US20170285158A1 | 2017-10-05 | |||
US20120280851A1 | 2012-11-08 | |||
US5252980A | 1993-10-12 | |||
US20130241764A1 | 2013-09-19 |
CLAIMS What is claimed: 1. A method for precision guidance and positioning or tracking of an object, comprising: transmitting, from one or more spatially displaced antenna elements, a sequence of transmitted pulses with a center frequency that varies per pulse; receiving the sequence of transmitted pulses at one or more receiving antenna elements; processing over a plurality of received signals, comprising: forming a complex valued range/Doppler map developed by correlating a received signal with a transmitted signal by matched filtering against a respective transmit key; coherently summing the sequence of center frequency varied pulses over the plurality of received pulses; applying a correction term representing a conjugate of a phase shift due to a frequency shift and a radial motion phase shift effect, thus achieving an increased effective bandwidth; and performing radar post processing and interferometric post processing to produce precision guidance and positioning or tracking data for an object, the data having an increased effective range resolution with reduced vulnerability to multipath bias effects. 2. The method according to claim 1, wherein the object is a passive target or an object comprising an RF receiver. 3. The method according to claim 1, wherein radar post processing comprises: performing pulse compression and decoding; forming a range/Doppler map for each phase center; identifying one or more detections in each range/Doppler map, wherein the detections are associated with a particular transmit/receive pair; removing ambiguities in range and Doppler in each range/Doppler map; and co-registering one or more detections across a plurality of range/Doppler maps. 4. The method according to claim 1, wherein interferometric post processing comprises: performing phase comparisons for each co-registered detection; and performing angle estimation of the position of the one or more objects. 5. The method according to claim 1, wherein matched filtering comprises two or more spatially disjoint receptions matched filtered against a single transmission. 6. The method according to claim 1, wherein matched filtering comprises a spatially coincident reception that is decomposed into two or more distinct signals by matched filtering against two or more distinct transmissions. 7. The method according to claim 1, wherein transmitting comprises a bandwidth step index that is proportional to a pulse index. 8. A method for precision guidance and positioning or tracking of an object, comprising: receiving a sequence of one or more transmitted pulses at one or more receiving antenna elements; processing over a plurality of received signals, comprising: forming a complex valued range/Doppler map developed by correlating a received signal with a transmitted signal by matched filtering against a respective transmit key; coherently summing the sequence of center frequency varied pulses over the plurality of received pulses; applying a correction term representing a conjugate of a phase shift due to a frequency shift and a radial motion phase shift effect, thus achieving an increased effective bandwidth; and performing radar post processing and interferometric post processing to produce precision guidance and positioning or tracking data for an object, the data having an increased effective range resolution with reduced vulnerability to multipath bias effects. 9. The method according to claim 8, wherein the object is a passive target or an object comprising an RF receiver. 10. The method according to claim 8, wherein the one or more transmitted pulses comprise a bandwidth step index that is proportional to a pulse index. 11. The method according to claim 8, wherein radar post processing comprises: performing pulse compression and decoding; forming a range/Doppler map for each phase center; identifying one or more detections in each range/Doppler map, wherein the detections are associated with a particular transmit/receive pair; removing ambiguities in range and Doppler in each range/Doppler map; and co-registering one or more detections across a plurality of range/Doppler maps. 12. The method according to claim 8, wherein interferometric post processing comprises: performing phase comparisons for each co-registered detection; and performing angle estimation of the position of the one or more objects. 13. The method according to claim 8, wherein matched filtering comprises two or more spatially disjoint receptions matched filtered against a single transmission. 14. The method according to claim 8, wherein matched filtering comprises a spatially coincident reception that is decomposed into two or more distinct signals by matched filtering against two or more distinct transmissions. 