Weather Radar
Eric OlbergAvionics TechnologySAIT Polytechnic
DISCLAIMER:The following information is for reference use only.The author shall not be liable for any damage that results from the misuse of the information provided herein.Always consult the manufaturer's most current reference material for the details of specific equipment.

Introduction

Weather radar is a type of radar system which provides information to the pilots relating to the weather conditions that are present ahead of the aircraft. It is important for the pilot to be aware of the weather that he or she is flying towards in order to ensure the safety of the aircraft and its occupants. Dangerous weather conditions account for 33% of all aviation incidents.
Thus, by displaying information suggesting that there is dangerous weather ahead, the pilot will therefore be able to fly around the storm, thus avoiding putting themselves, and their passengers, at risk.

History

The presence of weather ahead of the aircraft is accomplished through the use of RAdio Detection And Ranging equipment, or RADAR. Radar was first used for detecting enemy aircraft and vessels during WWII.

After the war ended, radar was introduced into the civilian aviation industry. Once radar was introduced to the industry, it quickly began evident that it could be used for weather detection. Thus, airborne weather radar systems were born.

Early weather radar systems consisted of many units, were heavy and cumbersome and provided only basic functionality. This is in stark contrast to the modern day units, which are small and compact. Today’s models also provide additional functionality such as color displays, weather mapping (detecting any electrical activity in oncoming storms) and terrain mapping.

Components

Airborne weather radar systems are as the name implies: airborne. In other words, weather radar systems are completely self-contained within the aircraft and do not rely on any ground equipment. Although weather related information does get transmitted through Air Traffic Control and on the ATIS frequency, this navigation relies on the VHF radio communication system and is not to be considered a component of weather radar systems.

The major component of any weather radar system, however, is known as the Radar Sensor. The Radar Sensor unit contains the radar’s transceiver, as well as circuitry required to move the antenna. The antenna is a second component of a weather radar system.

On modern day systems, the antenna is of the phase array type and is mounted in the nose of the aircraft. The “shell” that is around the antenna is known as the “radome”, and is there in order to protect the antenna. The following picture shows a radome that has been raised in order to expose the antenna.
Airborne_weather_radar_NASA.jpg
Figure 1http://fr.wikipedia.org/wiki/Fichier:Airborne_weather_radar_NASA.jpg
The job of the antenna is to both pick up any returning signal and transmit the radar energy in a very narrow, directional beam. Depending on the specific equipment being used, the antenna may be directly connected to the Radar Sensor, or it may be indirectly connected through the use of waveguides.

The third component of any weather radar system is the Indicator/Control Head. It is the job of the of Indicator unit to present the information being received by the Antenna/Radar Sensor assembly to the pilot as a top-down view of the approaching storm. This format is color coded in order to indicate the intensity of a particular region in a storm. For example, the Bendix IN-182A using the following colors: green – up to 4 mm/hr, yellow – 4 to 12 mm/hr, red – 12 to 50 mm/hr, magenta – over 50 mm/hr.

In some systems, the output of the “indicator” is actually sent to an EFIS screen or overlaid onto a GPS display. In this configuration, a stand-alone control head is required. The control head allows for varying functionality to be accessed, such as tilt, and mode controls. It also allows the pilot to choose how far ahead he or she wants the weather radar system to look.
in182a.gif
Figure 2http://www.seaerospace.com/king/kinggra/in182a.gif

Operation

As previously mentioned weather radar systems rely on the principles behind radio detection and ranging. The basic idea behind radar is that when an electromagnetic wave is transmitted, some of the energy will eventually strike an object and will be reflected back towards the transmitter. By detecting the reflected energy, the object’s position in relation to the transmitter can determined.

By transmitting microwave energy, the “objects” that weather radar is able to detect are water droplets. The frequencies that make this detection possible are in what is known as the C-band or X-band. The C-band range of frequencies is 4 to 8 GHz, while the X-band ranges in frequency between 8 to 12 GHz.

