The purpose of these reference lines is to compare slopes and signal levels of the different transmitters. The least squares fit for signal strength as a linear function of the log base 10 of the distance is also determined for each of the transmitter sites shown in Table 2. As a check of the system calibrations, expected signal levels at a specified distance from the transmitter can be compared against measured values. Cumulative distribution curves and histogram plots for deviation from the least squares fit are shown in Figures 16 - 33 . The same data points used to calculate the least squares fit are used for these plots.
Negative numbers represent signal strength occurring below the least squares fit.
When plotted against the log of the distance, most of the DGPS signal data appear relatively linear with a roll-off of 20 dB per decade. Deviations from this linearity appear in several cases. The cause of this is unknown but is probably due to excess noise. There are two regions of signal attenuation both of which are located physically in areas of rough terrain. It is interesting to note that the Sierra Nevadas behave much the same as an attenuator.