The real-time reflectivity is assigned the two letter code "FZ" (in-Field Reflectivity). The post-processed reflectivity is referred to as "DZ". Both parameters are derived from the radar received power, a directly measured quantity. FZ is determined using a linear approximation to an assumed received power calibration curve. The assumed calibration curve has an assigned slope and offset for the linear portion of the curve, and these assigned parameters may or may not be sufficiently accurate to allow a good approximation of FZ to DZ over this linear segment. In addition, since the true calibration curve is somewhat "S"-shaped, even a good linear approximation (one with an accurate slope and gain) will have some error near the range limits (both upper and lower) of the received power.
A comparison of FZ and DZ was done for data from the early part of the experiment; the residual quantity "FZ-DZ" was computed and plotted against occurrence. Results of the comparison are shown in a histogram. See the note, below, for details of the histogram. It is clear from the graph that FZ very closely approximated DZ for CP4 in WISP94.
The histogram referenced in this section of the report was produced using the ATD Radar EDITOR software. Areal coverage of a given reflectivity difference ( FZ minus DZ, a difference of logarithmic quantities) is plotted against that reflectivity difference; the "areal coverage" scale is self-adjusting. Reflectivity difference units are in dB.