Answer to Question #315 Submitted to "Ask the Experts"
Category: Microwaves, Radar, and Radiofrequency
The following question was answered by an expert in the appropriate field:
Q: What are the known health risks to microwave radiation such as that used on commercial aircraft weather radar. Assume everyday exposure 10 ft from radome in an occupational setting.
A: 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 to pilots, and definitely contribute to the safety and well being of all people who fly. An example of this type of system is the Rockwell Collins WXR-700 color weather radar (military designation FMR-2000-X). This system is widely used in commercial and military aircraft. This system provides full precipitation detection, turbulence detection, forward-looking windshear detection, and an additional skin paint mode capable of detecting tanker size aircraft out to 20 nautical miles. This system uses a coherent Doppler X-Band (9300 – 9500 MHz) transmitter with a nominal output of a few hundred watts of microwave power.
In general, these systems are not operating when the aircraft is on the ground. However, there may be circumstances, especially during maintenance and testing, that ground personnel may be exposed to the X-band radiation emitted by the system. It should be noted that X-band systems are widely used in military aircraft, especially in high powered fire control systems found for example on the F-15 and F-16 aircraft. Therefore, there is much experience on the evaluation of emitters in this frequency band. Commonly available broadband instrumentation used to measure microwaves is extremely accurate in this frequency band, primarily due to the short (~ 3 cm) wavelength of the emitted microwave radiation. There is a large database of calculated and measured hazard distances of X-band systems. The power output of weather radars, however, is many orders of magnitude lower than fire control radars. While fire control systems have the potential to overexpose personnel, it is a common misconception that any system found in the radome of an aircraft nose is dangerous. This is basically incorrect for modern weather radars.
It is quite correct to say that antennas operating in this frequency band produce very narrow, almost pencil-like beams at distances less than 10 feet from the antenna. Typically, these beams will also be scanning. Both these conditions will tend to minimize an individual’s exposure to the microwave radiation. In the X-Band region, the IEEE C95.1 Radiofrequency Radiation Standard(1999), as well as the ACGIH Threshold Limit Value (2000) has an exposure limit of 10 mW/cm2 for controlled (occupational) exposures, averaged over 6 minutes. For uncontrolled (essentially public areas), the IEEE has a exposure limit of 6.67 mW/cm2, with a slightly longer averaging time.
Measurements made on a typical WXR-700 system by the USAF in 1996 were unable to produce levels that are above either the controlled or uncontrolled limits recommended by IEEE. Therefore, in general, it is safe to assume that these systems are de minimis emitters that under normal operations on the ground, are incapable of overexposing personnel to recommended standards in wide use both in the United States and the rest of the world. In the case of Europe, the recommendations of the International Commission on Non-ionizing Radiation Protection(ICNIRP) are often used, and they are very similar to the IEEE/ACGIH limits. Of course, in operating these systems, it is important to follow the manufacturer recommendations, and never to place any part of the body in region of the antenna and the feed horn. This area is always potentially dangerous. However, it is unlikely, when the system is operating normally on the flight line, that levels exceeding IEEE, ACGIH, or ICNIRP will be encountered by ground personnel.
Older systems generally operating at lower frequencies (1000 – 2000 MHz) are becoming less common, but would have similar characteristics.
Reference:
Consultative Letter, AL/OE-CL 1996-015: Report of a Radio Frequency (RF) Radiation Hazard Survey of the C/KC-135 E/R Weather Radar System, WXR-700X, Radiofrequency Radiation Branch, Armstrong Laboratory, Brooks AFB, Texas (1996).
John Leonowich, PhD
Batelle Pacific Northwest National Laboratory
linky dink
