Patent Description:
As demand for satellite communications continues to grow, the use of spot beams in satellite systems has become increasingly popular. A spot beam is a modulated satellite beam focused on a limited geographic region of the Earth. By reducing the coverage area of the beam, a more directional antenna may be used by the satellite to transmit the beam to Earth. This higher gain associated with a spot beam may produce better signal-to-noise (SNR) ratio at a user terminal, which allows for higher rates of data transfer between the satellite and terminal. Also, the smaller size of spot beams allows for frequency reuse with limited inter-beam interference, thereby providing for even greater increases in data throughput at a satellite.

While spot beams can be very useful in areas of high demand, they may be susceptible to pointing errors. Satellite antenna movement within even a few thousandths of a degree may substantially change the coverage area of a spot beam on the Earth. Moreover, it is often the case that multiple spot beams are transmitted in a predetermined pattern from the satellite to various intended coverage areas. Thus, an antenna pointing error at the satellite may detrimentally reduce the quality of communications over multiple spot beams simultaneously.

In the foregoing circumstances and in other scenarios, therefore, there is a need for determining the pointing error of a steerable antenna system, with <CIT> detailing techniques for pointing-orienting-a satellite antenna. Among the many challenges associated with maintaining a correct pointing direction for a satellite antenna are the complexities and durations associated with the procedure(s) used for determining pointing errors and the signaling needed for making such determinations. <CIT> describes digital method of determining and correcting beam-pointing for a communications spacecraft that has a digital beam-forming architecture for defining multiple spot transmit and receive beams, the antenna system of the spacecraft including a receive antenna (DRA, AFR) having antenna elements providing respective antenna element signals, and wherein at least one of the uplink signals to the spacecraft includes a beacon signal, and wherein the method comprises digitally weighting components of said beacon signal present in antenna element signals, combining such weighted beacon signal components such as to derive beam-pointing error signals, and employing the error signals to adjust beam-forming weight values of the receive antenna, in order to adjust the pointing direction of at least one signal beam. The digital weights for the beacon signal define difference radiation patterns for x, y axes of the antenna which vary rapidly in a range corresponding to the pointing errors most commonly occurring.

Systems disclosed herein for pointing a steerable antenna system onboard a satellite exploit advantageous image-processing techniques that provide a computationally-efficient and accurate way of determining the pointing error of the steerable antenna system and determining corresponding pointing corrections. Received-signal power measurements for individual array elements in an antenna array of the steerable antenna system provide the basis for forming a power-distribution image that reveals where an uplink signal falls on the array, which in turn provides a basis for determining the appropriate pointing correction.

One embodiment comprises a satellite having a steerable antenna system and a control system. The steerable antenna system includes an antenna array and is configured to receive an uplink signal that illuminates a particular region of the antenna array in dependence on a current pointing direction of the steerable antenna system. The antenna array comprises a plurality of array elements arranged according to a feed grid. Correspondingly, the control system is configured to: (a) convert received-signal power measurements made for individual array elements of the antenna array during reception of the uplink signal into a power-distribution image comprising pixels arranged on a pixel grid derived from the feed grid and having pixel values determined in dependence on the received-signal power measurements made for corresponding ones of the array elements; (b) determine a center location of an illuminated region in the power-distribution image that corresponds to the uplink signal, the center location expressed in feed-grid coordinates; and (c) derive a pointing correction for the steerable antenna system in dependence on a difference between the center location and a reference location that also is expressed in feed-grid coordinates.

Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

<FIG> illustrate a satellite <NUM>, according to one example embodiment, with the satellite <NUM> including a satellite bus <NUM> and a payload <NUM>. The satellite bus <NUM> includes the electrical power system of the satellite <NUM>, along with other spacecraft infrastructure, while the payload <NUM> comprises the communications equipment and associated antenna systems, for relaying signals between respective terrestrial stations, such as gateway terminals and user terminals.

Example entities in the payload include communications circuitry <NUM> and one or more steerable antenna systems <NUM>. Each steerable antenna system <NUM> comprises, for example, an antenna assembly <NUM> and a corresponding antenna positioning module (APM) <NUM> that is operative to steer the antenna assembly <NUM>. "Steering" encompasses a variety of techniques for changing the "pointing" direction of the antenna assembly <NUM>, and each APM <NUM> comprises, for example, a motorized two-axis gimble or other steering element that performs commanded changes in the angular position of the antenna assembly <NUM> in two or more axes, e.g., to shift the geographic region on the surface of the Earth towards which the antenna assembly <NUM> is oriented.

The communication circuitry <NUM> carries communication signals in the forward direction-towards user terminals-and in the return direction-from the user terminals-and may comprise a plurality of transponders that provide signal pathways through the satellite <NUM>. Transponder functions include, for example, signal amplification, filtering, and frequency conversion, such as converting between frequencies used for uplink transmission and frequencies used for downlink transmission.

A control system <NUM> performs a number of operations, including determining pointing errors with respect to any one or more of the steerable antenna systems <NUM>. Pointing errors are reduced or eliminated by control circuitry comprised within the bus <NUM> translating the determined errors into corresponding steering adjustments and controlling the APM(s) <NUM> according to such adjustments, to perform antenna steering. Antenna steering may be understood as station keeping, wherein the satellite <NUM> compensates for changes in its attitude, to maintain a desired orientation of each antenna assembly <NUM>. In other embodiments or scenarios, steering commands to an APM <NUM> provide for purposeful reorientation of the involved antenna assembly <NUM>, e.g., to shift the terrestrial coverage area(s) provided by the antenna assembly <NUM>. One or more embodiments of the satellite <NUM> use a phased-array antenna for one or more of the antenna systems <NUM>, such that steering the antenna assembly <NUM> shifts the beams formed by the antenna assembly <NUM>. Such shifts can be understood as shifting or otherwise moving the terrestrial beam footprints of the beams, which changes the terrestrial areas illuminated by the beams.

An example control system <NUM> comprises processing circuitry <NUM> and associated storage <NUM>. The processing circuitry <NUM> comprises dedicated, fixed circuitry or programmatically-configured circuitry, or a mix of dedicated circuitry and programmatically-configured circuitry. For example, one or more microprocessors or other digital processors are specially adapted to carry out some or all of processing described herein for antenna steering, based on the execution of stored computer program instructions.

