DeFli Loop- A Novel Passive Radar for Drone Detection


3/1/20233 min read

At DeFli Networks we are continually working on passive radar models that are real-time, highly accurate, legally compliant and sensitive enough to detect micro-doppler shift from the rotation of blades on drones as well as identifying the RCS.

In addition to the operational requirements, we are also seeking to deploy passive radar techniques that enable the cost-effective, decentralized deployment of devices performing said passive radar operations so as to deliver on our “drone detection as a service” model.

Our work in the field has taken an exciting development in recent times and has led us to coining a new technology for passive radar known as “DeFli Loop”.

DeFli Loop utilizes existing infrastructure within our DeFli Devices to deliver a closed loop, Rx only Passive Radar system that can identify drones via their RCS and the micro-doppler. What follows is an explanation of the system.

Let’s start with what a traditional passive radar setup looks like:

Typically a high power commercial radio transmitter illuminates radar targets. Two antenna’s 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.

The issue for DeFli deploying such a model is the requirement to add in another antenna to our devices as well as an additional SDR. This pushes up the cost and prohibits use by people not living within range of a commercial radio antenna.

DeFli Loop

The DeFli Loop Passive Radar system utilizes the existing hardware infrastructure and the data we are already collecting, this is how.

Firstly we align the two channels (ADSB & L-Band) by using a common clock as reference for the downconversion stages and the analog to digital converters. This requires linking the ADSB SDR and the L-Band SDR.

The DeFli Loop is a full loop passive radar system meaning that both elements (ADSB and L-Band) act as both illuminator and receiver, this is how.

In the first loop, we use ADSB transmissions from aircraft to take the place of the radio broadcast antenna. These transmissions are used to illuminate our targets (drones). The L-Band antenna acts as the receiver antenna.

In the second loop, the roles are reversed. We use L-Band transmissions from aircraft to ground (ACARS) as well as iridium based downlinks to illuminate our targets. The ADSB antenna acts as the receiver antenna.

Signal Processing

We first need to get rid of the strong direct path signal so that we can observe the weaker echoes. We need to not only remove direct path signal from the transmitter, but also reflections from mountain sides and other large scatterers that might mask weaker signals of interest. This is done by using a well known statistical signal processing technique called linear least-squares estimation to deconvolve the phase and amplitude of the direct path signal and the echoes from mountains and other strong non-moving scatterers (also called radar clutter).

After the strong direct path signal and clutter has been estimated, it can be subtracted from the measured signal and the signal processing to estimate the weaker echoes from drones can be performed.

We make the assumption that the target has a scattering Doppler spectrum that doesn’t change over a long enough period of time to allow us to estimate the range-Doppler spectrum. This allows us to perform deconvolution on the autocorrelation function of the received echo. This is something that is called lag-profile inversion.

Delay Doppler Truth Using ADSB to Determine Likely Co-ordinates

Our collected ADSB data provides the latitude, longitude and altitude of aircraft. The bistatic range of each aircraft is computed using distance_rx_to_target + distance_tx_to_target - distance_rx_to_rx. The latitude, longitude and altitude is converted to ECEF coordinates which means distances can be computed with a simple norm. The bistatic Doppler by definition is the rate-of-change of the bistatic range.

By determining the delay doppler of the broadcast (ADSB from an aircraft) we can use the measured signal and RCS to determine an approximate co-ordinate for the identified drone which is then mapped within our DeFli UTM. We can also use this methodology to approximate velocity of travel.

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