15. A computer program product including one or more machine-readable mediums encoded with instructions that when executed by one or more processors cause a process to be carried out for guidance, positioning, or tracking of an object, the process comprising: forming a complex valued range/Doppler map developed by correlating a received signal with a transmitted signal by matched filtering against a respective transmit key; coherently summing a sequence of center frequency varied pulses over a plurality of received pulses; applying a correction term representing a conjugate of a phase shift due to a frequency shift and a radial motion phase shift effect, thus achieving an increased effective bandwidth; and performing radar post processing and interferometric post processing to produce precision guidance and positioning or tracking data for an object, the data having an increased effective range resolution with reduced vulnerability to multipath bias effects. 16. The computer program product according to claim 15, wherein the object is a passive target or an object comprising an RF receiver. 17. The computer program product according to claim 15, wherein radar post processing comprises: performing pulse compression and decoding; forming a range/Doppler map for each phase center; identifying one or more detections in each range/Doppler map, wherein the detections are associated with a particular transmit/receive pair; removing ambiguities in range and Doppler in each range/Doppler map; and co-registering one or more detections across a plurality of range/Doppler maps. 18. The computer program product according to claim 15, wherein interferometric post processing comprises: performing phase comparisons for each co-registered detection; and performing angle estimation of the position of the one or more objects. 19. The computer program product according to claim 15, wherein matched filtering comprises two or more spatially disjoint receptions matched filtered against a single transmission. 20. The computer program product according to claim 15, wherein matched filtering comprises a spatially coincident reception that is decomposed into two or more distinct signals by matched filtering against two or more distinct transmissions. |
[0052] Referring to FIG. 2, a diagram depicting multipath resulting from a low grazing angle geometry is shown. More specifically, for grazing incidence (d 1 , d 2 » H, H T ) the various signals arrive from similar directions (sin(θ) « 1), with similar ranges The multipath geometry involves a range of terms: (d 1 , d 2 » H,H T ), where di is the horizontal distance from the transmitter to the nominal ground bounce point M, and d 2 is the horizontal distance from the object to the nominal ground bounce point M. The object O can either be a passive target or a receiver - both cases will incur unwanted multipath contributions. Hr is the vertical height of the transmitter above the ground, H is the vertical height of the object above the ground, R, R 1 , R 2 are respective slant range distances for the direct path, transmit to bounce point, and bounce point to object O, respectively, and R’ is the total multipath distance. For the present application, the time delay between the direct and the first multipath return is typically considered in terms of the system’s effective range resolution (see e.g., FIG. 4C).
[0053] FIG. 3 depicts the various quantities involved in an interferometric estimate of object (e.g., target or receiver) position. More specifically, the diagram shows an object 58 in the presence of an FCR 48. In some cases, as described herein, the system operates as a conventional interferometric (Cl) system, as shown in FIG. IB, where a single transmitter 54 transmits a signal which is reflected back from an object 58 (i.e., a target) and the signal is detected by two or more receivers or transceivers 50, which are separated by a distance, or baseline 52. In one example, the receivers 50 are shown as a pair of receivers, however multiple receivers or receiver elements in an array are another embodiment. This is referred to as a two- way case. In another embodiment, the system operates as an orthogonal interferometric system such that an FCR illuminates an object 58 (i.e., a receiver). There, as shown in FIG. 1C, the receiver on the object receives signals transmitted by multiple transceivers 50, where transceivers are capable of both transmission and reception. This is referred to as a one-way case. [0054] The phase measurement for either conventional interferometry (CI) or orthogonal interferometry (OI) is as follows: represents the one or two way geometry and this expression assumes a relatively narrowband signal where the measured differential phase relates to the differential path developed by the object/receiver angle θ relative to the interferometers boresite While this expression is formally a narrowband relation, interferometric processing based on this expression can be employed over usefully wide bandwidths without noticeable degradation in the achieved precision. This relation between Δ ^ and ^ is then inverted to develop an angular estimate: [0055] Here, EST(θ o ) is the estimate of the root angle θ o which relates to the true angle ^ = by an offset involving n ambiguity wraps. Additional processing steps are required to estimate n. This additional processing is known to those of skill in the art and would be included in 27 of FIG.1A. [0056] According to the principles of the present disclosure, in one of the embodiments the use of orthogonal interferometric processing for guidance and positioning is based on RD map generation over one or more transmissions. After exploiting orthogonality, time, and frequency, etc. each RD