Correspondingly, in one or more embodiments, the storage <NUM> comprises one or more types of computer-readable media, such as a mix of volatile memory for use in program execution-working memory-and nonvolatile memory for longer-term storage of one or more computer programs <NUM> containing the aforementioned computer program instructions. The storage <NUM> in one or more embodiments also stores satellite provisioning information or other types of configuration data, such as antenna data <NUM>.

<FIG> depicts a particular example arrangement applicable to one or more embodiments, wherein control system <NUM> determines pointing errors, e.g., expressed in terms of azimuthal and elevational errors, and outputs corresponding error signaling to an antenna steering controller <NUM> comprised within the satellite bus <NUM>. The antenna steering controller <NUM> comprises, for example, a programmed microprocessor, an Application Specific Integrated Circuit (ASIC), a Field Programmable Array (FPGA), or other digital processing circuit, along with supporting power and interface circuitry.

Processing performed by the antenna steering controller <NUM> includes translating the determined pointing errors for a steerable antenna system <NUM> into corresponding adjustments, e.g., steering commands, which are then applied to the steering element(s) <NUM> in the involved APM <NUM>. As a non-limiting example, the steerable antenna system <NUM> includes a reflector having azimuthal and elevational angles that are controlled by stepper motors in a two-axis gimble. Steering-angle adjustments in this context comprise changing the reflector angle by commanding determined numbers of motor steps, which correspond to, e.g., millidegrees of angular adjustment.

<FIG> illustrates an example scenario of antenna steering, based on the satellite <NUM> receiving an uplink signal <NUM> from a terrestrial terminal <NUM>, which also may be referred to as a ground station <NUM>. The terrestrial terminal <NUM> comprises, for example, a satellite access node (SAN), which also may be referred to as a gateway terminal. SANs form part of the ground segment of a satellite communications system and interface directly or indirectly with external communication networks, such as the Internet or other Public Data Networks (PDNs), the Public Switched Telephone Network (PSTN), Public Land Mobile Networks (PLMNs), etc..

In at least one embodiment, the uplink signal <NUM> serves as a pointing reference for a steerable antenna system <NUM> onboard the satellite <NUM>. Particularly, the satellite <NUM> evaluates the current pointing direction of the steerable antenna system <NUM> by evaluating how the uplink reference signal impinges on the antenna assembly <NUM> included in the steerable antenna system <NUM>. <FIG> introduces example details that provide a context for such evaluations.

An example antenna assembly <NUM> included in each steerable antenna system <NUM> comprises, for example, an antenna array <NUM> and an associated reflector <NUM>. As seen in <FIG>, the uplink signal <NUM> impinges on the reflector <NUM>, which reflects it onto the antenna array <NUM>. As such, the portion of the antenna array <NUM> that is illuminated by the uplink signal <NUM> depends on the orientation of the reflector <NUM> relative to the antenna array <NUM>. As suggested in <FIG>, the reflector <NUM> may be motorized or otherwise adjustable around one or more axes. Changing the orientation of the reflector <NUM> relative to the antenna array <NUM> effectively changes the pointing direction of the antenna array <NUM>, and such changes therefore shift which portion of the antenna array <NUM> is illuminated by the uplink signal <NUM>.

<FIG> offers an example illustration of such details by depicting the antenna array <NUM> in a plan view, i.e., looking directly at the face of the antenna array <NUM>. As illustrated, the antenna array <NUM> comprises a plurality of array elements <NUM> arranged according to a feed grid <NUM>, which can be understood as defining the geometric arrangement of individual array elements <NUM>. As depicted, the feed grid <NUM> defines regularly spaced column lines and row lines and each row-column intersection represents a grid position <NUM> in the feed grid <NUM>.

If the horizontal distance spanned by the plurality of feed-grid columns depicted in <FIG> is taken as the X axis and the vertical distance spanned by the plurality of feed-grid rows is taken as the Y axis, any particular grid position <NUM> in the feed grid <NUM> is defined by its X-Y coordinate, expressed as {x, y}. Here, {x, y} represents a physical position or coordinate within the feed grid <NUM>.

The depicted embodiment of the antenna array <NUM> is based on a lattice arrangement of array elements <NUM> on the feed grid <NUM>, where every other grid position <NUM> going row-wise or column-wise is occupied by an array element <NUM>. In some embodiments, depending on involved signal frequencies and design requirements, all grid positions <NUM> are occupied by array elements <NUM>, and it will be understood that the physical spacing of the grid positions <NUM> depends on the wavelengths of the signal frequencies of interest.

Each array element <NUM> is a radiating or receiving element, or both, and has a corresponding transmit or receive signal chain associated with it. In one or more embodiments, measurement circuitry onboard the satellite <NUM> is configured to measure received-signal power on each array element <NUM>. In <FIG>, the reference number "<NUM>" denotes the particular region-area-of the overall antenna array <NUM> that is illuminated by the uplink signal <NUM> for a current angular orientation of the reflector <NUM> relative to the antenna array <NUM>-i.e., for a current pointing direction of the involved steerable antenna assembly <NUM>. "Illuminated" in this context refers to which array elements <NUM> in the antenna array <NUM> register received-signal power levels above some minimum threshold, with respect to the uplink signal <NUM>.

The reference number "<NUM>" denotes a reference location that represents the nominal center of the illuminated region <NUM>, if the steerable antenna system <NUM> was pointed correctly. That is, the current pointing error or alignment error of the steerable antenna system <NUM> is represented by the extent that the grid position <NUM> closest to the geometric center of the illuminated region <NUM> is not at the grid position <NUM> designated as the reference location <NUM>. The reference location <NUM> is expressed in the X-Y coordinates of the feed grid <NUM>.

An "imaging" technique disclosed herein offers both accuracy and efficiency in determining the pointing error. Effectively, the technique forms an image corresponding to the antenna array <NUM>, where pixels in the image correspond with array elements <NUM> in the antenna array <NUM> and are illuminated or not illuminated in dependence on the received-signal power registered on the corresponding array elements <NUM> during reception of the uplink signal <NUM>. The image is or represents a power distribution profile for the antenna array.

