RF Detection while a relatively cost effective option, it suffers from a few limitations. First is its inability to detect autonomous drones which are self-guided and follow a pre-programmed flight path. Therefore, they need not be dependent on the Ground Control Station (GCS) for navigation. This class of drones are more likely to be employed by the Military since they need to operate at extended ranges (few hundreds of kms to even a few thousand kms) from the GCS.
Some commercial drones also have an inherent capability to overcome failure of the RF communication links by incorporating a Return to Home (RTH) feature. This failure of the communication link could occur due to signal fading or even deliberate interference such as jamming. In such a situation the drone, activates the RTH feature and returns to it originating point by retracing its path since the outward journey control signals would have been stored in its memory.
One of the main drawbacks of RF detection is its inability to provide coordinates and range information of the drone when operating as a single station. To determine the coordinates of the drone there is a need to deploy at least three RF detectors in a baseline to take simultaneous Direction Fixes from the sensor. By geometric triangulation it is then feasible to deduce the coordinates of the drone and hence the range. However, the location fix is likely to be fairly imprecise due to the nature of RF wave propagation.
In such a situation there is a need to turn towards a technology that was developed in World War 2 – RADAR!
This technology has proved itself over the years as a reliable way to detect flying objects such as aircraft and helicopters. In brief, the RADAR transmitter irradiates the flying object with a burst of Microwave pulse energy. This pulse is reflected by the aircraft and is received by the RADAR receiver. Since these pulses travel at the speed of light, it is possible to determine the range by measuring the time difference between the transmitted pulse and the received pulse. When the RADAR tracks the moving object, it is also possible to measure the velocity of the target aircraft. It is possible to determine the coordinates of the target with a fair degree of accuracy when the angle information obtained from the Radar is also used.
The foregoing discussion indicates the importance of the reflectivity of the target surface. It is a function of the size of the target and the reflecting surfaces on the target. This parameter is measured in terms of Radar Cross Section or RCS. The difficulty in detecting drones is that they have a very low RCS in comparison to other flying objects. In fact, they can often be mistaken for birds. As a result the reflected energy from the drone is extremely low. Further, the use of plastic in the body of the drone again results in reducing its RCS.
Therefore, an additional principle of Physics, the Doppler Effect needs to be used to detect these objects.
The Doppler Effect postulates that there is a shift in frequency that is proportional to the velocity of the object from which it is emitted. Therefore, in addition to the amplitude of the received pulse, if the shift in frequency can be detected, it will be possible to detect the velocity of the object. In the case of multi copter drones, Micro Doppler effects are also observed due to the rapid motion of the rotor blades that provide lift to the drone. This helps in classification of the detected drone since every multi copter drone will have different Micro Doppler shifts associated with it.
By now, it would be quite apparent that RADAR detection of drones is feasible but its physical realization is certainly more difficult than in the case of RF detectors since there is an active transmitter pumping high amounts of energy to detect a tiny drone that can well be mistaken for a bird! Typically, RADAR operates in the S/C/X bands. This can cause difficulties of operation with co-located RF detectors since most commercial drones work in S/C bands. There can be Electromagnetic Interference between the RADAR and the RF Detectors. Deriving a multi sensor counter drone system is therefore a complicated proposition. This is precisely the topic of our next blog. Stay tuned for that!