When the transmitted microwave energy strikes the rain droplets, some of it gets refracted and some gets absorbed. Some of it, however, does also get reflected, as Figure 3 shows. The output power weather radar is often in the kilowatts in order to ensure fo the purpose of counteracting the extreme losses. The fact that not all energy is reflected by the water droplets, however, is in fact a desirable trait. It allows the weather radar to show weather activity further than the initial moisture. It is important to note this as it allows the weather radar system to show if there is actually a storm accompanying the initial rainfall, or if there are just clear skies. C-band frequencies have a better penetration ability, or resolution, than X-band frequencies do, though.
reflection.jpg
Figure 3http://www.gapan.org/ruth-documents/study-papers/Weather%20Radar.pdf

When the reflected energy gets back to the aircraft, the antenna picks up the signal and sends it to the Radar Sensor.
The circuitry in the Radar Sensor unit analyzes these returning signals in order to calculate the intensity and position of the rain. The unit knows the precise moments the microwave energy was transmitted and received. Therefore, the position of the storm can be determined by converting the time into a distance.

Determining the intensity of the water droplets is a little more complicated than just analyzing the amplitude of the reflected signal. Consider, for a moment, the analogy of throwing a ball against a wall. It is obvious that if you stand close to the wall, the ball will return at relatively the same strength at which you threw it. However, if you move further away from the wall, the ball will return at a fraction of the strength that you threw it.

This analogy relates to weather radar if you think of the wall as the water droplets and the ball as microwave energy. When the energy hits water droplet that are close to the transmitter, a great deal more energy will be returned than for water droplets than for those that are far away. Comparing amplitudes of these signals would suggest that a storm that is a vast distance away is less intense than a storm which is closer by, if we were to use the same threshold level. Hence, there must be a means for decreasing the threshold of signals which have travelled further.

This is accomplished by something known as “sensitivity time control” (STC). The comparison in Figure 4 show the difference between not using STC and using STC.
STC.jpg
Figure 4http://www.gapan.org/ruth-documents/study-papers/Weather%20Radar.pdf

After processing the signals, it produces a signal for the Indicator unit to decode. The Indicator uses this decoded signal to create a graphical visual representation of the sky directly ahead. The sky directly ahead would all that would be needed if aircraft fly in perfectly straight lines and if there was never any change in the weather.

Unfortunately, neither one of these statements are true. Therefore, the antenna scans back and forth in order to paint a wide representation of the skies ahead. The width of the “painting” may range anywhere between 90° and 240°. Also, if the aircraft is to enter a turn, the antenna can be tilted in either vertical direction in order to maintain a perfectly horizontal scan. The Radar Sensor unit performs control the antenna’s movement automatically. The pilot also has the ability to control the tilt of the antenna manually in order to check weather above or below his current altitude.

Terrain Mapping

The more modern systems have a mode of operation which allows the pilot to see a graphic representation of the ground below them if in close proximity. This allows pilots to see the features of the Earth below them such as any mountains, islands, or cities. This feature is only helpful to pilots who have experience interpreting such representations, but is helpful when in conditions of reduced visibility.

Doppler Radar

Some weather radar systems are also capable of detecting turbulent condition like wind shear. They can accomplish this task by using the Doppler shift principle. Doppler shift is the phenomenon which causes the frequency of an object moving away from you to sound different than when it is moving towards you.

This applies to weather radar systems because if a water droplet was moving straight down, the frequency of the reflected energy would be the same as the transmitted energy’s frequency. If the water droplet was being blown around by turbulent winds, the reflected and transmitted frequencies would not match. Circuits within the Radar Sensor see this difference and calculate the significance of the turbulence.

The inherent problem with this manner of detecting turbulent air is that it require water droplets to be in the air as no energy can be reflected from dry air.

Safety

As was previously mentioned, the power output of weather radar systems is extremely high. It is measured in kilowatts. For this reason, it is extremely important to exercise utmost caution around all weather radar systems. Adhere to all protocol, including keeping outside of the established maximum permissible exposure level (MPEL) boundary. When testing weather radar systems on the bench, ensure that a dummy load has been properly and securely fastened.
If the system is to be turned on while on the ramp, follow the following guidelines:
  • DO NOT, under any circumstances, turn the radar on when any aircraft in the vicinity is being refueled or defueled.
  • Do not turn the radar on while in a hangar unless proper wave absorbing equipment is in place.
  • Double-check that the radar is in fact off before applying power to an aircraft.
  • Ensure the aircraft is in an open area with the antenna pointed upwards if the weather must be turned on while on the ground.
Failure to heed these guidelines could result in severe physical injury and could spark the ignition of combustible materials.

