Polarisation radar techniques have the potential to provide more accurate rainfall estimates, but observations of differential reflectivity (ZDR) and specific differential phase shift (KDP) at individual gates made with an operational radar will be very noisy. Using KDP at a range of 100km if rainfall is to be estimted with an error of <40% and a spatial resolution of 2.4km then the rainrate (R) must be > 50mm/hr. Much significant rainfall occurs at lower rates, so we must use the ZDR, but ZDR must be known to an accuracy of 0.1 or 0.2dB at each gate if R is to be 25% accurate. This seems very difficult to achieve with an operational antenna. Instead we propose an alternative approach. Essentially, we are combining ZDR and Z to produce an estimate of the normalised drop concentration, Nw, which then fixes the value of 'a' in a Z=aR^1.5 relation. By analysing values of Z and ZDR over a region we show that it is possible to derive an accurate value of the mean Nw, and hence the mean value of 'a' which should be used to derive the rainfall rate from Z over that region.