Abstract:
A low phase error antenna. The antenna is adapted for use with the Global Positioning System and includes a spiral antenna for receiving electromagnetic energy at a first standard frequency (L 1 ) and a second standard frequency (L 2 ). The spiral antenna has a spiral element with a circumference greater than approximately one and a half times the wavelength of electromagnetic energy at the lowest frequency (L 2 ). The cavity of the antenna is unloaded and includes a balun adjusted for zero squint. The 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 received electromagnetic energy. The cavity is approximately ¼ of a wavelength deep at the positions along spiral that receive electromagnetic energy.

Description:
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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cut-away diagram of an antenna constructed in accordance with the teachings of the present invention. 
     FIG.  2 ( a ) is a cross-sectional diagram of an alternative embodiment of the present invention showing the unloaded cavity. 
     FIG.  2 ( b ) is a close up view of a portion of the diagram of FIG.  2 ( a ). 
     FIG. 3 is a top view of the spiral antenna of FIG. 2 showing tooling holes and the etched spiral element. 
     FIG. 4 is a cross-sectional diagram of a first alternative spiral antenna cavity constructed in accordance with the teachings of the present invention. 
     FIG. 5 is a cross-sectional diagram of a second alternative spiral antenna cavity constructed in accordance with the teachings of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a cut-away diagram of an antenna  10  constructed in accordance with the teachings of the present invention. The antenna  10  is constructed of aluminum or other suitable material. The antenna  10  has antenna elements or spiral arms  12 . The spiral arms  12  are fed by a conventional balun  14  having minimum squint. Minimum squint baluns help contribute to a desirable symmetric radiation pattern that does not lean to one side or the other. 
     In transmit mode, the spiral arms  12  radiate electromagnetic energy communicated to the arms  12  via balun  14 . In receive mode, the spiral arms  12  receive electromagnetic energy which is then communicated to the balun  14 . The spiral arms  12  are designed to accommodate electromagnetic energy having a frequency of approximately 1575.42 MHz (L 1 ) and electromagnetic energy having a frequency of approximately 1227.6 MHz (L 2 ). The L 1  and L 2  frequencies are the principle frequencies used for precision GPS surveying. 
     When the antenna  10  is in transmit mode, some electromagnetic energy will radiate outward from the antenna  10  as a transmitted wave. In the present specific embodiment, the transmitted wave is right hand circularly polarized. A left hand circularly polarized wave travels back into an unloaded antenna cavity  16 . The cavity  16  may be filled with a material other than air. The material may have a dielectric constant of approximately 1 and still be considered unloaded. The depth of the cavity  16  is approximately ¼ (90°) of the wavelength of electromagnetic energy being radiated or received. The left hand circularly polarized wave is out of phase with the transmitted circularly polarized wave by 180 degrees. When the left hand circularly polarized wave travels ¼ (90°) to the back wall  18  of the cavity  16 , it reflects off the wall  18  and switches to right hand circular polarization. The wave reflected off the back wall  18  travels another ¼ (90°) back toward the spiral element  12  and is in phase with the transmitted right hand circularly polarized wave, and has equivalent polarization. The reflected wave then adds with the transmitted wave, increasing the gain of the antenna  10  by approximately 3 dB. Typical spiral antennas have loaded cavities. A loaded cavity has an electromagnetic energy absorber or dielectric that is placed or loaded in the cavity  16  to increase the bandwidth of the antenna for broad band applications for which spiral antennas are known. Unloading the cavity for GPS applications allows for increased gain and a reduced radiation pattern backlobe which corresponds to better multipath rejection. 
     Spiral antennas are typically used for large bandwidth applications such as in military radar detection systems. Such antennas, however, have been overlooked for GPS applications due to a lack of general knowledge on the applicability of large diameter (versus wavelength) spiral antennas to GPS systems. 
     Those skilled in the art will appreciate that the balun  14  may be an infinite, printed-circuit, lumped-constant, or matrix excited balun, or another type of balun having low squint without departing from the scope of the present invention. 