The following procedures use the Bendix RDS-82 Weather Radar System as an example.

Ramp testing

Due to the safety concerns of operating weather radar systems, ramp testing is limited to just a operational test to ensure that the unit turns on, and the buttons work as they should. To perform a operational test, use the procedure as listed below:

1. Begin by turning the function switch from the OFF position to the TST position.
- This action should cause a test pattern to display on the indicator’s screen after a period of about 8 seconds.
- The test pattern should light up all alphanumerical information for mode and range, the word TEST, as well as a pattern for the 80-mile range that includes each of the four colors used by the system.

2. Adjust the BRT control and ensure that the screen brightness changes. Set the BRT control as desired.

3. When all personnel and equipment is out of harm’s way and all safety procedures have been followed, turn the function switch to the ON position.
- The display should change to indicate that the system is in Wx mode. The transmitter will now be sending out microwave energy at the frequency it has been designed to operate at. For the RDS-82, this is 9345 ±25 MHz.

4. Ensure that the mode selection buttons actually cause the system to change modes, by pressing the appropriate buttons.
- The indicator should change to display the current selected mode. If a NAV system is connected, the NAV pushbutton will cause any programmed waypoints to display, and the word NAV should appear in the lower left hand corner.
- Turning the function knob to LOG will display the flight log.
- If a NAV system is not installed, the words NO LOG and NO NAV will appear when that function or mode is selected.

5. Check to make sure that pressing the up and down RANGE pushbuttons actually does cause the range to increase or decrease, respectively.

6. Finally, press the left TRACK pushbutton to make a yellow track cursor move left and press the right TRACK pushbutton to make the cursor move right.
- Each time, observe the display to ensure that track cursor does actually move in the direction requested of it, and that the difference between the selected versus actual heading is displayed.

Turn the system off by switching to OFF position on the function selector. In order to allow time for the antenna to tilt downwards, it will take approximately five seconds for the system to completely power-down.

Bench Testing

It must be stressed again that weather radar is extremely dangerous if proper safety procedures have not been followed. Be sure to follow the manual exactly and check that all connections are correct.
The various weather radar systems are engineered to use one specific frequency. Therefore, the transmitter and receiver can only use this one particular frequency. As such, the test frequency of weather radar is therefore unique to each individual model. However, weather radar uses the C-band or in the X-band of frequencies and therefore the frequency will be somewhere between 4 GHz and 12 GHz.
Bench testing a weather radar system involves the use of the following pieces of equipment:
  • the unit under test
  • test harness
  • a dummy load, waveguide to coax adapter ,and crossguide coupler assembly (see figure 5)
dummy_load.jpg
Figure 5Bendix King RDS-82 Maintenance Manual Figure 2-2
  • a test box like the RD-300 (see figure 6)
1199851406.jpg
Figure 6http://www.equipmatching.com/uploads/images/1199851406.jpg
The specifications listed below should be tested using the test box and on the bench. The procedures listed in the manual for test are very detailed and therefore will not be repeated here. What is important to note is that each factor has its own limits mentioned specifically in the manuals.
  • Peak power output
  • Pulse Width
  • PRF
  • Receiver AFC Calibration
  • Receiver Sensitivity and Threshold at 240 miles, 80 miles and 10 miles
  • Sensitivity Time control at 10 miles
  • Manual gain control
  • False Alarm
  • Manual Tilt Alignment




References

IFR RD-300 User Manual
Bendix King RDS-82 Maintenance Manual
Aircraft Electrictrity and Electronics by Thomas Eisminn
http://fr.wikipedia.org/wiki/Fichier:Airborne_weather_radar_NASA.jpghttp://www.equipmatching.com/uploads/images/1199851406.jpg