Patent Description:
Radar apparatuses may be used in a variety of applications. For example, automotive radar apparatuses, such as frequency modulated continuous wave (FMCW) radar apparatuses, may be used for cruise control, collision warning, blind spot warning, lane change assist, parking assist, and rear collision warning, among other applications. Radar apparatuses may transmit a sequence of equally spaced chirps, which are frequency ramps, in a frame. The reflected signal is down-converted, digitalized and processed to estimate range and velocity of objects surrounding the radar apparatus.

These and other matters have presented challenges to efficiencies of radar implementations, for a variety of applications. <CIT> discloses a radar system is provided that includes a radar transceiver integrated circuit (IC) configurable to transmit a first frame of chirps, and another radar transceiver IC configurable to transmit a second frame of chirps at a time delay ΔT, wherein ΔT=Tc/K, and Tc is an elapsed time from a start of one chirp in the first frame and the second frame and a start of a next chirp in the first frame and the second frame, wherein the radar system is configured to determine a velocity of an object in a field of view of the radar system based on first digital intermediate frequency signals generated responsive to receiving reflected chirps of the first frame and second digital IF signals generated responsive to receiving reflected chirps of the time delayed second frame, wherein the maximum measurable velocity is increased by a factor of K. <CIT> discloses a method for determining distances and relative velocities of objects with using a radar includes transmitting a ramplike frequency-modulated transmission signal whose modulation pattern includes multiple sequences of ramps having an identical ramp slope, which alternately follow each other, the sequences having a frequency offset and a time offset with respect to each other; undersampling, and subjecting to a 2D Fourier transform, base band signals for the individual ramps; determining hypotheses for the distance and the relative velocity v of an object based on alternative distance velocity relationships and based on periodic ambiguous information about velocity; ascertaining degrees of the agreements of a phase relationship between spectral values of the spectra with phase relationships expected for the hypotheses between spectral values of the sequences; and determining unambiguous estimated values for the distance and the relative velocity by selecting a hypothesis having the maximum agreement.

Various example embodiments are directed to issues such as those addressed above and/or others which may become apparent from the following disclosure concerning determining a velocity of an object via a radar type apparatus.

In accordance with the invention, there is provided a radar method and an apparatus as defined by the appended claims.

Aspects of the present disclosure are believed to be applicable to a variety of different types of apparatuses, systems and methods involving determining a velocity of an object. In certain implementations, aspects of the present disclosure have been shown to be beneficial when used in the context of independent processing of multiple (e.g., two or three) interleaved chirp sequences, sent as a single set of chirps in a radar frame, to calculate a velocity of an object. While not necessarily so limited, various aspects may be appreciated through the following discussion of non-limiting examples which use exemplary contexts.

Accordingly, in the following description various specific details are set forth to describe specific examples presented herein. It should be apparent to one skilled in the art, however, that one or more other examples and/or variations of these examples may be practiced without all the specific details given below. In other instances, well known features have not been described in detail so as not to obscure the description of the examples herein. For ease of illustration, the same reference numerals may be used in different diagrams to refer to the same elements or additional instances of the same element. Also, although aspects and features may in some cases be described in individual figures, it will be appreciated that features from one figure or embodiment can be combined with features of another figure or embodiment even though the combination is not explicitly shown or explicitly described as a combination.