As such, the image will contain an illuminated region corresponding to the array elements <NUM> that registered more than some minimum level of received-signal power during reception of the uplink signal <NUM>. In this regard, multiple power measurements may be made with respect to each array element <NUM> during a reception interval, and averaged or otherwise filtered, to obtain a final power measurement value that is used as an input to the image generating process.

The pixel grid that defines the image has a defined correspondence with the feed grid <NUM>, meaning that the reference location <NUM> can be projected into the pixel grid and the center of the illuminated region in the image that represents the uplink signal <NUM> can be compared to the projected reference location to compute a pointing error of the steerable antenna system <NUM> being evaluated. The image-processing technique can be performed independently, for each steerable antenna system <NUM> onboard the satellite <NUM>.

<FIG> illustrates the foregoing example details by depicting an image <NUM> having an X' axis and a Y' axis and comprising a plurality of pixels <NUM> arranged on a pixel grid <NUM> that corresponds to the feed grid <NUM>. "Corresponds to" in this context means that there is a defined mapping or correspondence between each grid position <NUM> in the pixel grid <NUM> and each grid position <NUM> in the feed grid <NUM>. For example, the image <NUM> may have the same resolution as the feed grid <NUM>, meaning that there is one grid position <NUM> for each grid position <NUM> in the feed grid <NUM>. In at least one embodiment, the image <NUM> is a higher resolution or upscaled image, meaning that there are more grid positions <NUM> in the pixel grid <NUM> than there are grid positions <NUM> in the feed grid <NUM>-e.g., there may be four grid positions <NUM> for every grid position <NUM>. However, even with upscaling, there remains a defined mapping or correspondence between grid positions <NUM> in the pixel grid <NUM> and grid positions <NUM> in the feed grid <NUM>. As such, the numeric value of every pixel <NUM> in the image <NUM> depends on the received-signal power level registered on the corresponding array element(s) <NUM> in the antenna array <NUM> during the measurement interval used to obtain the image. The dependency may be a quantized relationship, e.g., a given pixel <NUM> may be considered as being "off" (not illuminated) or "on" (illuminated) as a function of whether the power level(s) registered for the corresponding array element(s) <NUM> satisfied some minimum threshold level.

Thus, the image <NUM> may be referred to as a "power-distribution image" and as seen in <FIG>, it contains an illuminated region <NUM> corresponding to the uplink signal <NUM>. Note that in this context, an illuminated pixel <NUM> has a numeric value resulting from the array element(s) <NUM> on which it depends having registered more than the minimum threshold level of received signal power during the interval in which the uplink signal <NUM> is received. The center of the illuminated region <NUM>, which may be computed geometrically, e.g., as the centroid of the illuminated region <NUM> is represented by a triangle shape in the figure, shown as "<NUM>" in <FIG>, and the reference location <NUM> as projected into the pixel grid <NUM> is represented by a star shape. The difference <NUM> between the center location <NUM> of the illuminated area <NUM> and the reference location <NUM> represents the current pointing error of the steerable antenna system <NUM>.

<FIG> illustrates a method <NUM> of operation by the control system <NUM> of the satellite <NUM>, consistent with the foregoing examples. Certain operations may be performed in an order other than suggested and the method <NUM> may be performed as part of ongoing satellite operations and repeated on a recurring basis, and may be carried out independently with respect to different steerable antenna systems <NUM> onboard the satellite <NUM>.

The method <NUM> includes receiving (Block <NUM>) an uplink signal <NUM> that illuminates a particular region <NUM> of an antenna array <NUM> in dependence on a current pointing direction of a steerable antenna system <NUM> that includes the antenna array <NUM>. The antenna array <NUM> comprises a plurality of array elements <NUM> arranged according to a feed grid <NUM>. Further, the method <NUM> includes converting (Block <NUM>) received-signal power measurements made for individual array elements <NUM> of the antenna array <NUM> during reception of the uplink signal <NUM> into a power-distribution image <NUM> comprising pixels <NUM> arranged on a pixel grid <NUM> derived from the feed grid <NUM> and having pixel values determined in dependence on the received-signal power measurements made for corresponding ones of the array elements <NUM>.

For example, with no upscaling or before performing upscaling, there is a one-to-one correspondence between the feed grid <NUM> and the pixel grid <NUM>, meaning that each grid position <NUM> in the pixel grid <NUM> maps directly to one grid position <NUM> in the feed grid <NUM>. If that grid position <NUM> is occupied, then the value of the pixel <NUM> depends on the received-signal power measurements made for the occupying array element <NUM>. If the grid position <NUM> is unoccupied, then the value of the pixel <NUM> is calculated, at least initially, in dependence on the values of the adjacent pixels <NUM>, corresponding to occupied grid positions <NUM> in the feed grid <NUM>. Of course, any initially calculated pixel values may be revised, e.g., as a consequence of filtering, upscaling, and binarization, any or all of which may be performed in some embodiments of image generation.

However the pixel values are finalized, the method <NUM> continues with determining (Block <NUM>) a center location <NUM> of an illuminated region <NUM> in the power-distribution image <NUM> that corresponds to the uplink signal <NUM>, where the center location <NUM> is expressed in feed-grid coordinates. For example, the grid position <NUM> in the pixel grid <NUM> that is closest to the computed centroid of the illuminated region <NUM> is taken as the center location <NUM> and that location is then translated into feed-grid coordinates according to the mapping from the pixel grid <NUM> to the feed grid <NUM>. From there, the method <NUM> continues with deriving (Block <NUM>) a pointing correction for the steerable antenna system <NUM> in dependence on a difference <NUM> between the center location <NUM> and the reference location <NUM>, which also is expressed in feed-grid coordinates.

The uplink signal <NUM> originates, for example, from a ground station <NUM> that serves as a pointing reference for the steerable antenna system <NUM> and the reference location <NUM> corresponds with a correct pointing direction of the steerable antenna system <NUM>.

