Compact cross-link antenna for next generation global positioning satellite constellation

An inter-satellite cross-link antenna for a communications satellite in a constellation of satellites in earth orbit. The complete cross-link system is an array of eight quadrifilar helix antennas with a new design which is eight times smaller than previous designs, and has superior inter-satellite communications performance. The quadrifilar helix antenna is designed with a length, diameter, helix pitch angle and ground plate connectivity which is matched to the UHF inter-satellite communication frequency to provided a toroidal radiation pattern with high signal strength in a direction normal to the antenna axis and very low signal strength in an axial direction. The array of eight quadrifilar helix antennas does not require interleaving with the L-band GPS antenna aperture on the satellite, and does not block or interfere with the earth-directed GPS signals.

BACKGROUND

Field

This invention relates generally to an antenna subsystem for a communications satellite and, more particularly, to a cross-link antenna for satellite-to-satellite communications in a constellation, where the cross-link antenna comprises eight quadrifilar helices situated in a ring around an L-band satellite-to-earth antenna, and the quadrifilar helices have a smaller and more effective design which improves both satellite-to-satellite and satellite-to-earth communications performance.

Discussion

Communications satellites are used to enable many different types of telecommunications. For fixed (point-to-point) services, communications satellites provide a microwave radio relay technology which is complementary to that of communication cables. Communications satellites are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, global positioning system (GPS), and for TV and radio broadcasting.

In one common implementation, many communications satellites are placed in low earth orbit (LEO) or medium earth orbit (MEO) in a constellation which circles the earth. The individual satellites in the constellation communicate with each other, and also communicate with users and communications providers on or near the earth's surface. The communications among the satellites in the constellation are handled by what are known as inter-satellite links (ISL) or cross-links.

Some satellite constellations use reflector-type antennas for ISL or cross-link, however these types of antennas and their control systems are expensive and bulky. It is desirable, where possible, to use simpler antennas for cross-link communications. However, the cross-link antennas must not only be effective in satellite-to-satellite communications performance, but must also not be detrimental to satellite-to-earth communications performance. These requirements have been difficult if not impossible to meet using past cross-link antenna designs.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed to a quadrifilar helical inter-satellite cross-link antenna is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the embodiments discussed below are described in the context of a constellation of global positioning system (GPS) satellites. However, the disclosed antenna may also be suitable for use in other types of satellites or other types of communications systems.

FIG. 1is an illustration of a constellation of satellites10circling earth. Three of the satellites10are shown inFIG. 1. In an actual constellation, many more of the satellites10would be used—possibly eight to ten, or more. The individual satellites in the constellation communicate with each other, and also communicate with users and communications providers on or near the earth's surface. The communications among the satellites in the constellation are handled by what are known as inter-satellite links (ISL) or cross-links.FIG. 1shows both satellite-to-satellite cross-link communications signals12and satellite-to-earth communications signals14.

Different types of cross-link antennas have been developed for satellite-to-satellite communications. One type of cross-link antenna uses a reflector to send and receive a highly directional communication signal. Reflector type antennas have good cross-link communications performance, but are bulky and expensive to deploy on a satellite, particularly due to the need to steer the reflector for precise aiming. Another type of cross-link antenna is an omni-directional, non-steerable design which can be much less expensive to construct and deploy.

FIG. 2is an illustration of a communications satellite20with a known design of non-steerable satellite-to-satellite cross-link antennas. The satellite20includes solar panels30mounted to a hub40, as would be understood by those skilled in the art. The hub40contains the communications and control systems onboard the satellite20. In particular, the hub40includes an L-band antenna50—which transmits radio signals “down” toward earth, and a cross-link antenna60—which transmits and receives UHF signals with other satellites in the constellation. In this example, the satellite20is a global positioning system (GPS) satellite; GPS signals typically fall within the L-band, defined by IEEE as the 1-2 GHz range of the radio spectrum. For various performance and packaging reasons, the cross-link antenna60shares space with the L-band antenna50on the earth-facing deck of the hub40.

In the known design employed on the satellite20, the L-band antenna50and the cross-link antenna60each consist of multiple sections. The L-band antenna includes a central aperture52and an intermediate ring54. The cross-link antenna60includes an inner ring62and an outer ring64. The inner ring62includes four quadrifilar helical antennas70, while the outer ring64includes eight of the quadrifilar helical antennas70. It can be seen inFIG. 2that the sections of the L-band antenna50and the cross-link antenna60are interleaved. That is, the central aperture52(L-band) is in the center, surrounded by the inner ring62(cross-link), which is surrounded by the intermediate ring54(L-band), which in turn is surrounded by the outer ring64(cross-link).

