Doppler navigation 3 : Airborne equipement.

 Airborne equipement.

1. Doppler navigation system uses directional beams to derive ground speed and drift as previously described this beams are arranged in a number of ways. 

2.The fore and aft beam are referred to as 'Janus' configuration (after the roman god of openings  And beginning, Janus, we could face in two directions at the same time).  

3.Three beams can be arranged in the form of greek letter lambda. 

4.The four beam arrangement in an X-configuration, only three beams are actually required,the  fourth provides a level of monitoring and redundancy. 

5.In the four beam arrangement, the fore and aft signals are transmitted in alternative pairs.

6. In the four beam arrangement, the fore and aft signals transmitted in alternative pairs.

Referring to the relationship:

               FD=2cosѲ x vf / c

FD: frequency diffrence

Ѳ: the angle between the beam and aircraft 

V=aircraft velocity

f=frequency of transmission.

C=speed of light

7. Frquency allocated is 13.25-13.4. Ghz. Some doppler system operate within 8.75-8.85ghz.

8.The doppler navigation system has been superseded for commercial airliner used by inertial and satellite navigation system. Rotary wing aircraft, however, use doppler sensors to provide automatic approach and stabilization during over manoeuvers, in this case the display would provide vertical displacement above /below the selected for altitude and lateral /longitudinal deviation from the selected for position.

 Advantages:

1.velocity and position outputs from the system are provided on a continuous basis.

 2.it requires no ground navigation aids. i.e.dead reckoning.

3. velocity output are very accurate navigation is possible over any part of the globe, including ocean and polar regions.

4. the system is largely unaffected by weather although certain rainfall conditions can affect the return .

5.The system doesn't require any preflight alignments.

Disdvantages:

1. It is dependent upon a directional  reference,  e.g. gyro- magnetic compass.

2. it requires a vertical reference to compensate for aircraft attitude.

3.Short term of velocity calculations can be inaccurate when flying over the tidal waters, the calculated aircraft velocity will be aired depending on the tides direction and speed .