Sensors

Sensors measure the states of a vehicle relative to a reference frame, process the data, and send the information to the cockpit display or the guidance computer for further action. They execute a distinct navigation task, contributing to the situational awareness of the vehicle. We restrict ourselves to sensors that establish the LOS to a point, be it an IP, aimpoint, or target. They are often referred to as seekers because they seek out the location of an object. At the risk of disappointing you, I will not discuss such navigation aids as star trackers, altimeters, angle of attack vanes, etc., but concentrate on microwave radar and electro-optical (EO) sensors found in aircraft and homing missiles.

Radar was conceived by the British during World War II to track incoming German aircraft. Ever since, it has been improved in power, accuracy, and flexibil­ity. EO seekers also had their crude beginnings during WWII in German guided missiles. They proved their deadly accuracy in the Gulf War. As we search the literature, we find many books on radar and optics, but few that relate to the task of sensing the LOS of an object. My favorite introductory text is by Hovanessian.20 He treats radar and EO sensors from an application-oriented standpoint. In addi­tion, I can recommend two handbooks that should be on your shelf as you wrestle with the modeling task of sensors: the infrared handbook by the company ERIM21 and the radar handbook by Skolnik.22

The main distinction between radar and EO systems is their operational fre­quency in the electromagnetic spectrum. The radar bands we consider are the X, Ku, and Ka bands. The EO wavelengths span the visible spectrum 0.38—0.76 ц. m

and the infrared spectrum 3—14 /im. Radars are active sensors, i. e., they emit the energy that they receive, whereas most EO systems are passive systems, relying on natural energy sources. An exception is the laser radar or ladar, which is an active sensor.

An aircraft or missile sensor has two distinct tasks: to acquire the target during the acquisition phase and to track the target during the tracking phase. Each phase makes different demands on the sensor. The acquisition phase requires the sensor to detect a target rapidly in a large search area, followed by accurate tracking possibly under adverse conditions.

Another distinguishing feature of sensors is the mounting type of the sensing element. A dish antenna would be mounted on a gimbal, but an array of wave guides can be mounted on a plate and bolted to the aircraft. EO seekers are usually placed on gimbals; however, advanced focal plane arrays lend themselves to body – fixed arrangements. Accordingly, we distinguish between gimbaled and strap – down sensors. Before we embark on these detailed models, let us take the kinematic approach of an ideal LOS pointing continually at the target—a simple but useful model.