Methods and apparatus to point a payload at a target

An example machine accessible medium having instructions stored thereon that, when executed, causes a machine to at least command a first actuator to move to a first corrected stroke position and a second actuator to move to a second corrected stroke position to point a payload at a target along a line of sight vector without verifying a target pointing direction of a base when the first and second actuators are positioned to the respective first and second corrected stroke positions and without using a feedback to verify the base being at the target pointing direction when the first and second actuators are positioned to the respective first and second corrected stroke positions.

FIELD

The present disclosure relates generally to payloads and, more particularly, to methods and apparatus to point a payload at a target.

BACKGROUND

Generally, a payload is to be substantially pointed at a target. For example, an antenna is to be substantially pointed at a target to enable communication between the antenna and the target. If the antenna points away from the target, the communication between the antenna and the target is affected. The antenna may be disposed on a satellite in orbit around Earth. Due to the distance between the satellite and a target on Earth, an alignment error of the antenna influenced by thermal distortion, machining tolerances, etc. may cause the antenna to point away from the target.

SUMMARY

An example machine accessible medium having instructions stored thereon that, when executed, causes a machine to at least: estimate a target pointing direction of a base supporting a payload to point the payload at a target, where the base is movable relative to a pivot about an azimuth angle and an elevation angle via a first actuator and a second actuator directly coupled to the base; determine a first estimated stroke position of the first actuator and a second estimated stroke position of a second actuator to position the base at the target pointing direction. The instructions further cause the machine to obtain a base orientation error. The instructions further cause the machine to determine a first stroke position error of the first actuator and a second stroke position error of the second actuator based on the base orientation error. The instructions further cause the machine to determine a first corrected stroke position of the first actuator based on a difference between the first stroke position error and the first estimated stroke position. The instructions further cause the machine to determine a second corrected stroke position of the second actuator based on a difference between the second stroke position error and the second estimated stroke position. Also, the instructions cause the machine to and command the first actuator to move to the first corrected stroke position and the second actuator to move to the second corrected stroke position to point the payload at the target along a line of sight vector without verifying the target pointing direction of the base when the first and second actuators are positioned to the respective first and second corrected stroke positions and without using a feedback to verify the base being at the target pointing direction when the first and second actuators are positioned to the respective first and second corrected stroke positions.

Another example tangible machine readable storage medium disclosed herein includes instructions that, when executed, cause a machine to at least receive a first command to move a payload relative to a first target. The instructions also cause the machine to estimate a base orientation of a base coupled to the payload to point the payload to the first target along a first line of sight vector. The instructions also cause the machine to determine a first estimated stroke position of a first linear actuator and a second estimated stroke position of a second linear actuator to position the base to the estimated base orientation. The instructions further cause the machine to modify the first estimated stroke position to a first corrected stroke position based on a base orientation error. Also, the instructions cause the machine to modify the second estimated stroke position to a second corrected stroke position based on the base orientation error. The instructions further cause the machine to command the first linear actuator to actuate to the first corrected stroke position and commanding the second linear actuator to actuate to the second corrected stroke position without verifying a final position of the payload after the first linear actuator is actuated to the first corrected stroke position and the second linear actuator is actuated to the second corrected stroke position.

Another example tangible machine readable storage medium disclosed herein includes instructions that, when executed, cause a machine to at least receive a command to actuate a first linear actuator and a second linear actuator operatively coupled to a base of a payload to orient the base at a first azimuth angle and a first elevation angle to point the payload at a target, the base being pivotably coupled to the payload via a joint and having only the first and second linear actuators to move the base about the joint relative to the azimuth angle and the elevation angle; determine a pointing error based on an experimentally determined second azimuth angle and an experimentally determined second elevation angle of the base to point the payload at the target; determine a first stroke position error of the first linear actuator and a second stroke position error of the second linear actuator based on the pointing error; determine a first corrected stroke position of the first linear actuator and a second corrected stroke position of the second linear actuator; and command the first linear actuator to move to the first corrected stroke position and the second linear actuator to move to the second corrected stroke position to point the payload at the target without verifying a final position of the payload after the first linear actuator is actuated to the first corrected stroke position and the second linear actuator is actuated to the second corrected stroke position

An example apparatus disclosed herein includes an instruction processor to receive a command to point a payload coupled to a base at a target, the base being movable relative to a pivot about an azimuth angle and an elevation angle via only a first linear actuator and a second linear actuator, the pivot, the first linear actuator and the second linear actuator being directly coupled to the base. In some examples, the apparatus includes a line of sight determiner to determine a line of sight vector between the payload and the target. In some examples, the apparatus includes a base orientation determiner to determine an estimated base orientation to point the payload at the target. In some examples, the apparatus includes an estimated stroke position determiner to determine a first estimated stroke position of a first linear actuator and a second estimated stroke position of a second linear actuator to orient the base at the estimated base orientation. In some examples, the apparatus includes a base orientation error determiner to determine a base orientation error. In some examples, the apparatus includes a stroke position error determiner to determine a first stroke positioner error of the first linear actuator based on the base orientation error and a second stroke position error of the second linear actuator based on the base orientation error. In some examples the apparatus includes a corrected stroke position determiner to determine a first corrected stroke position of the first actuator and a second corrected stroke position of the second actuator. In some examples, the apparatus includes an actuator controller to command the first linear actuator to move to the first corrected stroke position and the second linear actuator to move to the second corrected stroke position to point the payload at the target along a line of sight vector without verifying the target pointing direction of the base when the first and second linear actuators are positioned to the respective first and second corrected stroke positions and without using a feedback to verify the base being at the target pointing direction when the first and second linear actuators are positioned to the respective first and second corrected stroke positions.

Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. As used in this disclosure, stating that any part (e.g., a layer, film, area, or plate) is in any way positioned on (e.g., positioned on, located on, disposed on, or formed on, etc.) another part, means that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween. Stating that any part is in contact with another part means that there is no intermediate part between the two parts.

DESCRIPTION

Methods and apparatus to point a payload at a target are disclosed herein. An example apparatus disclosed herein includes a payload such as, for example, an antenna, a transmitter, a sensor (e.g., an infrared sensor), an optical device, a camera, and/or any other suitable payload coupled to a base that is rotatable about a joint. In some examples, the base is operatively coupled to a first actuator (e.g., a linear actuator such as, for example, a jackscrew) and a second actuator (e.g., a linear actuator such as, for example, a jackscrew). The first actuator and the second actuator may enable rotation of the base about the joint to adjust an azimuth angle and/or an elevation angle of the base. A controller may be in communication with the first actuator and the second actuator to control a first stroke position of the first actuator and a second stroke position of the second actuator.

To point the payload at a target, the controller may determine and compensate for a first stroke position error corresponding to the first actuator and a second stroke position error corresponding to the second actuator. The first stroke position error and/or the second stroke position error may be influenced by thermal distortion, machining tolerances, alignment errors, etc. In some examples, the first stroke position error and the second stroke position error are constant relative to an orientation of the base and/or an amount of rotation of the base. As such, the base may be rotated over a wide distance range (e.g., ten degrees of rotation or more) via the first actuator and the second actuator to point the payload at the target.

FIG. 1illustrates an example satellite100in accordance with the teachings of this disclosure. In the illustrated example, the satellite100is in orbit around Earth102. However, in other examples, the satellite100may be in orbit around another celestial body such as, for example, Earth's moon. In the illustrated example, the satellite100is in communication with a ground station104located on Earth102. In some examples, the satellite100is in communication with one or more other ground stations, satellites, and/or other targets. As described in greater detail below, the example satellite100includes a payload (FIG. 2), which may be employed to communicate information to the ground station104, receive signals, generate images, etc.

FIG. 2is a perspective view of an example payload assembly200disclosed herein, which may be used to communicate information from the example satellite100ofFIG. 1to a target (e.g., the ground station104), receive signals (e.g., optical signals) from the target, generate images of the target, etc. In the illustrated example, the payload assembly200includes a base202, a first actuator204, a second actuator300(FIG. 3), and a payload206such as, for example, an antenna, a sensor (e.g., an infrared sensor), a camera, an optical device, etc. In the illustrated example, the base202is a triangular plate. Thus, the base202includes a first corner208, a second corner210and a third corner212. The base202defines a substantially planar surface214onto which the payload206is coupled. In the illustrated example, the planar surface214is to substantially face the ground station104during operation of the example satellite100. In other examples, the base202is other shapes (e.g., rectangular, circular, a shape that defines a rounded or uneven (e.g., stepped) surface facing the ground station104, etc.). In the illustrated example, the payload206is disposed on the base202substantially normal to the planar surface214(i.e., the payload206is substantially perpendicular to the base202).

In the illustrated example, the base202is rotatably coupled to the satellite100via a pivot joint216(e.g., a ball joint) disposed at or adjacent the first corner208. The example joint216enables the base202to rotate at point A about a first axis (X-axis)218and a second axis (Y-axis)220. The example first axis218and the example second axis220intersect at point A. In the illustrated example, a third axis (Z-axis)222intersects the first axis218and the second axis220at point A. Thus, point A corresponds to coordinates (0, 0, 0). The above-noted axes218,220and222are merely examples and, thus, other axes may be employed in other examples. In the illustrated example, an elevation angle, ε, of the base202is an amount of rotation of the base202from coordinates (0, 0, 0) about the first axis218. An azimuth angle, α, of the example base202is an amount of rotation of the base202from coordinates (0, 0, 0) about the second axis220. Thus, a position or orientation of the example base202may be defined by the azimuth angle and the elevation angle of the base202.