As noted earlier, the steerable antenna system <NUM> may receive more than just the uplink signal <NUM> during the interval in which it makes received-signal power measurements for generation of the power-distribution image <NUM>. Consequently, there may be multiple illuminated regions within the power-distribution image <NUM>, with the locations of those regions being dependent on the respective angles-of-arrival of the signals. Thus, in at least one embodiment, the method <NUM> includes identifying the illuminated region <NUM>-i.e., the illuminated region corresponding to the uplink signal <NUM>-from among two or more illuminated regions that are present in the power-distribution image <NUM>, as a result of the steerable antenna system <NUM> receiving signal energy from more than one signal source while the received-signal power measurements are made.

In at least one embodiment, identifying the illuminated region <NUM> that corresponds with the uplink signal <NUM> comprises, for the two or more illuminated regions, comparing respective sizes of the two or more illuminated regions. This approach rests on the idea that the other signals are spurious and relatively weak, and therefore result in smaller illuminated regions in the power-distribution image <NUM>.

Converting the received-signal power measurements into the power-distribution image <NUM> comprises, for example, mapping the individual received-signal power measurements to a first set of pixels <NUM>. Each pixel <NUM> in the first set of pixels <NUM> corresponds to a populated grid position <NUM> in the feed grid <NUM> and has a pixel value corresponding to the received-signal measurement made for the array element <NUM> at that populated grid position <NUM>. Continuing this example, converting the received-signal power measurements into the power-distribution image <NUM> further comprises creating an expanded, second set of pixels <NUM> encompassing the first set of pixels <NUM> and additional pixels <NUM> that correspond to unpopulated grid positions <NUM> in the feed grid <NUM>. Each additional pixel <NUM> has a pixel value derived from one or more neighboring pixels in the first set of pixels <NUM>.

Converting the received-signal power measurements into the power-distribution image <NUM> further comprises, in at least one embodiment, creating an up-sampled image by generating multiple pixels <NUM> for each grid position <NUM> in the feed grid <NUM>. The method <NUM> in at least one such embodiment includes filtering the up-sampled image, to obtain the power-distribution image <NUM> used for determining the center location <NUM> of the illuminated region <NUM> in the power distribution image <NUM> that corresponds to the uplink signal <NUM>.

Further, in one or more embodiments, forming the "final" power-distribution image <NUM> for evaluation includes binarizing the power-distribution image <NUM>, and identifying, within the binarized power-distribution image <NUM>, the illuminated region <NUM> that corresponds to the uplink signal <NUM>. <FIG> suggests binarization, where each pixel <NUM> either is off-white in the figure- or is on-black in the figure.

In at least one embodiment of the method <NUM>, the power distribution image <NUM> before binarization comprises pixels <NUM> having individual pixel values-numeric values-that are proportional to the received-signal power measured for the corresponding array elements <NUM>, during an interval in which involved steerable antenna system <NUM> receives the uplink signal <NUM>. For example, individual pixels <NUM> have a "zero" value if the array element(s) <NUM> they correspond with have received-signal power measurements below a certain threshold. However, individual pixels <NUM> have a non-zero value that is proportional to the received-signal power levels measured on their corresponding array elements <NUM>.

Binarizing the power distribution image <NUM> means, with respect to each non-zero pixel <NUM> in the power distribution image <NUM>, deciding whether to change the pixel value to zero (off) or to a maximum value (fully on) in dependence on whether the pixel value is above or below a defined binarization threshold. Merely as a non-limiting example, consider an approach where defined pixel values range from <NUM> to <NUM>, with <NUM> corresponding to no received-signal power or received-signal power below some minimum power-level threshold, and <NUM> corresponding to received-signal power above some upper power-level threshold. Binarizing the power-distribution image <NUM> would then involve setting all pixels <NUM> having values below, say <NUM>, to <NUM>, and setting all pixels <NUM> having values above <NUM> to <NUM>.

Deriving the pointing correction for the steerable antenna system <NUM> in one or more embodiments of the method <NUM> comprises computing a horizontal offset in feed-grid coordinates between the center location <NUM> and the reference location <NUM> on a horizontal axis defined by the feed grid <NUM>, computing a vertical offset in feed-grid coordinates between the center location <NUM> and the reference location <NUM> on a vertical axis defined by the feed grid <NUM>, and translating the horizontal and vertical offsets into corresponding azimuthal and elevational pointing adjustments for the steerable antenna system <NUM>. These azimuthal and elevational pointing adjustments are, for example, servo commands for changing the angle of the reflector <NUM> of the involved antenna assembly <NUM>, which, as noted, effectively changes the pointing direction of the antenna array <NUM> included in the antenna assembly <NUM>. Thus, the method <NUM> in one or more embodiments includes actuating a steering mechanism of the steerable antenna system <NUM>, according to one or more actuator control signals determined as a function of the pointing correction.

As shown in <FIG>, the method <NUM> in one or more further embodiments includes the satellite <NUM> performing downlink beamforming via a steerable antenna system <NUM>, to provide a set of forward user beams <NUM> defining corresponding forward user beam coverage areas <NUM>. The forward user beam coverage areas <NUM> are the terrestrial footprints of the respective forward user beams <NUM>, and the pointing direction of the steerable antenna system <NUM> defines the geographical coordinates of an aggregate coverage area <NUM> defined by the set of forward user beams <NUM>.

In example operation, the satellite <NUM> uses a steerable antenna system <NUM> to perform downlink beamforming along the lines suggested in <FIG>, although the number of forward user beams <NUM> may be large, e.g., more than five hundred. Here, "forward" refers to transmission toward user terminals served by the satellite <NUM>, where such user terminals comprise set-top boxes or other data transceivers operating in respective ones of the beam coverage areas <NUM>.

<FIG> provides further example details for downlink beamforming, where transceiver circuitry <NUM> comprising signal chains on a per antenna element basis with respect to the antenna array <NUM> included in the involved steerable antenna system <NUM> provides power amplification of element signals <NUM>. Each element signal <NUM> corresponds to one of the array elements <NUM> in the antenna array <NUM> and is the same as the other element signals <NUM> except for having element-specific weighting in terms of amplitude and phase, such that the transmitted versions <NUM> of the element signals <NUM> form the forward user beams <NUM> in the far field, as a result of the patterns of constructive and destructive interference formed by the radiating signals <NUM>. The weighting may be performed onboard the satellite <NUM> or on the ground, using ground-based beamforming.

