# Development of a GPS Forest Signal Absorption Coefficient Index
## 4 Discussion  
In the first portion of this study, we presented the SAM. The SAM methodology is simplistic and, as such, future work associated with atmospheric modeling has been considered. Factors such as humidity, barometric pressure, clouds versus open sky, and fog could all be potential components in atmospheric modeling. However, many of these factors change rapidly and would require a very substantial series of photos and measurements of the different variables over short periods of time, thus we used the approach outlined above to develop the SAM. A primary objective of this research was to determine the parameters that influence signal attenuation. As such, during the modeling process, many other variables were considered to include the interaction of these variables. Parameters such as the leaf area index (LAI) (as calculated from gap light analyzer software), the density of the trees, average diameter at breast height, and average tree \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & \multirow{2}{*}{**City Vicinity**} & \multirow{2}{*}{**Tree Type**} & \multirow{2}{*}{**HT/STD (m)**} & \multirow{2}{*}{**DBH/STD (m)**} & \multirow{2}{*}{**SNR Index (dB) SFIM**} & \multicolumn{2}{c}{**SNR Index (dB) SFIM**} \\ \hline
1 & West Point, NY & Oak/Hickory & 23.4/3.4 & 0.30/0.18 & Fall & –3.74 & –3.75 \\  & & 100\% Deciduous & & Spring & –5.43 & –5.44 \\  & & & & Summer & –5.54 & –5.55 \\
2 & IMPAC & 100\% Pine & & & Winter & –4.37 & –4.38 \\  & Gainesville, FL & Needle Minimum & See Table 1 & See Table 1 & –3.31 & –3.32 \\  & Gainesville, FL & Needle Maximum & See Table 1 & See Table 1 & –4.30 & –4.31 \\
3 & Hougton Forest & 80\% Deciduous & 20.4/2.47 & 0.46/0.08 & –5.87 & –5.89 \\  & Gainesville, FL & 20\% Conifers & & & & \\
4 & Charlton, SC & 90\% Pine, 10\% Deiduous & 24.0/3.1 & 0.36/0.05 & –7.68 & –7.69 \\
5 & Alexandria, LA & 40\% Pine, 10\% Deiduous & 23.2/4.1 & 0.56/0.11 & –6.48 & –6.49 \\
6 & Cold Spring, TX & 80\% Pine, 20\% Deciduous & 19.5/4.7 & 0.52/0.13 & –6.03 & –6.06 \\
7 & Georgetown, TX & Cedex Elim and Live Oak & 6.3/1.1 & 0.42/0.11 & –5.15 & –5.16 \\  & & with Ash Juniper & & & & \\
8 & Cloudcroft, MN & Ponderosa Pine & 23.3/3.2 & 0.41/0.12 & –3.12 & –3.12 \\
9 & Flagstaff, AZ & Ponderosa Pine & 19.2/6.8 & 0.41/0.07 & –2.83 & –3.35 \\
10 & Gaudalupe, CA & Eucalyptus & 28.2/3.3 & 0.42/0.14 & –5.02 & –5.02 \\
11 & San Luis Obisp & Agridiation & 6.0/1.5 & 0.22/0.09 & –3.10 & –3.11 \\
12 & Davenport, CA & 75\% Redwood, 25\% & 54.0/6.3 & 1.20/0.56 & –10.78 & –10.80 \\
13 & Davenport, CA & 80\% Tanoak, & 18.7/1.4 & 0.28/0.08 & –8.05 & –8.07 \\
14 & Tahoe NF & Ponderosa Pine & 26.5/2.2 & 0.53/0.16 & –3.98 & –3.98 \\
15 & Nederland, CO & Aspen & 8.4/2.6 & 0.20/0.06 & –4.99 & –5.00 \\ \hline \hline \end{tabular} Note: Where HT is Tree Height, DBH is Diameter at Breast Height and STD is Standard Deviation.  
\end{table} 
Table 3: Absorption indexes of the forest study sites showing the results of the signal to noise forest index model (SFIM) and simplified signal to noise forest index model (SSFIM). height (to name a few) were considered. However, the parameters that make up the CCPM and SSFIM proved the optimum method. Many previous studies modeled forest structure. [PERSON] and [PERSON] explained the evolution of different canopy structure models as the assumption of uniformity of foliage was removed [22]. Building on this work, [PERSON] modeled forests with individual cylinder or parabolic crowns as trees with a uniformly distributed LAI density [23]. This work allowed for areas with no foliage and areas with overlap. The overlapping areas would cause a clumping effect. Other statistical models such as the Poisson, negative binomial, and Markov models predict the likelihood of a ray of light passing completely through forest canopy [24]. A great advantage of the models presented in this research is their simplicity. When comparing a single layer canopy to a triple layer canopy, for example, we simply obtain different CC values.