The L-band antenna50transmits a cone-shaped radiation pattern toward earth. The cone is typically required to cover +/−14°, or a 28° opening angle of the cone. Because of the interleaving of the sections of the L-band antenna50and the cross-link antenna60, and the relatively large size of the quadrifilar helical antennas70, the satellite20is known to suffer significant degradation in the L-band signal, especially in coverage areas which are not near the axis of the cone. This L-band signal degradation is due to the physical blockage of the L-band signal by the quadrifilar helical antennas70. The L-band signal degradation is undesirable, as it either results in poorer GPS receiver performance for users, or requires an increase in the number of satellites in the constellation in order to improve performance.

FIG. 3is an illustration of a new design of a quadrifilar helix antenna100which can be used for satellite-to-satellite cross-link communications. The quadrifilar helix antenna100has a design which provides better cross-link communications performance than previous designs, and also enables improved cross-link and L-band antenna packaging on a satellite—thereby improving L-band communications performance.

The antenna100is shown inFIG. 3with a local X/Y/Z coordinate frame, where the local Z axis is the axis of the helix and positive Z points toward earth when the antenna100is in position on an orbiting satellite. The antenna100is comprised of a center feed wire110, four end branches120, four helical filaments130and a ground plate140. The feed wire110is preferably a coaxial cable which connects the antenna100with a communications controller on the satellite.

The coaxial cable/feed wire110carries transmission signals from the communications controller to be transmitted by the antenna100, and carries received signals from the antenna100back to the communications controller. The feed wire110is split to connect to the four end branches120, each of which is connected to an end of one of the helical filaments130as shown. The feed wire110may be split such that one opposing pair of the end branches120is coupled to the inner conductor of the coaxial cable, and the other opposing pair of end branches120is coupled to the outer shield of the coaxial cable. The ends of the four helical filaments130opposite the end branches120are coupled to the ground plate140.

In one preferred embodiment, the quadrifilar helix antenna100has a height (in the Z direction—from the ground plate140to the end branches120) of 12″ (inches), and a diameter of 5″. The helical filaments130have a pitch angle of 20°, and are made of a wire with a diameter of 0.1″. As stated above, the filaments130are shorted to the ground plate140, which has a diameter of 10″. This embodiment has been designed for optimal performance in a satellite-mounted array of the quadrifilar helical antennas100as discussed further below.

FIGS. 4A and B are illustrations of old and new designs of quadrifilar helix antennas showing both size difference and difference in radiation patterns of the two designs.FIG. 4Ashows the quadrifilar helix antenna70ofFIG. 2, whileFIG. 4Bshows the quadrifilar helix antenna100ofFIG. 3. The antenna70has a height of 24″ and a diameter of 10″, while the antenna100has a height of 12″ and a diameter of 5″. Thus, the new design of the antenna100is half the height and half the diameter of the old design of the antenna70. Because the volume of a cylinder is a function of height times diameter squared, it can be seen that the new design of the antenna100has a volume which is one-eighth that of the antenna70. The much smaller size of the antenna100results in much less interference with the L-band transmission signal.

Furthermore, the reduced diameter of the antenna100causes a change in its radio signal radiation pattern compared to the antenna70. In the application described above, the cross-link communications between satellites in a constellation are in the UHF band at 260 MHz (megahertz). A 260 MHz signal has a wavelength of approximately one meter. The old design of the antenna70, with its circumference being close to the one meter wavelength value, operates in an axial or “end-fire” mode, where a radiation pattern72emanates predominantly from the open end of the antenna70. The new design of the antenna100, with its circumference being much smaller than the one meter wavelength value, operates in an omnidirectional normal mode, where a toroidal radiation pattern102produces substantially equal power in all directions perpendicular to the axis of the antenna100. The toroidal radiation pattern102of the antenna100not only delivers much more signal power in the 60°-120° elevation angle range where other satellites in the constellation exist, it also delivers almost no signal power toward earth where it is not wanted.

