Fig. 2. GPS satellite signal reception and rain forest canopy interference. (A) Despite favorable satellite geometry, attenuation of the satellite signal (SNR) by dense foliage makes data collection impossible. (B) Although six satellites are on the horizon, only four are usable, due to signal attenuation by the canopy. SNR from satellites passing near canopy gaps is optimal, but poor orthogonal positioning results in high PDOP, which makes data collection unreliable.