# Patent

## Title
Space-based lever arm correction in navigational systems employing spot beams

## Abstract

A method and apparatus for estimating a lever arm correction between the wide beam antenna and the spot beam antenna in a navigational satellite system is disclosed. A prescribed satellite maneuver is used to increase the accuracy of the predicted satellite attitude in determining the lever arm, and lever arm corrections are provided to navigation receivers using curve fit coefficients to permit long range estimates and to minimize data transmission requirements.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to the following co-pending and commonly assigned patent application(s), all of which applications are incorporated by reference herein:
Application Ser. No. 11/247,493, entitled “USER SEGMENT-BASED LEVER ARM CORRECTION VIA PRESCRIBED MANEUVER FOR HIGH-ACCURACY NAVIGATION,” by Jonathan A. Tekawy and Kevin M. O'Brien and filed on same date herewith; and
Application Ser. No. 11/247,640, entitled “CONTROL SEGMENT-BASED LEVER-ARM CORRECTION VIA CURVE FITTING FOR HIGH ACCURACY NAVIGATION,” by Jonathan A. Tekawy and Kevin M. O'Brien and filed on same date herewith.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to space-based navigational systems, and in particular, to a space-based lever arm correction system applicable to navigational systems employing spot beams.
2. Description of the Related Art
The Global Positioning System (GPS) is a satellite system that transmits navigation signals that are received by ground-based GPS receivers and used to determine the position of the GPS receiver to a high degree of accuracy. GPS currently provides standard service to commercial receivers, and a higher accuracy service to military receivers authorized to receive such signals.
In current systems, the GPS navigation signal is transmitted via a wide beam satellite antenna disposed on each GPS satellite. The wide beam antenna permits any GPS receiver having a line-of-sight to the GPS satellite to receive the navigational signal, and when the navigation signal from a sufficient number of GPS satellites has been acquired, the GPS receiver can determine its position via a precision clock and well-known triangulation techniques.
Because GPS signals are also used in military applications, countermeasures can be expected to be applied in an attempt to reduce the effectiveness of the GPS system. One such countermeasure is jamming. To increase the effectiveness of the GPS signals in a jamming environment, a steerable high gain antenna may be used to transmit high intensity GPS signals via spot beams to areas where needed.
One difficulty with this approach is that the high gain spot beam antenna is typically physically displaced from the wide beam antenna, and consequently, the phase center of each antenna is also displaced as well. This displacement is known as the “lever arm” between the antennas, and left uncorrected, can negatively affect the ability of the GPS receivers to determine their position. Without any correction, the lever arm between the wide beam antenna and a 7 meter diameter spot beam antenna can contribute up to 4.4 meters of user range error (URE). Depending on the GPS satellite constellation, this uncompensated URE can produce up to nine meters (RMS) of vertical (altitude) navigation error, which is a factor of 10 higher than the performance of the current GPS constellation of 0.9 meters (RMS). In civil aviation applications, such errors are sufficient to result in loss of life, and in military applications, they can result in increased collateral damage, and increased sortie and weapon consumption to perform the same mission.
Further exacerbating this problem is the fact that in order to maintain proper Sun and Earth pointing, the GPS satellites are required to perform attitude maneuvers. Such maneuvers can be very large, particularly about the yaw axis.
To achieve such high-accuracy navigation demanded from many missions, the GPS system must provide users with real-time corrections of the spot beam antennas phase center location relative to the Earth coverage antenna phase center, even while the spot beam antenna is moving to track specific terrestrial locations.
What is needed is an apparatus and method for computing a correction for lever arm related errors, and for incorporating this correction in navigation computations. The present invention satisfies these needs.
SUMMARY OF THE INVENTION
To address the requirements described above, the present invention discloses a method and apparatus for estimating a lever arm correction between a wide beam antenna and a spot beam antenna. The method comprises the steps of receiving a target spot beam position over the time period T in the satellite; generating a predicted position of the satellite over a time period T; generating predicted satellite attitude over the time period T from a prescribed satellite attitude maneuver profile over the time period T; generating predicted gimbal angles of the spot beam antenna over the time period T from the predicted satellite attitude and the target spot beam position over the time period T; predicted the lever arm correction between the wide beam antenna and the spot beam antenna over the time period T from the predicted gimbal angles of the spot beam antenna over the time period T and the predicated satellite attitude over the time period T; and transmitting the predicted lever arm correction from the satellite to a navigational receiver receiving the second navigation signal. The apparatus can be embodied in a navigational satellite, which comprises a first antenna transmitting a first navigational signal; a steerable second antenna transmitting a second navigational signal; a satellite receiver, for receiving data from a ground station, the data including a target spot beam position over a time period T; a lever arm correction module, for predicting a lever arm correction between the first antenna and the second antenna, and a navigation signal transmitter for transmitting the predicted lever arm correction to a navigational receiver.



BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
FIG. 1A is an illustration of a three-axis stabilized satellite;
FIG. 1B is an illustration of an embodiment of a gimbal assembly;
FIG. 2 is a diagram depicting a functional architecture of a representative satellite attitude control system;
FIG. 3 is a diagram depicting geometrical relationships between the satellite, the Earth and the Sun;
FIGS. 4A–4E are diagrams depicting the yaw noon maneuver at orbital midnight for a zero Sun angle;
FIG. 5 is a diagram depicting the yaw attitude angle as a function of the orbit angle and the Sun angle;
FIG. 6 is a diagram illustrating an exemplary embodiment of a technique for estimating the lever arm correction between the wide coverage antenna and the spot beam antenna;
FIG. 7 is a diagram illustrating architectural elements of a space-based navigational system;
FIGS. 8A and 8B are diagrams illustrating exemplary process steps that can be used to implement one embodiment of the invention, and how these process steps can be allocated to the architectural elements;
FIG. 9 is a plot of the yaw attitude angle of the satellite as a function of the orbit angle and the Sun angle for an exemplary prescribed satellite 100 maneuver profile; and
FIG. 10 is a diagram illustrating one technique that can be used to arrive at the prescribed maneuver profile.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
FIG. 1A illustrates a three-axis stabilized satellite or spacecraft 100. The satellite 100 has a main body 102 (which may be referred to as the “satellite bus”), one or more solar panels 104, one or more navigation beam antennas 106E and 106S, and a telemetry and command antenna 108 which is used to communicate with a control ground station. The satellite 100 may also include one or more sensors 110 to measure the attitude of the satellite 100. These sensors may include sun sensors, earth sensors, and star sensors. Since the solar panels are often referred to by the designations “North” and “South”, the solar panels in FIG. 1 are referred to by the numerals 104N and 104S for the “North” and “South” solar panels, respectively.
The three axes of the spacecraft 100 are shown in FIG. 1A. The pitch axis lies along the line of the solar panels' 104N and 104S mutual rotation axes. The roll and yaw axes are perpendicular to the pitch axis and lie in the directions and planes shown.
In the illustrated embodiment, the satellite 100 includes a first navigation beam antenna 106E and a second navigation beam antenna 106S. The first navigation beam antenna 106E is a wide-beam antenna which transmits a navigation signal with a beamwidth covering the widest range of the Earth's surface possible from that satellite's altitude at any time, and is directed toward the Earth along the yaw axis. Since this antenna 106E offers the widest coverage of the Earth's surface, it is typically not steerable. The navigation system uses a constellation of such satellites 100 to provide coverage anywhere on the Earth's surface by at least 4 different satellites at all times, thus permitting the navigational signal transmitted by the satillites to be used to determine the location of the receiver using triangulation techniques.
The second navigation beam antenna 106S is a steerable spot beam antenna that provides a second navigation signal in a much narrower navigation beam. This allows transmission of a higher-strength beam to selected points on the ground without requiring excessive transmitter power, thus reducing the effectiveness of countermeasures such as jamming. Since the required service area includes substantially the entire surface of the Earth and the beamwidth of the spot beam antenna 106S is not wide enough to cover the entire surface area, the boresight of the spot beam antenna 106S can be steered about to direct the spot beam where desired. Such steering can be accomplished mechanically, by use of a gimbal structure driven by gimbal motors, or electrically, using phased arrays, for example.
FIG. 1A also shows the phase center 112E of the wide beam antenna 106E and the phase center 112S of the spot beam antenna 106S. Since the spot beam antenna 106S is larger and offset from the wide beam antenna 106E, the phase centers 112S, 112E of the antennas are separated by an antenna lever arm 114, which is represented as a vector originating at the phase center 112E of the wide beam antenna 106E and extending to the phase center 112S of the spot beam antenna 106S. Due to motion of the satellite bus 102 and the spot beam antenna 106S and other factors, the antenna lever arm 114 does not remain fixed, but can very substantially over time. This variance is enough to add a significant uncertainty in the ability of a navigation receiver (such as a GPS receiver) to determine its location when a navigation signal is received via the spot beam.