     FIG. 2 is a cross-sectional diagram of an alternative embodiment  20  of the present invention. The antenna  20  includes an unloaded cavity  22  approximately ¼ wavelength in depth. A Marchand balun  24  is included and has no squint for achieving symmetrical radiation patterns. Techniques for incorporating Marchand baluns without squint are well known. The Marchand balun includes a coaxial portion  26  connected to a stripline  28  that directs electromagnetic energy to dual spiral feeds  30  of the spiral antenna  20 . As discussed below, the dual spiral feeds feed a dual spiral element etched on a surface  32  of the antenna  20 . Those skilled in the art will appreciate that the antenna  20  may be fed from the periphery rather than the center of the antenna  20  without departing from the scope of the present invention. 
     The spiral element is etched on the surface  32 . FIG.  2 ( b ) is a close up view of a portion  33  of the diagram of FIG.  2 ( a ). The surface  32  is supported by a thin dielectric substrate  36  of precision controlled thickness which has a dielectric reference surface  34 . The dielectric substrate  36  is supported by a precision machined metallic reference surface  37  which is part of an antenna housing  41 . The reference surface  34  and the metallic reference surface  37  are coplanar. 
     In the preferred embodiment, as shown in FIG.  2 ( a ), the dielectric reference surface  34  is also supported by a honeycomb material that has a dielectric constant of 1 and is precision machined to a thickness to occupy the cavity  22  to match the height of the metallic reference surface  37  (see FIG.  2 ( b )) and the dielectric reference surface  34 . This honeycomb material helps to consistently position the spiral element in elevation above the reference plane  34  and prevents undesirable sagging of the dielectric substrate  36 . This, in turn, provides for consistently manufactured spiral antennas. 
     To further aid in manufacturing consistency, the antenna  20  includes reference pins  38  used to consistently position the reference plane substrate  36  on the antenna  20 . The pins  38  are aligned with tooling holes (see FIG. 3) in the reference plane substrate  36 . This helps to ensure that the spiral element is centered over a master reference surface  39 . 
     The master reference surface  39  is a precision machined surface displaced a known distance from the metallic reference surface  37  (see FIG.  2 ( b )). This establishes a precision height of the spiral element above the master reference plane  39 . The master reference surface  39  is centered relative to the reference pins  38 . This keeps the spiral element on the surface  32  centered relative to the center of the master reference plane  39 . The centering of the master reference surface  39  relative to the tooling pins  38  is facilitated by bolt holes  43  in the master reference surface  39 . The bolt holes  43  may be accurately positioned relative to the reference pins  38  using conventional CNC machinery so that the master reference surface  39  is concentric with the dielectric surface  34 , the substrate  36 , and the spiral element feeds  30 . 
     The L 1  and L 2  frequencies will be received on the spiral elements (see FIG. 3) at different positions along the spiral elements. For example, electromagnetic energy received at the L 1  frequency will occur at a first diameter and electromagnetic energy received at the L 2  frequency will occur at a second diameter. The depth of the cavity  22  may be varied so that at the first diameter the depth of the cavity is approximately ¼ of the wavelength of the energy received at the L 1  frequency and at the second diameter, the depth of the cavity is approximately ¼ of the wavelength of the energy received at the L 2  frequency (see FIGS.  4  and  5 ). This will cause both phase centers corresponding to the received wave and the wave reflected off the back wall of the cavity to be coincident which will enhance the overall system performance. 
     FIG. 3 is a top view of the spiral antenna  20  of FIG. 2 showing alignment tooling holes  40 , feed tooling holes  46  and an etched spiral element  42 . The tooling holes  40 ,  46  provide for consistent manufacturing which results in antennas having similar receive patterns and low phase error properties. 
     The technique of observation differencing may be more effective at canceling any remaining phase errors when existing antennas are replaced by consistently manufactured spiral antennas. Conventional dual frequency patch or cross-dipole antennas typically lack convenient mechanisms to ensure very consistent manufacturing. 
     The spiral element  42  is an archimedian spiral and extends close to an edge  44  of the antenna  20 . Those skilled in the art will appreciate other types of spiral elements such as log spiral or equiangular spiral elements may be used for this purpose without departing from the scope of the present invention. Also, the spiral element may be a hybrid element  42 , such as a combination of an archimedian, log, and/or equiangular spiral. 
