The fundamental advantage offered by wide radar bandwidth is increased information about the presence, location, and identity of targets such as ships, aircraft, and the earth's surface features. Such increased information is produced by the additional, independent target reflectivity data that can be collected. For example, consider a narrowband pulsed radar designed for aircraft and ship surveillance, operating at a single transmitted wave polarization. Assume that aircraft or ships occupy only a small sector of the radar's antenna beamwidth and are unresolved in range so that each echo pulse is a measure of the reflectivity of the entire aircraft or ship at an instantaneous viewing angle. If the target's viewing angle were then changing due to either radar platform or target motion, the radar could be said to be able to collect target reflectivity data in one dimension: reflectivity versus viewing angle. The same radar operated over a wide frequency band, for example, by changing the transmitter frequency from pulse to pulse, collects target reflectivity data in two dimensions: reflectivity versus frequency and viewing angle. A wideband short-pulse radar collects reflectivity data versus range delay and viewing angle. To the extent that the additional dimension in either case provides additional independent samples of target reflectivity data, then there is increased information about the target's presence, location, and physical characteristics. With regard to echo sample independence, it is well known that microwave reflectivity of targets such as ships, aircraft, or the earth's surface features fluctuates rapidly with both viewing angle and frequency. Thus, data collected over a wide range of viewing angles or frequencies can be expected to contain a large number of independent samples of target reflectivity.
Target recognition of ships, aircraft, and objects in space is probably the best known type of information provided by high-resolution radar data. These types of targets, viewed over a wide range of frequencies and viewing angles, provide independent samples of their reflectivity related to their physical characteristics. Target amplitude and phase data collected versus frequency and viewing angle from such a target can be converted into reflectivity estimates in one or more dimensions of target space. Such data, called the radar target image, provides information about a target's identity and other characteristics
of interest.
A quantitative relationship between the available independent target echo data and target information probably cannot be defined in any general sense. However, a quantitative assessment of the benefits of radar bandwidth can be obtained by relating the available content of independent reflectivity data to radar bandwidth and data collection time without regard to the contribution of such data to target information. Consider the echo signal produced by a short, single-frequency transmitted pulse reflecting from an extended target illuminated by the radar's antenna beam at a fixed viewing angle. The echo signal can be thought of as a measure of the reflectivity of the target versus range delay. Temporal resolution of the echo signal, by way of proper receiver design, can approach that of the transmitted pulse duration. In terms of transmitted pulse bandwidth B, the temporal resolution is about 1/B. Unambiguous sampling of the carrier-free form of such an echo pulse received by a coherent radar requires, according to the Nyquist criteria, a sampling rate of at least 2B samples per second for a total of 2Bdt samples from an echo signal to be sampled over a range-delay extent dt. Sampling at the Nyquist rate will then produce 2Bdt independent samples of target reflectivity, assuming that reflectivity varies independently at the sample spacing. The total data content from the sampled echo signal, when quantized into m resolvable bits in amplitude, is 2mBdt bits. The three quantities determining the target signal's data content are transmitted signal bandwidth, sampled range-delay extent, and amplitude quantization. For a given level of amplitude quantization and a given range-delay extent to be sampled, the data content of a single echo pulse can be seen to be directly proportional to transmitted bandwidth.
Donald R. Wehner. High-Resolution Radar. Second Edition - Artech House, 1995