map will contain some level of multipath contamination/contribution onto the direct path segment of the respective RD maps. As described herein, aggregated bandwidth is one approach to mitigating these multipath contributions to these respective RD maps. In certain embodiments of the present disclosure, aggregated bandwidth refers to situations where both time and frequency are orthogonal. [0057] Referring to FIG. 4A and FIG. 4B, the diagrams show certain embodiments of signal processing characteristics of bandwidth stacking according to the principles disclosed herein. More specifically, FIG.4A is a plot of bandwidth stacking across frequencies according to one embodiment of the present disclosure. Here, the IBW is 50 MHz and BW total of 550 MHz. FIG. 4B is a plot showing improved range sidelobes as a function of increased levels of bandwidth stacking according to one embodiment of the present disclosure. [0058] Referring to FIG.4C and FIG.4D, the diagrams show how multipath components affect differential phase precision for interferometric angle estimation according to the principles of the present disclosure. More specifically, in FIG.4C an RD map with a direct path contribution at “X” and a multipath contribution (a wedge shape with leading point “O”) is depicted. In addition, the systems aggregate range resolution (with range sidelobes) 86 is depicted parallel to the “R” axis. Each phase center would have its own RD map which is depicted going “into the page” of the figure at 88. [0059] FIG. 4D depicts the pertinent complex phasors that result from the detection stage of FIG. 1A for a notional simulation of a two RD map example that is used to develop interferometric angular offset estimates. Each of these two complex phasors has a direct component 92, and a multipath component 90 which results in a final total phasor 94. The residual multipath contribution that “bleads through” the range sidelobes biases the total phasor and thus affects the accuracy of the phase measurement. FIG.4D shows delta phi as 96. [0060] FIG.4E depicts a simulation of three cases of coherent bandwidth aggregation, namely “1”, “1-3” and “1-N” referring to the aggregation of one, three, and N (20 in this case) fundamental bandwidth slices. The points of local maxima past a range offset of 2.5 m are marked on the plot with a “+” sign (at the tips of the arrows for 1, 1-3, and 1-N, respectively); the resulting autocorrelation shows that as the bandwidth is increased the autocorrelation develops increased resolution and additional multipath discrimination capability. [0061] FIG.4F depicts complex phasor quantities at these same “+” points shown in FIG.4E. In FIG.4F, the notation A and B are the respective phasor pair. The subscripts 1, 1-3, and 1-N are multipath contributions for the 1, 1-3, and 1-N cases, respectively. R_* is the resulting phasor sum. Only the phasor sum of the R1 case is drawn for the sake of clarity. The bandwidth (BW) aggregation benefit can be observed via the magnitude of the residual multipath components - note they become smaller with more slices (i.e., from 1, to 1-3, to 1-N (of 20 here)). These phasors represent notional complex values of what would be random phase components in an actual system. The impact of the reduced magnitude is best visualized as a circle (i.e., a uniform phase assumption). As depicted, the random phases from the simulation do not depict the worst case bias due to the multipath component, but rather are used for illustrative purposes. [0062] It is understood that Az/El estimation is strongly impacted by interference between direct and (comparable amplitude) ground-reflected signals. In rough ground situations, there could be many ground reflected signals. Bandwidth aggregation, as used herein, achieves sharper range sidelobes and smaller magnitude multipath interference terms resulting in increasingly accurate interferometric angular offset results. [0063] Referring to FIG. 4G, a time/frequency allocation map of a stepped frequency transmission scheme is shown. More specifically, the horizontal axis represents time (pulse index) and the vertical axis represents frequency offset (bandwidth step index). The bandwidth extent of a given pulse is defined as the interval between the first nulls. The spectrum of sinc(2π freq *T_pulse) yields nulls at frequency = ± 1/T_pulse). The time/bandwidth map for a notional orthogonal transmission scheme involving a single code key (A) is shown. The notation A(i,j) marks j th waveform within the i th CPI of the A code key. The pulse index u is the same as used in the Additional Algorithm Support section. [0064] Referring to FIG. 4H, a time/frequency allocation map for a notional orthogonal transmission scheme involving two keys (A and B) is shown. The notation A,B (i,j) marks j th waveform within the i th CPI of an A and B code key. The pulse index u is the same as used in the Additional Algorithm Support section. As depicted, the scheme would assume frequency orthogonality across respective pairs of A and B waveforms. Other configurations could be employed to reduce time/bandwidth density and achieve disjoint time as well as frequency segments. [0065] One aspect of the approach of the present disclosure is an integrated systems engineering approach that combines: 1) a stepped