Scentfc Research and Essays Vol. 6(6), pp. 1380-1387, 18 March, 2011 Avalable onlne at http://www.academcjournals.org/sre DOI: 10.5897/SRE11.122 ISSN 1992-2248 2011 Academc Journals Full Length Research Paper Detecton and parameters ntercepton of a radar pulse sgnal based on nterrupt drven algorthm Baraa Munqth Albaker* and Nasrudn Abd Rahm Unversty Malaya's Centre of Research for Power Electroncs (UMPEDAC) Research Centre/4th level Engneerng Tower/Faculty of Engneerng/Malaya Unversty/ 50603/ Kuala Lumpur/ Malaysa. Accepted 28 February, 2011 Ths paper presents the development of a new algorthm for radar pulse sgnal acquston and parameters ntercepton. The parameters are measured based on dgtzng ntermedate frequency nput stream and sgnal storng only on sgnal presence. In ths framework, two actvtes are mplemented. Frstly, the data acquston and preprocessng actvty, whch reads ncomng data stream, converts t nto vdeo sgnal, refnes ncomng data by applyng predefned double threshold nose gate, and stores radar pulse data upon sgnal exstence. Secondly, the data analyss actvty, whch measures the processed sgnal parameters. It ncludes the measurement of ntermedate frequency, pulse wdth, tme of arrval, tme of departure, pulse repetton frequency and pulse ampltude. The mplemented software conssts of three man unts connected through dfferent buffers herarchy. These unts are: the nput unt, dgtal sgnal processng unt, and the human computer nterface unt. These unts can operate n parallel allowng smple algorthms to be executed concurrently thereby fast radar sgnal parameters ntercepton. Key words: Radar sgnal detecton, sgnal dentfcaton, radar pulse analyss, data acquston system, radar sgnal processng. INTRODUCTION Radar systems use modulated waveforms and drectve antennas to transmt electromagnetc energy nto a specfc volume n space to search for targets. Objects wthn a search volume wll reflect portons of ths energy, echoes, back to the radar. These echoes are then processed by the radar recever to extract target nformaton such as range, velocty, angular poston and other target dentfyng characterstcs (Mahafza and Elsherben, 2004). Radars are most often classfed by the types of waveforms they use; t can be contnuous wave or pulsed radar. Pulsed radars use a tran of pulsed waveforms. In ths category, radar systems can be classfed on the bass of the pulse repetton frequency (PRF) as low PRF, medum PRF and hgh PRF radars. Low PRF radars are prmarly used for rangng whle hgh PRF radars are manly used to measure target velocty (Van, 2001; Mahafza and Elsherben, 2004). The radar *Correspondng author. E-mal: baraamalbaker@ymal.com. Tel: (006)13-3243516. system can be used n many applcatons those related to detecton and dentfcaton of targets. As an example, the radar system can be used n autonomous unmanned arcraft systems. A smart unmanned arcraft system requres the collson avodance capablty to automatcally sense and avods the statonary obstacles and non statonary movng objects along wth ts flght plan (Aryur et al., 2005; Kwag et al., 2007; Albaker and Rahm, 2009, 2010). The utlzaton of the radar n the unmanned arcraft s collson avodance system allows each arcraft to detect obstacle, upon exstence, whether t s cooperatve or not. Accordngly, pass t to conflct awareness to check for future potental conflct and thereby resolve t. To detect the conflct, the pulse parameters must be measured frst. Radar pulse parameters extracton s an mportant processng stage n the electronc warfare (EW) recevers (Schleher, 1986). The man objectve of radar sgnal parameters measurement s to perform one or more of the followng tasks: Sgnal sortng, to denterleave dfferent radar sgnals and collect pulses of each