     The circumference of the spiral element  42  enclosed by the edge  44  is approximately twice of the wavelength of electromagnetic energy to be received by the antenna. In the present embodiment the circumference is approximately twice the wavelength of electromagnetic energy received at the L 2  frequency. In the illustrative embodiment, the spiral element  42  has a diameter of at least 1¾ of the wavelength of the electromagnetic energy at L 1  or L 2 . At GPS frequencies, this diameter will typically be at least four inches. Those skilled in the art will appreciate that a larger antenna diameter may be used for this purpose without departing from the scope of the present invention. 
     When manufacturing the spiral antennas of the present invention, a series of steps are performed. First, the top surface (see  32  of FIG. 2) of the dielectric substrate  36  is manufactured from dielectric material. The reference surface is circular and flat having very small, predetermined tolerances. The precision machining of such flat, circular surfaces is well known in the art. 
     Next, a computer numerical control (CNC) drill is programmed to drill the tooling holes  40 ,  46  in the substrate  36 . The holes are drilled and the spiral element artwork is optically aligned and etched on the substrate  36 . 
     The dielectric substrate  36  is a copper coated dielectric material. The artwork includes a design of the spiral element with feed holes coinciding with the feed holes already drilled in the reference plane substrate  36 . The design is then optically aligned with the substrate  36  so that artwork feed holes are aligned with the feed holes already drilled in the substrate  36 . This optical aligning may be performed using conventional optical aligning techniques used in integrated circuit manufacturing processes. Next, the spiral element design is etched in the copper of the substrate  36  using conventional circuit board etching procedures, which include the application of photo-resist, ultraviolet light exposure, and etching. 
     Finally, the prepared reference plane substrate  36  is placed on a precision machined spiral antenna body (see  41  of FIG. 2) and aligned with tooling pins (see  38  of FIG. 2) thereon. The tooling pins coincide with the tooling holes  40  and are also positioned with the aid of a CNC machine. 
     A method for manufacturing a spiral antenna according to the teachings of the present invention comprises the steps of: 
     1. Machining the structural components of a spiral antenna including a cavity, and an antenna reference plane from aluminum. 
     2. Storing information relating to a desired position of the spiral element on the surface of the spiral antenna in a memory associated with a CNC drill. 
     3. Using the CNC drill to drill spiral element alignment tooling holes and element feed tooling holes in the surface in accordance with the information stored in memory. 
     4. Developing a mask of the spiral element to be etched on the surface of the antenna to fit the tooling holes. The mask is designed so that the spiral element has the necessary number of turns to radiate or receive at the L 1  or L 2  frequencies, or both. 
     5. Using the mask to etch the cement from a copper sheet using conventional integrated circuit manufacturing methods on the surface so that the spiral feeds are aligned with the feed tooling holes, and the spiral element is aligned with the alignment tooling holes. 
     FIG. 4 is a cross-sectional diagram of a first alternative spiral antenna cavity  50  constructed in accordance with the teachings of the present invention. The antenna cavity  50  has a conical back plane  52  causing the cavity  50  to vary in depth as a function of diameter. 
     Electromagnetic energy radiates from a spiral element at a different diameters for different frequencies of electromagnetic energy. At an L 1  diameter  54  at which electromagnetic energy having a frequency of approximately 1575.4 MHz radiates from the spiral element (see FIG.  3 ), the cavity is ¼ of the wavelength of the electromagnetic energy. Similarly, at a L 2  diameter  56 , the depth of the cavity is approximately ¼ of the wavelength of electromagnetic energy having a frequency of approximately 1227.6 MHz. 
     FIG. 5 is a cross-sectional diagram of a second alternative spiral antenna cavity  60  constructed in accordance with the teachings of the present invention. Like the cavity  50  of FIG. 4, the cavity  60  has a depth that varies so that the depth at a particular diameter is ¼ of the wavelength of electromagnetic energy radiated or received at that diameter. Since there are only two principle frequencies L 1 , and L 2  there is one step  62  in the cavity  60 . 
     Those skilled in the art will appreciate that the depth of the cavity  60  may be continuously varied across the entire antenna diameter so that at each position along a spiral element (see FIG. 3) the cavity  62  is ¼ of the wavelength of electromagnetic energy that may be radiated or received from that position, without departing from the scope of the present invention. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,