What is a drone radar altimeter?
A drone radar altimeter is an electronic device attached under an aircraft with the intention of measuring the altitude above the terrain presently under that aircraft. The device sends a radio wave to the ground which reflects back to it. The time taken by the wave to reach the altimeter is computed to calculate the distance from the ground to the aircraft. But the barometric altimeter’s point of reference is the sea level.
Components of the radio altimeter
- Transmission antenna
- Receiving antenna
- Central Processing Unit
The indicator has a needle, a height selection knob, reference height, and an altitude warning light. The height scale in the indicator is expressed in terms of hundreds of feet.
Principle of operation
The transmitter fitted on the altimeter sends a frequency modulated signal that varies frequency within a given frequency range over a given time. In its primitive stage, an LC tank was used with the employment of a tuning capacitor controlled by an electric motor. The final output is blended with the radio frequency carrier signal and sent out through the antenna.
Between the time taken by the signal to reach the ground and come back to the receiver, the frequency of the received signal differs from the transmitted signal. It’s the frequency mixer’s job to determine the difference between the two frequencies in order to find the altitude. The output value is measured in cycles per second.
Radar altimeters function mostly in three bands: E band, Ka band, and S band. The radar altimeters give an accurate measurement when taking measurements over a water body. This is why these altimeters are efficiently used in planes operating over oil rigs.
A real-life example to better explain the operation
The transmitting antenna emits a radio wave with a frequency of 4300 MHz. This wave touches the ground and reaches the receiving antenna. By the time the antenna receives this wave, the transmitting antenna is already emitting another wave with a different frequency, let’s say 4350 MHz. The processing unit compares the frequency of the received wave with the frequency of the wave that is being emitted. The difference in frequency allows these two waves allows the system to calculate the time taken by the signal to return, thus determining the current height. As the waves travel at the speed of light, this whole process occurs almost instantaneously which accounts for reliability.
The height indicated at every instant will be different even if the aircraft is flying at a level as all earth’s terrain is irregular. Even while landing, the reading may increase or decrease from time to time because of surface variations.
Reading the altimeter
The scale goes up to 2500 feet. If you fly above 2500 feet, the needle will either return to 0 or go out of visibility but this depends on the manufacturer and model.
Limitations of the radio altimeter
Although the radio altimeter is accurate and reliable, it has certain limitations. Since the waves are emitted downwards with respect to their mounting surface, there are pitch and bank limitations. During normal maneuvers, the altimeter can correctly determine the height of the aircraft. However, if the pitch and bank limits are exceeded, then the waves emitted by the system are not able to determine the current height of the aircraft. So there will be an error in the value given by the device. Typically, the pitch limit is around 30° and the bank limit is around 60° but it varies according to the design of the system.
Also, when flying near ground antennas, or having devices on board that use the same frequency band of the radio altimeter, they may interfere with the device. The 5G network band is capable of disrupting the calculation of a radar altimeter as it uses part of the frequency band of the radar altimeters.