Albaker and Rahm 1381 ndvdual radar; sgnal dentfcaton, to classfy the radar types from the collected pulses; jammng nformaton assgnment; and radar parameters extracton, to measure unknown radar parameters when only sngle radar s avalable n the concernng frequency band (Vaccaro, 1993; Stmson, 1998). Prevously, each parameters measurement crcut was bult from dscrete components and could only do a sngle task. Recently, advances n processng speed and memory, as well as the development of effcent dgtal sgnal processng technques, have allowed the computer to perform real tme processng even more effcently. The trend towards all dgtal has resulted n an everncreasng number of analogue functons beng replaced by dgtal one (Tsu, 1995; Dahnoun, 2000). In ths paper, two pulsed radar sgnal processng problems are handled. The frst problem focuses on how to acqure and preprocess the radar RF pulse sgnal. A superhetrodyne recever s utlzed to convert the rado frequency (RF) sgnal nto ntermedate frequency (IF) and accordngly to vdeo sgnal. A hardware nose gate s proposed n data acquston actvty to accurately capture pulse envelope and thereby avodng ncorrect nterrupts generated by false pulse shape reconstructon. After that the vdeo sgnal s stored based upon sgnal exstence. The second problem handles the measurements of the radar pulse parameters. A smple algorthm s mplemented n ths actvty to readout processed radar sgnal together wth parameter tmers and compute radar pulse parameters, whch ncludes: Rado frequency (RF), tme of arrval (TOA), tme of departure (TOD), pulse wdth (PW), pulse, PRF and pulse ampltude (PA). In Albaker and Rahm (2009), a smlar problem was consdered. However, the approach was based on contnuous dgtzng of the ntermedate frequency sgnal n the data acquston system. Ths wll result n an effcent radar data acquston n low pulse repetton nterval (PRI) but on the expenses of large amount of data samples to be stored and processed. On the contrary, the proposed approach reles on nterrupt-drven mechansm to nvoke the subsequent systems only when true radar sgnal detected. Due to that, acquston and processng of radar sgnal may be obtaned quckly allowng our approach to be mplemented n tme crtcal requrements. Moreover, the approach wll be much effcent than the prevous method n case of acqurng hgh PRI radar pulses. The remanng of the paper s organzed as follows: Frstly, the proposed dgtal recever together wth double threshold nose gate s descrbed wth the developed software archtecture. Secondly, new algorthm s ntroduced to extract radar sgnal parameters. Thrdly, the human computer nterface actvty s carred out and demonstrated to show the processng of dgtzed sgnal and to verfy the effectveness of the proposed algorthm. Fnally, bref summary and concludng remarks are presented. DATA ACQUISITION SYSTEM Dgtal electronc warfare recever operates at a wde frequency range. Analogue to dgtal converters (ADC) do not possess suffcent energy to drectly dgtze these nput sgnals. Therefore, the analogue to dgtal converter s usually used after a superheterodyne recever, whch down converts the RF sgnal nto an IF frequency. A superhetrodyne recever lnearly transforms, down converts, the nput RF sgnal nto an IF sgnal. A hgh speed analogue to dgtal converter s used to dgtze the IF sgnal. A vdeo detector wth double threshold nose gate s used to nterrupt the dgtal processor when an nput sgnal s present. Fgure 1 llustrates the proposed nterrupt-drven data acquston dgtal recever. In the dgtal recever, IF frequency band normally ranges from approxmately DC to at most one half the samplng frequency. The low pass flter of the down converter acts as an antalasng flter, and so t must have a very steep skrt (Vaccaro, 1993). For the above recever, the IF nput stream s dgtzed and stored only on sgnal presence. In ths approach, the dgtal data produced always contan a vald nput sgnal. However, t requres addtonal thresholdng hardware to generate and process nterruptons. For hgh PRI radar modes, our approach s consdered more sutable than that of contnuous dgtzaton approach. That s because of the lower requrements of storage sze and processng speed on the subsequent proceedng stages. However the tradeoff s the tme latency delay requred to recognze the nterrupton and swtchng tme to the nterrupt servce routne, whch s consdered as an added tme. Ths leads to be less effectve than contnuous dgtzaton approaches n case of hgh PRF data acquston. In the presence of multpath and hgh-level nose assocated wth nput sgnal, pulse envelope may be severely dstorted. If the nput sgnal s close to the recever threshold, the sgnal may trgger the recever many tmes. As a result, the recever wll report ncorrect parameters estmaton data. Ths wll make the dentfer n the next processng stage to dentfy many pulses from that of a sngle pulse. Fgure 