transmission frequency scheme - traversing two or more instantaneous bandwidths, (e.g., several GHz), while the ADC bandwidth might be a small fraction of the entire frequency sweep, 2) deliberate undersampling of the digitization with a) slight frequency shift keying to convey carrier offset for overpulsed, range ambiguous configurations, b) a scheme to build up the broadband data set over two or more pulses, and c) onboard processing to develop an ultra high range resolution source imagery of the transmission, and 3) methodologies to improve signal processing characteristics as mitigation performance against a particular geometry’s multipath bias. [0066] In certain embodiments, frequency-division multiplexing (FDM) is used to divide the total bandwidth available into a series of non-overlapping frequency bands, each of which is used to carry a separate signal. In some cases, time-division multiplexing (TDM) is used to transmit and receive independent signals over a common signal path by means of synchronized switches so that each signal appears only a fraction of time in an alternating pattern. In still other embodiments, orthogonal frequency-division multiplexing (OFDM) is used for digital signal modulation in which a single data stream is split across several separate narrowband channels at different frequencies to reduce interference and crosstalk. In one embodiment, improved signal processing characteristics include 1) variable frequency stepping and PRI jitter and/or 2) adaptive frequency selection to avoid fades or place them at less critical points along the trajectories. [0067] In one embodiment, if each sub-band index is keyed to a particular slow time index pulse index then the broader definition of a matched filter as a RD Map is as follows: (τo ,ω) = ∑u EJ {Δ(u) < τo + V rad T PRI u / c >} Λ(u,τ,ω) with RD map sub-band term Λ(u,τ,ω d ) = ∫-Δ/2 Δ/2 S * o(ρ+Δ(u)) Ro(ρ+Δ(u)+ ω d ) EJ { ( ρ+ ω d ) τ } dρ where each local sub-band (local index u) matched filter has its own frequency dependent phase shift. V rad is the radial velocity of a receiver relative to the transmitter, the velocity induced radial range shift is Δ R (u) = V rad u T PRI , with commensurate time delay Δ τ (u)= Δ R (u)/c , and the per pulse phase shift is Φ(u) = Δ τ (u) ω(u) = Δ(u) Δ τ (u) , (and in the linear stepped case of Δ(u) = uΔ, then Φ(u) = Δ V rad u 2 T PRI/c . (See, e.g., FIG. 4G and FIG. 4H). An Additional Mathematical Support section is presented below to simplify the discussion of the present disclosure. It is to be understood by someone versed in the art that this relative velocity can account for transmitter motion as well as receiver motion. [0068] The slow time index u is represented in the second power because of the stepped frequency index as well as the cumulative radial position of the receiver. If this pulse-wise phase progression term, EJ{Φ(u)}, is left uncorrected, the summation over sub-bands ∑u will not cohere. Hence, coherently aggregating the bandwidth in the presence of receiver motion requires the introduction of a pulse index dependent phase term into the per pulse ambiguity function operation: Λc(τo ,ω) = ∑u EJ { Δ(u) τo } EJ {- Φ(u)} Λ(u,τ,ω) which coherently aggregates the bandwidth for the moving receiver case. [0069] The pulse-wise phase correction operation represented by EJ{- Φ(u)} is similar to Synthetic Aperture (SA) processing, except that the algorithm definition allows for a sub-band narrow enough to allow for a single phase term Φ(u) to be applied within the entire Δ bandwidth at pulse u. [0070] It will be understood by practitioners versed in the art of radar processing that an estimation loop can be closed around assumed known parameters such as V rad, and/or nominal range to provide a running estimate of this parameter to support the mathematical operations described in the Additional Mathematical Support Section. [0071] In one embodiment of the system for extended bandwidth tracking for dynamic environments of the present disclosure in a single aperture RD map process example, radio frequency (RF) radiation impinges onto an aperture and induces voltages that are then digitized by an ADC. The resulting numeric voltages reside in computer memory and serve as the input, intermediate products, and the output values for all ensuing method steps. [0072] Additional Algorithm Support: [0073] The classical matched filter definition over the 2D parameter space (τ ,ω d ) can be expressed as a parameterized dot product of the signal template s o (t-τ) EJ{f d t} against the received signal r(t) as (notation : EJ(x) ≡ e jx ) [0074] This frequency domain casting of the 2D matched filter is a more natural basis for the bandwidth stacking method described herein. Now, we extend this standard RD map definition generation to include a moving receiver with slow time pulse index "u" processing and stepped frequency transmission. Assuming each slow time index has a pulse dependent frequency shift, then the general RD map steps above are revised to include explicit terms for Doppler and slow time processing per: The matched filter over the 2D parameter space (τ ,ωd ) which can be expressed as Λ(τ ,ω d ) = ∫ S * o (∨) R(∨ + ω d ) EJ { ( ∨ + ω d )τ } d∨ where the difference between the actual transmission frequency (ωa) and the