12.42.4. Describe the limitations and failure indications of a GNSS system.

– smaller GPS units can have reduced performance by the loss of signals when the view to the satellites is shielded by parts of the aircraft structure
– may have limited channels
– apparent error can be caused by trying to locate positions on a “non WGS 84 ” map

12.42.2. Describe the basic principles and operation of a GNSS (Global NavigationSatellite System). 

The owner and controlling authority of the GPS system is the US Department of Defence
– consists of a space segmnt of at least 24 solar powered satellites orbiting the earth at 20,200km altitude
– 4 satellites orbit in each of 6 circular orbital planes
– there are 3 satlellites as operational spares
– each of the 6 orbital planes is inclined at an angle of 55 degrees to the equator and each satellite takes approx 12 hours to complete an orbit
– the orbital planes are fixed in space – earth rotates at its normal rate within the constellation
– each satellite sends a signal wich contains a navigational message and a unique coded signal – repeats itself every millisecond

12.44. TCAS

12.44.2. Briefly describe the basic function and operation of TCAS.

12.40. Gyroscopic Instruments

12.40.16. Describe the indications of toppling.

12.40.14(b) . there is an indication of a power failure on an electrically-driven gyro. 

– the instrument may respond only slowly to changes in attitude or heading or indicate incorrectly
– loss of power could mean loss of the instrument
– toppling can cause insturments to become unusable

12.40.14(a) . the gyro rotor rpm is low; 

– the instrument may respond only slowly to changes in attitude or heading or indicate incorrectly
– loss of power could mean loss of the instrument
– toppling can cause insturments to become unusable

12.40.14. Briefly explain the errors likely to occur if:

12.40.12(c) . pilot checks for serviceability. 

Check power source of the HI prior to flight and when taxiing check the correct turn indications