The spot beam antenna 106S may be steered electronically (by appropriate phasing of elements in a scanning array) or mechanically (by use of a non-scanning antenna and a gimbal assembly), or a combination of both. FIG. 1B illustrates an embodiment using a gimbal assembly 152 having an inner gimbal 154A and an outer gimbal 154B. The inner gimbal 154A is associated with inner gimbal coordinate frame 156B while the outer gimbal 154B is associated with an outer gimbal coordinate frame 156C. Driven by gimbal motors or other devices (not illustrated), the inner and outer gimbals 154A, 154B angularly direct the antenna 106S boresight (which is represented in antenna signal boresight coordinate frame 156D) where desired to transmit the navigation signal spot beam using the antenna 106S. Both the inner and outer gimbals 154 typically include a potentiometer or other means to measure the gimbal angle.
To determine the angle at which the inner and outer gimbals 154 should be positioned to direct the spot beam antenna 106S to the desired scan locations (to perform a specific mission profile), the spacecraft 100 determines its attitude via the attitude sensors 110, which may be mounted on the satellite bus or body 102. Using the measured satellite attitude and the angular and translational displacement between the satellite body 102 and the spot beam antenna 106S (as expressed by coordinate systems 156A–156D), the satellite 100 can determine the appropriate gimbal angles, and commands the gimbal motors to move the gimbals 156 to the appropriate positions.
FIG. 2 is a diagram depicting the functional architecture of a representative attitude control system 200. Control of the spacecraft is provided by a computer or spacecraft control processor (SCP) 202. The SCP 202 performs a number of functions which may include post ejection sequencing, transfer orbit processing, acquisition control, station-keeping control, normal mode control, mechanisms control, fault protection, and spacecraft systems support, among others.
The SCP 202 may implement one or more processing modules such as antenna control module 276, which is used to control the satellite spot beam antenna drive 274 to slew the spot beam antenna 106S to the appropriate orientation and to transmit a navigation signal. Alternatively, the antenna control module 276 can be implemented in a different processor or in dedicated circuitry.
Input to the spacecraft control processor 202 may come from any combination of a number of spacecraft components and subsystems, such as a transfer orbit sun sensor 204, an acquisition sun sensor 206, an inertial reference unit 208, a transfer orbit Earth sensor 210, an operational orbit Earth sensor 212, a normal mode wide angle sun sensor 214, a magnetometer 216, and one or more star sensors 218.
The SCP 202 generates control signal commands 220 which are directed to a command decoder unit 222. The command decoder unit operates the load shedding and battery charging systems 224. The command decoder unit also sends signals to the magnetic torque control unit (MTCU) 226 and the torque coil 228.
Wheel torque commands 262 are generated by the SCP 202 and are communicated to the wheel drive electronics 240 which command the speed of the reaction wheels in reaction wheel assembly(s) 244. Typically, the spacecraft 100 includes four reaction wheels, at least one in each orthogonal direction, and one for redundancy purposes. The speed of the reaction wheels is also measured and fed back to the SCP 202 by feedback control signal 264. The SCP 202 also communicates commands and data 254 with command stations (further described in connection with FIG. 7) via a satellite transmitter/receiver (or transceiver) 258.
The foregoing describes an exemplary space stabilized satellite attitude control system. The present invention can be implemented with other attitude control system designs as well.
The SCP 202 may include or have access to memory 270, such as a random access memory (RAM). Generally, the SCP 202 operates under control of an operating system 272 stored in the memory 270, and interfaces with the other system components to accept inputs and generate outputs, including commands. Applications running in the SCP 202 access and manipulate data stored in the memory 270. The spacecraft 100 may also comprise an external communication device such as a satellite link for communicating with other computers at, for example, a ground station. If necessary, operation instructions for new applications can be uploaded from ground stations.
In one embodiment, instructions implementing the operating system 272, application programs, and other modules are tangibly embodied in a computer-readable medium, e.g., data storage device, which could include a RAM, EEPROM, or other memory device. Further, the operating system 272 and the computer program are comprised of instructions which, when read and executed by the SCP 202, causes the spacecraft processor 202 to perform the steps necessary to implement and/or use the present invention. Computer programs and/or operating instructions may also be tangibly embodied in memory 270 and/or data communications devices (e.g. other devices in the spacecraft 100 or on the ground), thereby making a computer program product or article of manufacture according to the invention. As such, the terms “program storage device,” “article of manufacture” and “computer program product” as used herein are intended to encompass a computer program accessible from any computer readable device or media.
