Radar (Radio Detecting And Ranging) systems are widely used to detect objects and to measure the distance between those objects and the radar system. As discussed in our previous blog post, radar systems are usually classified into two categories: active and passive. In this blog post, we will discuss in further detail some active radar parameters that affect the choice of data conversion devices (digitizers) in radar systems.
Active radar works by sending a very short burst of a modulated pulse, having width t at time t0, with sufficient energy to allow the echo pulse to be detected by the receiver. The radar receiver turns off while the system is transmitting the pulse to prevent high energy signals getting to the receiver. An echo pulse is received at time t1 at the receiver of the radar system. The range between the radar transmitter and the object is calculated as:
where ∆T is the total traveling time of the pulse and c0=3 × 108 m/s is the speed of light. This process is repeated at the pulse repetition interval (PRI), T, or pulse repetition frequency (PRF),1/T. The PRI determines the maximum unambiguous range as follows
It can be seen that the faster the PRI sent by the transmitter, the shorter the maximum unambiguous range and the higher the number of hits to the object on the radar screen. If the duration of the echo pulse is longer than the PRI, the system will see the next transmitted pulse as the previous associated echo pulse for determining the target range. This causes the system to determine an incorrect range or an ambiguous range. To avoid the ambiguous range, the next pulse must not be sent until the previous echo pulse has been received. Transmitted coded pulses can be sent more frequently because the receiver is able to decode the echo pulses correctly. Coded pulses provide better tolerance to interference from other radar systems and to jamming signals. However, coded pulses are less tolerant of the Doppler shift of a moving target because the matched filter in the receiver is sensitive to Doppler shift. If the transmitted pulse width, t, is longer than the total travelling time, ∆T, overlapping of the echo pulses can be observed in the receiver .
The minimum detectable range of the active radar system depends on the transmitted pulse width, t, and the recovery time,trcv , to turn on the receiver, as defined in the following equation:
As a result, the effective detectable range of the radar can be illustrated as shown in the following figure.
Figure 1: Effective Detectable Range of Active Radar
By understanding some of the important parameters of active radar, such as the transmitted pulse width, PRI, and coding type, the radar designer can figure out the best choice of RF components, ADC/DAC devices, signal processors, and data processors in the radar system. Modern radar transmitters using wide bandwidth pulses and wider modulation bandwidth require high speed DACs. Similarly, measuring low amplitude echo signals may require high sampling rate, high precision, and high dynamic range digitizers on the receiver side.
Nutaq’s µDigitizer series provides wide range of high speed digitizers from 125 MSPS to 5 GSPS, with 8-bit to 14-bit dynamic range, and 16-bit 1GSPS DACs that are able to cover data conversion requirements for low, medium and high PRF in active radar applications. A high speed AMC back end and a gigabit Ethernet interface allow digital signal post-processing to be done using either an embedded or an external processor.
|1.||Christian Wolff. Accessed 2013. “Radar Basics.” Radartutorial .eu. http://www.radartutorial.eu/01.basics/rb02.en.html.|
|2.||Agilent. 2013. “Agilent Radar Measurements – Application Note.” http://cp.literature.agilent.com/litweb/pdf/5989-7575EN.pdf.|