Radar has long been employed in applications such as air traffic control, fire control, navigation, etc. Due to the many advantages of radar usage in such applications, radar has also been the subject of continuous improvement efforts. One of the fundamental requirements of many types of radar is the implementation of some form of beam steering in order to conduct a sweep of a particular area in an effort to, for example, detect contacts, targets, navigation aids, etc. Conventional radars typically employed mechanical beam steering methods. For example, a commonly recognized image of a radar antenna is a parabolic antenna mounted on a rotating apparatus which steers the antenna. Such rotating radars often utilize complex mechanical mechanisms such as hydraulics, electric motors or hinge appendages in order to achieve the rotation that provides beam steering. However, mechanical apparatuses such as those listed above often require intensive maintenance in order to ensure optimal performance. Additionally, failure of a single element of rotating radars may render the entire apparatus unusable. Rotating radars also suffered limitations in scanning rates due to the mechanical rotation, which translated into limitations with respect to contact or target detection.
In order to overcome several of the disadvantages of conventional radars, electronic scanning antennas (ESAs) have been developed, which are also known as phased array radars. ESAs are a revolutionary type of radar whose transmitter and receiver functions are composed of numerous small transmit/receive (T/R) modules. ESA radars perform electronic beam steering which can be done without the limitations caused by physical rotation. Accordingly, ESAs feature short to instantaneous (millisecond) scanning rates. Additionally, since ESAs do not rotate, ESA radars have vastly simpler mechanical designs and require no complex hydraulics for antenna movement or hinge appendages that may be prone to failure. The ESA radar also occupies less space than a typical radar because ESAs have reduced infrastructure requirements as compared to rotating radars. The distributed nature of the transmit function in an ESA also eliminates the most common single-point failure mode seen in conventional rotating radars of lost ability to rotate. Given the improvements above, ESA maintenance crews are far less severely taxed, and the ESA radar is much more reliable than a comparable rotating radar. In addition to having much higher scanning rates than conventional radar, ESAs also typically have a much longer target detection range, higher capabilities in terms of the number of targets that can be tracked and engaged (multiple agile beams), low probability of intercept, ability to function as a radio/jammer, simultaneous air and ground modes, etc.
Although ESA radars represent a significant improvement over conventional radars, there is still a desire to improve the capabilities of ESA radars. Improvement among ESA radars is often achieved by reducing scanning rates, providing narrower (or more focused) beams, etc. In order to form focused beams with low pattern sidelobes, deep transmit and receive pattern nulls, good monopulse measurement capabilities, low time sidelobe levels, and high quality adaptive cancellation patterns, an antenna must be accurately characterized. Characterization includes accurate knowledge of the amplitude, phase, time and group delays of antenna components. The stationary or non-drifting components in the ESA antenna such as the T/R module electronics, analog signal paths, and manifolds are normally characterized at the factory, typically in a near field range. The ESA factory characterization is usually performed over a set of radiate/receive radio frequencies and operating temperatures. The characterization values are then stored in a tune table that is used by the beam steering computer to adjust the amplitude and phase values in the T/R module to form the desired beam pattern.
A desire for detection of low radar cross section targets in heavy clutter backgrounds in both ground and airborne applications may be met by radar systems with very low phase and amplitude noise stability and high dynamic ranges. Unfortunately, levels required to implement such radar systems may not be directly achievable at the component level, even with state of the art electronics. However, system improvements in stability and dynamic range may be achievable at the design level by paralleling the driving subsystem. For example, in the case of phase noise, paralleling an exciter such as distributed waveform generators (DWFGs), a master oscillator, and the receiver may assist in achievement of stability and dynamic range. Such paralleling may be achieved through distributing the DWFG and receivers on sub-arrays incorporated in the ESA architecture.
However, with the advent of DWFGs and distributed receivers to support a system requiring low noise, high stability, and high system dynamic ranges and the advent of technologies such as digital beam forming, performance would be enhanced by continually characterizing signal paths through the ESA rather than by simply relying on the factory characterization. Unfortunately, there is currently no mechanism by which parallel components within ESA architecture can be calibrated during normal operation using common signal injection paths between adjacent sub-arrays.