This entry will cover some of the basic principals of a passive radar system. Using an example system bellow the basic principles of a multi-static TDOA passive radar system will be explained. The basic outline provided in this post will be built upon in the future.
The above diagram details the proposed operation of a multi-static TDOA radar system. This system is passive as it uses transmitters of opportunity opposed to an active which transmitts its own signals. This example multi-static system consists of four transmitters, a target and a receiver. A single receiving station monitors the direct signals from the transmitters and indirect, multi-path signals from potential targets. The direct path signals 
propagate directly from every transmitter to the receiver. Multi path signals from each of the transmitters 
are reflected by the target 
and intercepted by the receiver. The path the reflected signal follows from the target to the receiver and is common for all transmitters 
.
The signals are modulated in such a way that signals from different transmitters are easy for the receiver distinguish. The time difference of arrival (TDOA) is the delay between the arrival of a multi-path signal and the arrival of the same direct path signal from the same transmitter. The timing is recovered from the signals through the correlation. Using these timings, solutions for possible target locations can be generated.
A basic break down of a multi-static TDOA radar’s sub-systems is presented bellow.
In the above diagram the system is broken down in to seven main components; Antenna array, reference antenna, antenna array signal conditioning, reference antenna signal conditioning, signal de-modulation, adaptive correlation and target plotting.
The antenna array intercepts the scattered signals from potential targets. This sub system is represented as an array as is it expected to employ some form beamforming. The beamforming will allow the antenna to be tuned to be more sensitive in a particular direction. This will allow the directional gain of the antenna to be optimized for multi-path signals from potential targets. Information on potential targets can be extracted from the antenna array by steering it towards the signal of a target and extracting the angle. The angle can be used in the plotting of potential targets. The Signal conditioning for the antenna array will optimize the intercepted signals for demodulation.
The reference antenna receives direct path signals from the transmitters. The direct path signals intercepted are used to aid de-modulation of the scattered signals from potential targets. TDOA information is extracted from the de-modulated scattered signals by cross-correlating them with the de-modulated direct-path signals. The target plotting sub-system takes TDOA information from multiple scattered signals from multiple transmitters to plot potential targets.
The next post will discuss target plotting using TDOA information and distinguishing clutter from targets of interest.