(1) Field of the Invention
The present invention relates to antennas and more particularly to a global positioning system (GPS) antenna.
(2) Description of the Prior Art
In the field of GPS technology, GPS receivers are used to determine the geographic location of the receiver by receiving microwave radio signals from a group of earth-orbiting GPS satellites. The geographic location of the receiver may be computed by calculating its distance from each satellite as the result of determining how long the signals take to travel from the satellite to the receiver. Typically, a flat GPS antenna element is utilized by GPS receivers to receive the signals transmitted. In order for the GPS receiver to compute its geographic location, the antenna element of the receiver must be oriented to receive an acceptable level of the signals. Optimally, the flattened surface of the GPS antenna element is righted against the force of gravity such that a maximum surface area of the antenna faces the satellites.
Present submarine communications with battlegroups or satellites utilize surface antennas for a variety of requirements including global positioning and communications. The use of surface antennas typically interferes with the covert operation of the submarine. For example, submarines obtaining position fixes using GPS must raise a mast containing an antenna which is oriented to receive the signals from the GPS satellites. The problem is that raising a mast renders the submarine vulnerable to either visual or radar detection, especially if the mast is raised in coastal or littoral areas.
Additionally, antennas used on the ocean surface are subjected to dynamic forces that act to cause the antenna to pitch, yaw and sometimes roll with the vessel under varying sea states. These antenna movements can easily re-orientate the receiving element of the antenna resulting in reception interruption. Varying sea states also cause a detuning effect that result in degradation of the patch elements of conventional GPS antennas. To minimize the effects of varying sea states, the submarine must operate in a station keeping status or must constantly adjust course headings.
One method of mitigating reception interruption of the antenna is to orient the flattened surface of the antenna to right itself or face “up” toward the sky irrespective of the movement of its supporting structure. In Ham (U.S. Pat. No. 6,292,147), an apparatus for maintaining a GPS antenna element at a predetermined orientation is disclosed. The apparatus includes a holder configured to support a GPS antenna element in which the holder includes a rectangular frame as a receiving portion of the dielectric substrate of antenna. The rectangular holder pivots on an axis in relation to gravity to the predetermined orientation even when the base structure to which the holder is coupled changes its orientation. While the disclosed reference allows a righting motion to the antenna element, the movement of the righting motion is limited to rotation around the axis of the pivot in which the rotation provides only one degree of freedom.
It is well known in the use of gyroscopes and in the use of compasses on ships, that a gimbal provides at least two degrees of freedom for either attached device by allowing a pivoting action on the axes of the gimbal in which the axes are rotatable at angles to each other. For example, the pivoting and rotating action of a gimbal used on a ship compensates for the roll and the yaw of the ship as well as the pitch of the ship thereby maintaining an accurate heading of a compass set in the gimbal.
As such, an improvement to the technology of GPS antennas would be to incorporate the degrees of freedom of a gimbal with a conformable GPS antenna in a manner that is suitable for use on a vessel or towed array as well as for use in any other situation that can require more than one degree of freedom in which the degree of freedom is needed to maintain the righting or facing up element of the antenna receiver. Such an improvement along with any other suitable improvements to the structure of the GPS antenna could act to minimize the reception interruptions and the detuning effects caused by varying sea states.