FIG. 3is a rear view of the example payload assembly200ofFIG. 2. In the illustrated example, the base202is operatively coupled to the satellite100via the first actuator204and a second actuator300, respectively. The example first actuator204is coupled to the base202at point B, which is adjacent the second corner210of the base202. The example second actuator300is coupled to the base202at point C, which is adjacent the third corner212of the base202. In the illustrated example, the first actuator204and the second actuator300are jackscrews. However, the first actuator204and the second actuator300may be implemented using any type of linear actuator. In the illustrated example, strokes of the first actuator204and the second actuator300are substantially parallel to the third axis222. For example, when actuated, a first arm302of the first actuator204and a second arm304of the second actuator300move substantially parallel to the third axis222, thereby moving the second corner210and/or the third corner212, respectively, toward or away from the satellite100). The example first actuator204and/or the example second actuator300may be actuated to rotate the base202approximately ten degrees about an axis of rotation (e.g., the first axis218and/or the second axis220). Other examples rotate other amounts (e.g., five degrees, forty five degrees, etc.) and/or about other axes.

In the illustrated example, the first actuator204and the second actuator300are in communication with a controller306. The example controller306may reside in the satellite100, within the ground station104, and/or in any other suitable location. In the illustrated example, the controller306controls a first stroke position, h1, of the first actuator204and a second stroke position, h2, of the second actuator300. In other examples, the first actuator204and the second actuator300are controlled via separate controllers. As described in greater detail below, the controller306determines and compensates for a first stroke position error corresponding to the first actuator204and a second stroke position error corresponding to the second actuator300to point the payload206at the ground station104.

FIG. 4is a top view of the example payload assembly200ofFIGS. 2-3. In the illustrated example, point B (i.e., where the example first actuator204is coupled to the base202) is a first distance L1from point A. Point C (i.e., where the example second actuator300is coupled to the base202) is a second distance L2from point A. In the illustrated example, the first distance L1and the second distance L2are substantially equal. In other examples, the first distance L1and the second distance L2may be different. The example first actuator204and the example second actuator300are spaced apart from each other such that the first actuator204and the second actuator300are substantially ninety degrees apart relative to the joint216(i.e., a separation angle ϕ between the first actuator204and the second actuator300is substantially ninety degrees). In other examples, the first actuator204and/or the second actuator300are in other positions relative to the joint216(e.g., the separation angle ϕ is greater than or less than ninety degrees, etc.).

In the illustrated example, to communicate information from the example satellite100to the ground station104, the example satellite100transmits one or more signals to the ground station104via the payload206. To facilitate transmission of the signal, the example payload206is pointed at or toward the ground station104. In other examples, the payload206is pointed at a target other than the ground station104to, for example, generate images of the target, receive signals from the target, take measurements (e.g., via a sensor), etc. Thus, while the following examples are described in conjunction with the example ground station104, the base may be oriented to point the payload206at any other suitable target in accordance with the teachings of this disclosure. In the illustrated example, the payload206is pointed at the ground station104by aligning the payload206with a line of sight (LOS) vector extending from the ground station104to the satellite100or from the satellite100to the ground station104(e.g., such that the signals transmitted via the payload206substantially propagate along the LOS vector). To align the payload206with the LOS vector, the controller306actuates the first actuator204and/or the second actuator300to adjust an orientation of the base202(i.e., the azimuth angle and the elevation angle of the base202) and, thus, a pointing direction of the payload206.

In some examples, the controller306and/or the ground station104determine the LOS vector from the satellite100to the ground station104and/or from the ground station104to the satellite100. Based on the LOS vector, an estimated base orientation (i.e., an estimated azimuth angle and an estimated elevation angle of the base202) to point the payload206along the LOS vector (e.g., such that a signal transmitted via the payload propagates substantially along the LOS vector to the ground station104) may be determined. To move (i.e., orient) the base202to the estimated base orientation, a first estimated stroke position and a second estimated stroke position of the first actuator204and the second actuator300, respectively, are determined, and the first actuator204and the second actuator300are actuated to the first estimated stroke position and the second estimated stroke position, respectively.

In some examples, the LOS vector is determined based on a position of the ground station104and a position of the satellite100. The position of the ground station104may be determined based on a position vector of the ground station104in an inertial frame such as, for example, an Earth-Centered Earth Fixed frame based on an Earth-Centered Inertial Frame. The position of the satellite100may be determined in a frame of the satellite100via an orbit frame. In some examples, the orbit frame is determined based on an orbit position vector of the satellite100in the Earth-Centered Frame. The LOS vector may then be determined based on a difference vector between the position of the ground station104and the position of the satellite100in the frame of the satellite100. Based on the LOS vector, the estimated base orientation to point the payload206at the ground station104may be determined.

