Conical Scan
How Conical Scan Generates an Angle Error
The defining feature of conical scan is that the antenna beam never points along its own mechanical axis. A feed offset, a tilted subreflector, or a rotating mirror displaces the beam peak by a fixed squint angle, and that squinted beam is then spun about the tracking axis at the scan rate. The locus of the beam peak traces a cone in space, which is where the name originates. A target on the cone axis is illuminated by the same point on the beam shoulder throughout each revolution, so the echo amplitude is constant. A target off axis moves the operating point up the beam slope on one side of the rotation and down on the other, impressing a sinusoidal amplitude modulation on the pulse train at exactly the scan frequency.
Recovering the pointing error is then an exercise in synchronous detection. The receiver envelope is multiplied by reference sine and cosine waveforms locked to the mechanical scan position, and the two products are low-pass filtered to yield an azimuth error voltage and an elevation error voltage. These voltages feed the antenna servo, which steers the axis until the modulation depth falls to zero. Because the error is built up from many pulses across at least one full scan revolution, the tracker needs the target return to stay coherent in amplitude over that interval, which is the root of its main weaknesses.
The squint angle is the central design parameter. Placing the crossover too close to the beam peak flattens the pattern slope and weakens the error sensitivity, while squinting too far reduces on-axis gain and raises crossover loss. The usual compromise sits near 0.4 of the half-power beamwidth, where the antenna pattern slope is near its steepest and the one-way crossover loss stays in the 1 to 3 dB range. Because a conventional conical-scan radar nutates on both transmit and receive, the round-trip penalty is roughly twice this figure.
Error Slope and Scan Geometry
G(θ) ≈ Gmax × exp[ −2.776 (θ / θ3dB)2 ]
Echo amplitude modulation for target offset ε at scan phase φ:
A(t) ≈ A0 [ 1 + ks ε cos(ωst − φ) ]
Error slope (normalized monopulse-equivalent sensitivity):
ks = (1 / G) × (dG / dθ) evaluated at the squint angle θq
Where θ3dB = half-power beamwidth, θq ≈ 0.41 θ3dB = squint, ωs = 2πfscan = nutation rate, ε = angular target offset, and ks = error slope. Crossover loss Lx ≈ 4.34 × 2.776 (θq / θ3dB)2 dB ≈ 2 dB at θq = 0.41 θ3dB.
Conical Scan Versus Other Tracking Methods
| Method | Pulses per estimate | Receiver channels | Crossover loss | Scintillation / jam resistance | Relative cost |
|---|---|---|---|---|---|
| Conical scan | Many (≥ 1 full scan) | 1 | 1 to 3 dB | Poor (AM jam, scintillation) | Low |
| Conical scan on receive (COSRO) | Many | 1 | 1 to 3 dB | Improved (scan rate hidden) | Low to moderate |
| Sequential lobing | 2 to 4 | 1 | 1 to 3 dB | Poor to fair | Low |
| Amplitude monopulse | 1 | 3 (sum, Δaz, Δel) | ~0.5 dB | Excellent | High |
| Phase monopulse | 1 | 3 | Low | Excellent | High |
Frequently Asked Questions
What squint angle and crossover loss are typical in a conical-scan radar?
The beam is squinted off the tracking axis by roughly 0.3 to 0.5 of the half-power beamwidth, where the pattern slope (gain change per unit angle) is steepest and angle-error sensitivity peaks. At that squint the on-axis gain sits about 1 to 3 dB below the beam peak; this one-way deficit is the crossover loss and is a permanent signal-to-noise penalty versus pointing straight at the target (roughly doubled on a round trip when both transmit and receive nutate). A squint near 0.41 × HPBW is a common compromise, placing the one-way crossover near 1.5 to 2 dB while keeping the error slope near maximum.
Why is conical scan vulnerable to amplitude jamming and target scintillation?
The angle error is extracted from the pulse-to-pulse amplitude modulation as the beam nutates, so the tracker cannot tell a true pointing error from any other amplitude variation. A target whose radar cross section fluctuates near the scan frequency injects a false error and the antenna tracks the scintillation. An inverse-gain (gain-inversion) jammer senses the scan rate and transmits an anti-phase amplitude-modulated reply that pulls the tracker off the target. Monopulse avoids this because it forms the error from a single pulse using sum and difference channels.
How does conical scan on receive (COSRO) differ from conventional conical scan?
Conventional conical scan nutates the transmit beam, so the scan modulation rides on the transmitted signal and an ESM receiver can read the scan rate, enabling gain-inversion jamming. COSRO transmits a fixed non-scanning beam and nutates only the receive pattern, typically by switching among offset feeds or phase states. The target sees no scan modulation, so the scan rate stays hidden and inverse-gain jamming becomes far harder, while the single-receiver simplicity of conical scan is retained.