The figure on the right shows the two dipole antennas and the mobile laboratory used to perform the measurement. In 1935, Sir Robert Watson-Watt and his colleague Arnold Wilkins performed the first aircraft tracking radar experiment by detecting echoes from a Heyford bomber aircraft illuminated by a BBC shortwave wave radio transmission. One of the first radar experiments could be classified as a passive radar.
While most radars systems we know nowadays have a dedicated transmitter, the idea of passive radar goes way back. This signal was then delayed so that it was out of phase and then used to subtract the direct path radio station signal from the other antenna, leaving mostly the radar echo from the aircraft. One of the antennas was used to pick up the direct path signal from the BBC short wave radio. For example, if a radar target doesn’t have a large enough radar cross-section on one frequency, there is a good chance that another frequency will work better.Ī depiction of the Watson-Watts and Wilkins passive radar experiment that detected a Heyford bomber aircraft at Daventry in 1935.
These transmitters cover a wide range of frequencies, which is really useful for radio remote sensing, as radio propagation characteristics and scattering properties of different media and radar targets can be highly frequency dependent. There are many radio transmitters out there: Television, FM radio, and AM radio are the most obvious ones, but cell phone towers and even satellites can be used for passive radar.With a multi-static system there is a greater chance that such a radar target is seen, as there are multiple different simultaneous illumination directions occurring at any given time. A good example is a specular meteor trail echo, which only has significant radar cross-section only when the illumination angle viewing angle combination is a specular reflection. There are many radar targets that have illumination and viewing angle sensitive radar cross-sections. This allows the three dimensional trajectories of radar targets to be estimated. A single station can multiple transmitters that can be in different locations. A passive radar system is inherently multi-static.Passive radar allows you to operate a radar with kilowatt or even megawatt class effective radiated power, without braking any laws or regulations. Being a pirate radar operator can sound exciting, but you really don’t want to transmit without an FCC license.You don’t need a transmitter, which is a good thing because high power radio transmitters are large, expensive, power hungry, and contain dangerous parts.The advantages compared with a normal radar are numerous: This can be your local radio or television station broadcasting with up to several megawatts of power. You rely on a radio transmitter of opportunity provided by somebody else to illuminate radar targets. Two antennas are used: one antenna measures what is transmitted by the radio transmitter and the other antenna is used to record the echoes from the radar targets.Ī passive radar is a special type of radar doesn’t require you to have a transmitter. A high power commercial radio transmitter illuminates radar targets, which in this case are an aircraft and a meteor trail that happens to be oriented in a way that allows specular reflection to be seen by the receiver. All of this may sound like a lot of effort, but don’t worry, making a passive radar isn’t too complicated.Ī simple passive radar setup. I’ll also go through strategies to determine that everything is working as expected. In later postings I will go through more detailed block diagrams of the different parts of a passive radar system, provide example data, and give some Python scripts that can be used to perform passive radar signal processing.
It will take a few posts to show how to build a radar capable of performing these types of measurements.
Because this has been a topic that keeps attracting inquiries, I’m going to explain how I did the experiment in more detail. It was mainly intended as a zany little side story for our radio telescope blog, but it ended up raising a lot of interest. I wrote about this in a very brief blog posting a few years ago. And get this, the radar system you’re going to build works by listening for existing transmissions that bounce off the targets being measured!
Below is an animation of a measurement of airplanes and meteors I made using a radar system that I built with a few simple easily available pieces of hardware: two $8 RTL software defined radio dongles that I bought on eBay, and two log-periodic antennas.