Abstract: In a satellite-based messaging system messages are exchanged between a ground station and mobile terminals over a wide geographic area covered by multiple satellite beams. Each beam contains one or more carrier frequencies. A message processing center is configured to provision a frame comprising a plurality of subframes to carry data over a forward link to the mobile terminals; hop between beams in the forward link and assign each subframe within a frame to a specific satellite beam; provision a transport medium in the form of virtual carriers each defined by one or more carrier frequency/beam hop combinations, wherein each virtual carrier is assigned a unique virtual carrier identifier; and notify the mobile terminals of carrier frequency/hop combinations for assigned virtual carrier identifiers.

Abstract: A quadrifilar helix antenna system with a finite ground plane has a pair of bifilar helical elements extending upwardly from the finite ground plane. A symmetrical array of monopole elements surrounds the lower portion of the pair of bifilar helical elements in the near field so as to load the lower portion and thereby raise the phase center of the antenna to improve the circularly polarized far-field radiation at low elevation angles.

Abstract: A method for determining that a GPS receiver is located within a predetermined geofence volume. An embodiment of the invention includes the steps of receiving global positioning system signals from a plurality of GPS satellites; calculating a solution set of possible receiver positions from the global positioning signals; determining whether any portion of the solution set of receiver positions is within the geofence volume; and determining that the receiver is outside the geofence volume if no portion of the solution set of receiver positions is within the geofence volume. The concept of exclusion space is introduced and used in methods for determining the probability that a receiver is within a geofence volume.