In many radar systems, such as in frequency modulated continuous wave (FMCW) radar systems, consecutive sequences of equally spaced chirps may be transmitted and received to generate radar signals. In certain such radar systems, after each sequence of chirps, there is an idle time for processing the resulting radar signals. The acquisition time of the sequence of chirps and the resulting idle time form a radar frame. The reflected signal is mixed with the transmitted signal to form an intermediate frequency signal that is filtered and digitized. Signal processing is performed on the digitized intermediate frequency signal to extract the range and velocity (and optionally the angle) of an object in view of the radar system. The maximum unambiguous velocity that may be measured is an estimation of the velocity of object moving at a velocity that is smaller than the maximum measurable velocity. The maximum unambiguous velocity may be erroneous in both magnitude and sign. As an example, a radar system with a maximum measurable velocity of <NUM> kilometers per hour (kmph) may estimate an object moving at <NUM> kmph as moving at a velocity of negative <NUM> kmph. Embodiments in accordance with the present disclosure are directed to determining a velocity of an object by radar using time interleaved chirp sequences, thereby increasing a maximum unambiguous velocity measure of the apparatus while maintaining a high frame rate. The interleaved chirp sequences may be generated by N chirp ramp sequences (N being an integer greater than one) in which a time-offset is located between the odd-indexed chirps with respect to the even-indexed chirps. A digital signal processing technique separates the odd and even chirps and creates two separate sequences (e.g., processing flows) which leads to two separate range and velocity estimations (e.g., direct range and velocity measurements) of the same scenario. The velocity estimations may be implemented by single or multimode radar apparatuses where different modes are active in the same radar system cycle.

Velocity of an object in view of the radar apparatus may be estimated by measuring a phase difference across consecutively received chirps. A large chirp time Tchirp (e.g., the elapse time from a start of a chirp to the start of the next chirp) may result in phase rollover that causes errors in the velocity estimate. A radar apparatus with small velocity resolution may require a long chirp time Tchirp and a large number of processed chirps Nchirp, such that <MAT> where TMeas = NchirpTchirp. With a large Tchirp, the radar apparatus unambiguously detect objects up to a velocity <MAT>, where λ is the wavelength corresponding to the center frequency of a chirp. As such the maximum unambiguous velocity measured by the radar Vmax is inversely proportional to the chirp time. Various factors limit the minimum achievable Tchirp, and thus the maximum Vmax. Example factors include the bandwidth spanned by a chirp and the slope of the chirp. For example, a high radar frame rate may limit the available time for the measurement TMeas and for processing TProc), such that TFrame = TMeas + TProc. Embodiments in accordance with the present disclosure involve use of two interleaved sequences of chirps, which are transmitted as a single sequence, such that the TFrame is not enlarged and the velocity ambiguity is resolved. More specifically, the velocity is determined by associating the phase difference between the detected peaks to an indirect velocity measurement. In some examples, the unambiguous velocity of an object is corrected by comparing the different velocity obtained by the direct and the indirect methods.

Turning now to the figures, <FIG> is an example radar apparatus, in accordance with the present disclosure. The apparatus <NUM> is or can include a radar system, such as an automobile radar system, having circuitry such as the illustrated front-end (radar) circuitry <NUM> and back-end (radar) circuitry <NUM>. The apparatus <NUM> may increase a maximum velocity measure while keeping a high frame rate through the use of two interleaved chirp sequences and processing of an additional (indirect) velocity measure.

The front-end circuitry <NUM> sends chirp sequences and receives responsive chirps. The front-end circuitry <NUM> may include transceiver circuitry and signal processor. The transceiver circuitry is arranged with an antenna to output chirp sequences and to receive the return chirps indicative of reflections from the output chirps, sometimes represented as frequency modulated continuous wave (FMCW) signal. As further described below, the chirp sequences transmitted in a frame may include two interleaved chirp sequences, with each of the interleaved chirp sequences being characterized by a common time spacing between respective chirps of the respective chirp sequence, and each chirp of one of the chirp sequences being offset by an amount of time that is different than the common time spacing. For example, the front-end circuitry <NUM> may be configured to generate and send the two interleaved chirp sequences as a single sequence of chirps via the radar transceiver. The offset may be associated with or cause a phase difference between detected peaks of resulting Doppler-spectrum data sets (e.g., Doppler maps generated using the chirp sequences and reflections).

