SubscribeRadar (RAdio Detection And Ranging) devices operate in the Radio Frequency (RF) band typically in UHF (300 MHz) through W-band (110 GHz) or higher. Radar detection devices are affected by water in the atmosphere, although radar to a much lesser extent than other detection technologies such as Laser, InfraRed and Video.
The biggest detection loss is due to rain in the atmosphere and is based on the droplet size and the radar transmission wavelength as shown in Figure 1.
 Figure 1: Attenuation and backscatter versus frequency at various rain rates |
For ground based radar systems detecting intruders, personnel and vehicles, the radars typically operate at the X-band (8 to 12.5 GHz), Ku-band (12.5 to 18 GHz) and Ka-bands (26.5 to 40 GHz).
Attenuation of the RF energy transmitted from the radar, reflected from the target and received by the radar affects the detection performance. This is considered the two-way absorption loss. For the X and Ku bands there is little difference in absorption performance. For the Ka band, there is much more significant absorption loss and this is why these ground based radars are typically used for much shorter range applications. For longer range (>3 kilometers) applications, using radars in the X and Ku bands is preferred.
Rain backscatter is transmitted RF energy from the radar that is reflected off the rain droplets back to the radar that competes with the signal-to-noise (SNR) ratio of the intruder. If the rain backscatter gets large enough then it can dominate the SNR of the intruder making the intruder undetectable. This rain backscatter is a function of the volume over which the intruder is being detected and is defined by the radar antenna, the range cell and the transmit frequency. The antenna defines the detection footprint of the radar and the range cell is the typical range bin or resolution cell of the radar. Radar antennas are inversely proportional to the beam width so the larger the antennas the smaller the beam width or detection foot print of the radar and the better the detection performance in rain. The range cell is defined by the pulse width for pulse Doppler type radar systems or the frequency sweep for FMCW radar systems and in general, the smaller the pulse width or larger the frequency sweep the better detection performance the radar will have in rain.
Figure 2 and Figure 3 show a comparison of the backscatter for two representative ground based radar systems. Figure 2 for an FMCW radar and Figure 3 for a Pulsed radar. Note the large differences in range cell size cause the difference in back scatter performance. Typical values of effective Radar Cross Section (RCS) for a walking person are on the order of 0.5 to 1.0 m2 and for a vehicle on the order of 10 m2. If the effective RCS is equal to the intruder size then the SNR is zero and the intruder is undetectable.
 Figure 2: Rain Backscatter versus Range for a representative FMCW Radar |  Figure 3: Rain Backscatter versus Range for a representative Pulsed Radar |
Other atmospheric obscurants such as sand, smoke, dust, snow, hail, etc. are either very small in relation to the wavelength or have less water content than rain and their effects over the detection ranges of the ground based radars is negligible.