In the illustrated example, once the estimated base orientation is determined, the first estimated stroke position of the first actuator204and the second estimated stroke position of the second actuator300to orient the base202at the estimated base orientation are determined. The first estimated stroke position, h1of the first actuator204and the second estimated stroke position, h2, of the second actuator300are a function of the estimated azimuth angle, α, and the estimated elevation angle, ε, of the base202as shown in the following equations:

In Equations 1 and 2, ϕ is the separation angle, and L is the first distance L1(i.e., the distance from point A to B) or the second distance L2(i.e., the distance from point A to point C). In the illustrated examples disclosed herein, L1=L2. In other examples, when L1is not equal to L2, calculations and/or equations described herein account for other variables. As a result, for example, Equations 1 and 2 become more complex when L1is not equal to L2. However, the calculations and/or equations disclosed herein may be configured or obtained when L1is not equal to L2. Equations 1 and 2 may be used for each of the first and second actuators to determine the estimated azimuth angle, α, and the estimated elevation angle, ε, of the base202. In the illustrated example, because the separation angle ϕ is substantially ninety degrees, Equation 1 and Equation 2 simplify as shown in Equations 3 and 4 below:

Based on Equations 3 and 4, the first estimated stroke position, h1, and the second estimated stroke position, h2, may be determined using the following equations:

Using the first estimated stroke position and the second estimated stroke position, the controller306communicates a command to the first actuator204and the second actuator300to actuate to the first estimated stroke position and the second estimated stroke position, respectively. In some examples, the payload206may not point at the ground station104when the first actuator204and the second actuator300actuate to the first estimate stroke position and the second estimated stroke position, respectively. Instead, the base202may be oriented at a resultant base orientation different than the base orientation at which the payload206points at the ground station104. A difference between the resultant base orientation and the base orientation at which the payload206points to the ground station104is a base orientation or pointing error. If not compensated for, the base orientation error may affect communication between the satellite100and the ground station104.

In the illustrated example, the base orientation error,

[d⁢⁢αd⁢⁢ɛ],
is a function of a first stroke position error, dh1, corresponding to the first actuator204and a second stroke position error, dh2, corresponding to the second actuator300as shown in the following equation:

FIGS. 5-6are plots500and600illustrating power levels of a payload signal detected at the ground station104.FIG. 5is a two-dimensional plot500of the power levels detected at the ground station104, andFIG. 6is a three-dimensional plot600of the power levels detected at the ground station104. In the illustrated example, the base orientation error is determined based on the power levels of the payload signal detected at the ground station104while the payload206is moved to scan (e.g., raster scan) a given area. When the example payload206is pointed at the ground station104, the ground station104detects a maximum power level. As the payload206points away from the ground station104, the ground station104detects a lesser power level. In the illustrated example, a first axis of each of the plots500and600corresponds to the azimuth angle of the base202, and a second axis of each of the plots500and600corresponds to the elevation of the base202. In the example plot500ofFIG. 5, the power levels are illustrated by rings502having sizes corresponding to an amount of power detected at the ground station104. In the example plot ofFIG. 5, a smallest ring504corresponds to a maximum power level detected at the ground station104, and a largest ring506corresponds to a lowest power level detected at the ground station104. In the example plot600ofFIG. 6, a third axis corresponds to the power levels detected at the ground station104.

In the example plots500and600ofFIGS. 5 and 6, coordinates (0, 0) and (0, 0, 0), respectively, correspond to the resultant base orientation (i.e., an orientation of the base202when the first actuator204and the second actuator300are actuated based on the first estimated stroke position and the second estimated stroke position, respectively). When the base202is oriented at the resultant base orientation, the ground station104detects the power levels of the signal communicated by the payload206. Then, the payload206is scanned (e.g., raster scanned) or moved to point to some or all positions illustrated in the plots500and600ofFIGS. 5-6while the power levels are detected (e.g., continuously, at predetermined intervals of time, etc.) at the ground station104. In the illustrated example, the ground station104detects the maximum power level when the base202is positioned at coordinates (0.02, 0.04). Thus, the payload206substantially points at the ground station104when the base202is positioned at coordinates (0.02, 0.04). The base orientation error is a distance between the orientation of the base202at which the ground station104measures the maximum amount of power (e.g., coordinates (0.02, 0.04)) and the resultant base orientation (e.g., (0, 0)) as shown in the following equation:

where the first term on the right labeled with subscript “measure” represents measured values of the base angles, the second term on the right labeled with subscript “cmd” represents the commanded values (e.g., estimated values) of the base angle, and the term on the left is the difference between the commanded based angles and the measured base angles, or the base orientation error.

In some examples, signals (e.g., radio frequency (RF) signals, etc.) are communicated from the ground station104(e.g., via ground-based beacon systems) to the satellite100to determine the base orientation error. When the satellite100receives the signals, the satellite100decodes and/or demodulates the signals to determine the base orientation error. Other examples employ other techniques to determine the base orientation error.

Based on the base orientation error and using, for example, the following equation, a first stroke position error of the first actuator204and a second stroke position error of the second actuator300may be determined:

where the term on the left represents estimated first and second stroke position errors and where

In the illustrated example, the first and second stroke position errors, dh1and dh2, respectively, are inherent in the payload assembly200(i.e., the stroke position errors are constant or invariable relative to the orientation of the base202and/or an amount of rotation of the base202via the first actuator204and/or the second actuator300). Thus, the controller306may compensate for the first and second stroke position errors to point the payload206at the target irrespective of a range of movement of the base202. In the illustrated example, the controller306compensates for the first and second stroke position errors by commanding the first actuator204and the second actuator300to actuate to the first and second corrected stroke positions. To determine the first and second corrected stroke positions, the first and second stroke position errors are subtracted from the first and second estimated stroke positions, respectively, as shown in the following equation:

where the first term on the right labeled with subscript “nonlinear” is the nominally calculated actuator travel based on the desired (or commanded or estimated) base angles and the second term on the right with subscript “est” estimate the actuator errors. When the first actuator204is actuated to the first corrected stroke position and the second actuator300is actuated to the second corrected stroke position, the base202is oriented such that the payload206points at the ground station104.

