A LOW COST, HIGH ACCURACY RADAR ALTIMETER Thi article decribe the development of a frequency modulated (FM) radar altimeter for meauring the height of flying object. The entire tructure comprie two part: a microwave unit that operate in the 4.3 GHz frequency band and a ignal proceing band that i baed on a reduced intruction et computing (RISC) microproceor with additional analog circuitry. The main feature of thi altimeter are it low cot contruction (< $2500), high dynamic range (70 db) and height meaurement accuracy (< 20 cm). Modern CAD technique 1 are ued to invetigate the radar ytem and circuit behavior. The radar altimeter i deigned uing Roger R4003 ubtrate material and modern urface-mount device (SMD) technology. Fig. 1 The radar altimeter functional block diagram. LINEAR MODULATOR LOWPASS FILTER 50 khz VCO 200 to 600 MHz MIXER 3.9 to 4.1 GHz An FM ignal in the region of 4.2 to 4.6 GHz i tranmitted toward the earth urface through a directive horn antenna, and the back reflected ignal i received by another directive horn antenna. 2 The received GAIN LIMITING AMPLIFIER GAIN DIGITAL COUNTER USING FAST ADC COUPLER 20 db POWER DIVIDER LOW NOISE AMPLIFIER HORN ANTENNA TRANSMIT 3.9 to 4.1 GHz MIXER ANALOG AND DIGITAL OSCILLATOR 2 GHz DOUBLER HORN ANTENNA RECEIVE ignal i proceed at microwave frequencie and then at baeband by a dedicated ignal proceor to determine accurately the frequency difference between outgoing and incoming ignal, which are proportional to the return path delay. Thee ignal then are ued to compute the ditance between the radar altimeter and the earth urface. The block diagram of the radar altimeter i hown in Figure 1. The ignal i tranmitted through a VCO, which i modulated by a triangular waveform that i generated in the ignal proceing ection. The ignal pae through a fifthorder hairpin bandpa filter and a coupler and i fed to the horn tranmit antenna. 3 The coupled ignal i fed to the firt downconverion mixer along with the local ocillator ignal. The local ocillator ignal i generated from it half frequency by mean of a nonlinear element and filtered to the expected frequency to be ued by the firt downconverting mixer. The ame ignal i ued by the econd downconverting mixer. Both local ocil- GIORGOS E. STRATAKOS, PAUL BOUGAS AND KOSTAS GOTSIS National Technical Univerity of Athen, Intitute of Communication and Computer Sytem Athen, Greece
Fig. 2 The tranmitted ignal output power pectrum. Fig. 3 The hairpin bandpa filter performance. AMPLITUDE (db) 0 10 20 30 40 50 3 S 21 S 11 4 5 FREQUENCY (GHz) Fig. 4 The horn receive and tranmit antenna. Fig. 5 The IF ection tructure. RF PART TRIANGLE WAVE GENERATOR FILTERS/ AMPLIFIERS SQ1 6 MCU ANALOG AMPLIFIER/ FILTER IF PART ANALOG SERIAL lator ignal are buffered. The important iue here i that the VCO doe not need to be table ince any frequency hifting i cancelled out by the microwave ytem architecture. In the receive path, a high directivity horn antenna i ued, followed by a low noie amplifier, to achieve increaed dynamic range. The DC power upply accept input from +13 to +28 V DC to produce a regulated poitive output voltage. Figure 2 how the output power pectrum of the tranmitted ignal. The frequency range of i covered to achieve the height reolution requeted. The output pectrum i limited to a 400 MHz bandwidth to eliminate any unwanted ignal that may deteriorate the altimeter performance. The repone of the microwave hairpin bandpa filter i hown in Figure 3. Thee filter are fifth-order tructure printed on Roger R4003 ubtrate material. The out-of-band rejection i greater than 40 db and the return loe exceed 16 db. Aide from the better enitivity compared to a homodyne architecture, one of the main reaon for the ue of the uperheterodyne architecture in the radar altimeter i the reduction of the low frequency triangular waveform that leak from the tranmitter to the receiver. Thi leakage i via the downconverting mixer, which do not have infinite iolation. 2,3 The leakage limit the altimeter height meaurement range to 700 m ince the expected reflected ignal from the ground become maked by the noiy triangular leakage. Uing digital ignal proceing to remove unwanted ignal component i one way to overcome thi problem. Horn antenna are ued for the ignal tranmiion and reception becaue of their gain and directivity characteritic. It i common to experience error in radar altimeter in low height meaurement where patch antenna are ued and the beamwidth i 50 to 70. Thi large beamwidth permit the receiver to track other obtacle cloe to the ground and, therefore, produce error in height meaurement. The horn antenna ha a gain of approximately 15 dbi, which i at leat 10 db more than the patch antenna equivalent. Thi gain add ignificantly to the receiver enitivity. In addition, the beamwidth i approximately 10, which reult in fewer error in height meaurement compared to the patch antenna counterpart. The only diadvantage i the large volume of the horn antenna, which in many application i not a problem. The horn receive and tranmit antenna are hown in Figure 4. THE PROCESSING SUBSYSTEM The ignal proceing ubytem amplifie the received baeband ignal by 