The foliage effect on the height time series from permanent GPS stations

The effect of deciduous trees growing above antenna height on data collected by permanent Global Positioning System (GPS) stations was investigated. Signal blockage due to foliage and branches was found to have the same effect as an increased elevation cutoff angle, i.e., there was a change in the computed position. Height estimates were affected the most, showing a decrease with tree growth. Empirical Orthogonal Function (EOF) analysis on the height-time series from five test sites and two stations surrounded by trees showed a similar EOF mode of signal. Signal availability, computed as the ratio of the complete to possible set of observations, decreased with increasing tree growth and showed seasonal variation, with the observation ratios being higher during the winter months when the leaves had fallen. A similar seasonal variation was observed in multipath error and signal attenuation due to foliage. The multipath error index MP2 was computed using the TEQC program and found to increase at a significant rate at sites with growing trees. Signal attenuation was analyzed using 1-σ uncertainties from the estimation process of daily GPS data processing. While 1-σ uncertainties did not show any seasonal variations at sites without trees, they were highly dependent on conditions related to the seasonal change of foliage when deciduous trees were near the antenna.