Doppler is a change in the frequency that takes place due to the motion between Transmitter and Receiver.
If the distance between the Transmitter and Receiver decreases, the Received frequency is more than the Transmitting frequency and vice versa.
Doppler Effect – Wherever there is a motion between the Transmitter and the Receiver, the received frequency is different from the Transmitted one.
If distance decreases Received frequency is more than Transmitted frequency & vice versa.
If angle of inclination is very high then Doppler shift will be low.
If angle of inclination is very low then Doppler shift will be high or the Transmitted wave may be reflected in some other direction.
This difference between the transmitting and receiving frequency is called Doppler. At 90° angle Doppler shift is 0.
Frequency- 8800 to 13,500 MHz. (SHF).
Principle- Doppler Shift.
For Drift calculations- 2 Aerials are placed at an equal angle from the longitudinal axis, one towards left side of the A/c and another towards right side of the A/c.
These 2 Aerials are rotated by a motor, so that both Aerials are aligned with the track and Ground Speed can be found correctly, also the angle by which these aerials rotate gives the Drift.
Ø » ½ the angle between 2 Aerials.
This Ø is affected by the Pitch attitude of the A/c, therefore to overcome this error, 4 Aerials are used, 2 located at the front and 2 located at the rear of the A/c.
These 2 Aerials Transmit ahead of the A/c and 2 Transmit behind the A/c. These Aerials are called Janus Aerial System located in an arrangement of
The Transmission of no. 1 and no. 3 Aerial are combined as well as the Transmission of no. 2 and no. 4 Aerial are combined and in a climb, the angle by which the front aerials move up, by the same angle the rear aerials move down giving accurate pitch indications.
Doppler is used by the A/c to calculate its Ground Speed and Drift.
It has 4 aerials located on the a/c, 2 Aerials are located ahead of the a/c and 2 Aerials are located behind of the A/c. these Aerials are called Janus Aerial system.
Limitations-
- Calm Sea- causes Wave to not sufficiently refract back to provide strong echoes to return to the Transmitter; therefore a minimum of 5 knots of wind velocity is required.
- Flying Low- causes Doppler shift to fall low, and totally miss the target, causing erroneous reading.
- Thunderstorm Error- causes Radar Wave to be deflected.
- Pitch & Roll Error- 20° Pitch & 30° Roll limit.
- Height Hole Effect- Radar wave = Pulse Recurrence Period
Errors-
- Latitude Error- N.M changes as Latitude changes.
Doppler Computer has constant value of 6080 feet.
- Aerial Misalignment Error- Aerial not aligned with Longitudinal Axis.
- Pitch Error- 4 Aerials Corrects this error. Janus Aerial System (2 Ahead & 2 Behind the A/c)
- Sea Movement- Due Sea Waves, Maximum when flying along Coastline
- Sea Bias Error- Radar Wave penetrates rarer medium such as water. When A/c is flying over Sea the Radar wave penetrates Sea and does not get reflected, therefore Ground Speed under reads.
Remedy- Land/Sea- Sea
When A/c is flying over Land with Sea selection on Land/Sea switch the Ground Speed Over reads.