2a shows the effects of multple trggerng a) Usng sngle detecton threshold and b) Usng double detecton thresholds. In ths stuaton, algorthms wll work neffcently. Introducng dual threshold levels for nose gatng wll resolve the problem. Such that, the nput sgnal must cross the upper threshold level to trgger the recever to acqure data; and drop below the lower threshold level to declare end of the sgnal and stop acqurng data. Fgure 2b shows the ntroducton of double threshold nose gate to resolve the mult-trggerng problem. Fgure 3 llustrates the dual threshold nose gate subsystem. The separaton between the two threshold levels can be arbtrary chosen, f the separaton s too small, the multple trggerng problem wll agan occurs. If the
1382 Sc. Res. Essays Fgure 1. The proposed dgtal recever wth front-end double threshold nose gate. Fgure 2. Demonstrates the Mult-trggerng problem. separaton s too large, the senstvty of the recever wll suffer because the upper threshold determnes the senstvty of the recever. In general, the separaton of 3dB s selected n ths paper. Sgnal presence can be sensed by montorng the processed vdeo sgnal. If the processed vdeo sgnal s hgher than hgher threshold, then sgnal s present. The dgtzaton and memory loadng cycle start when the leadng edge of the IF pulse s sensed by the hardware detector; and stops when the tralng edge of the pulse envelope s detected. Fgure 4 shows the presence of the sgnal through montorng the vdeo sgnal edges. The developed software archtecture s dvded manly nto three software unts. These unts are: nterrupt drven data acquston, pulse parameters extracton and human computer nterface. Fgure 5 llustrates the developed archtecture showng the nterconnectons and storage buffers between the actvtes. Each actvty s nvoked on specfc condtons ndependently on each another. Ths makes the algorthm to be executed concurrently allowng fast
Albaker and Rahm 1383 Fgure 3. The double threshold nose gate subsystem. Fgure 4. Shows sgnal presence through montorng the vdeo sgnal. processng of receved data stream. RADAR SIGNAL PARAMETERS EXTRACTION Ths actvty s used to extract pulse parameters of a sngle pulsed radar data obtaned from data acquston and preprocessng actvty. Pulsed radar waveforms can be completely defned by the followng: 1) carrer frequency whch may vary dependng on the desgn requrements and radar msson; 2) pulse wdth, whch s closely related to the bandwdth and defnes the range resoluton; 3) modulaton; and fnally 4) the pulse repetton frequency. Dfferent modulaton technques are usually utlzed to enhance the radar performance, or to add more capabltes to the radar that otherwse would not have been possble. Fgure 6 demonstrates the pulse radar sgnal of constant carrer frequency and the pulse parameters to be measured. The complete algorthm of parameters extracton s shown n Fgure 7. Some of the radar pulse parameters, these whch are measured n ths paper, are explaned n the followng subsectons:
1384 Sc. Res. Essays Fgure 5. Developed software archtecture of radar data acquston and parameters measurement. Fgure 6. Illustraton of a pulse radar sgnal wth ts parameters. RADIO FREQUENCY Ths nformaton s an mportant parameter for all aforementoned tasks. Many sortng and dentfcaton algorthms are based on t (Sanders, 2005). In early radar warnng recevers, the frequency of receved sgnals s determned by a channelzed recever followed by a crystal vdeo detector. Wth the advent of dgtal nstantaneous frequency measurement (DIFM) recevers and powerful dgtal processors to collect data; the frequency of each pulse could be measured and stored. Another way of modern EW frequency measurement recever could be mplemented usng a dgtal recever on the bases of a scannng superheterodyne recever wth sweep local oscllator, whch sweeps along the desred band followed by narrow band pass flter. If the nput sgnal s not a smple RF pulse, as an example a chrp radar sgnal, the startng and endng frequences are desrable. Whereas f the nput sgnal s phase modulated, the RF and the clock rate of the sgnal are desrable. In order to measure the IF sgnal frequences, the sampled data frame s converted from tme doman to frequency doman, usng fast Fourer transform (FFT). FFT have a great reducton of executon tme compared