LOW RANGE RADIO ALTIMETER (LRRA)

The low range radio altimeter (LRRA) is a landing radio altimeter that measures the distance from the aircraft to the ground during approach and landing phases of your flights, starting from 0 to 2,500 feet.

Basic Principle: Radio Height

Radio height is measured using the basic idea of radio ranging, i.e. measuring the elapsed time between transmission of an EM wave and its reception after reflection from the ground.

The height of the aircraft is determined by calculating the difference between the frequency of the reflected signal and the frequency of the reflected signal, termed as difference frequency, which is directly proportional to the time required for the transmission and reflection. This difference frequency is converted to distance by a microprocessor which then transmits the data to the equipment using ARINC 429 bus.

• Frequency Range: 4.2 – 4.4 GHz

• Modulation: FMCW (Frequency Modulated Continuous Wave)

  
  

The main difference between LRRA and other altimeters is that other altimeters shows height with respect to Mean Sea Level which might change invariably whereas LRRA shows height of the aircraft with respect to the reflected radio signals from the immediate ground below the aircraft using radio ranging.

Therefore, should a matter arise where both the altimeters are showing different values, the LRRA reading should be prioritized over the other altimeter readings.

The system has 3 transmitters and 3 receivers, whose antennas are interchangeable.

• Left TX & RX (1+1)

• Centre TX & RX (1+1)

• Right TX & RX (1+1)

If the transmitter is fitted at the receiver position, it will act as a receiver and vice versa, but keeping in mind the direction of the antenna.

It receives discrete inputs from the air ground relays and from the AIMS cabinet (B777) and sends data to TCAS, GPWC, AFDC, IMS.

Aircraft Installation Delay

An amazing fact about LRRA is its Line Replaceable Unit (LRU) which is placed in the Forward Cargo due to critical wire length and aircraft installation delay.

Aircraft Installation Delay (AID) is the elapsed time between the transmission and reception of the signals when the aircraft is at touchdown.

The main purpose of combining both the critical wire length and AID is to keep the impedance of the LRRA antenna and the wire equal, i.e. impedance difference should be zero.

When the LRRA is located forward of Main landing gear, due to pitch up condition of the aircraft on touchdown (flare of 3 degrees) the LRRA indication on the PFD will display a value in negative whereas when it is located aft of main landing gear, the indication will show a value as positive.

Maintenance

• Precautions:

  • There should be no paint on the radiation part of the antenna as it blocks the signals

  • The direction of the antenna should be kept in mind during installation or interchanging antennas.

  • No metallic object should be nearby during ground testing.

• The test stops if the the aircraft is moving at a speed of more than 40 knots or if the aircraft is in air.

• The XCVR stores internal faults in a non volatile memory segregated by flight segments

• Via the Central Maintenance Computer (CMC), under the ATA chapter 34 – Ground test, we can do a operational test of the left, center and right LRRA systems and their coaxial cables.