Low power, space combined, phased array radar

A plurality of mini radars that make the radar system conformable to a structure that it is attached or built into. A radar system includes a clock, a plurality of frequency modulated/continuous wave (FM/CW) radar units in signal communication with the clock and a processor in signal communication with the plurality of FM/CW radar units. Each of the plurality of FM/CW radar units includes a row of antenna elements.

BACKGROUND OF THE INVENTION

Current radar applications, in particular marine radar, are expensive, have significant weight, and have less than optimal range resolution. Some applications require a simple mechanically scanned array and some applications require electronic beam steering for high-speed review of a scanned volume. Also, these applications are ineffective at covering exceptionally large scanned volumes, such as on unmanned aerial vehicles (UAVs) where hemispheres of coverage are required with very low weight (a few pounds to a few tens of pounds) and very low aerodynamic drag. Previous design efforts have shown that active phased arrays (every element has its own transmit, receive, amplitude, and phase control) are expensive, complex, and heavy. Also, passive phased arrays with a central high-powered transmitter and passive low-pass phase shifters are less complex and have many advantages, but they are inefficient with RF transmission losses on both transmit and receive sides, despite all efforts to control losses in the power distribution network.

SUMMARY OF THE INVENTION

The present invention provides a radar that will permit: 1) a simple fixed beam; 2) electronic beam steering via coherent phase locked loop (PLL) phase shifts among the elements or subarrays; and/or 3) digital beam forming via digital phase adjustment and amplitude weighting of samples. Digital beam forming permits beam steering and the potential for multiple simultaneous beams.

The present invention includes a plurality of mini radars that make the radar system conformable to the structure that it is attached or built into. Phase errors caused by arbitrary curvature of a vessel or fuselage or vehicle, etc., can be corrected at each distributed mini radar. The phase error caused by the physical location of the mini-radar is compensated by a phase or frequency offset in addition to the nominal phase shift needed for beam steering and amplitude loss due to angular offset can be compensated by a digital amplitude multiplier.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates an exemplary radar system30that includes multiple radar units36(mini frequency-modulated/continuous-wave (FM/CW) radars or Linear FM Pulse Compression). Each of the radar units36is phase locked to a master timing oscillator (a clock34). Each radar unit36can have its transmitted modulation phase or FFT Processed receive signal adjusted (I&Q FFT Outputs multiplied by complex weight) such that a passive phase shifter function in a common phased array is performed. Also, output power from each of the radar units36is adjustable to allow amplitude taper on an array of antennas, to adjust beam steering, or both.

The system30includes a processor40that is connected to each of the radar units36and a display or other type of output device44that is in signal communication with the processor40. Adjusting the relative phase of digital phase lock loops within each of the radar units36permits electronic beam steering, electronic beam forming, or both. Also, an output level of transmitters within each of the radar units36is adjustable by electronic programming, such that an amplitude taper is applied across an antenna located within the radar units36to achieve low side lobe levels or multiple digitally formed beams.

FIG. 2illustrates details of the radar unit36. The radar unit36includes a multichip module (MCM)60that includes Pulse Compression or FM/CW radar components such as that described in U.S. Pat. No. 7,239,266, issued Jul. 3, 2007, which is herein incorporated by reference. The radar unit36also includes a direct digital synthesizer (DDS)62, a programmable logic device (PLD) or a field-programmable gate array (FPGA) controller64, an intermediate frequency (IF) strip72, an analog-to-digital (A/D) converter76, an external delay component70, a circulator66, and an antenna array68(see element or sub-array160inFIGS. 5-1and5-2). The FPGA controller64controls the DDS62and a digital attenuator80located within the MCM60. The external delay component70causes a delay of a signal produced by a voltage control oscillator (VCO)82and sends that signal to a first mixer84within the MCM60. The first mixer84within the MCM60combines the signal delayed by the external delay component70with a signal received by the antenna array68via the circulator66. The output of the first mixer84is sent to the IF strip72, that amplifies and applies a high pass filter and a band-limiting low-pass filter to the signal and sends the resulting signal to the A/D converter76, which converts the signal to digital and sends it to the processor40. The MCM60produces a transmission signal that is outputted by the antenna element or sub-array68,160via the circulator66. The clock34is connected to the DDS62and a second mixer86within the MCM40. The second mixer86adds the output of the DDS62to the clock signal to produce a signal that is sent to a Phase-Frequency Detector (PFD)88as a variable frequency reference after filtering. The PFD88compares the upconverted DDS signal with a divided-down signal from the output of the VCO82. The difference in frequency and or phase between the two signals is converted into an error voltage for delivery to a phased-locked loop (PLL) filter and buffer amplifier90that produces a control signal for the VCO82. If the physical size of the phase lock loop buffer amplifier90or any passive elements in filter are too large to be included in the MCM they may be added externally to the MCM.

