The Radio Altimeter is a radio wave based altimeter that calculates the height of the a/c when it is flying over a surface, or sea or a plain. It is a very essential aid for the aircraft on an approach and landing operation. It has two antennas, when in operation one antenna transmitter sends a continuous wave radio beam from an antenna that is located below the a/c, and the beams on hitting the ground beneath are returned and received on another antenna. The time taken by the beam to return is calculated with reference to time to calculate the height of the a/c.
The received returned signal is made to drive a motor that drives the radio altimeter indicator, which is located in the cockpit.
The circuit is supervised by a monitor circuit, and anytime it senses that the incoming signal is unreliable a warning flag is displayed on the Radio Altimeter indicator.
A system self test – button is also provided in the indicator, on pressing which a system test is carried out, checking all the electric monitor test circuit operation, and a satisfactory working of all the equipments is made by illumination of lights on a light panel, comprising different types of fault lights switches called Sensitivity light panel.
Frequency – 4200 to 4400 MHz, Frequency Modulation, and operation is on Continuous Wave, Primary Radar.
Height Indication – up to 2500 feet.
Collision Warning- 45 seconds before collision when a/c is descending @800 feet per minute.
Errors –
- Fixed Error- The indicator pointer moves in steps of 5 feet, which means an error of 2.5 feet is present any time the system is operational, which is very marginal and can be very easily ignored.
- Positive Height Error- This occurs when a/c is parked on the ground and the Radio Altimeter will still show some altitude, depending on the height set at DH bug at which it is made to read 0 feet. The height may vary depending on the installation of the antenna unit. This occurs because the Radio Altimeter is set to read 0 feet at Decision Height.
- Uneven Terrain Error- This error occurs because of uneven terrain beneath the a/c, it can cause Radio Altimeter to show erroneous reading instantly for a while, it is most prominent over high structural are followed by plain surface, and high buildings.
- Coastal Refraction Error- It is observed that the Radio wave traveling over rarer medium such as water, over river or sea reflects signal accurately than on any other surface. A dry surface or disturbed soil will cause erroneous reading, or in other words cause a low altitude reading than the actual height of the a/c.
Future developments- the newest equipments have replaced the previous equipments, and are very lightweight, since the mechanical needles and gears have been replaced by the microprocessors and integrated circuits and are also smaller in size.
The previous equipments required a different Radio Altimeter to be installed for the Radio altitude information; the newer modern equipments can directly feed this information to the (EFIS) Electronic Flight Instrument System.
These equipments are capable of providing direct information to Electronic Attitude Director Indicator as analogue and digital format.
Phantom RA’s are false RA’s (in TCAS).
Phantom RA is also known as PARROTS in TCAS.