<FIG> illustrates a more detailed example of beamformed coverage using a steerable antenna system <NUM> of the satellite <NUM>, where an aggregate coverage area <NUM> is formed by a plurality of beam coverage areas, depicted by the small squares in the diagram. The current boresight of the involved steerable antenna system <NUM> is shown, as indicated by the unfilled circle in the diagram. The filled circle indicates the nominal or intended boresight, and the method <NUM> provides the satellite <NUM> with an efficient and accurate mechanism for determining the pointing error.

As noted, the satellite <NUM> may have multiple steerable antenna systems <NUM> onboard, and the satellite <NUM> may perform the method <NUM> with respect to each steerable antenna system <NUM>, based on receiving a respective uplink signal <NUM> for each steerable antenna system <NUM>. That is, each steerable antenna system <NUM> may provide service coverage in a different geographic region and there may be a ground station <NUM> in each geographic region that serves as the pointing reference for the respective steerable antenna system <NUM> onboard the satellite <NUM>. A further point regarding the method <NUM> is that the control system <NUM> onboard the satellite <NUM> may repeat the method <NUM> on a recurring or triggered basis, with respect to each steerable antenna system <NUM>-repeating the operations of receiving an uplink signal <NUM>, which may be received on a recurring basis, converting the corresponding power measurements into a power-distribution image <NUM>, determining the pointing error from the power-distribution image <NUM>, and deriving pointing corrections based on the determined pointing error.

<FIG> details a method <NUM> of generating a power-distribution image <NUM>, and may be performed as part of the method <NUM>. Image generation according to the method <NUM> includes mapping (Block <NUM>) uplink power to feed coordinates and generating the initial image. "Mapping" means associating the received-signal power measured for each array element <NUM> for reception of an uplink signal <NUM> with the corresponding grid positions <NUM> in the feed grid <NUM>. Generating the initial image comprises converting the measured powers into pixel values for the pixels <NUM> occupying respective grid positions <NUM> in a pixel grid <NUM> corresponding to the feed grid <NUM>. The pixels <NUM> may be arranged in a matrix or other data structure, where the ordering or arrangement of the pixels <NUM> represents the pixel grid <NUM>.

The method <NUM> continues with applying (Block <NUM>) filtering to pixels <NUM> in the initial image, e.g., a smoothing filter, upscaling/resizing (Block <NUM>) the image, applying (Block <NUM>) to the upscaled/resized image, and then binarizing (Block <NUM>) the image. The image as output from the binarizing operation is then used for identifying the illuminated region <NUM> of the pixel grid <NUM> that corresponds to the uplink signal <NUM>, and then calculating (Block <NUM>) the center of the illuminated region <NUM>, e.g., using a centroid formula. The center location <NUM> can then be expressed in feed-grid coordinates and compared with the reference location <NUM>, which may also be expressed in feed-grid coordinates, to determine the pointing error of the involved steerable antenna system <NUM>.

<FIG> illustrates a method <NUM> performed by the satellite <NUM>, with the method <NUM> including choosing (Block <NUM>) a ground station <NUM> to use as a pointing reference for a steerable antenna system <NUM> onboard the satellite <NUM>, and verifying (Block <NUM>) that all of one or more conditions for determining pointing corrections with respect to the pointing reference are fulfilled. Condition monitoring includes, for example, checking for one or more fault conditions that interfere with or prevent checking and correcting the pointing direction of the involved steerable antenna system <NUM>, which also may be referred to as "antenna tracking. " Assuming the absence of fault conditions, the method <NUM> continues with determining (Block <NUM>) the pointing error-e.g., according to the method <NUM>-and applying (Block <NUM>) the pointing correction-e.g., commanding one or more servos of other positioning controls according to the determined pointing error.

<FIG> illustrates a method <NUM> performed by the satellite <NUM>, with the illustrated processing representing an example approach to initialization. The processing may be performed as an initial part of the method <NUM> or performed in advance of performing the method <NUM> and it is based on example scenario involving three steerable antenna systems <NUM>, each including a movable reflector R. Hence, "R1" denotes the reflector in a first one of the steerable antenna systems <NUM>, "R2" denotes the reflector in a second one of the steerable antenna systems <NUM>, and "R3" denotes the reflector in a third one of the steerable antenna systems <NUM>. Each reflector R is associated with corresponding antenna array <NUM> having array elements <NUM> arranged on a feed grid <NUM> having X and Y dimensions.

The initialization method <NUM> involves performing a series of uploading operations (Blocks <NUM>, <NUM>, <NUM>, and <NUM>), to upload a set of configuration parameters, including: (<NUM>) X, Y feed-grid coordinates for each reflector R1-R3; (<NUM>) the reference location <NUM> to be used for steering each one of the three steerable antenna systems <NUM>, expressed in the corresponding feed-grid coordinates; (<NUM>) the beam deviations factors applicable to each steerable antenna system <NUM>; (<NUM>) and the out-of-bounds limits applicable to each steerable antenna system <NUM>. Uploading operations further include uploading (Block <NUM>) measurement schedule information that defines times for measuring uplink signal power for pointing-error determinations. Once the parameters and scheduling information are uploaded or otherwise configured on the satellite <NUM>, it is ready to carry out antenna tracking (Block <NUM>).

In an example embodiment, the antenna array <NUM> included in each steerable antenna system <NUM> has a defined number of rows and columns, e.g., <NUM> rows and <NUM> columns defining a <NUM> x <NUM> matrix of array elements <NUM>. Each array element <NUM> may be associated with producing a forward user beam having a beam number and a feed-grid position defined by a Y position expressed in inches and an X position expressed in inches, with the position defining the location of the array element <NUM> / beam number within the involved feed grid <NUM>. Similarly, the reference location <NUM> for each steerable antenna system <NUM> may be expressed in X inches and Y inches.

