Chirp-based method and apparatus for performing distributed network phase calibration across phased array antenna

[00002] The present invention relates to subject matter disclosed in our co-pending U.S. patent application Ser. No. 10/603,843, filed Jun. 25, 2003, entitled: “Chirp-based Method and Apparatus for Performing Phase Calibration Across Phased Array Antenna” (hereinafter referred to as the '843 application), assigned to the assignee of the present application, and the disclosure of which is incorporated herein.

[00003] The present invention relates in general to communication systems and subsystems therefor, and is particularly directed to a new and improved, distributed chirp-based arrangement for deriving a very accurate measure of phase variation through respective sections of a reference frequency transport cable of a relatively physically large phased array antenna architecture, such as a spaceborne synthetic aperture radar system.

[00004] Relatively large phased array antenna architectures, such as but not limited to spaceborne, chirped synthetic aperture radar systems, typically contain a multiplicity of transmitters and receivers distributed across respective spaced apart arrays. In such systems, a common, very precise reference frequency signal is customarily supplied to both the transmit and receive array portions. As such, there is the issue of how to take into account phase shift associated with variations in the substantial length of signal transport cable that links the reference frequency source, which is customarily installed in one location of the array, with the remaining portion of the array.

[00005] Because terrestrial open loop calibration of the system suffers from the inability to take into account variation in temperature along the transport cable due to changes in sun angle, and variations in obscuration by components of the antenna support platform in the antenna's space-deployed condition, it has been proposed to perform temperature measurements at a number of locations along the cable and provide phase compensation based upon the measured values. A drawback of this approach stems from the fact that there are non-linearities within the cable, so that over different temperatures it is necessary to employ a larger number of values in the calibration table. In addition, because this technique performs multiple measurement points along the cable, it introduces associated variations in loading which, in turn, produce separate amounts of phase shift to the reference frequency signal.

[00006] In accordance with the invention disclosed in the above-referenced '843 application, this transport cable-based phase variation problem is effectively obviated by injecting an RF chirp signal into the signal cable from the remote end thereof, and correlating the returned chirp that is reflected from the reference source end with a delayed version of the injected chirp, to derive a measure of the phase delay through the cable between its opposite ends.

[00007] Although this approach works quite well for a single length of cable, it can become cumbersome when applied to a multinode system, wherein the reference signal is to be delivered to a plurality of spatially separated array sites. One straightforward approach would be to implement a star-configured architecture, with each spoke of the star containing its own dedicated chirp generator and associated processing circuitry. Unfortunately, such an approach is hardware intensive, and costly to implement.

[00008] In accordance with the present invention, this problem is effectively obviated by employing a distributed network to connect multiple array nodes with a single source of the reference frequency signal, and injecting a single chirp from a far end node of the distributed reference frequency transport medium toward the reference frequency source node. The source of the reference frequency signal is coupled to the reference frequency signal transport medium by way of a bandpass filter, which is centered on the output frequency of the reference frequency signal generator.

[00009] A chirp signal, such as that produced by a direct digital synthesizer, is injected onto the reference frequency signal transport medium at a downstream-most end of the cable. The chirp signal propagates ‘up’ the cable in a ‘forward’ direction and is extracted at each of a plurality of sites or nodes to which the reference frequency signal is distributed, before being reflected from the bandpass filter and returning back ‘down’ the cable in a ‘reverse’ direction.

[00010] Each reference frequency utilization location along the cable is configured to extract the upstream-directed chirp signal and the reflected and downstream-directed return chirp signal. These two chirp signals are coupled to respective inputs of a mixer, the difference frequency output of which is coupled to a frequency domain operator, such as a Fast Fourier Transform (FFT)-based operator. The FFT operator is operative to process the difference frequency content of the output of the mixer to derive a measure of the electrical distance between that respective site and the reflective termination at the reference frequency signal source end of the cable. Given this electrical distance the array signal processor for that site determines the amount of phase shift which the reference frequency undergoes in traversing the section of cable between the reference frequency signal source end and the site or node of interest.

