WX system will transmit microwave energy
By Jim Sparks
Microwave energy is a term that often instills fear in the mind of aviation personnel. After all many of us have seen the result of placing a bratwurst in the microwave oven. The effects of these unseen forces can be startling as you watch what you thought was lunch proceed to explode. When working around weather radar I often contemplate the ill fate of my brat prior to approaching the energy emitting area.
Even though radar systems manufactured in recent years use only a fraction of the power of their predecessors caution is warranted. U.S. Federal Aviation Administration Advisory Circular 20-68B addresses radiation safety precautions for ground operation of weather radar systems.
The prime mission of airborne weather radar is the detection, processing, and display of weather phenomena with the objective of generating advance warning of potentially threatening weather. A typical state-of-the-art Doppler radar is capable of providing real-time surveillance and advance warning of potentially severe weather systems and definitely contributes to the safety and well-being of all people who fly.
"Radar" is an abbreviated term for "radio detecting and ranging" and the principle involves the transmission of a burst of energy followed by monitoring for a reflection.
This echo occurs when something such as precipitation is in the direct path of the transmitted signal. The velocity of the transmission is known so the time lapse until the echo is detected will directly relate to the distance the signal traveled. This calculation is used to interpret the distance to the target. By using a narrow beam width coupled with an antenna capable of sweeping an arc in front of the aircraft an accurate location of hazardous conditions can be portrayed to the flight crew. It was in the 1950s that commercial aircraft began to depend on onboard weather detecting systems and the system has been evolving ever since.
Radar systems are widely used throughout aviation and can detect various types of precipitation, turbulence, and even wind shear. Some units can detect other aircraft. This can all be accomplished using a Doppler X-Band (9,300-9,500 MHz) transmitter with a nominal output of microwave power. There are five frequency ranges that have been used for radar and they include "L band" 1,000-2,000 MHz, "S band" 2,000-4,000 MHz, "C band" 4,000-8,000 MHz, "X band" 8,000-12,500 MHz, and "K band" 18-26.5 GHz.
Radar may also be used as a stand-alone navigation system and can map the terrain below the aircraft. System components may include a flight deck display that may be dedicated to weather information or can even be part of the electronic flight instrument system (EFIS), a receiver transmitter (R/T), and an antenna. In many modern systems the R/T and antenna are included in one device. In systems where the R/T and antenna are separated, a device called a Wave-Guide provides the connection between the two units. A weather controller is also either part of the radar display or can be separate and in many cases there may be a controller for each pilot.
Since the introduction of weather radar in aviation two designs have been used and in either case energy pulses are generated and then radiated by an antenna. The presence of an obstacle such as precipitation will reflect some of this energy back to the antenna and receiver. In addition to the time and direction calculation the receiver will assess the amount of return energy to evaluate the magnitude of the target. Analog systems would accomplish this by generating output of 5,000 watts or more. The sensitivity on these devices was relatively low so the only way to ensure penetration of storm cells was to use a strong radiated signal. Newer systems using digital technology can achieve very high sensitivity in the receiver requiring power output in the 24-watt range. Capability and range are predicated on more than just a sensitive receiver.