Defining an effective point of radiation for a microstrip patch antenna

In many high-precision GPS applications, accuracies on the cm level are required. However, due to the nature of GPS measurements, the signal transmitted by the satellite traverses the ionosphere and troposphere, along a different path for each incident (observation) angle, before it is detected by the receiving antenna on earth. Thus, in addition to the delay the GPS signal experiences associated with the path length between the satellite and the receiver, the signal also undergoes an additional timing delay due to the complicated nature of the propagation medium [1]. While the effects of these delays (phase errors) are typically minimized by using differential GPS techniques [1], an additional timing delay that is often neglected is associated with the variation in the antenna's effective point of radiation (“phase center”) as a function of the observation angle. Even with differential correction of the ionospheric and tropospheric phase errors, the timing delay associated with the phase center variation can degrade the positional accuracy of precision GPS measurements. Hence, a thorough characterization of the effective point of radiation for a GPS antenna is important.