Antenna Theory and System Analysis
i. Low frequency SAR design aspects
ii. Useful bandwidth for ground imaging SAR
iii. Internal and external additive noise and modulated signal impact
iv. SAR imaging principles
v. Antenna electromagnetic modeling
vi. Wide band waveforms
vii. Emission adaptation
viii. SAR imaging of moving objects
ix. Comparison with collected SAR data
Based on the scattering theory thus established, achievable resolution when using large fractional bandwidth is investigated and SAR theory for large fractional bandwidth developed.
A significant concern is antenna implementation. High directivity is not a requirement for meter wave SAR, but there are strict conditions for controlled antenna patterns. These are hard to make compliant with the demands for wide band operation and the aerodynamic restrictions imposed by aircraft integration. Satellites would allow more antenna integration flexibility but have to be ruled out because of ionospheric interference in the lower frequency part of band considered. A particular concern is the sheer size of meter wave antennas making integration into small platforms, e.g. drones, difficult. A remedy is that at meter waves exterior noise is dominant over internal noise implying that significant antenna gain is of no particular advantage. For this reason the book develops theory for electrically small antennas, and in particular of such designs that fulfil the requirements for meter wave SAR installed on a small platform. The small antenna development has been proved experimentally by the CARABAS III system.
The qualitatively differences from microwaves of transfer characteristics (e.g. due to electrically small antennas) and internal and external noise, all call for compensatory measures. Standard design rules, e.g. the radar equation, take a different shape. Also challenges, not present for microwaves, enter at meter wave – foremost the issue of cohabitation with communication signals for the fractionally wide band exploited by meter wave radar. Communication interferes may be extremely strong coming from e.g. nearby commercial broadcast transmitters. Adopting emerging technologies for waveforms and transceiver design, their negative impact can be largely reduced. In particular active cancellation of the wide band interference of digital television has been demonstrated feasible. In all during the extensive data collection campaigns conducted the mitigation methods adopted the impact of interference has been demonstrated to be relatively minor.
Equally crucial is the impact of radar transmissions on the multitude of communication frequency allocations within the adopted meter wave band. The typical requirement legitimating radar operation is that the radar signals should be notched down to noise level across these. Considerable effort has gone into developing required technology. Foremost is the development of continuous wave radar, providing the high spectral control required of radar transmissions. Also techniques have been developed to, by mere phase control of a constant amplitude transmit signal, attain spectral notches- a prerequisite since amplitude modulation of the transmitted signal is an unwanted complication.