Warning! Radar Operating
By Jim Sparks
May-June 1998
The primary purpose of weather radar is to detect storms along the flight path and give the pilot a visual indication of rainfall intensity, and with doppler radar, possible turbulence. This enables the crew to navigate around potentially hazardous areas. Another capability is to "ground map." In this mode, the flight crew receives a topographical picture which can be of assistance for navigation and position reference. In some cases, weather radar systems will detect aircraft in their path; however, these systems are not intended as collision avoidance.
A typical radar system includes a flight deck display, receiver transmitter, and an antenna. The cockpit indicator is referred to as a plan position indicator (PPI) as it indicates both distance and direction to the target. This can be a stand alone cathode ray tube (CRT), or in aircraft using electronic flight instruments (EFIS), a multi-function display (MFD) can be used to paint the radar image.
Control panels for weather radar contain a function switch, which in addition to selecting the unit "OFF," can be positioned to "standby" (STBY or SBY). This allows warm up with the antenna not scanning and the transmitter inhibited. Most recently developed systems require about 90 seconds to become operational, while older units require several minutes. Some installations have what is known as a "forced standby." This is a situation where the radar will automatically stop transmitting and the antenna will stop sweeping. One common means of introducing this forced stand by is to have the aircraft in a "weight on wheels" configuration or by activating the approach mode in a flight guidance system.
It's important to investigate possibilities of forced standby prior to performing maintenance. Jacking some aircraft with avionic equipment powered up may result in activation of the radar unit. With Electronic Flight Instruments (EFIS), separate radar controls may be available to both pilot and copilot. In this case, both will need to be selected to STBY or OFF. When in doubt, pull all radar circuit breakers prior to turning on electrical power.
After the warm-up is complete, selection to the "weather" (WX) mode causes the transmitter portion of the receiver transmitter (RT) to deliver high power pulses to the antenna. Between two transmissions, the RT serves as a receiver processor and configures the return signal into a format for useful display to the crew. Most of today's weather radars are "X"-Band and operate on a frequency of around 9345 Mhz, and power outputs of 8 to 10 kilowatts. This enables a viewing range of up to 300 nautical miles. The length of the pulses may vary depending on the requirements of the system, with one to two microseconds being nominal values. Pulse repetition frequency (PRF) will vary depending on the range being monitored. For long distances, the rate must be slow enough to allow the return to be processed before the next transmission. This is determined by the range selector position on the radar controller panel.
The antenna is supplied by the transmitter and radiates the electromagnetic energy into the air. It then becomes the receptor of the echoes from the targets. These devices come in sizes from 10 to 24 inches and the larger the diameter the narrower the beam width.
Radar antenna platforms are rather complex as they have to rotate and pitch to accurately scan the area in front of the aircraft.
"Scan" or "sweep" control will determine the total area in front of the aircraft to be observed. In most systems, the rotation of the antenna will encompass a 120 degree arc. This "azimuth" is displayed in either 15 or 30 degree increments. When an area of weather is detected and the crew wishes to observe it in more detail, the scan can be reduced to a 60 degree sweep. In most cases when the area observed is reduced, the "looks per minute" will increase. Normally, in an 120-degree arc there are 12 looks per minute. With a 60- degree sector being observed, the scan rate goes to 24 looks per minute. This provides the crew with the most up to date and accurate view of the target.
"Tilt" is another selection available to the crew and is used to set the "up, down" position of the antenna beam relative to the horizon. Most systems provide a range of 15 degrees above zero to 15 degrees below zero. "Altitude compensated tilt" (ACT) is a means where the antenna tilt is automatically adjusted as the aircraft changes altitude. This capability is available in many new technology systems and requires an input from an air data computer. Another automatic function is "stabilization," which uses a vertical gyro to provide aircraft turn and bank information so the radar antenna can remain in the same azimuth and elevation relative to the ground. The pilot has the option of disconnecting this stabilization by activating a switch on the radar control panel. Differences in some antenna installations may require stabilization trim adjustments. Airframe manufacturers' maintenance manuals, as well as radar manufacturers' procedures, should always be closely observed when making any adjustment.
