It is the most common type of altimeter. Barometric type altimeter determines altitude by measuring air-pressure at altitude and comparing this pressure concerning sea level air-pressure. As the altitude increases, air pressure decreases and vice-versa. However, air pressure is also affected by the weather.
It is an electronic device that is capable to measure the height of the drone above terrain immediately below the drone. Radio altimeter uses radio or electromagnetic waves to measure altitude value by using the principle of electromagnetic reflection. To measure altitude it transmits radio waves and calculates the time difference of reflected wave. Although they are affected by irregularities in the terrain and it generates deviations in radio waves, the radio altimeter provides a precise and reliable altitude value.
GNSS (Global Navigation Satellite System):
Global Navigation Satellite System receivers can also determine by using tri-lateration with four or more satellites. It is a kind of GPS system of European Union.
Laser altimeter works by using laser light (it is an electromagnetic wave within the visible range of the spectrum). Its working is the same as the radio altimeter. Again, the time taken is calculated for the emitted signal to travel from the transmitter to the surface and again back to the transmitter. It uses the principle of LIDAR (Light Detection and Ranging).