This is good stuff!
Building your own SDR-based Passive Radar on a Shoestring
two $8 RTL software defined radio dongles and two log-periodic antennas.
And get this, the radar system you’re going to build works by listening for existing transmissions that bounce off the targets being measured!
A passive radar is a special type of radar doesn’t require you to have a transmitter. You rely on a radio transmitter of opportunity provided by somebody else to illuminate radar targets. This can be your local radio or television station broadcasting with up to several megawatts of power.
A passive radar system is inherently multi-static. A single station can multiple transmitters that can be in different locations. This allows the three dimensional trajectories of radar targets to be estimated.
There are a large number of non-military applications for passive radars. I’m interested in the use of passive radar for geophysical and astronomical radio remote sensing.
The Manastash Ridge radar used FM radio for passive radar, which is probably the easiest to start with, because the bandwidths of individual stations are relatively modest. With FM radio, you can also typically observe not only ionospheric irregularities, but also meteor trails and movements of airplanes flying within a 100-600 km radius of yourself.
One of the antennas is pointing towards the FM radio transmitter and used to obtain a measurement of the waveform transmitted by the radar. The other is pointing towards the opposite direction and is used to measure echoes
The signals from these two antennas need to be recorded in a coherent manner. This means that we need two channels with samples that are aligned. This is typically achieved by using a common clock used as a reference for the downconversion stages and the analog to digital converters.
A word of warning before rushing to implement the RTLSDR approach: there are subtle tweaks that need to be done in order to make everything work
Passive radar with $16 dual coherent channel rtlsdr dongle receiver