The beam deviation factors are, for example, a 2x2 matrix for each steerable antenna system <NUM>, expressing a delta azimuthal value and a delta elevational value. As a more detailed example, a methodology disclosed herein, such as in the embodiment shown in <FIG>, finds the X,Y coordinate representing the illumination center of an uplink signal impinging on an antenna array <NUM> comprised in a steerable antenna system <NUM> of the satellite <NUM>. The difference between that location and a reference location-e.g., a location that would attain if the steerable antenna system <NUM> was pointed correctly-gives delta x and delta y values. With offset fed reflectors, coordinate moves in x and y translate into beam moves in millidegrees, and the beam deviation factors may be expressed in degrees per inch. As such, the beam deviation factors allow the determined delta x and delta y values to be translated into angular adjustments for the reflector <NUM>.

As for the applicable limits, they too may be expressed per steerable antenna system <NUM>. Example limits include a lower limit on the signal level usable for antenna steering-i.e., a minimum signal level for the uplink signal <NUM> to be used as the pointing reference. The limits also may include an out-of-bounds error limit for each steerable antenna system <NUM> that prevents responding to a calculated pointing error if that error is too large according to defined error-size limit.

<FIG> illustrates a tracking method <NUM> performed by the satellite <NUM> with respect to one of its steerable antenna systems <NUM>, according to an example embodiment. The illustrated operations may be carried out by the control system <NUM> of the satellite payload <NUM>, for example.

Processing begins (Block <NUM>) with the satellite <NUM> in a READY state, such as depicted in <FIG>. There may be conditions or times during which antenna tracking is not enabled and the satellite <NUM> thus checks whether tracking is enabled (Block <NUM>). If tracking is not enabled ("NO" from Block <NUM>), processing advances to Block <NUM>, in which corresponding tracking status information is sent to a telemetry (TLM) buffer, and a bus interface subprocess (Block <NUM>) may report the status information to the bus <NUM>. In the case that tracking is disabled, for example, the interface subprocess may indicate that state to the bus <NUM>.

On the other hand, if tracking is enabled ("YES" from Block <NUM>) and uplink (UL) power measurements for a received uplink signal are available for the steerable antenna system <NUM>, the method <NUM> continues with calculating the pointing error (Block <NUM>). The pointing error is expressed as an azimuthal error (Az) and an elevational error (El) for the angular settings of the reflector <NUM> included in the involved steerable antenna system <NUM>, and the computation of the pointing error relies on the image-generation method <NUM>, using the configuration data detailed in <FIG>. Block <NUM> may further include calculating an "uplink sum" by summing the per-element received signal power measurements used in computing the pointing error, to ensure that the measured uplink signal had sufficient power for use as a pointing reference. Here, note that a power measurement subprocess (Block <NUM>) runs according to the uploaded scheduling information-i.e., it performs uplink signal power measurements for the steerable antenna system <NUM> at scheduled times and stores those measurements in a memory that is read from, for carrying out the calculations in Block <NUM>.

If the calculated pointing error is within defined limits and the uplink sum satisfies a defined threshold power level ("YES" from Block <NUM>), then the computed pointing error (Az-El error) is sent to the TLM buffer (Block <NUM>), and the interface subprocess (Block <NUM>) sends a corresponding Az-El error request to the bus <NUM>, with a corresponding antenna steering controller <NUM> of the bus <NUM> translating the Az-El error request into adjustments (control signaling) for revising the pointing direction of the steerable antenna system <NUM>.

<FIG> illustrates an example "initial" image obtained in the process of generating a power-distribution image <NUM>, where there is an initial set of pixels <NUM>. Each pixel <NUM> corresponds to a grid position <NUM> in the feed grid <NUM> of the involved antenna assembly <NUM>, and its pixel value is a digital value representing the received-signal power measured on the corresponding array element <NUM> in the antenna array <NUM>. Hence, the pixels <NUM> corresponding to unoccupied/empty grid positions <NUM> have a zero value. The pixel values may be based on converting analog measurements of received-signal power on each array element <NUM> to a digital value using an <NUM>-bit analog-to-digital (A/D) converter, for example.

<FIG> illustrates a detailed example according to one embodiment, for processing the initial image shown in <FIG>, to obtain a final power-distribution image <NUM> that is used to evaluate the pointing error. Although <FIG> illustrates particular filter types and filtering parameters, such details shall be understood as an example configuration. Other filter types or parameterizations may be used. Indeed, one or more embodiments include fewer filtering operations or omit filtering. Further, rather than implement the gray scaling and binarizing operations depicted in <FIG>, one or more embodiments perform "color" image processing, such as where the different uplink power measurements made on a per-element basis are mapped into power ranges that correspond to different colors. Such an approach may be understood as generating a color "heat map" image, for analysis.

In any case, the illustrated processing includes measuring (Block <NUM>) uplink (UL) power during a scheduled interval-e.g., during a quiescent interval during which the only signal purposefully received by the involved steerable antenna system <NUM> is an uplink signal <NUM> originating from a ground station <NUM> that serves as a pointing reference for the steerable antenna system <NUM>. Of course, the steerable antenna system <NUM> may receive one or more spurious signals during this interval, which may be defined according to the corresponding playlist uploaded to the satellite <NUM>, along with the other relevant configuration data.

Processing continues with digitizing (Block <NUM>) the power measurements and storing them (Block <NUM>), for use in building an initial image (Block <NUM>), such as the one shown in <FIG>. Once the initial image is generated, a series of processing operations to smooth the image and increase its resolution-i.e., upscaling is performed. However, although upscaling creates a pixel grid <NUM> containing more pixels <NUM> than there are grid positions <NUM> in the feed grid <NUM>, there remains a defined mapping that translates grid positions <NUM> into the feed grid <NUM> and vice versa, e.g., every grid position <NUM> in the feed grid <NUM> is represented by grid positions <NUM> in the (upscaled) pixel grid <NUM>.

A first operation applied to the basic or initial image from <FIG> is a first filtering operation (Block <NUM>) applied to the pixels <NUM>, using a filter "disk" having a radius of one. The disk filter is a two-dimensional (2D) filter exemplified by the below table:.