The pointing direction of the payload206may be subsequently adjusted to keep the payload206pointed at the ground station104(e.g., if the satellite100moves relative to the ground station104), point the payload206at another target, etc. In such examples, the controller306determines an updated estimated base orientation to point the payload206at the ground station104(or other target) based on an updated LOS vector from the satellite100to the ground station104(or the other target) and/or from the ground station104to the satellite100. The controller306then determines a first updated estimated stroke position and a second updated estimated stroke position based on the updated estimated base orientation. To compensate for the first and second stroke position errors, the controller306determines first and second updated corrected stroke positions. To determine the first and second updated corrected stroke positions, the controller306subtracts the first and second stroke position errors from the first and second updated estimated stroke positions, respectively. The controller306communicates a command to the first actuator204and the second actuator300to move to the first and second updated corrected stroke positions, and the first actuator204and the second actuator300actuate to point the payload206at the ground station104(or the other target).

In Equation 17, L is the first distance L1or the second distance L2when L1=L2. In some examples, when L1is not equal to L2, equation 17 becomes more complex and is not further described herein for simplicity. Thus, in some examples, a variation of Equation 17 may be used when L1is not equal to L2. Using, for example, Equation 18 below, a sequential estimator may be used to estimate the alignment errors in Equation 17:

[dh1dh1d⁢⁢ϕd⁢⁢L]est,k=x_k=x_k-1+Kk⁡([d⁢⁢αd⁢⁢ɛ]measure-C⁢⁢x_k-1)Equation⁢⁢18
where the subscript “est” represent the estimated or commanded values and the subscript “measure” represent the measured values. In other examples, other estimators are used. In Equation 18, K is an update gain matrix at a k-th step. The gain matrix may be designed using any suitable method(s) such as minimum-variance, Kalman filter, fixed gain observers, etc. Thus, in the illustrated example, an amount of thermal distortion along the first distance (or any other suitable portion of the base202) may be determined. Based on the alignment errors determined using Equations 17 and 18, corrected actuator assembly parameter values (e.g., the separation angle ϕ, the first distance L1, etc.) may be determined as follows:

{ϕest=ϕ+d⁢⁢ϕestLest=L+d⁢⁢LestEquation⁢⁢19
where values with subscript “est” represent estimated values. Because the alignment errors affect a determination of the estimated stroke positions, corrected estimated stroke positions may be determined based on the corrected actuator assembly parameters values as show in Equation 20 below:

[h1h2]cmd=[h1⁡(α,ɛ,ϕest,Lest)h2⁡(α,ɛ,ϕest,Lest)]nonlinear-[dh1dh2]estEquation⁢⁢21
Thus, once the corrected stroke positions are determined, the controller306may communicate with the first actuator204and the second actuator300to actuate to the corrected stroke positions to point the payload206at the ground station104. In such examples, the corrected stroke positions compensate for the stroke position errors and the alignment errors.

FIG. 7is a block diagram of the example controller306ofFIG. 3. In the illustrated example, the controller306includes an instruction processor700, a LOS information determiner702, a base orientation determiner704, a memory706, an estimated stroke position determiner708, a base orientation error determiner710, a stroke position error determiner712, an alignment error determiner714, a corrected stroke position determiner716, and an actuator controller718.

The example instruction processor700ofFIG. 7receives instructions from the ground station104and/or a flight computer720disposed on and/or in communication with the satellite100. In some examples, the instructions include a command to determine a, LOS vector to a target, adjust a pointing direction of the payload206, actuate the first actuator204and/or the second actuator300, transmit a signal to the ground station104via the payload206, decode and/or demodulate signals received via the satellite100, etc. These instructions may be executed via the example controller306, stored in the memory706, communicated to one or more components of the satellite100and/or the ground station104, etc.

The example LOS determiner ofFIG. 7determines the LOS vector between the satellite100and a target (e.g., the ground station104). In some examples, the LOS vector is based on the position of the satellite100and the position of the target in one or more frames of reference. The LOS information determiner702may access and/or utilize position information related to the satellite100and/or the target from the memory706and/or the flight computer720. In some examples, in response to the flight computer720indicating relative movement between the satellite100and the ground station104, instructions to point the payload206at a different target, etc., the LOS information determiner702determines an updated LOS vector from the satellite100to the target and/or from the target to the satellite100. In some examples, some or all of the LOS information is determined and/or provided via the flight computer and/or the target.

The example base orientation determiner704ofFIG. 7determines the estimated base orientation to point the payload206at the target (e.g., orient the payload206to enable signals transmitted and/or received via the payload206to propagate substantially along the LOS vector to the target). In the illustrated example, the base orientation determiner704determines an estimated azimuth angle and an estimated elevation angle of the base202to point the payload206at the target.

