Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
1997-08-25
2001-03-13
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
C343S792500
Reexamination Certificate
active
06201513
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to the Global Positioning System (GPS). Specifically, the present invention relates to low phase error antennas for receiving GPS signals.
2. Description of the Related Art
The Global Positioning System is used in a variety of demanding applications ranging from geological surveys, to military positioning applications. Such applications require accurate antennas to precisely determine distances and positions with sub-millimeter accuracy.
The Global Positioning System includes a constellation of satellites equipped with GPS transmitters. A ground receiver receives signals from the satellites. By measuring signal travel time from the satellites to the phase center of the ground receiver's antenna, the position of the ground receiver may be determined. The phase center of the antenna corresponds to the point at which the antenna appears to receive a spherical wavefront. The phase center may be different than the physical center of the antenna.
Often, the phase center of the antenna does not correspond the physical center of the antenna due to multipath errors and/or phase errors. Typical GPS antennas are either dual frequency patch antennas or cross dipole antennas which are particularly prone to phase and multipath errors. Multipath errors occur when signals transmitted from the GPS satellites reflect off hills or objects and combine. The combined signal is received by the ground receiver and results in an effective electrical position that erroneously moves with satellite transmit location. An antenna with a receive pattern that extends well below horizontal may more readily detect such combined reflected signals. An antenna with such a receive pattern is said to have a large backlobe and is more susceptible to multipath problems.
Phase errors are inherent in certain antenna element designs such as patch antenna designs. Other phase errors occur due to manufacturing tolerance such as in cross dipole designs. Phase errors cause the phase center of a stationary ground antenna to move with satellite position. The effective phase center of patch antennas and cross dipole antennas often vary with GPS satellite position due to antenna structure and manufacturing error respectively.
To reduce multipath errors choke slot groundplanes were developed. Choke slots are highly reactive devices at the design frequency which when installed on a GPS antenna reduce antenna surface currents and re-radiation. The reduced surface currents may result in a decreased antenna backlobe and reduced multipath errors. The antenna is said to have improved multipath rejection. GPS antennas that employ choke slots are often large and expensive as a result of structural limitations.
To reduce phase errors associated with existing GPS antennas a method known as observation differencing was developed. Observation differencing involves canceling phase errors through the introduction of compensation variables. This method requires antennas in the GPS system to be of the same make and model. The method relies on the assumption that antennas of the same make and model behave similarly. The lack of consistency between such antennas limits the effectiveness of observation differencing in canceling phase errors. This lack of consistency is partially due to manufacturing inconsistencies due to difficult tooling procedures.
Hence, a need exists in the art for a cost effective, compact antenna that minimizes phase and multipath errors. There is a further need for antenna that provides for tooling procedures that result in antennas with similar and consistent performance.
SUMMARY OF THE INVENTION
The need in the art is addressed by the low phase error antenna of the present invention. In the illustrative embodiment, the inventive antenna is adapted for use with a global positioning system and includes a spiral antenna for receiving a signal at a first frequency and/or a second frequency. The spiral antenna has a spiral element with a circumference greater than approximately one and one-half times the wavelength of the signal received at the lowest frequency.
In a specific embodiment, the first frequency and the second frequency are the standard L
1
and L
2
frequencies respectively. The cavity of the spiral antenna is unloaded and includes a Marchand balun adjusted for no squint. The spiral antenna element is either a logarithmic spiral or an archimedian spiral.
In the illustrative embodiment, the spiral antenna includes a cavity having a depth which varies in accordance with the radiated GPS frequencies. The cavity is approximately ¼ of a wave deep at the position along the spiral that receives the radiated electromagnetic energy.
The novel design of the present invention is facilitated by the use of tooling markers on the surface of the antenna which ensure consistent manufacturing and performance.
REFERENCES:
patent: 3555554 (1971-01-01), Kuo
patent: 4287603 (1981-09-01), Moser
patent: 4319248 (1982-03-01), Flam
patent: 4905011 (1990-02-01), Shea
patent: 5508710 (1996-04-01), Wang et al.
patent: 0080804 (1982-05-01), None
Connolly Peter J.
Ow Steven G.
Benman William J.
Ho Tan
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