TECHNOLOGY WEATHER RADAR

BY TONY BAILEY The Magic Within

adar. The very word tends to the radar screens were covered in a entice feelings of Super Sci- gray haze ... and planes could not be Rence and Magic. In order to detected. While scientists tried to get understand weather radar, we must around the problem, weather forecast- have a little background. ers quickly saw the advantages of the Radar, an acronym for Radio De- new invention. Weather radar was tection and Ranging, was patented by born! British scientist Sir Robert Watson- Basically, radar works by sending Watt for meteorological applications The first magnetron out a radio wave at a very high fre- in 1935. Since practical applications quency. When the radio signal hits for airborne microwave radar had not During World War II, large-scale re- raindrops part of the signal bounces been developed before World War II, search at MITʼs Radiation Laboratory back to the radar. The signal travels at the government of England requested was devoted to the rapid development the speed of light (over 350,000 kilo- assistance from the U.S. National De- of microwave radar. Projects included meters per second). Knowing exactly fense Research Committee (NDRC) to physical electronics, microwave phys- how fast the signal is traveling, means develop this capability. Britainʼs secret ics, electromagnetic properties of mat- that a computation can be made to Tizard Mission, named after its orga- ter, and microwave communication tell how far away the rain is by tim- nizer Sir Henry Tizard, rector of the principles. The “RadLab” designed ing how long it takes for the signal to Imperial College of Science and Tech- almost half of the radar deployed in travel to the rain and then bounce back nology and chairman of the govern- World War II, created over 100 dif- to the radar. This happens so fast that mentʼs key scientific committee on air ferent radar systems, and constructed most radars send out about 1,000 sig- defense, was dispatched to Washing- $1.5 billion worth of radar. At the nals (called pulses) each second. ton, D.C. in September 1940 to intro- height of its activities, the RadLab So it makes sense that radar sys- duce the 10-centimeter cavity magne- employed nearly 4,000 people work- tems would utilize a transmitter to tron. A book written by Robert Buderi ing on several continents. What began send out the radio waves and a re- published in 1996 by Simon & Schus- as a British-American effort to make ceiver to collect the returned signal all ter titled “The Invention that Changed microwave radar work, evolved into channeled through an antenna and a the World: How a small group of radar a centralized laboratory committed to display. Older airborne radar systems pioneers won the Second World War understanding the theories behind ex- were plagued with huge components and launched a technological revolu- perimental radar while solving its en- often connected with miles of wiring. tion,” portrays the efforts of Eddie gineering problems. Not so anymore. The newer systems Bowen in his efforts to bring Britainʼs One of the evolutions of radar hap- are often contained in one neat little most closely guarded secret, the Reso- pened during the war when units in package connected to a display which nant Cavity Magnetron, to American England were trying to get advance can perform many different functions shores and also provides some insights warning of Nazi air strikes during the (multi-function) or smaller compo- as to the design and principles of radar Battle of Britain utilizing Radar sys- nents closely located in the nose of the systems. tems. They noticed that when it rained aircraft.

