Basic Radar Operation
The simplest mode of radar operation is range-finding, performed by time-of-flight calculation. The unit transmits a radar signal, i.e., sends radar waves out toward the target. The waves hit the target and are reflected back in the same way that water waves are reflected from the end of a bathtub. The returning wave is received by the radar unit, and the travel time is registered. Basic physics tells us that distance is equal to rate of travel multiplied by the time of travel. Now all electromagnetic waves travel at the same speed in a vacuum—the speed of light, which is 3.0 × 108 m/s. This speed is reduced by some small amount when the waves are traveling in a medium such as air, but this can be calculated. If the radar system sends a pulse out toward a target and records the amount of time until the return pulse is received, the target distance can be determined by the simple equation d = vt, where d is distance, v is velocity, and t is time.
A basic radar unit consists of: a frequency generator and timing control unit; a transmitter with a modulator to generate a signal; an antenna with a parabolic reflector to transmit the signal; a duplexer to switch between transmission and reception mode; an antenna to gather the reflected signal; a receiver to detect and amplify this return; and signal processing, data processing, and data display units. If the transmitter and receiver are connected to the same antenna or to antennas in the same location, the unit is called monostatic. If the transmitter and receiver antennas are in very different locations, the unit is known as bistatic. The frequency generator/timing unit is the master coordinator of the radar unit. In a monostatic system, the unit must switch between sending out a signal and listening for the return reflected from the target; the timing unit controls the duplexer that performs the switching. The transmitter generates a radio signal that is modulated, or varied, to form either a series of pulses or a continuously varying signal. This signal is reflected from the target, gathered by the antenna, and amplified and filtered by the receiver. The signal processing unit further cleans up the signal, and the data processing unit decodes it. Finally, the data is presented to the user on the display.
Before target range can be determined, the target must be detected, an operation more complicated than it would seem. Consider radar operation again. A pulse is transmitted in the direction that the antenna is facing. When it encounters a material that is different from the surrounding medium (e.g., fish in water or an airplane in the air), a portion of the pulse will be reflected back toward the receiver antenna. This antenna in turn collects only part of the reflected pulse and sends it to the receiver and the processing units where the most critical operations take place. Because only a small amount of the transmitted pulse is ever detected by the receiving antenna, the signal amplitude is dramatically reduced from its initial value. At the same time, spurious reflections from non-target surfaces or electronic noise from the radar system itself act to clutter up the signal, making it difficult to isolate. Various filtering and amplification operations help to increase the signal-to-noise ratio (SNR), making it easier to lock on to the actual signal. If the noise is too high, the processing parameters incorrect, or the reflected signal amplitude too small, it is difficult for the system to determine whether a target exists or not. Real signals of very low amplitude can be swamped by interference, or "lost in the noise." In military applications, interference can also be generated by reflections from friendly radar systems, or from enemy electronic countermeasures that make the radar system detect high levels of noise, false targets, or clones of the legitimate target. No matter what the source, interference and signal quality are serious concerns for radar system designers and operators.