The estimated stroke position determiner708determines a first estimated stroke position and a second estimated stroke position of the first actuator204and the second actuator300, respectively, to orient the base202at the estimated base orientation. In some examples, the estimated stroke position determiner708determines the estimated stroke positions based on the estimated base orientation by, for example, using Equations 1-8 and/or Equations 9-12 above. If the example alignment error determiner714determines that one or alignment errors are present, the estimated stroke position determiner708determines corrected estimated stroke positions to compensate for the alignment errors.

The example base orientation error determiner710ofFIG. 7determines the base orientation error. In some examples, the base orientation error determiner710determines the base orientation error experimentally. For example, the base orientation error determiner710may instruct a transmitter722(e.g., an antenna transmitter) to transmit a signal when the first actuator204is in the first estimated stroke position and the second actuator300is in the second estimated stroke position. At the ground station104, a power level of the signal is detected, and the base orientation error determiner710may instruct the actuator controller718to move (e.g., span) the payload206. At the ground station104, power levels of the signal are detected while the payload206is moved. Based on the power levels detected at the ground station104, the base orientation error is determined by the base orientation error determiner710. In some examples, the base orientation error is determined via the ground station104and communicated to satellite100. In other examples, signals (e.g., radio frequency (RF) signals, etc.) are communicated from the ground station104(e.g., via ground-based beacon systems) to the satellite100, and the base orientation error determiner710determines the base orientation error by decoding and/or demodulating the signals.

The example stroke position error determiner712ofFIG. 7determines a first stroke position error of the first actuator204and a second stroke position error of the second actuator300. In the illustrated example, the stroke position error determiner712determines the first and second stroke position errors based on the base orientation error. In the illustrated example, the stroke position errors are inherent in the payload assembly200(i.e., the stroke position errors are constant or invariable relative to the orientation of the base202and/or an amount of rotation of the base202via the first actuator204and/or the second actuator300). Thus, the controller306may compensate for the first and second stroke position errors irrespective of an amount of movement of the base202to point the payload206at the target.

The example alignment error determiner714ofFIG. 7determines alignment errors of the example actuator assembly such as, for example, an alignment error of the first actuator204and/or the second actuator300, actuator assembly parameter value errors influenced by thermal distortion (e.g., thermal expansion, thermal contraction, bending influenced by a temperature gradient, etc.), machining tolerances, etc, and/or other alignment errors, etc. In some examples, the alignment error determiner714determines corrected actuator assembly parameter values such as, for example, a corrected separation angle, a corrected distance between point A and point C (i.e., a length between a first point about which the base202rotates via the joint216and a second point where the second actuator300is coupled to the base202). The corrected actuator assembly parameter values may be used by the estimated stroke position determiner708to determine corrected estimated stroke positions.

The example corrected stroke position determiner716ofFIG. 7determines a first corrected stroke position of the first actuator204and a second corrected stroke position of the second actuator300based on the first and second stroke position errors, respectively. In some examples, the first corrected stroke position and the second corrected stroke position compensate for the first and second stroke position errors, respectively, and the alignment errors. When the first actuator204and the second actuator300are actuated to the first corrected stroke position and the second corrected stroke position, respectively, the payload206points at the target.

The actuator controller718controls the first actuator204and/or the second actuator300. In the illustrated example, the actuator controller718instructs the first actuator204to actuate to a given stroke position such as, for example, the first estimated stroke position, the first corrected stroke position, and/or any other stroke position. The example actuator controller718may instruct the second actuator300to actuate to a given stroke position such as, for example, the second estimated stroke position, the second corrected stroke position, and/or any other stroke position. In some examples, the actuator controller718instructs the first actuator204and/or the second actuator300to move the payload206to scan (e.g., raster scan) an area.

The example memory706(e.g., volatile memory, non-volatile memory, etc.) stores information such as, for example, a position of the ground station104, a position of the satellite100, actuator assembly parameter values, corrected actuator assembly parameter values, estimated stroke positions, stroke position errors, payload alignment errors, and/or any other information. The information stored in the example memory706may be accessed by one or more components of the example controller306, the example satellite100, the example ground station104, etc.

