**3. Obstacle detection and analysis**

There are two types of obstacle detection in NOE flights:-


#### **3.1. Obstacle detection using passive sensors**

A passive sensor is a microwave device developed to collect and measure natural emissions produced by components of the earth's surface and its ecosystem [2, 3]. The passive sensor measures the output as a combination of environmental temperature, surface roughness, surface composition, and other physical properties. The measurements of passive sensors determined with the help of molecular resonance by fixing the radio frequency bands. These frequencies no longer alternate and statistics could not duplicate in other frequency bands. The passive sensors could able to detect low-quality emissions particularly very sensitive multitude emissions on the ground both from the frequency band wherein measurements made and from out-of-band. Spaceborne passive sensors offer the- capacity to acquire all-climate, day and night, worldwide observations of the earth and its environment [2, 3].

#### **3.2. Obstacle detection using active sensors**

An active sensor measures the signals that reflected, refracted or scattered from the earth's surface or its atmosphere. Spaceborne active sensors worked on the radar principle and used in many applications related to atmospherelogy, meteorology [2]. For example, Doppler radar analyses the electromagnetic echo from a moving object and define the absolute velocity of the object; mapping radars use synthetic aperture radar to scan the sizeable geographical area for geography. There are some specific radar systems to identify the human density in the forest as well as the construction area by measuring the reflective waves. Navigation radars commonly employed in ships for collision avoidance and it works by sensing reflections. Spaceborne active sensors used in the meteorological-satellite communication services. Sensor frequency allocations recurrently shared with supplementary radar systems; as such, systems frequently well matched with the operation of the sensors [2, 3].