As may be appreciated, a signal processor is configured and arranged with the transceiver circuitry (at least a portion of which may be part of the front-end circuitry) to mix the output chips with the responsive chirps (e.g., the reflections) and to generate a digital stream of input data from the mixture. The signal processor can include an analog-to-digital converter (ADC) and a timing circuit. In specific embodiments, the front-end circuitry may include a transmitter (TX), receiver (RX), one or more antennas, a timing circuit and/or an ADC, such circuitry may further include logic/computer circuits for carrying out the related activities (transmitting, receiving, timing control, analog-digital signal conversion, etc.) for each.

The sequence of chirps is sometimes referred to as a frequency ramp signal, are transmitted, hit an object, and comes back to the transceiver circuitry with a given delay. This delay represents the time-of-flight and is used to detect objects and, optionally, estimate their distance from the apparatus <NUM>. For example, both the internal receiver signal and external delayed received signal (e.g., responsive chirps as received by the receiver antenna) may be mixed by a mixer within the receiver (e.g., via receiver mixer or downconverter), and an intermediate frequency (IF) signal may be created, representing the distance of the target.

In certain embodiments, a chirp generator (not shown) such as a phase locked loop (PLL circuit) may be used to generate the appropriate chirp sequence in accordance with the instant disclosure. The chirp sequence is provided/passed along both the transmission and reception paths of the transceiver (e.g., transmitter circuitry and receiver circuitry of the front-end circuitry <NUM>). A chirp in such a sequence may be considered a frequency signal that represents or corresponds to a FMCW signal which has a sweep beginning at the start frequency (F1) and ending at the stop frequency (F2). The ADC circuitry converts the IF signal, which is an analog signal, to the digital data stream which is provided to the back-end circuitry <NUM>.

The apparatus <NUM> further includes the back-end circuitry <NUM> that processes the digital data stream using the processing circuitry <NUM>. The back-end circuitry <NUM> includes the processing circuitry <NUM>, such as a microcontroller, that is used to derive Doppler-spectrum data sets. The Doppler-spectrum data sets may include Doppler information and/or a target map based on the Doppler information or similar/equivalent aggregated or intepreted summary of the digital data stream (e.g., raw radar data) from the responsive (e.g., reflections) chirps. A target map can include information indicative of detected objects and, optionally, an estimated distance from the apparatus <NUM>. In specific embodiments, the target map can be generated using two dimensional fast Fourier transform (2D-FFT) and which shows, for each object, the position and velocity of the object (sometimes herein interchangeably referred to as "a target") relative to the apparatus <NUM>. The target map may be transferred to a display for the user, and/or evaluated by a driver assistance system or an autonomous driving system.

FFTs can be computed for a variety of purposes, such as for deriving a range Doppler image in a FMCW radar system. The FFT execute so called FFT-butterflies. Inside the butterflies, the input data is multiplied by constant coefficients. A large number of multiplications can be computed, as many butterflies are computed per FFT and a number of FFTs are computed for each range-Doppler image. In FMCW radars, the range-Doppler images are computed at a high frame rate.

FMCW radar systems typically have two or more FFTs, one for range and the other for velocity estimations of targets. In some embodiments, FMCW radar systems can have a third FFT that is computed for each range-Doppler bin in the two-dimensional spectrum to obtain an angle of arrival of the signal. This third FFT can have a size corresponding to the number of virtual antennas (referring to at least one physical antenna configured to act as multiple antennas in this radar context). The FMCW system can operate by a timing engine generating a periodic start signal. After each start signal, the TX produces a frequency ramp, e.g., a sinusoidal wave signal with a linearly increasing frequency signal (e.g., a chirp) between e.g., <NUM> and <NUM>. This signal is propagated to the antenna and transmitted. The transmitted signal is reflected by the targets and picked up by the RX antenna after it is down-mixed in the RX front-end circuitry. As radio waves travel at the speed of light and the target is at a certain distance, there is a time delay between the signal transmitted and the one received. After mixing of the received signal with the transmitted signal, the time delay results in a sinusoidal wave signal with a particular frequency. This signal is called the beat signal. After sampling of this beat signal by an ADC it is further processed in the digital domain using a number of steps. A range FFT is computed on the samples of each chirp. The resulting FFT output contains peaks if targets are present at different distances (e.g., ranges). In certain applications, targets that are below the noise floor at this stage and a second FFT is needed to provide sufficient processing gain (hence, this state is not <NUM>% correct). The results of a number of ranges FFTs are stored in a memory. After the range FFT results are stored in the memory, the data in the memory is transposed. After transposition, velocity FFTs are computed. The results of the velocity FFTs are again stored in memory. The resulting data matrix is called the range Doppler map because it contains range and Doppler information about the targets. The range Doppler map is processed, and the peaks are identified as targets and a target list is generated as output of the apparatus <NUM>.

