Introduction

Radar stands for radio detection and ranging. Unlike active radar, passive radar does not use active transmitters to illuminate its target. In this blog post, we introduce common terms used in the radar field and explain how Nutaq’s PicoDigitizer can be used to effectively implement passive radar technologies.

Definitions

Bistatic or multistatic radars employ multiple antennas in different locations for transmitting and receiving functions. Monostatic radars use the same antenna for receiving and transmitting at one node. Historically, radars were all bistatic or multistatic. Today’s radar is mainly monostatic, with the exception of passive radars. A passive radar system is one without a transmitter.  A Bistatic Passive Radar (BPR) uses one or more antennas to receive existing signals. However, those systems generally have more than one receiving antenna, for multiple reasons. The bistatic radar uses existing transmitters such as FM broadcasting stations, for example, to use as a source of RF waves for detecting its targets.

The basics

A radar works by processing a received signal, comparing it with the transmitted one, and analysing the differences to find the location, speed, and direction of one or many targets. In the case of active radars, the transmitted signal is generated by the radar system itself. Passive radars are opportunistic and use existing RF signals generated by a friendly source, a neutral one, or an enemy source. A friendly target is called cooperative and a neutral one or enemy one is called non-cooperative. A TV broadcasting station, for example, is a neutral source. The third-party source is called an illuminator because, like a light bulb that illuminates objects so you can see them, the RF source illuminates the target so that radar can detect it.

Illumination sources

Passive radars can operate anywhere in the spectrum.  Most of today’s passive radars use broadcast lower frequencies that are high power and allow a good detection range. The modulation schemes of the signals depend on the type of non-radar source (illuminator) used: HF broadcast, VHF FM radio signals, UHF analogue TV signals, digital audio and TV broadcasts, and cellphone networks (GSM, 3G) are a few examples. Some of the more recent passive radar systems may also use satellite-borne signals like XM or navigation (GPS) signals.

The performance of the radar system may be computed by the ambiguity function. Range is also an important parameter and the system’s performance greatly depends on the illumination source used. Satellite signals typically have a weak power when they reach the ground and consequently,  radar systems that use them have a limited range.

Advantages

Interest in passive radar has increased in recent years due to its inherent advantages. First, it is stealth because no signal is emitted. This makes it less sensitive to counter-measures like anti-radar missiles (ARMs). Jamming is also less likely because most jammers are designed to interfere with active radars at the frequencies used by them (active radars use mainly the EHF band, or millimeter waves). UHF and VHF illuminators of FM radio signals and analogue TV have high power that allows them to spread over huge distances, providing the radar system with long ranges.

Another great advantage of passive radars is that they are not affected by spectrum regulations and can be used in any territory by changing the illumination source. Active radars, on the other hand, use defined spectrum space and are subject to regulation. However, a downside of the  passive radar is that it depends on the activity of an external source of RF waves that could be turned off at any time.

Why the PicoDigitizer?

One of the main challenges behind BPR is that the system needs to distinguish between different signals that appear very similar. Because the illuminator and all the targets emit or reflect very similar signals at very low power levels, the BPR finds itself in a situation where it needs to detect very weak signals in an environment where interference is very strong. Therefore, it is essential that the A/D hardware have very good specifications regarding linearity, noise figure, and dynamic range.

Nutaq’s PicoDigitizer platform is an excellent choice for those who are looking to sample a signal for radar purposes and program an algorithm in an FPGA or record the digitized data (up to 5.7 GB/s of sustained recording). The PicoDigitizer’s hardware and software is designed to help you to achieve these goals in a quick amount of time.

The spurious free dynamic range (SFDR) of the PicoDigitizer 125 is 74.6 dBFS (typical). Its noise figure at the same frequency is 66.1 dBFS (typical) and the total harmonic distortion (THD) is 82.5 dBFS (typical). These values were obtained using a 65-k-point FFT. The measurements were made using the onboard 125-MHz oscillator as a source for the sampling frequency.

The PicoDigitizer is equipped with 16/32-channel 125 MSPS A/D sampler (14-bit resolution), enabling it to cover a theoretical maximum analog bandwidth of 62.5 MHz, according to the Nyquist theorem. Once digitized, the signal can be processed on a powerful Virtex-6 FPGA where, for example, the direction of arrival (DoA) could be computed by a multiple signal classification (MUSIC) algorithm.

Future plans

Nutaq was present at the IEEE Radar Conference in Ottawa in 2013 and is planning to be present in Cincinnati this year (2014). We are working with partners to develop new technology for beamforming and DoA, so stay tuned to learn more about what we can offer you for your radar system.