SYNTHETIC APERTURE RADAR IMAGING USING A UNIQUE APPROACH TO FREQUENCY-MODULATED CONTINUOUS-WAVE RADAR DESIGN

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1 SYNTHETIC APERTURE RADAR IMAGING USING A UNIQUE APPROACH TO FREQUENCY-MODULATED CONTINUOUS-WAVE RADAR DESIGN Gregor L. Charvat, MSEE, Leo C. Kempel, Ph.D. Department of Eletrial an Computer Engineering Mihigan State Universit 0 Engineering Builing East Lansing, MI 4885 ABSTRACT A uniquel inepensive solution to Frequen-Moulate Continuous-Wave (FMCW) raar was evelope, using low ost Gunn osillator ase mirowave transeiver moules. However these transeiver moules have stailit prolems ausing them to e unsuitale for use in preise FMCW raar appliations, when just one moule is use. In orer to overome this prolem, a unique raar solution was evelope whih uses a omination of transeiver moules to reate a preise FMCW raar sstem. This FMCW raar sstem was then use in a small Sntheti Aperture Raar (SAR) imaging sstem. The SAR imaging sstem was ompose of a foot long linear tra to whih the FMCW raar sstem was mounte. The FMCW raar sstem woul traverse the linear tra, aquiring ata to e use for prouing SAR imager. The omination of the small aperture length, narrow anwith transmit hirp, an overall frequen instailit of the FMCW raar sstem reate a numer of SAR imaging prolems whih were unique in this appliation. However, it was foun that when these issues were properl aresse it was possile to reate SAR imager on a low uget. Kewors: FMCW, Gunn Osillator, SAR, Linear SAR, Raar Imaging, Small Aperture SAR. Introution A unique approah to FMCW raar was previousl introue in [] an []. Using this raar sstem, the implementation of two SAR imaging algorithms was stuie. These inlue a range staing algorithm, an Polar Format Proessing ase methos. Setion eplains the theor an operation of the unique approah to FMCW raar. Setion 3 esries the SAR sstem use to aquire range profile ata use in testing the algorithms. Setion 4 is a isussion of the range staing algorithm, its implementation, an resulting imager. Setion 5 is a isussion on using a Polar Format Proessing ase imaging metho an its implementation. Conlusions an potential future wor are isusse in setion 6.. The Unique Approah to FMCW Raar The unique approah to FMCW raar esign is ase entirel aroun the use of two inepensive mirowave transeiver moules. These moules are Gunn ioe ase, an are more ommonl nown as Gunnpleers. The mirowave transeiver moule in use for this sstem is the M/A-Com moel MA877- X-an mirowave transeiver moule. The MA877- is ompose of three major omponents, a voltage ontrolle osillator (VCO), mier, an irulator as shown in figure 3. The VCO is fe into port of the irulator. Port of the irulator is onnete to the WR-90 waveguie flange input/output port of the transeiver. Port 3 of the irulator is onnete to the RF input of the mier. Some power is ouple off the VCO an fe into the Loal Osillator (LO) port of the mier. The IF output of the mier is onnete to a small soler terminal on the outer ase of the transeiver. Figure 3: MA877- lo iagram. VCO is a varator ontrolle Gunn ioe osillator. A varator ioe is plae insie of a avit Gunn osillator as shown in figure 4. A ias voltage on the varator ioe etween, roughl, 0 an 0 V ontrols the frequen of the Gunn osillator. A seon ias voltage of approimatel 0 V is neee to ause the Gunn ioe to osillate at the frequen of the avit that it is plae in.

2 Figure 5: Simplifie lo iagram of the unique FMCW raar solution. Figure 4: MA877- Phsial Laout. CPLR is a smoli representation of the oupling ation that ours etween the Gunn osillator ioe an the Shott mier ioe plae within lose proimit (see figure 4). MXR is reate the ouple power from the Gunn ioe osillator. This ouple power auses the Shott mier ioe to swith on an off. This swithing ation auses the Shott mier ioe to operate as a single alane mier. CIRC is a ferrite irulator plae insie of the resonant waveguie avit that ontains VCO an MXR. CIRC is asiall a large magnet preisel plae insie of the resonant avit. CIRC auses RF power from VCO to eit the input/output port, an auses RF power oming into the input/output port to e transferre into MXR. When looing at figure 3, it appears as though one transeiver moule alone an e utilize as an FMCW raar sstem. However, it was foun in la tests that the pass an of the IF port on MXR starts to roll off aroun MHz, ausing little to no response at auio frequen, whih is were most eats from a short range FMCW raar sstem will e loate. The transeiver moule s reeiver wore most effiientl at IF frequenies aove 30 MHz, where the loss ue to the mier was foun to e the least. The la of an aeptale low frequen to near DC response from MXR reners one iniviual transeiver moule useless for most short range FMCW raar appliations. This prolem is ommon for most mirowave transeiver moules of this tpe. Regarless of its shortomings, when two MA877- (or similar) transeiver moules are use, the unique FMCW raar esign solution an e otaine. A simplifie lo iagram of the unique FMCW raar solution is shown in figure 5. XCVR is entere at frequen f an FM moulate with a linear hirp, f, where volts seon =. The output of XCVR is represente with the equation: TX t = A! f t! () ( ) [ ] os + f The output of XCVR is fe into the transmit antenna. The transmitte signal is reflete off of the target. The target is situate at a range R an moving at a veloit v (if it is moving). The range R an veloit v orrespon to a time ifferene an Doppler shift etween the original transmit signal an that whih was pie up the reeive antenna an fe into XCVR. This time ifferene orrespons to a eat frequen ifferene as was proven in setion. Thus, the reflete signal from the target is represente the equation: TX ( t) = A [! f t +! f f t] os +! XCVR is set to a fie frequen of f. XCVR is raiating a fie frequen arrier at that frequen whih an e represente the equation: t = A os! f t (3) ( ) [ ] TX As eplaine earlier, the IF output of eah transeiver moule is a prout of its VCO frequen an an RF power that is oming into the input/output port of the moule. Beause of this, the IF output of XCVR an e alulate: ( t) TX ( t) TX ( t) IF = f ()