Embodiments in accordance with the present disclosure include circuitry which processes the responsive chirps (e.g., the reflections) of each of the interleaved chirp sequences independently from one another to produce respective Doppler-spectrum data sets and to calculate a velocity of the object based on the respective Doppler-spectrum data sets. The circuitry may process each of the plurality of respective sequences as waveforms, associated with the responsive chirps of each of a plurality of respective sequences, as a sample of a Doppler component. The chirps are staggered due to the time offset, which decreases the Tchirp time, compared to transmitting two separate sequences, thus increasing Vmax without decreasing the frame rate. The offset is associated with or causes a phase difference between detected peaks of the respective Doppler-spectrum data sets.

As further described below, calculating the velocity of the object may include generating a direct estimate of a velocity and a distance of the object for each of the chirp sequences using the responsive chirps, and providing an indirect estimate of the velocity using a phase difference between the two interleaved chirp sequences. The circuitry may separate the chirp sequences, at <NUM>, calculate an FFT (e.g., a 2D-FFT) for each chirp sequence, at <NUM> and <NUM>, detect peaks in the FFTs for each chirp sequence, at <NUM> and <NUM>, and using the detected peaks, estimate the velocity at <NUM> (while FFT blocks <NUM> and <NUM> refer to DM and IM (respectively for direct and indirect measurements, both FFT blocks yield a direct measurement and indirect measurements may be obtained via the phase difference between both direct measurements) Although the apparatus <NUM> illustrates the circuitry as including the components of the back-end circuitry <NUM>, the circuitry may include components of the front-end circuitry <NUM> and/or the back-end circuitry <NUM>. For example, the circuitry includes a radar transceiver (of the front-end circuitry <NUM>) to send the two interleaved chirp sequences and receive the responsive chirps, and the processing circuitry <NUM> to process the responsive chirps and calculate the velocity. In other examples, the circuitry includes a radar-signal FMCW transmitter and/or radar-signal FMCW receiver of the front-end circuitry <NUM> to send the two interleaved chirp sequences and receive the responsive chirps, and the processing circuitry <NUM> to process the responsive chirps and calculate the velocity. In some embodiments, components of the front-end circuitry <NUM> may process the responsive chirps to generate beat signals and the processing circuitry <NUM> further processes the beat signals to estimate the velocity, in accordance with the embodiments described herein.

The Vmax of the radar apparatus may be increased by use of the two interleaved chirp sequences which, in interleaved form, may be referred to as Nchirp. In Nchirp, a time offset T<NUM> is placed between the odd-indexed and even-indexed chirps (e.g., in the time after the even-indexed chirp and before the odd-indexed chirp), such that the repetition time (e.g., the time from the start of a first even-indexed chirp to the start of a second even-indexed chirp) is T<NUM> + <NUM>TACQ + <NUM>TD, with TACQ being the time for transmitting one chirp (e.g., time for acquiring sample data) and TD being the dwell time (e.g., processing). The total measurement time is TMeas = Nchirp(T<NUM> + <NUM>TACQ + <NUM>TD) and the velocity resolution corresponds to <MAT>, where λ is the wavelength associated to the system carrier frequency (note: for the duration, the last dwell time may be discounted). An illustration of such chirp sequencing is shown by way of <FIG>.

