velocity computations are performed by the SDC?
Along heading velocity (Vx); Across heading velocity (Vy); vertical velocity (Vz)
What does the Air Training System Computer generate for the
CDU?
The software for the Emulated Control Display Unit (CDU)
What are the inputs for the Doppler system?
Antenna system radiates transmitter-generated RF signals
Receiver mixes reflected signal with transmitted frequency, passes the freq difference to the SDC
SDC converts the mixed signal into drift angle and ground speed. Sends it to the ATSC database
SDC also performs Doppler system validity checks and sends status to the ATSC
What are the outputs for the Doppler system?
System info from ATSC to all emulated CDUs
Drift and groundspeed displayed in the Doppler area of DID
Mode control signals from CDU to ATSC, which sends them to Doppler hardware via SDC
What are the drift and groundspeed limits for the Doppler system>
Drift: 20° maximum on flag display
G/S: 25 to 650 kts (666 kts during self test)
What are the power requirements for the Doppler system?
28V DC
115V AC
26V AC @ 400 Hz
Review Questions
List the major components of the AHRS
Remote Compass Transmitter - Flux Valve
AHRS control panel
AHRU
Review Questions
What is the function of the Flux Valve?
Detects orientation of the aircraft in the earth’s magnetic field (H component)
Review Questions
List the outputs of the AHRS.
Flight instruments: EFIS (ADIs & HSIs), pilot and co-pilot RMIs
Flight guidance computers
Air data computers
Advisory displays
Review Questions
What does the memory light on the Doppler indicate?
A loss of valid returns on two or more beams