26 AVIONICS NEWS • MAY 2005 TECHNOLOGY

ed Sensitivity Timing Control (STC) through the harmonic filter into the control circuit, WX Random Access port circulator. The circulator prevents Memory (RAM), radial sum circuit, the received energy from passing to- circuit RAM, and the serial data trans- wards the magnetron and allows the mitter. The digitized video from the energy to pass to the receiver. video A/D is passed onto the lower ad- From the circulator, the received en- dress bits of the WX ROM. Additional ergy passes through the diode limiter. address bits of the WX ROM are used The diode limiter is used to protect for weather compensation control. The the receiver from the high amount of final address bits of the WX ROM are energy produced by the magnetron. used for extended STC. Although the circulator provides iso- Bendix/King RDR 2100 Example The extended STC increases the lev- lation to the receiver from the magne- el of the returned signal at a linear rate. tron, there is still a very large amount The transmitter portion of the ra- The extended STC provides additional of energy which is allowed to pass. dar usually consists of a high voltage STC range, but not at a true calibration The diode limiter is self biasing when a power supply, pulse transformer, mag- rate. The compensated video from the large amount of energy enters the input netron, magnetron isolator, circulator, WX ROM is then clocked into the WX port. The limiter is then biased on and harmonic filter and the antennas. The RAM at the range clock rate. The WX allows only a portion of the power out high voltage power supply and the RAM consists of different pages. Each the output port. The received signal is pulse transformer provide the magne- page holds bytes of compensated video of low enough energy level to prevent tron with a pulse for a set amount of from one radial with numerous radials self-biasing of the limiter. Once the re- time, depending upon the model. The involved. At approximately 4 msec. ceived signal passes through the lim- timing of the pulse is generated by the after the magnetron fires, the radial iter, it is then passed on to the receiver pulse timing circuit on the logic board. summation starts. The radial summa- module. Once the magnetron fires, the Radio tion sums all radials and then divides The receiver module consists of a Frequency (RF) pulse passes through the sum by its number to obtain an low noise amplifier, mixer, local oscil- a magnetron isolator. This isolator is average compensated video. The aver- lator, and a unity gain buffer. The re- necessary to protect the magnetron age compensated video is transferred ceived signal is amplified, mixed, and from being pulled in the case of a bad to the circuit RAM at approximately 5 the buffered intermediate frequency Voltage Standing Wave Ratio (VSWR) msec. after the magnetron fires. Dur- (IF) is then sent to the preamp. The from either the antenna or radome. ing the transfer, the average compen- pre-amplifier consists of a one stage From the magnetron isolator the RF sated video is converted to a color lev- amplifier with a noise gain and a tem- pulse then passes through a port circu- el. Once the data has been sent to the perature sensor. The temperature sen- lator. This circulator allows the mag- circuit RAM, the microprocessor then sor is used for all receiver calibration netron energy to pass through to the sends a request to the circuit transmit- operations. From the pre-amplifier the antenna and not on to the receiver. ter to start transmitting data to the indi- received signal is then passed to the IF From the circulator, the RF pulse cator. The data format of the transmis- section of the logic board. The IF sec- then passes through a harmonic filter sion follows the modified ARINC 708 tion consists of a band pass filter, two which is necessary to reduce the power Data Specification with compatibility stages of gain, an active detector, a low in the second harmonic of the magne- to the Radar Distribution Switchboard pass filter, and a video amplifier. Also tron in order to pass radiated harmonic (RDS) series radars. in the IF section of the logic board specifications. The RF pulse is then As for the configuration of the aver- is the Automatic Frequency Control radiated by the antenna into the atmo- age receiver, the RF pulse travels from (AFC) sample amplifier which is used sphere with a gain that is dependent the antenna at a rate approximately to supply a strong signal to the AFC upon the antenna size. equal to the speed of light to a weather circuit during the time the magnetron The weather data accumulator cir- target and then is reflected back to the is firing. There are different stages of cuit consists of video Analog to Digi- antenna. The antenna provides a gain gain in the IF section used to control tal Converter (A/D), Weather Transmit to the received energy depending upon the calibration and noise level of the (WX) Read Only Memory (ROM), the antenna size. The received en- receiver.

weather compensation and an extend- ergy from the antenna is then passed Continued on following page