FIG. 5is a graph150showing radio signal gain curves as a function of elevation angle for the new quadrifilar helix antenna100, for a range of helix pitch angles. The horizontal axis represents elevation angle θ, while the vertical axis represents gain (dBi). Curves152,154,156and158show the gain for helix angles of 10°, 20°, 30° and 40°, respectively, as indicated by the legend and the line fonts. The curves on the graph150are based on measured data for the quadrifilar helix antenna100with the size and properties described above (12″ length; 5″ diameter; filaments shorted to ground plate140).

The first thing to notice about the curves on the graph150is that they all drop off substantially below about 20° elevation angle (where the 0° elevation angle is straight “down”—toward earth). This is because, as discussed above, the antenna100is designed as a normal mode antenna with the toroidal radiation pattern102. The fact that very little radio signal power emanates from the end of the antenna100is expected and is desirable. The quadrifilar helix antenna100is designed to have a gain in the normal direction which is at least 40 dBi greater than in the axial direction.

It can also be seen on the graph150that antenna performance varies dramatically with helix pitch angle. Specifically, the 10° pitch helix (curve152) exhibits a large dip in signal gain at an elevation angle of about 110° (from 80°-140°), which overlaps with the visibility window to other satellites in the constellation; this translates to an undesirable reduction in satellite-to-satellite communication performance. Similarly, the 30° pitch helix (curve156) exhibits a large dip in signal gain at an elevation angle of about 65° (from 20°-120°), which also overlaps with the visibility window to other satellites in the constellation and translates to an undesirable reduction in satellite-to-satellite communication performance.

However, the 20° pitch helix (curve154) exhibits no dip in signal gain in the elevation angle range of interest. In addition, the 20° pitch helix provides the highest gain of any pitch angle in the 80°-100° elevation angle range of primary importance. Higher pitch angles, such as 40° (curve158) and higher (not shown) delivered less performance in the 80°-100° elevation angle range. Thus, the 20° pitch helix is chosen as optimal for the design of the quadrifilar helix antenna100.

FIG. 6is an illustration of a communications satellite200with the new design of the quadrifilar helix antenna100which improves both satellite-to-satellite cross-link and satellite-to-earth communications performance. As did the satellite20ofFIG. 2discussed previously, the satellite200includes solar panels210and a hub220. The hub220includes an earth-facing deck222, on which an L-band aperture230and a cross-link array240are mounted. The L-band aperture230—which could instead be any other type of satellite-to-earth antenna—is a single circular element, as opposed to the central aperture52and the intermediate ring54of the satellite20. The simplification of the L-band aperture230is enabled by the smaller footprint of the cross-link array240owing to the smaller number and size of the antennas100, which is in turn enabled by the improved performance of the quadrifilar helix antennas100.

The cross-link array240includes eight of the quadrifilar helix antennas100arranged in a ring surrounding the L-band aperture230. Only eight of the quadrifilar helix antennas100are required on the satellite200, where twelve of the helix antennas70were required on the previous satellite20. Considering the smaller number of the quadrifilar helix antennas100, and their smaller size (each antenna100has 8× less volume than the antenna70), it is apparent that the satellite200offers over an order of magnitude reduction in cross-link antenna volume, while at the same time providing improved cross-link communications performance.

The eight quadrifilar helix antennas100in the cross-link array240communicate with a communications controller (not shown) in the hub220via coaxial cable, as discussed previously. A simple splitter/combiner can be used to terminate the eight coaxial cables at the controller. Alternatively, in some cases it may be advantageous to provide a separate connection for each of the eight coaxial cables to the controller, where the eight cables could carry transmission signals with different phasing or other differences.

The satellite200, with the L-band aperture230and the cross-link array240of eight quadrifilar helix antennas100, demonstrates superior performance in every way as compared to legacy systems. First, the L-band signal directed toward earth by the satellite200suffers less interference than with previous designs, which are known to cause an L-band signal degradation of more than 1.5 dB. In contrast, the L-band signal from the new design of the satellite200has negligible degradation. The L-band signal improvements of the satellite200are due to three factors—the reduced physical size of the quadrifilar helix antennas100, the elimination of interleaving between L-band and cross-link arrays, and the reduction of undesirable UHF radiation directed toward earth—all of which are made possible by the new design of the quadrifilar helix antennas100.