<FIG> illustrates the effect of the filtering applied in Block <NUM>.

Image filtering continues with the application of a median filter (Block <NUM>), e.g., a nine-point media filter as depicted below:.

<FIG> illustrates application of the median filter of Block <NUM>.

Image processing continues with rescaling the pixel data (Block <NUM>), e.g., based on a minimum pixel value of <NUM> and a maximum pixel value of <NUM>, resizing (Block <NUM>) the image, e.g., upscaling by a factor of four, and applying (Block <NUM>) a radius-<NUM> disk filter to the upscaled image. An example disk (circular) filter appears below:.

<FIG> illustrates the image after the processing of Blocks <NUM>, <NUM>, and <NUM>, with the resulting image then processed according to the processing of Block <NUM>, which involves a grayscale determination. The right-side image shown in <FIG> illustrates the results of gray-scaling (Block <NUM>).

<FIG> illustrates the results of binarizing (Block <NUM>) the grayscale image. A notable aspect of <FIG> is that it illustrates that there may be more than one illuminated region in the binarized image. This possibility is handled in the processing of Block <NUM>, which involves locating the illuminated regions within the binarized image, and finding the center (centroid) of the largest one among the illuminated regions (Block <NUM>). This logic can be understood as taking the largest one among the two or more illuminated regions in the binarized image as representing the uplink signal <NUM>.

Once the center location <NUM> of the largest illuminated region is determined, the difference between the center location <NUM> and the reference location <NUM> applicable to the feed grid <NUM> associated with the subject steerable antenna system <NUM> is determined and used to calculate the pointing correction. The pointing correction according to Block <NUM> comprises determining the delta Az (azimuthal) and delta El (elevational) adjustments for the reflector <NUM> of the subject steerable antenna system <NUM>.

Thus, the Az/El determination process represented by <FIG> can be understood as: (<NUM>) receiving an uplink signal <NUM> during a "special time" when no other transmissions from the ground are present (in the involved frequency band); (<NUM>) using one or more radiofrequency (RF) power detectors to measure the received-signal power on individual array elements <NUM> of the antenna array <NUM>, where the detection bandwidth may be narrowband or wideband, and continuous wave or modulated waveforms may be involved; and (<NUM>) the control system <NUM>, which may include or comprise a "payload processor," collecting the power measurements and using an algorithm that converts the measurements into a power-distribution image <NUM>, to determine the "uplink location" in feed-grid coordinates.

The azimuth and elevation error is then determined as: <MAT> and <MAT> where XR and YR are the coordinates of the reference locationg <NUM>, and where BDF = beam deviation factors. The beam deviation factors are based on the reflector geometry. Once the adjustments are determined, they are used either to actuate antenna or spacecraft body mechanisms, to obtain the calculated pointing correction.

Consider the below table, which illustrates an example pointing correction:.

With the above example details in mind, a satellite <NUM> according to one or more embodiments comprises a steerable antenna system <NUM> that includes an antenna array <NUM> and is configured to receive an uplink signal <NUM> that illuminates a particular region of the antenna array <NUM> in dependence on a current pointing direction of the steerable antenna system <NUM>. The antenna array <NUM> comprises a plurality of array elements <NUM> arranged according to a feed grid <NUM>, and the satellite <NUM> further includes a control system <NUM> that is configured to convert received-signal power measurements made for individual array elements <NUM> of the antenna array <NUM> during reception of the uplink signal <NUM> into a power-distribution image <NUM> comprising pixels <NUM> arranged on a pixel grid <NUM>. The pixel grid <NUM> is derived from the feed grid <NUM>, e.g., either a one-to-one correspondence or an upscaled correspondence. In either case, the pixels <NUM> have pixel values determined in dependence on the received-signal power measurements made for corresponding ones of the array elements <NUM>. The control system <NUM> is further configured to determine a center location <NUM> of an illuminated region <NUM> in the power-distribution image <NUM> that corresponds to the uplink signal <NUM>. Still further, with the center location <NUM> expressed in feed-grid coordinates, the control system <NUM> is configured to derive a pointing correction for the steerable antenna system <NUM> in dependence on a difference <NUM> between the center location <NUM> and a reference location <NUM> that also is expressed in feed-grid coordinates.

As shown in the introductory example depiction of <FIG>, the control system <NUM> in one or more embodiments comprises processing circuitry <NUM>, which may include or be associated with storage <NUM>. In at least one embodiment, the processing circuitry <NUM> comprises one or more microprocessors or other digital processors that is/are specially adapted to carry out the image-generation and pointing-correction determinations described herein-i.e., to convert received-signal power measurements into a power-distribution image <NUM> that represents the antenna array <NUM> and corresponding feed grid <NUM> within the antenna assembly <NUM> of a steerable antenna system <NUM>, and to use that image to determine pointing corrections for the steerable antenna system <NUM>. As shown, such pointing corrections may comprise azimuthal and elevational adjustments to a reflector <NUM> that effectively controls the pointing direction of the involved antenna array <NUM>.

Broadly, the control system <NUM> in one or more embodiments is configured to perform any one or more of the operations detailed in any one or more of the methods <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. For example, the storage <NUM> stores one or more computer programs <NUM> comprising program instructions that, when executed by the one or more microprocessors or other digital processors comprising the processing circuitry <NUM>, cause such processor(s) to perform the method operations.

Thus, in at least one embodiment, the control system <NUM> comprises processing circuitry <NUM> that is configured according to the execution of computer program instructions held in storage <NUM>. However, whether implemented in fixed circuitry, programmatically-configured circuitry, or a mix of both, in one or more embodiments, the control system <NUM> is configured determine the center location <NUM> of the illuminated region <NUM> in the power-distribution image <NUM> by calculating the centroid of the illuminated region <NUM>. As noted, the control system <NUM> may be configured to determine the center location <NUM> based on identifying the illuminated region <NUM> from among two or more illuminated regions that are present in the power-distribution image <NUM> as a result of the steerable antenna system <NUM> receiving signal energy from more than one signal source, while the received-signal power measurements are made. For example, the control system <NUM> is configured to identify the illuminated region <NUM> that corresponds with the uplink signal <NUM> by, for the two or more illuminated regions, comparing respective sizes of the two or more illuminated regions.