As described above, it is desirable to correct for navigation errors caused by the lever arm 114 between the wide beam antenna and the steerable spot beam antenna. One approach taken to correct for such errors is to compute the antenna lever arm 114 in the satellite coordinate frame, and transmit this information to the GPS receivers in the navigational message. The GPS receiver then uses this information to compute corrections in the GPS (e.g. Earth Centered Earth Fixed) coordinate frame. However, this technique is complicated by the maneuvers that the satellite 100 must undergo to direct the solar panels 104 at the Sun and the wide beam antenna 106E at the Earth. Depending on the Sun angle, this requires a rather substantial yaw maneuver twice each orbit (at orbit noon and midnight).
FIG. 3 is a diagram depicting the geometrical relationships between the satellite 100, the Earth 308 and the Sun 306. Each satellite 100 of the GPS system is in an orbit 302 around the Earth 308, thus defining an orbital plane 304. The angle θ between the orbital plane 304 and the Sun 306 is the Sun angle. The satellite 100 orbits the Earth 308, always directing the antenna 106E at the Earth 308. The satellite 100 attitude and solar panels 104 are adjusted to keep the solar panels 104 directed at the Sun 306 and the antenna 106E directed at the Earth 308 at the same time. Typically, this requires the satellite 100 to perform a large rotation about the yaw axis at the orbital midnight (0 degree orbit angle) and the orbital noon (180 degree orbit angle). The smaller the Sun angle θ, the larger the required rotation about the yaw axis.
FIGS. 4A–4E are diagrams depicting the yaw turn maneuver at orbital midnight for a zero Sun angle (wherein the Sun 306 is in the orbital plane 304). FIG. 4A illustrates the satellite 100 attitude and solar panel 104 orientation as the orbital midnight is approached. Note that the solar panels 104 are canted at an angle to direct them to be perpendicular to the Sun's rays. Shortly before the Sun 306 begins to be eclipsed by the Earth 308, the satellite 100 begins its maneuver and yaws counter clockwise as shown in FIG. 4B through FIG. 4D. When the Sun 306 fully emerges from the eclipse, the satellite 100 has yawed 180 degrees from its initial position.
FIG. 5 is a diagram plotting the ideal yaw angle as a function of the orbit angle. At higher Sun 306 angles, the yaw angle movement is gradual, as shown in trace 502. However, at low Sun 306 angles, the yaw angle abruptly transitions 180 degrees at orbital angles of 0 and 180 degrees, with resulting high yaw angular rates and/or accelerations, as shown in trace 504. Such low Sun angle conditions (e.g. less than about five degrees) occur over a significant portion of the year. For example, the eclipse season occurs twice a year for each orbital plane. Since a GPS constellation typically includes six orbital planes, the constellation typically experiences eclipse conditions in one of its planes at all times of the year.
FIG. 6 is a diagram illustrating an exemplary embodiment of a technique for estimating the lever arm 114 correction between the wide coverage antenna 106E and the spot beam antenna 106S. In block 602, a prediction of the satellite 100 position over a time period T is generated. Block 604 illustrates the generation of a prediction of the satellite 100 attitude over the time period T from a profile of a prescribed navigational satellite attitude maneuver. Finally, block 606 illustrates predicting the lever arm correction between the wide coverage antenna and the spot beam antenna over the time period T using the predicted satellite attitude over the time period T and a target position on the Earth of the spot beam signal.
FIG. 7 is a diagram illustrating architectural elements of a space-based navigational system 700. The navigational system 700 comprises a space segment 704 having a plurality of navigational satellites 100 (hereinafter alternatively referred to as spacecraft) in orbit around the Earth. Each of the satellites 100 includes a first transmitter 710, which transmits a first navigational signal using the wide beam antenna 106E and a second transmitter 712, which, using the spot beam antenna 106S, transmits a spot beam 719 having a second navigational signal to one or more of the navigational receivers 716 that together form the user segment 706. The spot beam 719 covers an area 715 on the Earth's surface, and is centered on the target spot beam position 717. Both the first navigational signal and the second navigational signal include information that can be used to determine the location of the navigational receiver 716. In a jamming environment, a navigational receiver may lose lock on the first navigation signal and will not be able to produce any positioned solution. The second navigational signal, however, is typically of greater power by virtue of being transmitted via a spot beam, and, will be used for positional determination using the navigational receiver 716 in the jamming environment. The satellites 100 in the space segment 704 are controlled via commands from the ground segment 702, which comprises one or more ground-based command stations 707, each including a command center 708 and a communicatively coupled antenna 714 for transmitting and receiving information and commands to and from the satellites 100 of the space segment 704.
