Are YOU concerned about your exposure to electromagnetic radiation? Most of us are ... Artificial sources of microwave radiation are often of greatest concern because we are generally unfamiliar with radiation of this type. Are radar systems that are routinely used for mapping the earth potential hazards? Let us look at the levels of microwave radiation exposure that we receive from the Canadian RADARSAT-1 synthetic aperture radar satellite to see what hazard potential exists. Dr. Chuck Livingstone, one of our radar experts at CCRS, came up with an amusing scenario and a 'down to earth' explanation of the effects of microwaves from satellites.

Physics is fine, but most of us are happier if we can relate unfamiliar situations to every day life. So lets look at a more familiar source of radiation as a parallel to what we can expect to experience from RADARSAT.

As the primary effect of microwave exposure on the human body is heating, let us look at the thermal radiation exposure that we experience from an ordinary 100 Watt incandescent lamp and try to match it to radiation from a RADARSAT overpass.

If the lamp is reasonably well worn (like the front porch light at home) we can expect that only about 10% of the power supplied to it is radiated as electromagnetic energy in the form of heat and light. If we assume that this radiation is isotropic (radiated uniformly in all directions) then our radiation exposure is just the radiated power times the solid angle defined by us and our distance from the light.

How far from the light bulb must we be to have a radiation exposure equivalent to our exposure from RADARSAT? This distance is just the square root of the radiated lamp power times our surface area divided by 4 pi and divided by the power that we intercept from RADARSAT. This would be approximately 300m.

In order to experience the full thermal effect of the light bulb "equivalent", you would have to coat your naked body in soot (to help absorb the radiation). Then, expose yourself to the light bulb, positioned 300 metres away, for three quarters of a second!

• you will not notice the heat
• you will impress the neighbours
• you may have trouble explaining your actions.

RADARSAT Precision Transponder
The RADARSAT calibration transponders are precision instruments that receive radar pulses from the satellite as the radar beam passes through the transponders antenna pattern. Data are extracted from the received signals to measure the radar performance, and a precisely scaled version of the received signals is retransmitted to the satellite for use in RADARSAT SAR calibration. Three transponders have been commissioned, one at Resolute Bay, one in Nepean, Ontario, and one in Fredericton, New Brunswick. The one pictured here is located in Nepean, Ontario.

Mapping radars transmit pulses of microwave energy towards the ground and receive the signals reflected back from the earth to form radar images. The RADARSAT transmitter provides its antenna with a 6000 Watt pulse of 5.3 GHz signal, 30 microseconds long, more than 1400 times per second. The antenna focuses these transmissions into a radar beam that illuminates the earth as the satellite moves along its orbit at 7 kilometres per second. As the signal travels from the satellite to the earth, its power density decreases as the square of the distance traveled. For RADARSAT, this distance is greater than 940 km for all beams used for radar mapping. Thus, the microwave radiation density at the ground is quite small.

By looking at the energy distribution in the radar beam and taking the distance squared effect into account we can calculate the power density in Watts per square metre (W/m²) on the ground. This power density determines our radiation exposure level. The problem with such calculations is that people who rarely use them are not comfortable with the results. In the RADARSAT case we don't need to rely on this calculation to determine the ground power density, as the signal strength at the ground is measured directly by the equipment used to calibrate the radar.

An example of the measurement results is shown in the graph below. This graph displays the strength of the satellite signal as measured by a precision calibration transponder for one pass of the satellite over the calibration site. Each point on the horizontal scale represents one transmitted pulse. The vertical scale is logarithmic and so each decade represents a change of a factor of 10 in received power. The units of the vertical scale are 10 times the logarithm of the ratio of the received power to 1 milliwatt per square metre (dBm/m²). At the peak of the curve we have a received power of -33 dBm per square metre or 5 x 10^-3 (5/1,000) milliwatts per square metre. At all other points in the curve the received power is smaller. The shape of the curve is the shape of the radar beam component in the direction of satellite motion. The received power curves can be translated into power density by compensating for the transponder antenna and transmission line properties. Lana Teany of CCRS, has been measuring the performance of the RADARSAT radar and reports that the maximum power density on the ground is 11 dBm/m².

The radiation exposure time is the time required for the satellite beam to pass over a point on the earth (approximately 0.75 seconds at the closest approach of the satellite) and the power absorbed by a person is the radar cross section of the person (effective surface area exposed - about 0.5 square metre on average) times the power density. Combining these results we see that the radiation exposure of a person during a RADARSAT pass is always less than 3.8 x 10^-6 Watts for 0.75 seconds.

Experts in industrial safety agree that the primary danger from microwave energy is a result of heat transferred to body tissues. At the RADARSAT frequency, most of the energy absorbed by the person is dissipated in the outer couple of centimetres of their body. Various safety standards are in use around the world. All of these recognize that dangerous levels of microwave exposure change at different microwave frequencies and different exposure times. The most conservative standard defines a hazard threshold of 2 Watts per square metre for exposures of one hour at the RADARSAT frequency. Radiation exposure from RADARSAT is more than 200,000 times smaller than this threshold and the exposure time is 4800 times shorter. At these very low exposure levels no short term or accumulative effects are reported.