78 db after filtering 4 the paband to 200 Hz to 100 khz. The output i driven to a Schmitt trigger tage that produce pule for the input to a counter. A triangular waveform i produced in the analog part of the ignal proceing ubytem a well a the ubytem mater clock, which i ued to ynchronize it digital part. A coherent pule with the triangular waveform i ued to begin the pule count. The meaurement lat for at leat 10 m once the trigger pule begin. Five meaurement are taken (one every 20 m), and the mean value of the meaurement i ent to the RS-232 erial port. At the ame time, an analog output i produced uing a pule width modulation (PWM) controller. The accuracy of the meaurement i 10 bit. If an error occur during a meaurement, the PWM controller a well a the mean value take into account the previou meaurement. If five error occur in a period of five ucceive meaurement, an error ignal i ent at the erial output. The IF ection, hown in Figure 5, amplifie the ignal, detect the valid part of the ignal and meaure it frequency. Uing an etimate of the frequency, the ditance from the ground i calculated and can be read via RS-232 from the erial output or a a voltage proportional to the altimeter height from the analog output. The IF ection alo generate the triangle wave for the linear modulation. The IF part doe not ue frequency domain analyi to etimate the frequency but, intead, employ low cot time domain technique. A a reult, two of the main feature of the IF ection are it low cot (only $20) and implicity. The IF component of the device comprie four part: a triangle wave generator, an analog output, IF ignal filter and amplifier, and a micro controller unit (MCU) that meaure the ignal frequency. The output of the RF ection i filtered, amplified
SQ1 Fig. 6 The triangle wave generator. AMPLITUDE ADJUST Fig. 7 The amplifier and filter configuration. INPUT 200 Hz 110 khz OFFSET ADJUST + TRIANGLE WAVE 20 20 CLOCK 1 110 khz 20 200 Hz 110 khz 200 Hz 110 khz 110 khz and fed to the MCU, which meaure the frequency of the ignal and produce a digital erial output and PWM output. The PWM i filtered and buffered to produce a 0 to 5 V analog output. THE TRIANGLE WAVE GENERATOR The triangle wave generator comprie a quare wave generator, integrator and amplitude/offet adjut unit. The output of the integrator i a 25 Hz triangle wave. Fine-tuning of the triangle wave offet and amplitude i neceary to properly bia the VCO output in the deired frequency region. The triangle wave generator i hown in Figure 6. For every period of the triangle wave, a new meaurement cycle begin. The beginning of each cycle i ignaled by the quare wave SQ1. SQ1 tranition indicate the change of the triangle wave lope and are ued by the MCU to locate the valid region during the IF ignal frequency meaurement. Fig. 8 The MCU. TIMER COUNTER Fig. 9 The oftware architecture. CLEAR THE TIMER AND THE COUNTER. WAIT FOR AN SQ1 TRANSITION. WAIT FOR VALID DATA. BEGIN COUNTING AFTER A TRANSITION FOF THE INPUT. OPEN THE CLOCK. TRANSMIT THE PREVIOUS VALUE, UPDATE THE PWM. WAIT FOR A 10 m INTERRUPT. MCU UART PWM SERIAL PWM WAIT FOR THE SAME TRANSITION OF THE. FREEZE THE CLOCK. READ THE TIMER AND THE COUNTER. CALCULATE THE HEIGHT. IF FILTERS AND AMPLIFIERS The IF ection contain one buffer, three cacaded amplifier and five cacaded filter, a hown in Figure 7. The IF output of the mixer pae through a paive econd-order bandpa filter, 4 which i ued to eliminate any DC offet. The ignal i then fed to a number of cacaded filter and amplifier. The overall gain i approximately 78 db. A teep filter lope i neceary to band limit the ignal and prevent DC offet error or poible aturation due to leakage from the triangle wave. The output of the lat amplifier i filtered and converted to a pule uing a Schmitt trigger. The deign i extremely imple yet efficient ince the meaurement of the frequency require only the number of ignal tranition. Therefore, uing expenive component to amplify and filter the ignal i not neceary. THE MCU In the cae of the altimeter a wide range of frequencie mut be meaured. In order to obtain high accuracy acro the deired frequency range, the architecture hown in Figure 8 i ued. An SQ1 tranition ignal the beginning of a new meaurement cycle. The counter count the incoming ignal pule N and a timer meaure the time T uing a high frequency clock f C. The MCU read the timer and counter and calculate an etimate of the frequency N ƒ = ( 1) T Since the frequency i directly proportional to the altimeter height, the MCU eaily derive a height etimate. Thi etimate i erially tranmitted via the MCU Univeral Aynchronou Receiver Tranmitter (UART). Another form of output i a 10-bit PWM. The output i fed to a lowpa filter and buffer to output a 0 to 5 V analog output proportional to the altimeter altitude. THE SOFTWARE ARCHITECTURE The altimeter oftware architecture i hown in Figure 9. At the beginning of each meaurement, the timer and
