How do you mitigate the effects of multi-path interference on GNSS signal accuracy?

Multipath interference occurs when GNSS (Global Navigation Satellite System) signals, such as those from GPS, reach the receiver via multiple paths. Instead of traveling directly from the satellite to the receiver, some signals bounce off of nearby surfaces (buildings, trees, the ground) before arriving. These reflected signals travel a longer distance, causing a delay and distortion of the signal. This interference degrades the accuracy of the UAV's position calculation. Several techniques can be employed to mitigate the effects of multipath interference. One method involves using antennas with good signal selectivity and multipath rejection capabilities. These antennas are designed to preferentially receive signals arriving directly from the satellites while minimizing the reception of reflected signals. Another approach is to use advanced signal processing techniques within the GNSS receiver. These techniques can analyze the characteristics of the incoming signals and identify and reject multipath signals based on their delay, amplitude, and direction of arrival. Techniques like code correlation and carrier phase smoothing can reduce the impact of multipath. Real-Time Kinematic (RTK) and Differential GPS (DGPS) are also used. These techniques involve using a base station with a known location to provide corrections to the UAV's GNSS receiver, which can significantly improve accuracy and reduce the effects of multipath. The base station measurements are used to model and remove common errors, including some multipath effects. Flying in open areas away from buildings and other reflective surfaces helps minimize multipath interference. Areas with dense vegetation or tall buildings are prone to significant multipath effects. Finally, implementing altitude constraints can help. Multipath signals often arrive from below the horizon, so maintaining a sufficient altitude can reduce their impact. For example, when mapping a construction site with many reflective surfaces, using a high-quality antenna, RTK correction, and maintaining a reasonable altitude will help mitigate multipath interference and improve the accuracy of the resulting map.