Undefined GPS Spacecraft Sidelobes and Radio Occultation

GPS satellites are distributed in six circular orbital planes and orbit the earth once every 12 hours.  They continuously transmit at two L-band frequencies, roughly 1575 MHz and 1227 MHz towards the earth’s surface.   Radio Occultation (RO) occurs when a GPS satellite rises or sets across the limb of the earth (horizon) with respect to a LEO satellite equipped to receive those signals.  A signal, passing through the atmosphere is refracted to a degree as a result of atmospheric density.  This source of atmospheric information is rapidly becoming an important to the meteorological and climatological communities.  Off-axis energy from the GPS L-band transmitting antenna is not defined as a requirement and can change as vendors for the GPS spacecraft change.  How this possible change of off-axis gain might affect the GPS RO community will be discussed in this paper.