While an example manner of implementing the controller306ofFIG. 3has been illustrated inFIG. 7, one or more of the elements, processes and/or devices illustrated inFIG. 7may be combined, divided, re-arranged, omitted, eliminated and/or implemented in any other way. Further, the instruction processor700, the LOS information determiner702, the base orientation determiner704, the memory706, the estimated stroke position determiner708, the base orientation error determiner710, the stroke position error determiner712, the alignment error determiner714, the corrected stroke position determiner716, the actuator controller718, the ground station104, the flight computer720, the transmitter722, the first actuator204, the second actuator300and/or, more generally, the example controller306ofFIG. 7may be implemented by hardware, software, firmware and/or any combination of hardware, software and/or firmware. Thus, for example, any of the instruction processor700, the LOS information determiner702, the base orientation determiner704, the memory706, the estimated stroke position determiner708, the base orientation error determiner710, the stroke position error determiner712, the alignment error determiner714, the corrected stroke position determiner716, the actuator controller718, the ground station104, the flight computer720, the transmitter722, the first actuator204, the second actuator300and/or, more generally, the example controller306ofFIG. 7could be implemented by one or more circuit(s), programmable processor(s), application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or field programmable logic device(s) (FPLD(s)), etc. When any of the apparatus or system claims of this patent are read to cover a purely software and/or firmware implementation, at least one of the instruction processor700, the LOS information determiner702, the base orientation determiner704, the memory706, the estimated stroke position determiner708, the base orientation error determiner710, the stroke position error determiner712, the alignment error determiner714, the corrected stroke position determiner716, the actuator controller718, the ground station104, the flight computer720, the transmitter722, the first actuator204, the second actuator300and/or, more generally, the example controller306ofFIG. 7are hereby expressly defined to include a tangible computer readable medium such as a memory, DVD, CD, Blu-ray, etc. storing the software and/or firmware. Further still, the example controller306ofFIG. 7may include one or more elements, processes and/or devices in addition to, or instead of, those illustrated inFIG. 7, and/or may include more than one of any or all of the illustrated elements, processes and devices.

FIGS. 8-9depict example flow diagrams representative of methods or processes that may be implemented using, for example, computer readable instructions. The example processes ofFIGS. 8-9may be performed using a processor, a controller (e.g., the example controller306ofFIG. 7) and/or any other suitable processing device. For example, the example processes ofFIGS. 8-9may be implemented using coded instructions (e.g., computer readable instructions) stored on a tangible computer readable medium such as a flash memory, a read-only memory (ROM), and/or a random-access memory (RAM). As used herein, the term tangible computer readable medium is expressly defined to include any type of computer readable storage and to exclude propagating signals. Additionally or alternatively, the example process ofFIGS. 8-9may be implemented using coded instructions (e.g., computer readable instructions) stored on a non-transitory computer readable medium such as a flash memory, a read-only memory (ROM), a random-access memory (RAM), a cache, or any other storage media in which information is stored for any duration (e.g., for extended time periods, permanently, brief instances, for temporarily buffering, and/or for caching of the information). As used herein, the term non-transitory computer readable medium is expressly defined to include any type of computer readable medium and to exclude propagating signals.

Alternatively, some or all of the example processes ofFIGS. 8-9may be implemented using any combination(s) of application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)), field programmable logic device(s) (FPLD(s)), discrete logic, hardware, firmware, etc. Also, one or more operations depicted inFIGS. 8-9may be implemented manually or as any combination(s) of any of the foregoing techniques, for example, any combination of firmware, software, discrete logic and/or hardware.

Further, although the example processes ofFIGS. 8-9are described with reference to the flow diagrams ofFIG. 8-9, respectively, other methods of implementing the processes ofFIGS. 8-9may be employed. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, sub-divided, or combined. Additionally, one or more of the operations depicted inFIGS. 8-9may be performed sequentially and/or in parallel by, for example, separate processing threads, processors, devices, discrete logic, circuits, etc.

FIG. 8is a flowchart representative of an example method800that can be performed to point the payload206at a target such as, for example, the ground station ofFIG. 1. The example method800ofFIG. 8begins by the LOS information determiner702of the controller306determining line of sight information (block802). In some examples, determining the line of sight information includes determining a line of sight vector from the target (e.g., the ground station104) to the satellite100and/or from the satellite100to the target. In some examples, the ground station104and/or the flight computer720determines and/or provides some or all of the LOS information. Based on the line of sight information, the base orientation determiner704determines an estimated base orientation to point the payload206at the target (e.g., the ground station104) (block804). In some examples, the estimated base orientation is defined by an estimated azimuth angle of the base202and an estimated elevation angle of the base202to point the payload206at the target.

At block806, the estimated stroke position determiner708determines a first estimated stroke position of the first actuator204and a second estimated stroke position of the second actuator300to orient the base202at the estimated base orientation. At block808, the actuator controller718communicates a command to the first actuator204and the second actuator300to actuate to the first estimated stroke position and the second estimated stroke position, respectively. In some examples, as a result of thermal distortion, machining tolerances, etc., the payload206does not point at the target when the first actuator204and the second actuator300actuate to the first estimated stroke position and the second estimated stroke position, respectively. In such examples, the base202moves to a resultant base orientation. At block810, the base orientation error determiner710determines a base orientation error. The base orientation error may be a distance from the resultant base orientation to the position at which the payload206points at the target. In some examples, the base orientation error is determined by detecting power levels of a payload signal (e.g., transmitted via the transmitter722) at the target. In other examples, the base orientation error is determined by decoding and/or demodulating signals received via the satellite100and/or via any other suitable technique.