[00013] Before describing in detail the distributed chirp-based phase calibration arrangement of the present invention, it should be observed that the invention resides primarily in a modular arrangement of conventional communication circuits and components and an attendant supervisory controller therefor, that controls the operations of such circuits and components. In a practical implementation that facilitates their being packaged in a hardware-efficient equipment configuration, this modular arrangement may be implemented by means of an application specific integrated circuit (ASIC) chip set.

[00014] Consequently, the architecture of such arrangement of circuits and components has been illustrated in the drawings by a readily understandable block diagram, which shows only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustration is primarily intended to show the major components of the invention in a convenient functional grouping, whereby the present invention may be more readily understood.

[00015] Attention is initially directed to the FIG. 1, wherein an embodiment of the distributed chirp-based cable calibration arrangement of the present invention is diagrammatically illustrated. As shown therein, a reference frequency signal generator 10, such as a very stable oscillator that drives a remote antenna array 20, is coupled to a bandpass filter 30, which is centered on the output frequency of the reference frequency signal generator. Bandpass filter 30 is coupled to a first end 41 of a length of cable 40, which serves to supply the reference frequency signal produced by generator 10 to a plurality of remote array sites 50-1, 50-2, . . . , 50-N distributed along the cable.

[00016] As pointed out above, one or more portions of the reference frequency signal distribution cable 40 can be expected to be subjected to temperature variations (and accompanying variations in cable length/transport delay) due to changes in temperature, such as those associated with changes in sun angle, and obscuration by components of the antenna support platform. The present invention solves this problem and provides an accurate measure of respective sections of cable transport delay, by injecting a chirp signal from a second or downstream-most end 42 of the cable. When so injected by a chirp generator 60 (such as, but not limited to a direct digital synthesizer (DDS)), the chirp signal propagates up the cable in a ‘forward’ direction and is extracted at each of the distributed-sites 50-i, before being reflected from the bandpass filter 30 and returning back down the cable in a ‘reverse’ direction.

[00017] Each location 50-i contains a pair of forward and reverse couplers 51 and 52, that are respectively operative to extract the upstream-directed chirp signal shown at 45 in the frequency vs. time diagram and the reflected and downstream-directed return chirp signal shown at 46. The forward chirp signal processing path from coupler 51 is coupled through an amplifier 61 to a first input 71 of a mixer 70. The reverse chirp signal processing path from coupler 52 is coupled through amplifier 62 to a second input 72 of mixer 70. The output of the mixer is coupled to a low pass filter 80, which is operative to couple the difference frequency output of mixer 70 to a Fast Fourier Transform (FFT) operator 100.

[00018] FFT operator 100, shown in detail in FIG. 2 to be described, is operative to process the difference frequency content of the output of mixer 70 to derive a measure of the electrical distance between site 50-i and the reflective termination (bandpass filter 30) at the reference frequency signal source end 41 of the cable 40. Given this electrical distance the array signal processor 90 for site 50-i may readily determine the amount of phase shift which the reference frequency undergoes in traversing the section of cable between reference frequency signal source end 41 and the site or node of interest.

[00019] Referring now to FIG. 2, a non-limiting example of an implementation of the FFT operator 100 is shown as comprising an analog-to-digital (A/D) converter 110 that is coupled to sample the difference frequency output of the low pass filter 80. The sampled difference frequency data is subjected to an FFT 120, so as to provide a relatively coarse measurement of the electrical distance between the reference frequency signal source termination 41 and the node of interest. The output of FFT 120 is then subjected to a centroid finder 130, which reduces the relatively coarse electrical distance measurement to a relatively fine electrical distance value. The electrical distance value produced by centroid finder 130 is then converted into a phase offset value for that node's cable delay by means of a unit converter 140.

[00020] It should be noted that the rate of change of cable length is considerably slower relative to the processing time associated with the operation of the invention. As noted previously, in an environment, such as a spaceborne application, changes in cable length due to temperature are ambient effects, such as sun angle and obscuration by components of the antenna support platform. Such changes are very slow relative to the high signal transport and processing speeds associated with the generation of the chirp and correlation processing of the chirp return, which may be in the pico to microsecond range.

[00021] While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art. We therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.