| dc.description.abstract | The end game guidance and control of an interceptor for neutralization of a hypersonic vehicle or a maneuvering aerial target is a challenging problem. This problem can be addressed in two ways: through faster autopilot coupled with faster lateral error correction and through the use of better on-board homing sensor. The former demands not only sophisticated control and guidance algorithms, but also faster control mechanisms such as larger control surfaces driven through faster and bigger actuators, bulky reaction control systems or thrust vector control systems. All these options lead to increase in the weight of the system, which is not only undesirable with respect to weight penalty, but also makes the vehicle performance sluggish. The latter method helps to increase the homing duration via advances in sensor technology, which will lead to less dependence on heavier control systems. This transforms to a need of a seeker with larger lock-on and tracking range in order to cater to higher closing velocities.
Active Radio Frequency Seekers are generally used as the homing sensor in long range interceptors, as they give all-weather applicability. For intercepting long range hypersonic targets, or highly manuvering aerial targets at longer ranges, Phased Array Seekers (PAS) are the promising homing sensor, as they give longer tracking ranges, even against targets with lower Radar Cross Section (RCS). Target tracking, and Line-Of-Sight (LOS) estimation through the PAS have poor efficacies, mainly due to the beam quantization, arising because of the phase angle digitization, resulting due to the use of Digital Phase Shifters (DPS). The limited space, as well as the constraint of weight, put a serious limit on the antenna aperture and the transmit power of the seeker. To achieve longer tracking ranges and better target parameter estimation, the seeker beam is required to position its boresight near to the target true LOS. This demands the placement of PAS beams in close vicinities, which is in contrast to the PAS capability, as the use of DPS limits the close placement of beams. A few papers in the existing literature propose the placement of beams in close vicinities through the use of truncated phase angles on to the PAS elements, incurring beam pointing errors. The pointing errors are proposed to be compensated through in-lab measurements and calibrations. This thesis proposes methods of high fidelity beamforming to reduce such calibration needs. We propose novel methods for reducing the beam pointing quantization in PAS, and derive their complete mathematical model. We also propose suitable strategies for effective target tracking through the PAS, which uses the proposed beamforming methods.
In the first part of the thesis, we introduce novel methods for high fidelity beamforming with reduced beam quantization. In the first set of methods, named as Phase Angle Bunching (PAB) methods, we propose bunching of DPS digitized phase angles to be assigned to PAS elements. These methods are able to achieve error-free closely spaced beam pointing. Another set of methods, named as Phase Angle Round-Off (PARO) methods, has been proposed for the purpose, which gives lower beam pointing steps by using rounding-off of phase angles. An Optimal Rounding-Off realization has been derived to minimize beam pointing errors. Another novel method, named as Composite Beamforming (CB) method, has been proposed for partially reducing the step size by forming additional beams in-between the ideal feasible beams. The mathematical formulations for the beamforming, and the monopulse characterization for the CB method have also been derived. An Off-Axis scan philosophy over the composite beams has been proposed for the Line-Of-Sight (LOS) estimation, and the electronic beam steering. The CB method gives mathematically tractable partially reduced quantized beams, which makes the implementation of Off-Axis scan based on the mathematical model feasible, relaxing the need of extensive in-lab calibrations of the physical seeker. We have employed the proposed methods on Uniform Linear Arrays (ULA) to demonstrate their efficacies. We propose unique strategies, to assign the relative phase angles to the ULA elements, for each of the proposed methods and their realizations. The implementation aspects and approaches for the proposed methods, and when they are to be used for implementation on to the seeker, have also been discussed in the thesis.
In the second part, we explore options for beam scanning and target tracking. First, the On-Axis scan method, which is in-general used in the on-board seeker, has been briefly discussed. Then, we have formalized an Off-Axis scan method based on the monopulse error characteristics of the PAS beams. We propose to use the Off-Axis scan, with the CB method, for the efficient tracking of target through the on-board PAS. The parametrization and characterization of the composite beams, for target Direction-Of-Arrival (DOA) estimation, have been carried out in the thesis. We have proposed strategies, to be used for the Off-Axis scan, to suppress the estimation noise dependent jitters, and to activate LOS dynamics depended beam switchings.
In addition, to demonstrate the target tracking efficacies, we have carried out the implementation of the proposed Composite Off-Axis scan philosophy, to engage the target during the homing phase. The Proportional Navigation (PN) guidance scheme has been used for the engagement. We have carried out extensive Monte-Carlo simulations, to demonstrate and compare the performances of the four proposed target tracking strategies, for target tracking during the homing phase and interception.
To summarize, the thesis contributes by developing high fidelity beamforming methods for forming closely spaced beams for active RF phased array seekers, and also proposes strategies for beam selection and beam switching during the target tracking. | en_US |