The operations illustrated in FIG. 6 can be performed on-board the satellite 100, the ground segment 702 controlling the satellite 100, by the navigational receivers 716 making up the user segment 706, or any combination thereof.
However, while a fixed antenna lever arm 114 can be computed in the satellite coordinate frame 156 and transmitted to the GPS receiver in the navigational message assuming an idealized yaw steering profile, this technique has serious disadvantages. First, this approach also cannot be applied to GPS systems that use steerable spot beam antennas, because the GPS receivers do not know the satellite 100 attitude, very accurately, and thus have insufficient information to compute an accurate estimate of the lever arm 114 at all times. Second, even if the GPS receivers were provided with the satellite 100 attitude, this technique requires that the GPS receivers perform the majority of the correction processing. Since the GPS receivers are often required to be very small, lightweight, and low power consumption devices, such processing can be a significant problem. Also, because of satellite attitude control system 200 limitations, the high yaw rate maneuvers that occur at low Sun 306 angles (such as shown in FIG. 5) add substantial uncertainty to the antenna lever arm 114 corrections transmitted to the GPS receivers.
One solution to these problems is to compute discrete lever arm 114 corrections in the GPS coordinate frame in the satellite 100 and periodically transmit the corrections to the GPS receivers. Since the correction data is in the GPS coordinate frame, it can be applied to the satellite orbital motion data already provided by the GPS system, and thus requires only minimal GPS receiver processing. However, such discrete corrections do not provide solutions in between the discrete points, and adding discrete points would consume a prohibitively large portion of the transmission bandwidth in the navigation message.
Finally, a problem with either of the foregoing correction approaches is that the corrections must be extrapolated for some period into the future. The first approach (that of transmitting the fixed antenna lever arm position in the satellite 100 coordinate frame in the navigational message and relying upon the GPS receivers to use this information along with a satellite yaw steering maneuver to enable computation of the corrections in a GPS coordinate frame), relies on an idealized (and often inaccurate) prediction of the satellite 100 attitude for all relevant times. The second approach makes predictions at least 15 minutes into the future, and during those 15 minutes, without additional information of constraints, this solution can yield large errors.
FIGS. 8A and 8B are diagrams illustrating exemplary process steps that can be used to implement one embodiment of the invention, and how these process steps can be allocated to the architectural elements (e.g. the control segment 702, the space segment 704, and the user segment 706).
Referring first to FIG. 8A, the predicted position of the satellite 100 over a time period T is computed, as shown in block 802. The predicted position of the satellite 100 can be determined using a variety of methods well known in the art, and can include parameters to model satellite maneuvers (whether for mission specific or station keeping purposes). Also, for a navigational satellite such as a GPS satellite, the parameters for predicting satellite position are normally present on-board the satellite as an integral part of the navigational system. The predicted position of the satellite 100 can be determined from this information.
Next, as shown in block 806, a prediction of the satellite's attitude over the same time period T is computed. This is accomplished using the computed satellite position (from the computation shown in block 802), and from a prescribed satellite maneuver profile 808 that can be obtained either from the ground segment 702 or the memory of the satellite 100 in the space segment 704.
FIG. 9 is a plot of the yaw attitude angle of the satellite 100 as a function of the orbit  angle 310 and the Sun 306 angle for an exemplary prescribed satellite 100 maneuver profile. The prescribed maneuver profile can be defined such that, when accounting for the capabilities and limitations of the satellite's attitude control system 200, the satellite 100 can execute the prescribed attitude maneuver within a specific angular tolerance, the selection of such tolerance is a function of the desired lever arm estimation accuracy. In one embodiment, the value of this tolerance, expressed as the difference between the predicted satellite attitude and the actual satellite attitude to less than 0.2 degrees. The prescribed maneuver profile can be defined so as to limit the maximum angular velocity and/or acceleration about all axes (pitch, roll, and yaw) not to exceed maximum values or such that the maximum angular velocity and/or acceleration does not exceed maximum values in any one of the axes (for example, in the yaw axis).
