**4. Radar altimeter**

The radio altimeter does not depend on radar standards beyond the way that it reacts to reflected signs. The radio altimeter used to show targets roughly around 300miles. The time gap between the transmitted and received signals analyzed by radar altimeter and that determined the distance between the destinations. A radio altimeter communicates an FM tweaked continuous wave 4-GHz signal. Their frequency balanced on "entrance ramp" with the goal that it changes direction. The spectrum between the transmitted and received signals ceaselessly analyzed to show altitude above the terrain [2, 4]. The radio altimeter does not depend on radar standards beyond the way that it reacts to reflected signs. It used to show targets roughly 300miles. The time gap between the transmitted and received signals analyzed by radar altimeter and determined the distance between the destinations. A radio altimeter communicates an FM tweaked continuous wave 4-GHz signal. Their frequency balanced on "entrance ramp" with the goal that it changes direction. The spectrum between the transmitted and received messages ceaselessly analyzed to show altitude above the terrain radar altimeter essentially projected to display the target locations within 2500feet. It is inactive when the aircraft fly above 2500feet [2]. A typical radio altimeter system on an airplane utilizes two antennas namely transmitting and receiving antenna. The transmitting and receiving antennas arranged as close to the point of the hinge as could expect under the circumstances, generally under the fuselage between the wings. Positioning and installation of the antenna in flight vehicle is very critical and harsh work. The antenna should never be situated at the nose or tail because the angle of attack could then contribute genuine mistakes [2, 5]. Low altitude flying airplanes use radio altimeter technology for escaping from ground radar detection. This technology is very famous in military based flying vehicles like helicopters. Terrain-following radar also uses this technology for allowing a plane to fly at high speeds over varied topography [1].

#### **4.1. Working principle and applications**

Radar altimeter works on one of the two methods. They are,


#### *4.1.1. Pulse modulated radar*

Pulse modulated radar consists of a series of discrete pulsed radiation. In this method, the distance between the targets identified by analyzing the reflected transmitted radio waves.-

#### *4.1.2. Frequency modulated continuous wave (FM-CW) radar*

Frequency modulated continuous wave radar (FM-CW) is capable of determining distance and is a short-range measuring radar set. This increases the reliability of the system. When more than one reflected wave arrives at the radar antenna, providing distance measurement along with speed measurement is essential for identification. This kind of radar used to measure the exact height during the landing procedure of aircraft. It also used as early-warning radar and proximity sensors. The difference in frequency between that received from the ground and that transmitted is a measure of the time delay [2, 6, 7].