Calculating the velocity of the object may include generating a direct estimate of the velocity of the object for each of the interleaved chirp sequences using the responsive chirps and providing an indirect estimate of the velocity using a phase difference between the interleaved chirp sequences. For example, the even and odd-indexed chirps may be processed separately (e.g., two separate processing flows), resulting in two separate range velocity estimations of the same scene (e.g., a first estimate of the velocity and range for the object using the even-indexed chirps and a second estimate of the velocity and range using the odd-indexed chirps). More specifically, two Range-Doppler images are created, one for each of the interleaved chirp sequences. The number of processed chirps per sequences are <NUM>Nchirp, as the sequence includes two interleaved chirp sequences. Two Range-Doppler images can be created for the two chirp sequences using 2D-FFT, such as described above, and objects appear in the same range-velocity bins for both processed chirp sequences. Since the measurement time TMeas remains the same, the velocity resolution of both the sequences is <MAT>. In the Range-Doppler images, the range measurement is unambiguous. Considering an unambiguous range measurement, the velocity measurement <MAT> can have ambiguities since the maximum detectable unambiguous velocity is <MAT>, with DM standing for a direct measurement of the velocity associated to a moving target.

The relative time offset of TOFFSET = T<NUM> + TACQ + TD is present between the two sets of chirps. The time offset can be associated with a phase difference between the detected peaks of the two different Range-Doppler images. The phase difference ΔØ between the detected peaks of the two Range-Doppler images may be used to create an indirect measurement of the velocity <MAT>. This velocity measurement can be also ambiguous <MAT>.

The velocity of the object may further be calculated by comparing the direct measure(s) to the indirect measure. The resolution of the indirect estimation (IM) depends on the precision of the phase estimation of the two peaks associated with the same object. For example (with "std" representing standard deviation): <MAT> and where ØRD<NUM> and ØRD<NUM> correspond to even indices and odd indices, respectively. As such, there are two velocity measurements of the same targets available. If, for a specific object, the two estimated velocity components are equal or within a threshold of one another ( <MAT>), the true velocity for the specific target corresponds to <MAT>. In such embodiments, either of the velocity measures may be used as the velocity estimate of the object. For example, the even and odd-indexed chirps may be processed and provide two identical (or near identical) measures of the velocity and range, with either or both (e.g., a weighted average) being used as the velocity estimate.

If, for a specific object, the two estimated velocity components are not equal or are outside the threshold of one another ( <MAT>), meaning that one (or both) the velocity measurements are ambiguous. In such embodiments, an unfolding-based velocity estimation is used to estimate the velocity of the object. The unfolding-based velocity estimation technique is used to expand the estimated velocity beyond the boundaries imposed by the time interval between two odd-indexed chirps. The direct/indirect velocity estimates (which are different if the velocity is ambiguous) can be expanded considering the different repetition interval. Unfolding may be performed by adding integer multiples of the velocity range to the initial ambiguous estimate (e.g., 2x the maximum velocity as given by the equation earlier). The velocity range is the difference between the biggest (positive) velocity and the small (negative) velocity. This range is different for the direct and indirect measurements. Then two sets of possible velocities are generated. The difference between all combinations of both sets is calculated and the combination corresponding to the smallest difference may be assumed to be the true velocity. The unfolding is done until a certain maximum velocity, e.g., <NUM>/h is reached.

As specific examples, for the velocity component estimated using the Range-Doppler map, the unfolded velocity is: <MAT> where k = ∓<NUM><NUM><NUM>. For the velocity component estimated using the phase interferometry (or phase-difference) technique, the unfolded velocity is: <MAT> where n = ∓<NUM><NUM><NUM>. , and n ≠ k
If for a specific k = K̂ and n = N̂ results that: <MAT> Than the true velocity is <MAT>, where α = <NUM><NUM><NUM>. , is an integer number that considers the possible frequency drift of the phase interferometry technique due to a low signal to noise ratio (SNR) values of the detected peaks.