The FPGA controller64includes memory and or computational capability to generate the desired modulation waveforms. The FPGA controller64connects to the DDS62and the digital attenuator80via a high-speed serial or parallel data bus.

The antenna element or sub-array68,160is directly integrated with the other components of the radar unit36in order to provide a radar signal with a very low voltage standing wave ratio (VSWR) (less than 1.2:1) across an entire operating bandwidth. Also, phase noise of the radar unit36is not worse than approximately −100 dBc/Hz at 100 KHz offset from the transmit frequency. The phase noise is cancelled at the first mixer84, due to the output of the external delay component70. In other words, the time of arrival of energy reflected from an input port of the antenna element or sub-array160arrives at virtually the same time as the local oscillator signal (signal outputted by the VCO82within the MCM60).

The phase noise of the clock34is less than −145 dBc/Hz at 100 KHz from the clock frequency. The low phase noise of the clock34assures that any multiplication of this phase noise within the MCM60remains acceptably low and that the A/D clock supplied by a frequency divider from the master clock34is achieved with exceptionally low jitter, thus assuring maximum possible signal-to-noise ratio.

The DDS62provides a sweep reference frequency and includes a digital-to-analog (D/A) converter having at least twelve bits for the lowest possible phase noise.FIG. 3illustrates a radar system100formed in accordance with an alternate embodiment of the present invention. The radar system100includes one or more radar units102that are connected to a common clock, synthesizer, and controller104. Like the radar system30, the radar units102are connected to the processor40, which is then in signal communication with the output device44.

As shown inFIG. 4, each of the radar units102includes the MCM60, the external delay unit70, the circulator66, the antenna array68, the IF strip72, and the A/D converter76. The connections of these components are similar to that of the radar unit36shown inFIG. 2. The clock, synthesizer, controller unit104includes the master clock34, the DDS62, and the FPGA controller64that are connected in a similar manner as that described with regard to the radar units36, as shown inFIG. 2. Because the radar units102share one DDS and FPGA controllers with other radar units102, digital steering of radar signals produced by the plurality of radar units102cannot be performed like they can in the radar system30described above. This configuration is used where a transmitted beam steering is not required but multiple digitally formed receive beams are required. The single source of modulation and clock reference produces a single beam that may be divided into sub-regions by multiple receive beams formed as shown inFIG. 6or8.

FIGS. 5-1,5-2, and5-3illustrate an exemplary layout of thirteen radar units168coupled to corresponding vertical antenna arrays158. In this embodiment, each of the vertical antenna arrays158includes four antenna elements160(i.e., microstrip patch elements). The vertical antenna arrays158are separated by an isolation wall162. An exemplary isolation wall is formed of a carbon fiber material or a comparable material for performing 25 to 30 dB isolation between the vertical antenna arrays158. The antenna elements160and the isolation walls162are mounted to an antenna circuit board164.

Each of the radar units168is mounted to radar circuit boards. The radar circuit boards are mounted to the antenna circuit board164on a side that is opposite the antenna elements160. Located above the radar units168is a circuit component172. Electrical traces connect the antenna elements160through the antenna circuit board164to their respective radar units168or the circuit component172. The circuit component172includes the master clock34, such as that shown inFIGS. 1 and 2, or includes the master clock34, the DDS162, and the FPGA controller64, such as that shown inFIGS. 3 and 4.

Multiple transmit beams may also be formed simultaneously. The multiple transmit beams are formed by combining subsections of the available overall array to form individual beams. For example, if there are 12 array elements in azimuth fed by 12 “mini-radars” then one beam is produced on the left from the left most 6 elements and another beam is produced on the right with the other half. A beam will be formed independently for each subset of associated modules.

Beam transmission can take on a dynamic quality. For example, for a short period one beam is transmitted then in the next moment two beams are transmitted independently. The available power associated with each transmit beam is reduced in direct proportion to the number of beams that are formed. The receive beams that can be formed are constrained to exist within the illumination of each transmit beam. So the larger the number of transmit beams the greater the beamwidth and the more scanning volume is available within the beam. Technically the digital beam forming can create a beam that points in any desired direction, however if no transmitter power is radiated in the steered direction then no target power can be received from that direction. There may be other reasons for steering to look where no signals were transmitted. For example, it is possible to locate the direction of a jammer by locating the max detected power, or to listen to a broadcast data transmission from a source that is not a radar. The present invention allows for simultaneous datalinking and radar operations.