At block812, the alignment error determiner714determines an alignment error of the base202. In some examples, the alignment error includes an error in an alignment of the first actuator204and/or the second actuator300, an error in an actuator assembly parameter value such as, for example, the separation angle ϕ, the first distance L1, the second distance L2, etc. In some examples, the alignment error determiner714determines one or more corrected actuator assembly parameter values based on the alignment error. Based on the base orientation error and the alignment error, the stroke position error determiner712determines a first stroke position error and a second stroke position error corresponding to the first actuator204and the second actuator300, respectively (block814). At block816, the corrected stroke position determiner716determines a first corrected stroke position and a second corrected stroke position based on the first stroke position error and the second stroke position error, respectively. In the illustrated example, the first and second corrected stroke positions compensate for the alignment error and the stroke position errors. At block818, the actuator controller718communicates a command to the first actuator204and the second actuator300to actuate to the first corrected stroke position and the second corrected stroke position, respectively. When the first actuator204and the second actuator300actuate to the first corrected stroke position and the second corrected stroke position, respectively, the first actuator204and the second actuator300orient the base202such that the payload206points at the target.

FIG. 9is a flowchart representative of an example method900that can be performed to point the payload206at a target. The example method900ofFIG. 9begins by determining if the payload pointing direction is to be adjusted (block902). In some examples, the flight computer720monitors a position of the satellite100relative to the target. If the flight computer720senses relative movement of the satellite100, the flight computer720communicates instructions to the instruction processor700to adjust a pointing direction of the payload206to point the payload206at the target. In some examples, the controller306may be instructed to adjust the pointing direction of the payload206to point the payload206at a different target. If the payload pointing direction is to be adjusted, the LOS information determiner702determines line of sight information (block904). In some examples, the flight computer720and/or the target provides and/or determines some or all of the LOS information. In some examples, the line of sight information includes a line of sight vector from the target to the satellite100and/or from the satellite100to the target. Based on the light of sight information, the base orientation determiner704determines the estimated base orientation to point the payload206at the target (block906). At block908, the estimated stroke position determiner708determines a first estimated stroke position of the first actuator204and a second estimated stroke position of the second actuator300to orient the base202at the estimated base orientation. In some examples, the first estimated stroke position and/or the second estimated stroke position may be determined based on previously determined corrected actuator assembly parameter values such as, for example, a corrected separation angle, etc. The previously determined corrected actuator assembly parameter values accessed via the memory706.

At block910, the corrected stroke position determiner716determines a first corrected stroke position and a second corrected stroke position of the first actuator204and the second actuator300, respectively. To determine the first corrected stroke position, the example corrected stroke position determiner716compensates for a previously determined first stroke position error (e.g., by subtracting the previously determined first stroke position error from the first estimated stroke position). To determine the second corrected stroke position, the example corrected stroke position determiner716compensates for a previously determined second stroke position error (e.g., by subtracting the previously determined second stroke position error from the second estimated stroke position).

At block912, the actuator controller718communicates a command to the first actuator204and the second actuator300to actuate to the first corrected stroke position and the second corrected stroke position, respectively. When the first actuator204and the second actuator300are actuated to the first corrected stroke position and the second corrected stroke position, respectively, the base202is oriented such that the payload206points at the target.

FIG. 10is a block diagram of an example computer1000capable of executing the instructions ofFIGS. 8-9to implement the controller306ofFIG. 7. The computer1000can be any suitable type of computing device.

The computer1000of the instant example includes a processor1012. For example, the processor1012can be implemented by one or more microprocessors or controllers from any desired family or manufacturer.

The processor1012includes a local memory1013(e.g., a cache) and is in communication with a main memory including a volatile memory1014and a non-volatile memory1016via a bus1018. The volatile memory1014may be implemented by Synchronous Dynamic Random Access Memory (SDRAM), Dynamic Random Access Memory (DRAM), RAMBUS Dynamic Random Access Memory (RDRAM) and/or any other type of random access memory device. The non-volatile memory1016may be implemented by flash memory and/or any other desired type of memory device. Access to the main memory1014,1016is controlled by a memory controller.

The computer1000also includes an interface circuit1020. The interface circuit1020may be implemented by any type of interface standard, such as an Ethernet interface, a universal serial bus (USB), and/or a PCI express interface.

One or more input devices1022are connected to the interface circuit1020. The input device(s)1022permit a user to enter data and commands into the processor1012.

One or more output devices1024are also connected to the interface circuit1020. The output devices1024can be implemented, for example, by a transmitter (e.g., the transmitter722). The interface circuit1020, thus, may include a graphics driver card.

The interface circuit1020also includes a communication device (e.g., communication device56) such as a modem or network interface card to facilitate exchange of data with external computers via a network1026(e.g., a bus, coaxial cable, RF signal transmitter, etc.).

The computer1000also includes one or more mass storage devices1028for storing software and data. Examples of such mass storage devices1028include hard drive disks, compact disk drives and digital versatile disk (DVD) drives. The mass storage device1028may implement the local storage device62.

The coded instructions1032ofFIG. 10may be stored in the mass storage device1028, in the volatile memory1014, in the non-volatile memory1016, and/or on a removable storage medium such as a CD or DVD.

Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this disclosure is not limited thereto. On the contrary, this disclosure covers all methods, apparatus and articles of manufacture fairly falling within the scope of the claims.