Requirements for Radar receivers
Figure 1: Receiver and Signal Generator of the ASR–E ATC–radar (Manufacturer: EADS)
The ideal radar receiver is required to:
- amplify the received signals without adding noise or introducing any form of distortion;
- optimise the probability of detection of the signal by its bandwidth characteristics;
- provide a large dynamic range to accommodate large clutter signals;
- reject interfering signals so that the required information can be optimally detected.
Minimum Detectable Signal (MDS)
The minimum receivable power (Pemin) for a given receiver is important because the minimum receivable power is one of the factors which determine the maximum range performance of the radar. The sensitivity level MDS has got a value of 10 -13 Watts ( -100 dBm) for a typical radar receiver.
All receivers are designed for a certain sensitivity level based on requirements. One would not design a receiver with more sensitivity than required because it limits the receiver bandwidth and will require the receiver to process signals it is not interested in. In general, while processing signals, the higher the power level at which the sensitivity is set, the fewer the number of false alarms which will be processed. Simultaneously, the probability of detection of a “good” (low-noise) signal will be decreased.
One of the most important factor is receiver noise. Every receiver adds a certain amount of noise to its input signal, and a radar receiver is no exception. Even with very careful design, noise due to thermal motion of electrons in resistive components is unavoidable. The amount of such thermal noise is proportional to receiver bandwidth.
Therefore, bandwidth reduction is a possible solution to the problem of receiver noise. However, if the bandwidth is made too small the receiver does not amplify and process signal echoes properly. A compromise is required. In practice, the receiver bandwidth of a pulse radar is normally close to the reciprocal of the pulse duration. For example, a radar using 1 µs pulses may be expected to have a bandwidth of about 1 Mhz.
The receiver system must amplify the received signal without distortion. If a large clutter signal sends the system into saturation, the result is a modification to the spectrum of the signal. This change in spectral content reduces the ability of the signal processor to carry out Doppler processing and degrades the MTI improvement factor. Furthermore, if the receiver enters saturation, then there can be a delay before target detection is restored. In principle, the dynamic range of the receiver must exceed the total range of signal strength from noise level up to the largest clutter signal. In practice dynamic ranges of 80 dB’s or so meets system requirements.
The clutter power confirms this requirement as it averages:
- Rain clutter up to 55 dB
- Angels to 70 dB
- Sea clutter to 75 dB
- Ground clutter to 90 dB.
These figures are reduced somewhat by the application of swept gain.