Maritime radars are required to have a very good target detection capability. Detection and tracking of small targets in harsh weather conditions can help authorities in efficient monitoring of illegal activities.
It is well known that rain can significantly reduce detection ranges of maritime radars, especially on small targets. This paper explains the causes for this and how adverse effects of rain can be reduced by proper choice of radar scanner parameters.
Introduction
Raindrops absorb and scatter radar signals, so less energy reaches the target and even less returns to radar as an echo. Hence, precipitation (rain, snow, hail etc.) in general and rainfall in particular affect the propagation of electromagnetic (EM) waves (radar signals) in two ways: first, there is absorption of electromagnetic energy by water drops and vapour which causes radar signal attenuation (loss). Second, there is a returned signal from the rain which ‘clutters’ the radar return and can mask targets. This phenomenon is known as rain clutter.
Precipitation losses
The absorption of the EM energy is mainly dependent on precipitation rate, its type and extent, radar operating frequency and its polarisation. The attenuation increases with both frequency and the rainfall rate and is usually expressed in decibels per kilometre. Table 1 gives representative values for rain attenuation at S- and Xbands of frequencies, customarily used in maritime radars.
The numbers shown indicate that the loss at 10GHz is one to two orders of magnitude higher than at 3GHz, and that a long path in heavy rain at higher frequency could produce large attenuation. Rainfall rates along the radar/target path cannot be easily determined, partly because the rainfall is not uniform along the path. For example, the rainfall over the ground, where the radar station is based, is usually different from the rainfall out at the sea, where the target is (offshore rates tend to be lower). Furthermore, heavy rainfall mainly occurs in areas occupying only a part of the total radar/target path length. Having said that, one should bear in mind that heavy rain represents very high navigational hazard and the radar performance calculations should be based on the worstcase assumption of the whole-path precipitation. Whether this assumption is being used or not should be clearly stated, as the effect on long range detection performance can be considerable, especially for small targets.
The world-wide statistics on precipitation intensity is available from International Telecommunications Union (www. itu.int): ITU-R Recommendation P.837-4, ‘Characteristics of precipitation for propagation modelling’. The document describes a numerical model which can estimate the rainfall rate exceeded for a given percentage of time a year. For example, there is probability of 0.01% (about one hour a year) of precipitation exceeding 40mm/hour on the coast of Portugal.
Rain clutter
Rain reflects some of the energy transmitted by radar back to the radar receiver as rain clutter (Figure 1). The rain clutter is a volumetric phenomenon so the amount of clutter returned back to the radar will be directly proportional to the volume of rain illuminated by the radar beam (Figure 2). Obviously, the narrower the radar beam the smaller the clutter. The beam should be narrow in both azimuth and elevation. The reduction in the clutter returns as function of elevation and azimuth beamwidths is given in Table 2.
When circularly polarised radar scanner illuminates rain, internal reflections within perfect spheres of water would return the circularly polarised signal of the opposite hand, which perfectly polarised scanners would reject. The raindrops are not perfect spheres, however, and neither the scanners have perfect circular polarisation. Even so, good circularly polarised radar scanners reduce rain clutter by about 20dB, which enables substantial improvement of radar performance in rain (Figure 3). Unfortunately, radar cross section of targets is slightly reduced for circular polarisation (hence, slightly reduced radar ranges), so for optimum detection the operator should have a capability of polarisation switching: circular polarisation in rain and linear polarisation in clear weather.
The pulse duration which the radar transmits also affects the level of rain clutter returns: the longer the pulse duration, the more clutter is received by the radar scanner. Reducing the pulse duration lowers the clutter energy but the energy on target is also reduced and the situation when the energy is not sufficient to ensure detection has to be avoided.