AVIONICS NEWS • MAY 2005 27 WEATHER RADAR is produced. In order for the IF to remain the microprocessor drives the local os- Continued from page 27 at a set level, the AFC circuit must ad- cillator by smaller frequency steps. The The first stages of the amplification just the local oscillator periodically to local oscillator drive circuitry consists are the STC and the Automatic Gain correct for magnetron frequency drift. of D/A converters. The output from Control (AGC). The STC is used to cali- The AFC circuitry consists of three basic these converters is summed together brate the receiver to compensate for the parts—the frequency sample circuit, the and sent to the receiver as the LO drive. two way loss of energy through the at- microprocessor, and the local oscillator The converters consist of the course, mosphere. This gain level will vary with voltage drive circuit. During the mag- fine and offset. The offset is adjusted by time starting with the magnetron fire and netron fire, a small amount of energy is the microprocessor in the AFC software ending at a set nautical mile distance, allowed to pass into the receiver and is procedure. The offset is also controlled depending upon the size of the antenna. mixed with the local oscillator. by the microprocessor and accounts for Once the STC time is complete, the fi- The IF signal is then amplified by the the frequency pulling of the local oscil- nal gain level will be the AGC gain. The preamp and sent on to the IF section of lator by the magnetron. second amplifier is the Manual Gain the logic board. In the IF, the AFC am- A key component of the radar system control. The gain of this amplifier will plifier increases the magnetron frequen- is the gyro and antenna stabilization vary depending upon the gain setting cy sample to a level which activates the circuit. The gyro and antenna stabiliza- selected by the pilot. After the received discriminator circuit. The output from tion circuit consists of three parts—the signal is amplified to a calibrated level, the discriminator circuit is then passed analog gyro inputs, the azimuth and tilt it is then detected by an active detector on to a unity gain amplifier to the dis- motor drive circuits, and the Hall effect with a gain. From the detector, the sig- criminator A/D. “Samples” from the sensors. The analog gyro inputs con- nal is once again filtered by a selectable discriminator A/D are saved in the STC sist of the 400 Hz reference, pitch and low pass filter. This switching of filters RAM and recalled by the microproces- roll. There are numerous gyro systems increases the receivers performance by sor for AFC control. which the radar system must interface reducing the noise of the signal. From The microprocessor reads the samples to. Each of these systems have varying the low pass filter, the signal is buff- from the memory and determines if the parameters from aircraft to aircraft. The ered through a unity gain amplifier and intermediate frequency is either too gyro reference input can vary depend- passed on to the video A/D. high, too low or on frequency. If the lo- ing upon specific component ranges. Included in the receiver is the AFC cal oscillator is off by a large amount, The gyro reference input therefore has which is used to control the frequency either high or low, then the micropro- a switchable gain amplifier so that the of the local oscillator in the receiver cessor drives the local oscillator at large same level of reference voltage may module. The local oscillator is mixed frequency steps. Once the IF is within a be applied to the detector circuit. The with the received signal and a frequency pre-determined range from desired, then pitch and roll inputs are applied to fixed gain amplifiers and then detected. The detected outputs from the three signals TERMS AND DEFINITIONS are then integrated and multiplexed to a programmable amplifier. The gain val- TERM ...... DEFINITION ues for the programmable amplifier are RAM ...... Random Access Memory stored in the configuration module and ROM ...... Read Only Memory applied to the amplifier when the appro- RADAR ...... Radio Detection and Ranging priate input is selected. NDRC ...... National Defense Research Committee The pitch and roll data is multiplexed RF ...... Radio Frequency by the microprocessor to the A/D con- VSWR ...... Voltage Standing Wave Ratio verter and then read by the microproces- A/D ...... Analog to Digital Converter sor. The pitch and roll data is then used IF ...... Intermediate Frequency to calculate the appropriate tilt of the WX ...... Weather Transmit antenna in accordance with the current STC ...... Sensitivity Timing Control azimuth setting. Once the tilt angle has RDS ...... Radar Distribution Switchboard been calculated, then the microproces- AFC ...... Automatic Frequency Control sor determines if the tilt of the antenna AGC ...... Automatic Gain Control should be increased or decreased from its current setting. The microprocessor,

28 AVIONICS NEWS • MAY 2005 Bendix/King RT 2100 RADAR Safety Boundary in a fixed interrupt time, instructs the tion from the magnetron to the antenna) azimuth drive circuit and the pitch drive during operation. This also applies when circuit to step their motors. The azimuth a wave guide has been damaged. Take motor drive circuit and the tilt motor the time to read FAA Advisory Circular drive circuit are located on the power AC 20-68B, Recommended Radiation supply/modulator/motor drive board Safety Precautions for Ground Opera- due to the heat generated by the driver tion of Airborne Weather Radar. Your circuit. These drivers receive commands safety may depend upon it. from the microprocessor to either step That pretty much sums up the basic the motors up or down or not to step operation of an airborne weather radar the motor at all. The azimuth and pitch system. The truth is, it is a combination motors are usually stepper motors and of super science and magic, although step the antenna in a measured degree the experts may disagree. From an avi- of steps. The two Hall effect sensors are onics technician standpoint, the magic used to determine the position of the an- involved comes from the science of a tenna in both the tilt and azimuth axis. specific installation doing funky stuff! As far as troubleshooting a problem That is the radar we really see. At least with any radar system, it is imperative now, the science makes sense. Clear as that you utilize the proper maintenance mud, right? ❑ manual and observe all of the safety precautions. Radar is dangerous! Some Reference Material: FAA Advisory Circular common sense rules apply and each AC 20-68B, Recommended Radiation Safety Precautions for Ground Operation of Air- manufacturer has set guidelines that borne Weather Radar; The Invention that must be followed for your safety. One Changed the World: How a small group of common safety element while working radar pioneers won the Second World War with radar systems is that the area in and launched a technological revolution by front of the aircraft or radar antenna must Robert Buderi published in 1996 by Simon & Schuster; ARINC 708 Data Specification; remain clear during transmit or WX op- Honeywell Bendix/King ART 2100 Color eration. Another rule is to never, never, Weather Radar Antenna/Receiver/Transmit- expose any part of your body to an un- ter Manual Number 006-05390-0001Revi- terminated wave guide (connecting por- sion 1, June, 2000.

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