<FIG> show an example of two interleaved chirp sequences generated by an apparatus of the type implemented in a manner consistent with one or more apparatuses disclosed herein in accordance with the present disclosure (such as consistent with <FIG>). More specifically, the first time line <NUM> shows an example timing diagram of two interleaved chirp sequences having the measurement TMeas. The two interleaved chirp sequences include even-indexed chirps <NUM>-<NUM>, <NUM>-<NUM> and odd-indexed chirps <NUM>-<NUM>, <NUM>-<NUM> which respectively are associated with one of the two interleaved chirp sequences.

As shown by the second timeline <NUM> and third timeline <NUM>, the space between chirps of the same chirp sequence is the same for both chirp sequences, thus the interleaving of the chirps is asymmetrical. For example, the second timeline <NUM> shows the even-indexed chirps <NUM>-<NUM>, <NUM>-<NUM> and the third timeline <NUM> shows the odd-indexed chirps <NUM>-<NUM>, <NUM>-<NUM>. The repetition time for both the second and third times lines <NUM>, <NUM> is T<NUM> + <NUM>TACQ + <NUM>TD, with the offset time T<NUM> being placed between the odd-indexed chirps <NUM>-<NUM>, <NUM>-<NUM> and the even-indexed chirps <NUM>-<NUM>, <NUM>-<NUM>. In one example (not necessarily corresponding to the example shown in <FIG>), the offset time is placed after the end of an even-indexed chirp and before the start of an odd-indexed chirp, and is not placed after the end of the odd-indexed chirp (and before the start of the next even-indexed chirp). For example, the time between the end of the first even-indexed chirp <NUM>-<NUM> and the start of the first odd-indexed chirp <NUM>-<NUM> is TD + T<NUM>. The time between the end of the first odd-indexed chirp <NUM>-<NUM> and the start of the second even-indexed chirp <NUM>-<NUM> is TD.

<FIG> is an example of processing the responsive chirps, in accordance with the present disclosure. As shown by <FIG>, the chirps of each sequences are separately processed, with a first 2D-FFT <NUM> being generated for the odd-indexed chirps and a second 2D-FFT <NUM> being generated for the even-indexed chirps <NUM>.

As previously described, a timing engine generates a periodic start signal. After each start signal, the TX of the front-end circuitry produces a chirp which may be provided to the antenna for transmission. The ensuing transmitted signal is reflected by objects and before being picked up by the receive antenna after it is down-mixed in the front-end circuitry. After mixing the received signal with the transmitted chip, the time delay results in a sinusoidal wave signal with a particular frequency. This signal is called the beat signal. After sampling the beat signal by an ADC of the front-end circuitry, it is further processed in the digital domain. From the beat signals, at <NUM> and <NUM>, 2D-FFTs for each sequence are generated at <NUM> and <NUM> (e.g., range FFTs and velocity FFTs, resulting the range Doppler map). The resulting FFT output from each of the interleaved chirp sequences, at <NUM> and <NUM>, contains peaks if objects are present at different distances (ranges). Peaks are identified for both the even and odd-indexed chirps, at <NUM> and <NUM>, as well as signal-to-noise estimation may optionally be performed.

At <NUM>, velocity estimation (or fine-distance/velocity estimation) may be performed, for example, based on a calculation or estimation of VDM (direct measure of velocity) being performed provided from block <NUM>/<NUM>. At <NUM>, an interpolation function may also be performed to extract most-accurate distance and velocity information from the range-Doppler map(s). While the velocity estimation may be provided as above, the indirect measure may be calculated by comparing the phase of the identified peaks. At <NUM>, if the indirect measure and the direct measure of velocity match or are within a threshold, the direct measure of velocity is used as the velocity estimate for the object. If not, at <NUM>, an unfolding-based velocity estimate is used to determine the velocity estimate.