3 A = os A + os [! f t +! f t +! f t +! f t] [! f t +! f t +! f t "! f t] The higher frequen term an e roppe. This is a pratial onsieration sine the IF output port of the transeiver moules is not apale of prouing X-an mirowave signals. Thus, the IF output of XCVR an e simplifie as: IF A + (4) ( t) = os[! f t +! f t +! f t "! f t] Simultaneousl, some power from XCVR is ouple into XCVR, taing avantage of a oupling prolem that woul otherwise limit a tpial FMCW raar sstem. Power from XCVR is elieratel ouple out using CPLR an output through ATT, CIRC, an into CPLR. The ouple power injete into CPLR is fe into XCVR. The resulting frequen response at the IF port of XCVR is alulate using the equation: A = os A + os ( t) TX ( t) TX ( t) IF = [! f t +! f t +! f t] [! f t "! f t "! f t] Lie XCVR, the higher frequen term an e roppe. Thus, the IF output of XCVR an e simplifie as: IF ( t) A ( t) = os[! f t "! f t! f t] " IF is fe into the input port of a limiting amplifier, LIM. The output of LIM is use as the LO rive of MXR. IF ( t) is fe into the input port of an LNA, whih is represente LNA. The output of LNA is fe into the RF input port of MXR. IF ( t) an ( t) + IF are multiplie together in MXR. The IF output of MXR is amplifie a vieo amplifier. The resulting prout from MXR an e represente the equation: = t IF t (8) IF Vieo Output ( ) ( ) (5) (6) (7) two sinusoial signals. Therefore the vieo output of the raar sstem an e epresse as: A Vieo Output os[! f t] 4 4 = (9) It is lear from the equation aove, that the vieo output is the eat frequen ifferene f ue to istane from target R an veloit of target v. Thus, we have an FMCW raar sstem using two inepensive mirowave transeiver moules. 3. SAR Sstem Implementation The unique solution to FMCW raar evie use for this paper is shown in figure 6. This sstem has the following speifiations: Center Frequen = 0.5 GHz Chirp Banwith = 70 MHz Transmit Power = 0 Bm Front en Noise Figure = 0 B Figure 6: The unique solution to FMCW raar. This raar was mounte on a arrier whih traverse a foot long linear SAR tra. The transmit an reeive antennas were irete perpeniular to the tra towars the image sene (see figure 7). A piture of this setup is shown in figure 8. The IF port of MXR is not apale of reprouing the high frequen terms resulting from the multipliation of

4 Figure 7: Eperimental setup of linear SAR imaging sstem S05 S79 S9 S53 S66 S7 S40 S S4 8 Figure 0: 0 Bsm target loate at a istane of 5 ft, an a 30 Bsm target loate at 30 ft from the linear SAR tra. Figure 8: Piture of SAR eperimental setup. This partiular eperiment was performe inoors. Shown in the foregroun is the linear SAR tra, an in the agroun are two stanar raar targets. Data aquisition was performe using a National Instruments PCI604 ata aquisition ar. A Laview VI was programme to ontrol ata aquisitioning an some pre-proessing. Data aquisition was triggere two inputs. The first input results from the VI prompting the user to move the raar inh own the tra. After this is performe, the user hits enter. The VI then waits for an eternal harware trigger from a pulse generator. On the rising ege of this trigger, the ata aquisition ar simultaneousl moulates the raar with a linear ramp an igitizes the vieo output of the raar unit storing the range profile in a -D ata file. 000 samples of the vieo output of the raar unit are igitize at a rate of 00 KSPS. This proess is performe ever inh until the raar has traverse the entire foot long tra. 4. Range Staing Algorithm A range staing algorithm was implemente iretl from [3]. The sstem setup an ata aquisition proeure esrie in setion 3 was use to aquire ata inoors on two stanar raar targets. One target was a 30 Bsm triheral orner refletor loate 30 feet from the tra. The other target was a 0 Bsm triheral orner refletor loate 5 feet from the tra. Resulting SAR imager is shown in figure 9. SAR imager was suessfull reate using a range staing algorithm. However, it ma e possile to reate a more fouse image tring a more avane tehnique. 5. Polar Format Proessing Polar format proessing ase algorithms from [4] were stuie. Most notal, a narrow eamwith an narrow anwith assumption algorithm was implemente. An more preise algorithms were researhe. The vieo output of the raar sstem was igitize at inh intervals aross the entire foot long linear SAR tra. Vieo ata was aquire at 00 KSPS, with a total of 000 samples for eah range profile. Coherent integration was performe 0 times efore the raar was move to the net position. In orer to implement this algorithm, omple ata (I an Q) ha to e etrate from the vieo output of the raar sstem. Unfortunatel, ue to the unusual struture of the LO signal hain in the raar sstem, using a low ost I an Q emoulator was not an option. Therefore, the vieo output was over sample a fator of 0, allowing a software I an Q emoulation sheme to e implemente. This software I an Q emoulator was one using simple isrete multipliation an filtering tehniques [5]. The resulting omple ata was then eimate a fator of 0 in orer to ease omputation loa. A efine sene enter at 30 own range an 6 ross range was efine, an use to reate a mathe filter. The ata was then multiplie the mathe filter.

