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Active cancellation stealth techniques have been in the planning phase for many years, and may now be put into practice with the availability of modern RF/microwave and digital electronics technologies. Military forces constantly seek tactical advantages; when properly implemented, active stealth methods can provide these. When developing cancelling waves and signals, errors in amplitude and phase must be minimized and various other factors must be considered to achieve true stealth performance.

The concept of active cancellation stealth has actually existed since the 1960s, with the basic idea being to produce a train of coherent waves that will reduce the scattering echo of a radar target, rendering the target “invisible” to a radar system. Active cancellation methods have several advantages compared to passive stealth techniques: They do not require changing the shape of a target, or spraying it, or coating it with absorbing materials.

Field amplitude ratio curve

They also can be used in many different frequency bands, as well as adjusted to the parameters of an incident radar wave signal. But active cancellation becomes more difficult with increasing frequency, making it appear most suitable for lower-frequency radar-cross-section-reduction (RCSR) applications, where passive stealth techniques have yielded poor results.1

Active cancellation stealth is believed to be a smart and adaptive technique that produces a artificial radiation field. Such a field has equal amplitudes and same-frequency, but opposite phase from the target’s scattering field. The enemy radar receiver is always located in the synthetic pattern zero, thereby suppressing the target echo signal received by enemy radar and ultimately achieving stealthy aim. The availability of modern microelectronics technologies is enabling more practical active cancellation stealth solutions.

According to the definition of RCS,2 it can be expressed as Eq. 1:

Eq. 1

where:

(σ)0.5 = the complex root of the RCS scatterer;

λ = the wavelength;

k = 2π/λ = the wave number;

e$r = a unit vector aligned along the electric polarization of the receiver;

R = the distance between the radar and the scatterer;

Es = the vector of the scattered field; and

Ei = the electric field strength of the incident wave impinging on the target.

In theory, the RCS of a target (as it appears in different directions) can be measured accurately, as can the transient characteristics of a radar’s incident field. A radar system’s electromagnetic (EM) waves share characteristics with acoustic waves, and can be coherently stacked or cancelled in a similar way. In theory, achieving active cancellation is possible with a radar system by generating a wave with opposite phase.

According to ref. 3, any radar echo that satisfies the conditions described by Eq. 2 can be cancelled out completely:

Eq. 2

where:

Δa = amplitude error;

Δf = frequency error; and

Δφ = phase error.             

Unfortunately, a number of extraneous and often uncontrollable factors can yield errors when attempting to generate precise radar cancellation signals, defeating attempts at achieving active cancellation stealth effects. Minimizing these errors thus requires some analysis. Since waves reflecting from a complex target can be resolved into a collection of N discrete scatterers or scattering centers, Eq. 1 can be written in the form of Eq. 34:

Eq. 3

where:

σn = the RCS of the nth scatterers and

φn = the relative phase of the scatterer’s contribution due to its physical location in space.

Many factors can impact the effectiveness of active cancellation efforts, with frequency error the most influential of these factors. Frequency errors can destroy signal coherent and cause a signal beat phenomenon, with loss of ideal cancellation effects. Even when the frequency difference is small and the signal is in pulse mode, new problems can arise. To simplify the radar cancellation model, the initial values of phase differences between two carriers can be set at odd times of π. The pulse train signal is represented by the sampling function:

nδn(t – nTt)

The synthesis of the field amplitude can be expressed by means of Eq. 4:

Eq. 4

where:

Tt = the pulse repetition period;

a = Δf/Ωt;

Ωt = 2π/Tt, and E is given by Eq. 5: 

Eq. 5

The coefficients of Eqs. 4 and 5 reach 0 at the same time if (and only if) variable a is an integer, which is when the stealth cancellation effect is at its best. When the number of pulse accumulations by the stealth system is large, such as 100, the cancellation effect is almost minimal. Figure 1 shows amplitude ratio versus parameter a (with a corresponding pulse accumulation number of 10).

Normalized RCS versus phase error

As can be seen, cancellation effects are achieved only when the value of a is close to an integer. This frequency tolerance must be extremely tight in any active cancellation stealth approach.

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