<FIG> is an example illustration of an unfolding-based velocity estimation, in accordance with the present disclosure. As shown, the unfolding-based velocity estimation <NUM> includes assessing the indirect measure <NUM> and the direct measure <NUM> of velocity to identify a velocity at which the indirect measure <NUM> and the direct measure <NUM> match or are within a threshold, as shown by <NUM> (e.g., the closest match available is selected as the velocity estimate).

<FIG> and <FIG> and <FIG> and <FIG> show examples of emulated experiments and related experimental results as may be conducted in connection with more-detailed specific embodiments in a laboratory/simulation setting (e.g., an anechoic chamber using a target simulator) for purposes directed to such experiments. In such experiments, the maximum unambiguous velocity realized corresponds to ±<NUM>/h. The example experiments validate the ability of the embodiments described herein to detect ambiguous and non-ambiguous velocities.

More specifically, <FIG> and <FIG> illustrate an example of estimating velocity, in accordance with the present disclosure. In the example, the target is emulated at <NUM> distance with an unambiguous velocity of <NUM>/h. The output of the two range-Doppler maps are visible in <FIG>, while the unfolded Doppler Frequency of the direct measurement (DM) and the indirect measurement (IM) obtained using the interferometric processing is depicted in <FIG>. The detected true Doppler frequency corresponds to <NUM> which represents a velocity of about <NUM>/h.

In <FIG> and <FIG>, another example of estimating velocity is more specifically illustrated in accordance with the present disclosure. In the further example, the target is emulated at <NUM> distance with an ambiguous velocity of <NUM>/h. The output of the two range-Doppler maps are visible in <FIG>, while the unfolded Doppler Frequency of the direct measurement (DM) and the indirect measurement (IM) obtained using the interferometric processing is depicted in <FIG>. The detected true Doppler frequency corresponds to <NUM> which represents a velocity of about <NUM>/h.

Terms to exemplify orientation, such as upper/lower, left/right, top/bottom and above/below, may be used herein to refer to relative positions of elements as shown in the figures. It should be understood that the terminology is used for notational convenience only and that in actual use the disclosed structures may be oriented different from the orientation shown in the figures. Thus, the terms should not be construed in a limiting manner.

Claim 1:
A radar method for determining a velocity of an object, the method comprising:
generating and sending two interleaved chirp sequences (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) towards an object, each of the interleaved chirp sequences (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) having a common time spacing between respective chirps of the respective chirp sequence, and each chirp of one of the chirp sequences being offset by an amount of time (T<NUM>) that is different than the common time spacing;
receiving responsive chirps corresponding to reflections of the chirps of the two interleaved chirp sequences from the object;
processing responsive chirps of each of the two interleaved chirp sequences independently from one another to produce respective Doppler-spectrum data sets; and
calculating the velocity of the object based on the respective Doppler-spectrum data sets,
characterized in that calculating the velocity of the object includes generating a direct estimate (VDM) of the velocity of the object for each of the interleaved chirp sequences (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) using the responsive chirps, and providing an indirect estimate of the velocity using a phase difference between the interleaved chirp sequences, the direct estimate being generated by separating the interleaved chirp sequences (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) into even-indexed chirps (<NUM>-<NUM>, <NUM>-<NUM>) and odd-indexed chirps (<NUM>-<NUM>, <NUM>-<NUM>), calculating an FFT for each chirp sequence and detecting peaks in the FFTs for each chirp sequence, the indirect estimate being obtained from a phase difference between direct estimates for the even-indexed chirps (<NUM>-<NUM>, <NUM>-<NUM>) and odd-indexed chirps (<NUM>-<NUM>, <NUM>-<NUM>).