|2. PHASE_1 data are spot measurements using frequencies that emphasize the VHF band and environments that emphasize irregular terrain. One feature is that many of the paths appear in threes with the receiving antenna at the "principal" location or at an "alternate vertical" or an "alternate horizontal" location. The principal location is assigned from a map, and should be fairly random. The alternate locations simulated "siting improvement".
For the alternate vertical location, the receiving vehicle was driven slowly over available roads within a 100 metre radius of the principal location while observing vertically polarized 100 MHz test signals with the 3 metre antenna height. The new location is that at which the received power is a maximum. The alternate horizontal location is determined in the same way, except that horizontally polarized signals are used.
For each of the three measurement sets there are three data files: A ..PARS file, a ..PATH file, and a ..DATA file. There may also be a ..XDATA file as described later on. The "PARS" file provides values for the pertinent parameters of the measurement system. The "PATH" file gives coordinates of both terminals together with the computed distance between them.
Finally, the "DATA" file provides the corresponding measured values. They are given as the observed "attenuation relative to free space" (also called the "excess path loss"). If a space is blank there was no corresponding measurement--often because of equipment failure. If the signal was below system sensitivity there is a different indication. The maximum attenuation that would have been observable is computed, and then the value used is that computed attenuation plus the number 500. For example, the value 560 means that the corresponding attenuation was unobservable but was evidently greater than 60 dB.
The three data sets correspond to three different terrain types: the plains, and then the mountains of central Colorado, and the hills of northeast Ohio. The basic measurements are nominally at 20, 50, and 100 MHz, and the last frequency at both vertical and horizontal polarization. There are additional measurements according to the data set involved.
In Colorado (for both the plains and mountains data sets) there are two additional measurement sets. The first used a second transmitter at 100 MHz. This was a commercial FM broadcast station having the call letters KLIR and located about 40 km southeast of the basic transmitting site in a fairly flat region. The second set of additional measurements concerned the cross-polarized signals at 100 MHz. The vertically polarized transmissions were received on the horizontally polarized antenna and the horizontally polarized transmissions on the vertically polarized receiving antenna. The results are recorded in the files ..XDATA. Note that the original data analysis, as repeated in the reference reports, used the wrong receiving antenna gains.
In the present files there has been an attempt to correct the data. This correction, however, has introduced a further variability of about 3 or 4 dB.
In Ohio, there were six transmitting sites--a "central" site and five "peripheral" sites. The latter were evenly spaced on the circle 50 km from the central site. Receiving sites were all within this same circle. In the present files, the central site has been called T0 and the peripheral sites T1,...,T5.