Second, the satellite200provides better cross-link communications performance than previous designs, due to the optimization of the toroidal radiation pattern102from the quadrifilar helix antennas100to deliver the greatest signal strength in the 80-100° elevation angle window where it is needed. Because of the normal mode antenna characteristic and the toroidal radiation pattern102, fewer of the quadrifilar helix antennas100are needed on the satellite200than on previous designs. Finally, the cross-link communications performance of the satellite200is extremely robust with respect to azimuth angle.

FIG. 7is a diagram showing the layout of the eight quadrifilar helix antennas100in the cross-link array240, on the satellite200ofFIG. 6. As described above, the antennas100have a diameter of 5″. In a preferred embodiment, the eight antennas100in the cross-link array240are arranged in a circular pattern (circle250) with a diameter of 66″. The eight antennas100are positioned at 45° intervals around the circle250. Recall from the discussion ofFIG. 3above that each of the antennas100includes four end branches120in two opposing pairs, and that the two opposing pairs may be wired differently. Thus, it may be important to control the orientation of the end branches120—which in turn controls the orientation of the helical filaments130.

InFIG. 7, the antenna100at the top-center of the page has its end branches120labeled with a “north” (N) direction pointing toward the top of the page, where the north direction may correspond, for example, to one of the end branches120which is coupled to the inner conductor of the coaxial cable. In one embodiment of the satellite200, each of the antennas100is oriented according to its clock position on the circle250—for example, such that the relative “north” of the end branches120is always pointed radially outward from the center of the circle250, as shown inFIG. 7. In another embodiment, the antennas100are all identically oriented—such that the relative “north” of the end branches120are all pointed in the same direction (parallel). The orientation of the end branches120and the corresponding orientation of the helical filaments130may be chosen in a particular array design to achieve the optimum azimuth angle variation in signal strength around the satellite—where the optimum may be minimum variation around the 360° of azimuth angle, or the optimum may be a shaped pattern based on positions and orientations of the satellites200in their constellation.

FIG. 8is a graph260showing radio signal gain as a function of elevation angle for the quadrifilar helix antenna cross-link array240ofFIG. 6. Curve262shows that the signal gain drops off dramatically at 0° elevation angle (toward earth). This is desirable and expected performance for the cross-link array240, as discussed above. The curve262also shows singularity points where the signal strength drops at about 30° and at about 150° elevation angles. These elevation angles are of no interest in cross-link communications between satellites. However, most significantly, the curve262also exhibits a broad shoulder of high signal gain in the 70°-90° elevation angle range (and on both sides of this range). It is the 70°-90° elevation angle which is most important in satellite-to-satellite cross-link communications in a constellation of satellites—as can be seen in the illustration ofFIG. 1. The quadrifilar helix antennas100and the cross-link array240have been designed to provide the desired performance shown inFIG. 8—with very high signal strength in the 70°-90° elevation angle range where adjacent satellites in the constellation are located, and very low signal strength directed toward earth.

FIG. 9is a graph270showing radio signal gain as a function of azimuth angle for the quadrifilar helix antenna cross-link array240ofFIG. 6. The graph270shows gain vs. azimuth angle at an elevation angle of 70°—which is of primary interest for cross-link communications in a satellite constellation, as discussed above. Curve272depicts signal gain variation around one half-circumference of the satellite (0°-180°). It can be seen that the gain varies cyclically through four cycles around the half-circumference. This is expected performance, as the array includes eight of the quadrifilar helix antennas100; thus, the signal gain rises and falls according to alignment with or between the four antennas100in the half-circumference. Most important inFIG. 9are the actual gain values on the vertical axis. Azimuth gain variation is less than one dB, which is a very small amount, as desired. For comparison, note that the elevation gain variation ofFIG. 8covers a span of about 60 dB. The quadrifilar helix antennas100and the cross-link array240have been designed to provide the desired performance shown inFIG. 9—where signal strength variation is very small as a function of azimuth angle.

The inter-satellite cross-link antenna system described above provides numerous advantages over previous systems. These advantages include smaller and fewer quadrifilar helical antennas for cross-link communications, better cross-link communications performance due to normal mode antenna operation, simpler design of both the L-band and cross-link antenna systems due to elimination of interleaving, and better L-band transmission performance due to less interference from the cross-link antennas. This combination of features enables communication satellites to provide better performance while being made less expensive and less complex—all of which are favorable for telecommunications and other companies which employ communications satellites, and ultimately for the consumer.