Radar Basics

Definition: Radar signature

Radar Signature

A radar signature consists of information about characteristic echo signals of a reflecting object (target). Like a fingerprint, it is a way to determine the type of target.

The radar signature does not only concern the aspect-dependent amplitude changes of the Radar Cross-Section, (RCS) but also the spectrum of the Doppler frequencies[1], their characteristic modulation or harmonics in the echo signal. Although the radar signature also contains information about the effective reflection surface of the target, it cannot be equated with it. Radar signatures are determined empirically and collected in databases. They play a major role in target characterization, especially in military radar equipment.

Figure 2: Jet engine modulation of a propeller aircraft and of a wind power plant.

Figure 2: Jet engine modulation of a propeller aircraft and of a wind power plant.

Radar Cross-Section

The Radar Cross-Section is very strongly dependent on the direction from which the reflecting object is illuminated by the radar. Here characteristic changes are significant, which can also contain information about the attitude of the target relative to the radar. For the radar signature, this amplitude value usually only has a referencing function, i.e. for the measurement of the amplitude difference between unmodulated and modulated components.

A frequency shift caused by the radial speed (Doppler effect) must be deducted in order to obtain a normalized signal that can be compared (correlated) with the database entries.

Jet Engine Modulation

Much more important for the respective radar signature are special Doppler frequencies through parts moving at or in the target, e.g. rotor blades or compressor blades of the engines, which also gave these modulations their name: Jet Engine Modulation (JEM). These Doppler frequencies depend on the number of parts and their rotational speed. Due to the rotation, these parts constantly change the instantaneous radial velocity in relation to the radar and are concealed at certain angles of rotation. Their rotation thus produces a characteristic pattern of Doppler frequencies. All Doppler frequencies must be measured and correlated with the image in the database. Like a fingerprint, these can be used to identify the aircraft type.

Harmonic waves

If the transmitted pulse of the radar hits a boundary layer in the material which has a non-linear characteristic similar to a diode, harmonics are generated in the echo signal. For military applications, this effect is of little significance, since aircraft manufacturers are already making sure that such boundary layers do not occur. For the receiving of these harmonics, an extremely broadband receiver is required also, which becomes an obstacle for the interference protection of the radar.

For another application, these harmonics have a decisive significance: the so-called Harmonic Radar. This is used, for example, in mountain rescue to find people buried by avalanches. The harmonics are generated here at a small dipole with a semiconductor diode between the two dipole halves. This dipole can be part of a special ski pass or entrance ticket. By the manufacturers of special clothing for ski sports or mountain hikes, such reflectors can also be specifically incorporated into the laundry. The radar can now very well distinguish the radar signatures from the passive echo signal, for example of a rock and a buried motionless person.