Fig. 10 The radar altimeter. TABLE I THE RADAR ALTIMETER Operation principle FM-CW with linear modulation Frequency weep range (GHz) 4.2 to 4.6 Microwave power output (at the antenna +16 input) (min) (dbm) Antenna gain (tranmit, receive) (dbi) 15 Altitude meaurement range (m) 1.5 to 700 Altitude meaurement accuracy (cm) 15 Meaurement rate (mea/) 10 Connector type counter are cleared. The oftware then wait for a new SQ1 tranition and a pecific time when the ignal value i valid. The count of the incoming pule begin with a poitive or negative tranition of the ignal. At the ame time, the timer i tarted and et to produce an interrupt after 10 m (T min ). Until SMA male Size (mm) 160 90 60 Power conumption (W) 8 Voltage upply (V DC) +14 then there i enough time to end the reult of the previou cycle through the erial port and update the PWM. After thi T min time period, the oftware continuouly check the counter for a new incoming ignal pule. When a new ignal pule arrive, it freeze the timer and counter. The ytem carrie on with the diviion of N /T to obtain the etimate of the frequency and further calculate the height. The ue of thi particular technique combine the advantage of high accuracy with low cot. In the altimeter cae the error of the meaurement f i equal to ƒ ƒ 1 = ƒ c T ƒ ƒ 1 1 = ƒ T A 1 MHz clock i ued and the time T i in the 10 to 15 m range, thu the maximum error i limited to 0.01 percent. Conequently, the maximum error in the meaurement of ditance i limited to 0.01 percent. A photograph of the ytem i hown in Figure 10. The pecification of the radar altimeter are lited in Table 1. SYSTEM EVALUATION The radar altimeter wa evaluated in a tatic environment. The procedure followed ued an overhead crane approximately 8 m tall. The ytem wa placed on the crane and, through computer control, change in altitude were meaured imultaneouly with a common meter and comparion were performed. 5 Ten different height meaurement were made and the accuracy obtained wa approximately ±10 cm. The ytem require further verification for height up to 700 m, which i the altimeter highet altitude capability. Verification of the ytem enitivity at ( 2) thi altitude wa performed by mean of a variable power attenuator, and the experiment meaured a enitivity of approximately 85 dbm. Thi power level correpond to a height meaurement of at leat 700 m. CONCLUSION The novel deign and development of a microwave radar altimeter have been decribed. The radar can be ued for highly accurate height meaurement in the 1.5 to 700 m range. The unique feature incorporated in the altimeter include highly directive receive and tranmit antenna with a high iolation between them and a highly integrated lightweight microwave tranceiver with very table operation. The altimeter utilize advanced ignal proceing technique, which provide very high altitude meaurement accuracy. The device i houed in a ingle active box with very low height and two horn antenna, making intallation very eay. Further improvement to the ytem are poible, including the ue of an automatic gain control to improve the ignal-to-noie ratio at the IF band and, thu, the enitivity of the receiver of the altimeter. Thi increae in enitivity will improve the range of the height meaurement ignificantly with the ame power level at the tranmitter output. Digital ignal proceing alo can improve the fale meaurement ince the leakage of the triangular waveform limit ytem performance. Thee improvement are already under invetigation. Reference 1. HP-EEof Serie IV oftware manual. 2. Skolnik, Introduction to Radar Sytem, McGraw Hill. 3. David K. Barton, Modern Radar Sytem Analyi, Artech Houe, Norwood, MA. 4. Bernard Sklar, Digital Communication, Fundamental and Application, Prentice Hall International Edition. 5. Henri Sauvageot, Radar Meteorology, Artech Houe, Norwood, MA.
Giorgo E. Stratako received hi BSc and PhD degree in electrical engineering from the National Technical Univerity of Athen (NTUA). He i currently a enior reearcher in the Microwave and Fiber Optic Laboratory of the Intitute of Communication and Computer Sytem at NTUA. He ha alo been employed at INTRACOM SA in the R&D department ince 1992. Stratako reearch interet include microwave and mm-wave telecommunication ytem, radar, microwave CAD linear and nonlinear technique, MIC and MMIC deign, conformal array ytem, adaptive antenna and automated microwave meaurement technique. He can be reached via e-mail at george@ed.ece.ntua.gr. Paul Bouga initiated hi tudie at the National Technical Univerity of Athen (NTUA) in 1992. Since then, he ha worked on embedded ytem deign and data acquiition technique. The ubject of hi diertation wa the development of a digital electrocardiograph ytem and a program deigned to complement the ytem by diplaying, proceing and toring the acquired data. Currently, Bouga i a PhD tudent at NTUA and a reearcher in the Microproceor and VLSI Laboratory. Hi main activitie involve the development of embedded ytem a well a educational material and practice in the area of microproceor and microcontroller both for high chool and undergraduate tudent. Bouga can be reached via e-mail at paul@microlab.ntua.gr. Kota Goti i an electronic engineering tudent at the National Technical Univerity of Athen. Hi main reearch interet i low cot digital electronic deign and embedded ytem deign. Goti can be reached via e-mail at Kgoti @microlab.ntua.gr.