Auxiliary Channel
Understanding Auxiliary Channels
In a sidelobe canceller (SLC), auxiliary antennas are positioned around the main antenna to intercept jammer signals that enter through the main antenna's sidelobes. The adaptive processor adjusts complex weights on each auxiliary channel so that the weighted auxiliary signals, when subtracted from the main channel, cancel the jammer components. The main beam is unaffected because the low-gain auxiliary antennas have negligible signal contribution in the main beam direction. This architecture has been the primary anti-jam technique for large radar systems since the 1960s.
The number of auxiliary channels directly determines the system's jamming suppression capability. Each auxiliary provides one complex degree of freedom that can place one null in the adapted pattern. A system with 4 auxiliary channels can simultaneously cancel 4 independent jammers from different directions. The adaptive weight computation uses algorithms like Sample Matrix Inversion (SMI), which computes the optimal weights from the inverse of the interference covariance matrix, or Least Mean Squares (LMS), which iteratively converges to the optimal weights over multiple pulses.
Optimal Weight Computation
wopt = Raa-1 × ram
Where:
Raa = Auxiliary-auxiliary covariance matrix (N × N)
ram = Cross-correlation vector between auxiliary and main channels
N = Number of auxiliary channels
Cancellation Ratio (single jammer, N auxiliaries):
CR = (1 + JNR × |ρ|2) / (1 + JNR × (1 - |ρ|2))
Where JNR = jammer-to-noise ratio, ρ = correlation between main and auxiliary jammer samples
Minimum Training Samples for SMI:
K ≥ 2N (Reed-Mallett-Brennan rule)
Auxiliary Channel Architectures
| Architecture | Aux Channels | Jammers Cancelled | Complexity | Application |
|---|---|---|---|---|
| Single-Channel SLC | 1 | 1 | Minimal (1 weight) | Legacy radar, simple jamming |
| Multi-Channel SLC | 2 to 8 | 2 to 8 | Moderate (N×N matrix) | Military radar, SATCOM |
| Generalized SLC (GSLC) | 4 to 16 | 4 to 16 | Higher (blocking matrix) | Phased array with partial adaptivity |
| Fully Adaptive Array | All elements | N-1 (max) | Very high (full N×N) | Advanced AESA, 5G massive MIMO |
Frequently Asked Questions
How does a sidelobe canceller use auxiliary channels?
A sidelobe canceller uses one or more low-gain auxiliary antennas positioned near the main antenna to sample jammer signals entering through the sidelobes. The processor computes complex weights that, when applied to the auxiliary channels and subtracted from the main channel, cancel the jammer. The weights are computed adaptively using LMS or SMI algorithms to minimize residual output power. A key design constraint is that the auxiliary antenna pattern must have higher gain than the main antenna sidelobes in the jammer direction so the auxiliary provides a clean copy of the jammer signal. Cancellation typically achieves 20 to 40 dB of jammer suppression.
How many auxiliary channels are needed to cancel multiple jammers?
N auxiliary channels can place N independent nulls in the antenna pattern, cancelling N separate jammer sources. Each auxiliary provides one complex degree of freedom (amplitude and phase), so 4 auxiliaries can cancel 4 independent jammers from different directions. However, wideband jammers or jammers with frequency-dependent characteristics may consume more than one degree of freedom each, reducing the effective number that can be cancelled. Adding more auxiliaries than jammers improves convergence speed and robustness. The Reed-Mallett-Brennan rule requires at least 2N independent training samples for reliable weight estimation via SMI.
What is the difference between auxiliary channels and fully adaptive arrays?
An auxiliary channel system uses a fixed main beam and a small number of auxiliary elements (2 to 8) for interference cancellation only. The main beam shape is preserved, and auxiliaries modify only the sidelobe region. A fully adaptive array applies independent weights to every element (potentially hundreds), optimizing the entire radiation pattern. Fully adaptive arrays offer superior performance but require inverting an N-by-N covariance matrix, which is computationally expensive and needs at least 2N training samples. Auxiliary channel systems are computationally lightweight and are the standard approach for large mechanically-steered radar antennas.