Receiver sensitivity needs to be properly calibrated to eliminate background noise, yet provide for reception of even the weakest reflected signal. The "gain" control is a useful tool for weather analysis and ground mapping. In the mapping mode it is possible to reduce the level of the typically very strong returns from ground targets, while in the weather mode, calibrated sensitivity can be increased to allow very week targets to be observed.
In some later technology weather radars, it can be difficult to distinguish where the receiver/transmitter (RT) stops and the antenna begins. This type of device is referred to as a receiver transmitter antenna (RTA).
In earlier systems the antenna and the RT were connected by a radar wave guide. This is a hollow, usually rectangular metal conduit that would allow passage of the ultra high frequency (UHF) signal. The transmitted radio wave departing the RT could not escape through the walls of the wave guide, so it will then flow to the end where the antenna could radiate the signal through the air. Wave guides are sealed and frequently pressurized to prevent moisture ingress as any contamination has the potential to distort the radar signal.
One of the numerous cautions in dealing with radar systems is to avoid an open end of a wave guide while the radar is operating. Severe eye damage can result. In fact, any technician who is involved in maintenance of radar equipped aircraft should obtain a copy of Advisory Circular 20-68B "Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar," and become thoroughly familiar with its contents.
Weather radar antennas are located in some forward facing section of the aircraft. These areas may include a wing leading edge or most frequently the nose section, which incorporates a radar dome (radome). The radome is a covering whose primary purpose is to protect the radar antenna from the elements. This component has to be strong enough to withstand the aerodynamic loads of the aircraft, yet made of material that will allow free passage of the transmitted output as well as the return signal.
The construction of a radome is usually in one of two methods. The "thin wall" which is useful with low frequency systems and in areas where aerodynamic and structural loads will allow, and "sandwich" style radomes that are constructed of two or more skins separated by a nonconductive core. This can include foam-filled or hollow chambers such as with honeycomb. Radome manufacturers frequently experiment with new exotic materials, including quartz, to deliver a unit with the ultimate in electrical and structural properties.
Both physical and electrical thickness are important factors in the design of a radome. Electrical thickness is related to physical thickness and is based on mathematical equations that factor in operating frequencies, materials used, and the type of construction. A very small variation in physical thickness can have a major effect on electrical thickness. For this reason radomes are fabricated for specific types of radar. If a radome designed for a "C" band radar was installed on an aircraft using "X" band, the performance will be degraded. Paint or other coatings such as abrasion shields need to be of a material that will not interfere with the transmitted signal and cannot be an excessive thickness.
Electrical bonding is another area of genuine concern. Airframe manufacturers typically have detailed procedures for maintenance and repair while frequent inspections are an excellent way to prevent radar discrepancies. When viewing, always check for cracks, erosion, delamination and condition of the "static strips." Anytime a radome is repaired or is a suspect in causing poorly operating radar, several tests should be conducted. These include "transmissivity," which is the ability of a radome to pass radar energy, "reflection," which is the amount of returned energy that did not pass through the radome back into the antenna, and "diffraction," which is the bending of the radar wave as it goes through the structure.
When any of these electrical properties are not within prescribed tolerances, signal loss will occur. The targets may become cluttered and distorted.
The most frequent damage to radomes are holes caused by electrostatic discharge. Regardless of size, these holes can cause significant damage by allowing moisture in. Structural damage will result when a quantity of trapped moisture freezes within the walls of the radome and causes delamination. The accumulation of water can also cause a reflection of radar energy that may result in a radar image displayed to the flight crew where nothing really exists. Electrostatic bonding tests on radomes are required by most aircraft manufactures. These checks should also be done anytime small discharge holes are revealed.
Many dangers can result from a radar operating at an inopportune time. With the high voltages and the escape of x-rays that could occur, most radar maintenance is best left to those who are well qualified. Avoid walking or working within the safety zone of an operating radar (usually seven feet). Anytime "power-ON" maintenance is to be performed on a radar-equipped aircraft, take the extra time needed to make sure the system is OFF.