wth conventonal dscrete Fourer transform (DFT). Ths reducton s ganed from the symmetry property nherent n the computaton algorthm of DFT. The frequency of the nput sgnal can be measured from fndng the locaton, F LOC, of the correspondng maxmum fast Fourer transform output sample. Gven samplng frequency (f s ), local oscllator frequency (f LO ) and frame length (F), the frequency of the sgnal can then be determned from Equaton 1: RF f LOC * f s = f LO + (1) F TIME OF ARRIVAL AND TIME OF DEPARTURE TOA can be used to generate the pulse repetton nterval. It provdes a tme reference to all of the receved pulses. Also t can be used to nterrupt the processor to begn data acquston phase, as used n our
Albaker and Rahm 1385 Ths module s nvoked whenever there s an nterrupt. Set the desred threshold levels Enter maxmum peak to peak ADC nput voltage; Read Parameter Tmers; TOA=Counter_TOA_Loc/fs; TOA_Count = sze of (Counter_TOA_Loc); TOD = Counter_TOD_Loc/fs; TOD_Count = sze of (Counter_TOD_Loc); f (TOA_Count>1) loop to (TOA_count-1): PRI_vector= TOA_vector (+1)-TOA_vector (); else Dsplay (No enough data to compute PRI); endf f (TOA_Count> = 1 && TOD_Count> = 1) loop to (TOD_count): PW_vector= TOD_vector ()-TOA_vector (); else Dsplay (No enough data to compute PW); Endf PRF_Vector = 1/PRI_Vector; Read data frame; Loop to (F/sum (PW_samples)): Pulse_Ampltude = Max (Magntude of (data samples)*v pp/2 K ); FFTOut = FFT (data frame); f LOC = locaton of maxmum FFTOut; frequency = f LOC* fs/f; RF = f LO + frequency; Parameters = (TOA, TOD, RF, PW, PRI, PA); Dsplay Parameters; Fgure 7. Interrupt-drven data analyss. proposed approach. It s a useful sortng parameter, especally aganst radars wth stable pulse repetton frequences. TOA and TOD nformaton vectors can be measured by comparng each sample s ampltude wth threshold level. If the prevous sample ampltude s less than the hgher threshold level and the current sample ampltude s greater than the hgher threshold level; a postve edge of the pulse s detected (that s, low goes hgh edge). In ths locaton tme of arrval locaton s recorded n TOA-locaton tmer (Counter_TOA_Loc). The process s repeated for the entre radar data. Furthermore, each recorded event wll cue the DSP processor and data storng wll be ntated. If the prevous sample ampltude s greater than the lower threshold level and the current sample ampltude s less than the lower threshold level; a negatve edge of the pulse wll be detected (that s, hgh goes low edge). In ths locaton, tme of departure locaton s recorded n tme of departure locaton vector (Counter_TOD_Loc). The process wll be repeated for the entre data buffer. In addton, the hgh goes low event wll stop the process of radar data storng and trgger the data acquston to acqure buffered data. Pulse wdth Ths nformaton can be used n all above mentoned tasks. However PW s an unrelable sortng parameter because of multpath transmsson problem. PW can range from few nanoseconds to contnuous wave (CW). In most recevers, maxmum PW s programmable (that s, a short pulse s measured wth fne tme resoluton whereas long pulse s measured wth coarse tme resoluton). In the early years, a pulse s passed through a hgh pass flter, ths results n a postve spke at leadng edge and a negatve spke at the tralng edge. By usng the postve spke to start count and the negatve spke to stop the count the PW can be measured wth a great accuracy. Wth the advent of fast analogue to dgtal converters, PW s sampled at a hgh rate, allowng complete shape of the pulse to capture dgtally. Thus, after computng TOA and TOD vectors, PW-vector becomes extremely smple to extract, as gven n Equaton 2: PW = TOD TOA ; : [0,N ] (2) Where N p s the length of PW vector Pulse repetton frequency The measurement of ths parameter can also be used for all above mentoned tasks. In the early years, PRF of the p
