# How a Radar Altimeter Work

A Radar Altimeter is a height measurement device found in many aircraft. The Radar Altimeter device is also known as a Radar Altimeter. The abbreviation for Radar Altimeter is RA. The Radar Altimeter device indicates the aircraft’s altitude above the Earth’s surface. In other words, this device is used to measure the aircraft’s instantaneous height above the ground while it is in flight. How a Radar Altimeter Work ?

## How a Radar Altimeter Work ?

A Radar Altimeter is located beneath the fuselage of aircraft. The device is equipped with two antennas, one of which serves as a transmitter, while the other functions as a receiver. In addition to these antennas, there is a processing unit, and in the cockpit, there is a measurement display or electronic indicator to provide the data. How a Radar Altimeter Work ?

### The Operating Principle of the Radar Altimeter System

The transmitting antenna sends a radio wave from the aircraft towards the Earth’s surface. We know that radio waves travel at the speed of light. Once this radio wave reaches the Earth’s surface, it reflects back to the aircraft and is received by the receiving antenna. Then, the processing unit calculates the time it took for the wave to travel to and from.

Indeed, this is the basic operational principle, but modern radar altimeter devices work slightly differently. The signal frequency sent by the transmitting antenna continuously varies between 40 and 240 megahertz. In this system, the actual time it takes for the wave to travel is not directly measured; instead, the phase shift, or in other words, the difference in wave frequency, is measured. Since we know how rapidly the wave frequency changes, the time it takes for the wave to go and return can be indirectly determined.

This may seem too complex to you, let’s examine it with an example for a clearer understanding;

In this case, let’s assume that the transmitting antenna is emitting a radio wave at a frequency of 40 to 300 megahertz. This wave reaches the surface and reflects back to the aircraft. Simultaneously, as the radio wave is being reflected back to the aircraft, the transmitting antenna is emitting a different radio wave. The system measures and compares the frequency difference between these two waves. Thanks to this frequency difference between the two waves, the system calculates the time it takes for the wave to travel back and forth, thus allowing the aircraft to determine its current altitude relative to the Earth’s surface. As previously mentioned, radio waves travel at the speed of light, making this process almost instantaneous. This speed makes the system highly accurate and reliable.

However, there are some important considerations with this system, one of which is altitude.

In reality, the altitude displayed by the system does not correspond directly to the distance between the antenna and the ground. This is because when the aircraft touches down on the ground, the system shows the altitude as zero feet. The system is calibrated to display the height above the ground as measured from the landing gear to the ground. Therefore, the term “remaining altitude” is used to refer to the height difference between the antenna and the ground.

However, in some aircraft, depending on the design of the landing gear, different altitudes can be involved. One example is the Boeing 747 model. In such cases, during landing, the main landing gear tilts, resulting in a relatively higher remaining altitude. When the landing gear makes contact with the runway, it compresses, reducing the remaining altitude difference.

Since the Radio Altimeter is calibrated to measure altitude based on the remaining altitude, it can display negative altitude values when the aircraft has touched down on the ground. Additionally, there are other factors to consider with Radar Altimeter systems.

#### Radar Altimeter Measurements on Rough Terrain

If an aircraft is flying over uneven terrain, the altitude measurement can vary significantly. In such a situation, the aircraft may appear to be flying at a constant altitude, but the altitude indicator constantly changes due to irregularities on the Earth’s surface. As a result, even during descent, the altitude reading can increase.

For example, let’s consider an aircraft that has initiated its descent and is initially at an altitude of one thousand feet above the ground. During the descent, the altitude may drop to around five hundred feet due to the descent. However, due to uneven terrain, the aircraft continues to descend, and the altitude indicator may show six hundred feet, even though the aircraft is still descending.

This phenomenon occurs because the Radar Altimeter measures the altitude above the ground directly beneath it. As the aircraft encounters changes in terrain elevation, the altitude reading can fluctuate, leading to variations in the displayed altitude even when the aircraft’s actual altitude relative to sea level is decreasing.