In one or more embodiments, the control system <NUM> is configured to convert the received-signal power measurements into the power-distribution image <NUM> by mapping the individual received-signal power measurements to a first set of pixels <NUM>, each pixel <NUM> in the first set of pixels <NUM> corresponding to a populated grid position <NUM> in the feed grid <NUM> and having a pixel value corresponding to the received-signal measurement made for the array element <NUM> at that populated grid position <NUM>. Further, in at least one such embodiment, the control system <NUM> is configured to create an expanded, second set of pixels <NUM> encompassing the first set of pixels <NUM> and additional pixels <NUM> corresponding to unpopulated grid positions <NUM> in the feed grid <NUM>, each additional pixel <NUM> having a pixel value derived from one or more neighboring pixels <NUM> in the first set of pixels <NUM>. Still further, in at least one embodiment, the control system <NUM> is configured to create an up-sampled image by generating multiple pixels <NUM> for each grid position <NUM> in the feed grid <NUM>. Creating new, additional pixels <NUM> comprises, for example, interpolating and extrapolating pixel values from the existing, neighboring pixels <NUM>.

Of course, for smoothing purposes, the control system <NUM> in one or more embodiments is configured to filter the up-sampled image, to obtain the power-distribution image <NUM> used for determining the center location <NUM> of the illuminated region <NUM> that corresponds to the uplink signal <NUM>. Such processing may also include binarizing the up-sampled image, e.g., after smoothing and gray-scaling operations, to produce a distinct and relatively smooth on/off boundary defining the illuminated region <NUM>.

Having a regularly-shaped illumination region <NUM> that is defined by a clean on/off pixel boundary aids in accurate determination of the center location <NUM> of the illuminated region <NUM>. Correspondingly, in one or more embodiments, generating the power-distribution image <NUM> may comprise the following processing operations: (<NUM>) obtain received-signal power measurements for occupied grid positions <NUM> of the feed grid <NUM>-i.e., grid positions <NUM> that have an array element <NUM>; (<NUM>) create an initial image having one pixel grid position <NUM> for each feed grid position <NUM>, where the pixels <NUM> at pixel grid positions <NUM> corresponding to occupied feed grid positions <NUM> have a digitized value corresponding to the power measurement made for that position and where pixels <NUM> at pixel grid positions <NUM> corresponding to unoccupied feed grid positions <NUM> have a zero value ("null" pixels); (<NUM>) use the non-zero pixel values to interpolate/extrapolate values for the null pixels <NUM>; (<NUM>) perform initial smoothing (filtering) of the resulting intermediate image; (<NUM>) upscale the intermediate image to increase pixel resolution; (<NUM>) smooth the upscaled image and gray-scale it; and (<NUM>) binarize the gray-scaled image, with the resulting "black-and-white" image, where each pixel <NUM> is "on" or "off," taken as the power-distribution image <NUM> to use for identifying the illuminated region <NUM> corresponding to the uplink signal <NUM>.

In one or more embodiments, the control system <NUM> is configured to derive pointing corrections for the steerable antenna system <NUM> on a recurring basis, based on recurring receptions of the uplink signal <NUM>. See, for example, the slot playlist information uploaded to the satellite <NUM> as configuration information in Block <NUM> of <FIG>. That is, there may be special slots defined by a schedule, wherein the uplink signal <NUM> is the only uplink signal transmitted to the involved steerable antenna system <NUM> during the special slots, so that the uplink signal <NUM> is cleanly discernable in the generated power-distribution image <NUM>.

Once the power-distribution image <NUM> is generated, the control system <NUM> according to one or more embodiments is configured to derive the pointing correction for the steerable antenna system <NUM> based on computing a horizontal offset in feed-grid coordinates between the center location <NUM> and the reference location <NUM> on a horizontal axis defined by the feed grid <NUM>, computing a vertical offset in feed-grid coordinates between the center location <NUM> and the reference location <NUM> on a vertical axis defined by the feed grid <NUM>, and translating the horizontal and vertical offsets into corresponding azimuthal and elevational pointing adjustments for the steerable antenna system <NUM>. Further, the control system <NUM> is configured to actuate, or initiate actuation of, a steering mechanism of the steerable antenna system <NUM>, according to one or more actuator control signals determined as a function of the pointing correction.

In at least one embodiment, the communication circuitry <NUM> of the satellite <NUM> performs downlink beamforming via the steerable antenna system <NUM>, to provide a set of forward user beams <NUM> defining corresponding forward user beam coverage areas <NUM>. Here, the pointing direction of the steerable antenna system <NUM> defines the geographical coordinates of an aggregate coverage area <NUM> defined by the set of forward user beams <NUM>.

Claim 1:
A satellite (<NUM>) comprising:
a steerable antenna system (<NUM>) that includes an antenna array (<NUM>) and is configured to receive an uplink signal (<NUM>) that illuminates a particular region of the antenna array (<NUM>) in dependence on a current pointing direction of the steerable antenna system (<NUM>), and wherein the antenna array (<NUM>) comprises a plurality of array elements (<NUM>) arranged according to a feed grid (<NUM>); and
a control system (<NUM>) configured to:
convert received-signal power measurements made for individual array elements (<NUM>) of the antenna array (<NUM>) during reception of the uplink signal (<NUM>) into a power-distribution image (<NUM>) comprising pixels (<NUM>) arranged on a pixel grid (<NUM>) derived from the feed grid (<NUM>) and having pixel values determined in dependence on the received-signal power measurements made for corresponding ones of the array elements (<NUM>);
determine a center location (<NUM>) of an illuminated region (<NUM>) in the power-distribution image (<NUM>) that corresponds to the uplink signal (<NUM>), the center location (<NUM>) expressed in feed-grid coordinates; and
derive a pointing correction for the steerable antenna system (<NUM>) in dependence on a difference (<NUM>) between the center location (<NUM>) and a reference location (<NUM>) that also is expressed in feed-grid coordinates.