1386 Sc. Res. Essays Fgure 8. The processng of dgtzed sgnal graphcs mplemented n the human computer nterface. pulsed sgnals was measured usng dgtal flter. Today, after dgtal capturng of the sgnal, PRI s measured by usng TOA data. A tmer s utlzed to count the tme nterval between two successve TOA data. Pulse repetton frequency s computed by just fndng the recprocal of the PRI values. Usng only TOA-vector data, PRF can be calculated easly usng Equaton 3: be used n dfferent applcatons. PA can be measured by a vdeo detector, ADC converter to dgtze vdeo sgnal, and a comparator to compare current sample wth prevous one by fndng maxmum ampltude of samples n a sngle pulse wdth. Let a j be the ampltude of the vdeo data sample j and L be the length of samples n a sngle pulse wdth, then: PRF 1 = ; : [0, M ] TOA TOA + 1 (3) PA max( a j ); j : j [1, L] = (4) Where M s the length of TOA-vector. Pulse ampltude The PA parameter s computed by takng the peak value of the processed data n a gven pulse. It can be used to generate the scan pattern of some radar, a sortng parameter to predct the scan pattern of radar, and predct radar to an EW recever approxmate range. Accurate pulse ampltude measurement may be helpful n generatng some effectve range estmaton that can THE HUMAN COMPUTER INTERFACE UNIT The man objectve of ths unt s to pass user commands, dsplay measured pulse parameters and draw the processed radar sgnals n clear and effcent way. The mplemented algorthm ncludes nput/output user dalogs and graphcs. The user nput/output dalogs allow the user to set requred threshold levels, and frame length. Furthermore, dsplay the extracted radar sgnals parameters n an effectve way. The Graphcs explan processng steps carred on the dgtzed nput data and tme and frequency drawngs, as shown n Fgure 8:
Albaker and Rahm 1387 CONCLUSION Ths paper descrbes drect and effcent method of calculatng the sgnal parameters of sngle pulse radar n an electronc warfare dgtal recever. In our nterruptdrven data acquston approach, the data stream s dgtzed and stored only on sgnal presence. Therefore, the data produced for the subsequent processng stages always contan vald sgnal. Ths wll result n reducng the processng power requred to analyze and extract radar parameters. Allowng processor to be nvoked just when the data segment contan vald sgnal. A double threshold nose gate s utlzed n the data acquston and preprocessng module to resolve mult-trggerng problem that occur n multpath stuaton. Thereby, false nterrupts and ncorrect dentfcaton of pulse shape that results n ncorrect parameters extracton s avoded. Ths approach wll be much more effectve n case of extractng the parameters of hgh PRI radar types as compared wth contnuous dgtzaton approaches. That s because of the low storage and processng power requrement, just to store and process vald data not the entre data stream. However, the approach wll be less effectve than contnuous methods n low PRI stuatons due to nterrupt latency tme. Important factors, such as computatonal tme, parallel executons and smplcty of mplementaton have been taken nto consderaton to measure pulse parameters. The ndependence of the software permts the ablty to process each software actvty alone. Ths makes a smultaneous sgnal processng allowng the three unts to be executed n parallel. REFERENCES Albaker BM, Rahm NA (2009). Sgnal Acquston and Parameters Estmaton of RF Pulse Radars usng Novel Method. IETE J. Res., 55(3): 128-134. Albaker BM, Rahm NA (2010). Unmanned Arcraft Collson Avodance System Usng Cooperatve Agent-Based Negotaton Approach. Int. J. Smulaton, Syst. Sc. Technol., 11(4): 1-8. Aryur KB, Lommel P, Enns DF (2005). Reactve Inflght Obstacle Avodance va Radar Feedback. In the proceedngs of IEEE 2005 Amercan Control Conference. USA, pp. 2978-2982. Dahnoun N (2000). Dgtal Sgnal Processng mplementaton usng the TMS320C6000 TM DSP platform. Prentce Hall, USA. Kwag YK, Cho MS, Jung CH (2007). UAV based collson avodance radar sensor. In the proceedngs of IEEE Geoscence and Remote Sensng conference. Hawa, USA, pp. 639-642. Mahafza BR, Elsherben AZ (2004). Matlab Smulatons for Radar Systems Desgn. Chapman & Hall/ CRC Press Company, USA. Sanders H (2005). Detecton and measurement of radar sgnals: a tutoral. 7 th annual ISART, (NTIA nsttute for telecommuncaton scences). Schleher DC (1986). Introducton to Electronc Warfare. Artech House, London. Stmson GW (1998). Introducton to Arborne radar. ScTech Publshng, USA. Tsu J (1995). Dgtal Technques for Wdeband Recevers. Artech House, London. Vaccaro DD (1993). Electronc Warfare Recevng Systems. Artech House, London. Van Trees HL (2001). Detecton, Estmaton, and Modelng Theory, Part III. Wley & Sons, Inc., New York.