received frequency (ω ) is the Doppler frequency ωd = ω- ωa. The frequency shift is due to radial velocity Vrad and wavelength λ(ω) = ω/c resulting in ωd = 2π V rad /λ(ω) [0075] Defining a bandwidth step Δ, a pulse index u the relation between a local frequency variable ρ and large frequency ∨ , ∨= ρ+Δ(u) and a delay that depends on pulse index u , τ(u), the wideband frequency interval ∫ d∨ can be factored into N u sub-bands where Soo(ρ;u)=So(ρ+Δ(u)) and Roo(ρ;u)=Roo(ρ+Δ(u)) are local functions centered at ω= Δ(u). We can now define a bandlimited likelihood function centered at the local stepwise center frequency Δ(u) as shown, below: Λ(u,τ,ωd) = ∫-Δ/2 Δ/2 S * oo(ρ;u) Roo(ρ+ωd;u) EJ { ( ρ+ ωd ) τ } dρ [0076] As a function of slow time (u), the moving receiver delay can be defined as τ(u) ~ τo + Vrad TPRI u / c, where τo = Ro/c is the nominal delay at the start of the observation CPI, T PRI is the pulse interval, and c is the speed of light. The aggregate bandwidth likelihood function can then be recast as a function of a nominal delay τo , radial velocity Vrad , and pulse repetition interval: where the pulse dependent delay is τ(u) = τo + Δτ(u) = τo + Vrad TPRI u / c and the pulse dependent frequency is ω(u). In the case of a linear stepped frequency ω(u) = u Δ. [0077] The resulting per pulse phase shift is the product of the cumulative frequency shift and delay Φ(u) = Δτ(u) ω(u) = Δτ(u) Δ(u) . In the case of a linear stepped frequency, Δ(u) = uΔ, then Φ(u) = Δ Vrad u 2 T PRI/c and contains the expected u 2 term due to the stepped frequency effect as well as the cumulative radial position of the receiver. Accounting for this pulse-wise phase progression term EJ { Φ(u) } is a requirement to maintain coherency across the span of slow time measurements (u ⊂ 1 ⋯ Nu ) being aggregated. [0078] In order to coherently aggregate the signal bandwidth in the presence of receiver motion we introduce a pulse-wise correction (to remove the EJ {Φ(u)} term) into the per pulse ambiguity function prior to summation over u, the phase correction term is applied according to: Λ u,c (τ o ,ω d ) = Λ(u,τ,ω d ) * EJ {- Φ(u)} Then the resulting pulse-wise motion corrected RD map, is the following. This coherently aggregates the overall bandwidth equivalent for the moving receiver case. [0079] Flowcharts according to the principles of the present disclosure are as follows. [0080] Single aperture RD map process
[0081] Multiple Aperture Interferometry
[0082] The computer readable medium as described herein can be a data storage device, or unit such as a magnetic disk, magneto-optical disk, an optical disk, or a flash drive. Further, it will be appreciated that the term "memory" herein is intended to include various types of suitable data storage media, whether permanent or temporary, such as transitory electronic memories, non-transitory computer-readable medium and/or computer-writable medium. [0083] It will be appreciated from the above that the invention may be implemented as computer software, which may be supplied on a storage medium or via a transmission medium such as a local-area network or a wide-area network, such as the Internet. It is to be further understood that, because some of the constituent system components and method steps depicted in the accompanying Figures can be implemented in software, the actual connections between the systems components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings of the present invention provided herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention. [0084] It is to be understood that the present invention can be implemented in various forms of hardware, software, firmware, special purpose processes, or a combination thereof. In one embodiment, the present invention can be implemented in software as an application program tangible embodied on a computer readable program storage device. The application program can be uploaded to, and executed by, a machine comprising any suitable architecture. [0085] While various embodiments of the present invention have been described in detail, it is apparent that various modifications and alterations of those embodiments will occur to and be readily apparent to those skilled in the art. However, it is to be expressly understood that such modifications and alterations are within the scope and spirit of the present invention, as set forth in the appended claims. Further, the invention(s) described herein is capable of other embodiments and of being practiced or of being carried out in various other related ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having," and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items while only the terms “consisting of” and “consisting only of” are to be construed in a limitative sense. [0086] The foregoing description of the embodiments of the present disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto. [0087] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the disclosure. Although operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. [0088] While the principles of the disclosure have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the disclosure. Other embodiments are contemplated within the scope of the present disclosure in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present disclosure.