FIG. 6illustrates a plurality of subarray antennas202similar to those described above, each of which is connected to a separate FM/CW or Pulse Compression radar204with a master clock206that sends a clock signal to each of the FM/CW or Pulse Compression radars204. This is similar to the radar system100shown inFIG. 3. A phase and amplitude controller208(e.g., DSP Controller) sends phase and amplitude control signals to each of the FM/CW or Pulse Compression radars204, similar to that shown inFIGS. 3 and 4at Point A. The outputs of the FM/CW radars204are sent to a processor (e.g., the processor40). The processor performs a fast-Fourier transform (FFT) of the received signals from the FM/CW radars204to produce spectral I and Q values. The processor then performs one or more digital beam-forming processes (see blocks212-216) that electronically steer the beam by re-using the original FFT I&Q data with appropriate complex weights and summation.

The quadrature baseband I and Q values can be used to represent a radio signal as a complex vector (phasor) with real and imaginary parts. Two components are required so that both positive and negative frequencies (relative to the channel center frequency) can be represented as follows:
s(t)=x(t)+j y(t)s(t) is the complex baseband signalx(t)=i(t) is the real party(t)=−q(t) is the imaginary partj is √{square root over (−1)}.

For beamforming, the complex baseband signals are multiplied by the complex weights to apply the phase shift and amplitude scaling required for each antenna element.
wk=akej sin(θk)
wk=akcos(θk)+j aksin(θk)wkis complex weight for the kthantenna elementakis the relative amplitude of the weightθkis the phase shift of the weight (i.e., differential phase shift), θk=360(d/λ)sin(θ),d is the spacing between antenna elements,λ is the freespace wavelength, andθ is in the desired scan angle in degrees.

The amplitude weight of each element (k) is determined by a desired taper function. There are many amplitude tapers that are used across an array. A simple example is a Cosine on a pedestal—where “0” is the center of the array with a max weight of 1 plus the pedestal offset and the remaining values are scaled as from cos(φ) plus the pedestal offset: Ampk=cos (φ)+offset pedestal.

A general-purpose digital signal processor (DSP) can implement the complex multiplication for each antenna element:
sk(t)wk=ak{[xk(t)cos(θk)−yk(t)sin(θk)]+j[xk(t)sin(θk)+yk(t)cos(θk)]}

FIGS. 7 and 8illustrate a radar system280that performs elevation monopulse, thereby allowing more accurate elevation detection of targets. The radar system280includes subarray antennae, FM/CW or Pulse Compression radars, a Master clock similar toFIG. 6. The radar system280also includes a combiner288for each pair of antenna elements160that is located in each antenna element array158. The system shown inFIGS. 7 and 8can perform EL monopulse. The phase and amplitude control is relative to each adjacent subarray column not within the each small sub-array160. The output of each the power combiners288is submitted to a 180-degree hybrid component290that produces an elevation delta channel signal and a summation channel signal. The sum channel signal is also connected to a circulator (e.g. the circulator66) of the radar system280. Another receiver (not shown) receives the output of the Elevation Delta Channel. The Sum Channel is used to both transmit the signal and receive the main sum signal. The hybrid is located within the radar unit168. A processor performs target detection using the collection of elevation summation beams from the sum channels of the 180-degree hybrid components290. Also, the processor performs target tracking using an azimuth monopulse tracking via digital beam forming algorithm during receive processing and elevation monopulse tracking by comparing phase and amplitude of the signal in the summation channel with the phase and amplitude of the elevation delta channel.

The Azimuth and Elevation Monopulse Beam former algorithm can be expressed mathematically for Elevation Monopulse as follows: vector sum from 0 to N/2 of the upper half of all antenna elements minus the vector sum of N/2 to N of the lower half of the elements shifted by 180 degrees. Similarly, the Azimuth monopulse tracking beam is formed by the vector sum of the left half of the array and subtracting the vector sum of the right half of the array elements phase shifted by 180 degrees.

As shown inFIG. 9, several fix-mounted, electronically scanned antennas350coupled to a processor, as described above, are placed around a pilot house of a (marine or land-based) vehicle320to provide panoramic or 360-degree coverage with very high range resolution of a few feet to identify small skiffs at sea and alert security details onboard, etc. Combatants cannot readily see the simple thin active antenna structures that do not mechanically move and do not attract attention. Combatants are known to shoot at and attempt to destroy visible satellite antennas, rotating marine radar antennas. Other exemplary radar applications include, but are not limited to: 1) covert littoral small craft operations; 2) marine barge radars used within the very narrow confines of (e.g. European and U.S.) rivers, locks, and canals where marine radar carriage is mandated; and 3) antipirate applications onboard cargo ships.