5 A Fourier transform was taen in the ross range omain. The frequen ata was then plotte in the wave numer omain to eamine the viailit to tae an evenl spae -D Fourier transform for image formation. The an wave numer omain plot is shown in figure. From [4], the wave numer equations were use: " f = = os! = sin! n n (0) Where: f = instantaneous transmit frequen of raar unit = spee of light In orer to ahieve image formation, a -D Fourier transform of this ata must e taen. However, the ata is not evenl spae. Therefore interpolation tehniques oul e use. However, the ata shown in figure is not planar, thus not allowing a -D Fourier transform to e performe on the entire arra of ata at one time. Onl small huns of ata an auratel e proesse. For these reasons, it was eie to use the narrow eamwith an narrow anwith approimation to otain some initial imager [4]. This assumes that the transmit anwith is relativel narrow ompare to the enter frequen of the raar, whih is true for this ase. It also assumes that the aperture anwith (the anwith) is also narrow. This is not true, ut for this ase it was eie to attempt a rue image. The resulting image is shown in figure, where a 30 Bsm triheral orner refletor was plae at 60 ft ownrange, an a 0 Bsm triheral orner refletor was plae at 30 ft own range. Data for this image was aquire outoors. Figure : an wave numer omain plot of aquire ata, units in ra/m. As shown in figure, the ross range omain oupies a wie anwith, approimatel -80 to 80 ra/m. This fairl wie aperture anwith is ue to the small aperture size of this raar sstem, whih in this ase is onl feet long. The ownrange wave numers oup a narrow anwith of approimatel ra/m ut at a ver large wave numer of approimatel 45.5 ra/m. This is ue to the narrow 70 MHz transmit anwith of the raar sstem entere at a frequen of 0.5 GHz. Due to the la of frequen stailit of the Gunn ioe ase mirowave evies, the enter frequen of 0.5 GHz is onl an approimation, an will often rift ue to temperature variations an age of the osillator s avit. Consequentl, this is alwas a ause of unertaint with an SAR image proessing using the unique approah to FMCW raar. Figure : Raar imager reate using polar format proessing an the narrow anwith an narrow eamwith assumptions. This imaging tehnique prove to e suessful. However, the resulting image in figure suffers from some smearing in the ownrange. There is also a potential antenna ringing prolem ourring lose to the linear SAR tra. This is liel ue to the front en eletronis an parasiti oupling. Further results an attempts to improve the imager will e presente at the onferene. 6. Conlusions an Future Wor The unique approah to FMCW raar has een proven suessful at reating raar imager using various

6 algorithms. Frequen stailit, an la of transmit anwith are a ontinue prolem ut one that must e ealt with in orer to use suh a low ost tehnolog. Caliration of this sstem shoul e researhe. Some potential future wor for this projet woul e to evelop ommerial appliations for this low ost raar imaging sstem. 7. Referenes [] Charvat, G.L., Kempel, L. C., A Unique Approah to Frequen-Moulate Continuous-Wave Raar Design, AMTA Smposium 004. [] Charvat, G.L., A Unique Approah to Frequen- Moulate Continuous-Wave Raar Design. East Lansing MI: A thesis, sumitte to Mihigan State Universit, 003, in partial fulfillment of the requirements for the egree of Master of Siene. [3] Stimson, G.W., Introution to Airorne Raar. El Seguno, California: Hughes Airraft Compan, 983. [4] Soumeh, M., Sntheti Aperture Raar Signal Proessing With MATLAB Algorithms. New Yor: John Wile & Sons, In., 999. [5] R. E. Ziemer, an W. H. Tranter, Priniples of Communiations, Sstems, Moulation, an Noise, 4 th Eition. New Yor: John Wile & Sons, In., 995.