Seasonal Effect of Tree Foliage on GPS Signal Availability and Accuracy for Vehicular Navigation

The seasonal effec! of deciduous tree foiiage on GPS signal availibility and accuracy for vehicular navigation is investigated. GPS signals in the upper UHF part of the spectrum are line-of-sight and suffer much attenuation under tree branches and their leaves. An important question is to whether significant variations will occur between seasons due to the presence or absence of deciduous foliage. Two types of performance variaticns are investigated, namely signal availability and accuracy .In the presence of leaves, . . signal availability is expected to decrease. Since multipath is expected to increase, accuracy is expected to decrease as well. In order to investigate this aspect and possible correlation with different receiver technologies tests were conducted at normal cruising speeds along selected tree-lined streets of Calgary under both seasonal conditions with multichannels wide and narrow correlator spacing C/A code receivers. Tine tests were conducted in DGPS mode to assess effectively repeatable positioning accuracy. Test results are presented and analysed. They reveal that the effec! of foliage on signal availability is a function of the number of channels available and, to some a lesser extent, on receiver characteristics. Foliage is also found to have a significant effect on positioning accuracy. Narrow correlator spacing technology produces significantly better positioning accuracy under foliage conditions due to its better multipath rejection capability.