As was true in FIG. 5, trace 502 illustrates yaw angle profile for a high Sun 306 angle θ. At high Sun 306 angles θ, the required yaw rates and accelerations are relatively small. However, at low Sun 306 angles θ, the required yaw rates and accelerations are much higher, as shown in trace 504. These yaw rates are high enough so that the satellite's attitude control system 200 cannot follow the ideal profile, and thus, the deviation from the expected satellite 100 attitude angle and the actual satellite 100 attitude angle is significant enough to negatively impact the estimation of the lever arm 114. Also, the maneuver is initiated based upon sensor (e.g. Sun, Earth, and/or star sensors) measurements, hence the time the maneuver is initiated is another source of unpredictability. To prevent this, the satellite is commanded to perform a prescribed satellite maneuver 808.
This prescribed satellite maneuver profile 808 can be defined such that, when accounting for the capabilities and limitation of the satellite's attitude control system 200, the satellite 100 can execute the prescribed attitude maneuver to within specified tolerances. The maneuver profile can be arrived at by (1) limiting the satellite attitude angular rate and/or acceleration in all axes (e.g. pitch, roll, and yaw) or a single axis (e.g. yaw only) to a maximum value, (2) limiting the error between the predicted satellite attitude and the actual satellite attitude (when attitude control system 200 limitations are considered), (3) and/or placing other suitable restrictions upon the commanded satellite maneuver. For example, the prescribed satellite maneuver profile 808 can be defined so as to limit the error between the predicted satellite attitude and the actual satellite attitude to less than 0.2 degrees and/or limit the yaw attitude rate and acceleration to a particular value.
An exemplary profile is illustrated in FIG. 9 as trace 902. Note that the difference between the ideal maneuver (trace 504) and the prescribed maneuver (trace 902) is small enough so that the solar panels 104 remain adequately directed at the Sun, yet by commanding the satellite attitude control system 200 to perform the prescribed maneuver instead of the ideal maneuver, the resulting actual satellite 100 attitude becomes predictable enough to allow the lever arm to be accurately predicted. A description of how the profile can be computed is discussed further below.
Returning to FIG. 8A, spot beam gimbal angles are computed, as shown in block 816. This is accomplished using the predicted satellite 100 attitude over the time period T (computed in block 806), the gimbal geometry 810, and the target position of the spot beam antenna (e.g. the where the spot beam is aimed to direct it at one or more terrestrial targets) over the time period T, as computed in block 814. The position of the target spot beam over time period T is computed from a mission profile. The mission profile, which describes the mission that one or more of the satellites 100 in the navigation system 700 is to perform (for example, which areas on the Earth's surface the spot beam antenna will be directed at), is preferably obtained by transmission from the ground segment 702 to the space segment 704, as illustrated in block 812.
The gimbal geometry 810 (which includes, for example, a description of the dimensions and orientation of the gimbal structures and joints used to rotate the gimbals to direct the spot beam antenna 106S or equations describing the relationship between pointing direction and gimbal angles) can be loaded into a memory of the satellites 100 of the space segment 704, but can also be uplinked to the space segment 704 from the ground segment, if desired. The geometry can include, for example, positional vectors from the wide coverage antenna 106E phase center to the spot beam antenna the rotational axis of the first spot beam antenna 106S gimbal motor, and from this point to the rotational axis of the second spot beam antenna 106S gimbal motor, and to the second spot beam antenna 106S gimbal motor to the spot beam antenna 106 phase center.
Turning to FIG. 8B, the spot beam gimbal angles are used to predict the lever arm 114 vector in a satellite bus-based coordinate system, as shown in block 820. In block 828, the lever arm 114 vector

LASV(x(t),y(t),z(t))

is transformed into Earth-centered Earth Fixed ECEF coordinates so that this information may be used by the navigational receivers 716 without further conversion. Since each spacecraft 100 is aware of its attitude, this operation is preferably performed on the spacecraft 100.

Because the lever arm 114 vector is time dependent, the transmission of the lever arm correction at all times to the navigational receivers 716 would consume a significant amount of available transmission bandwidth. In a preferred embodiment of the invention, this difficulty is overcome by using curve-fitting techniques (including linear and non-linear regression) to identify closed form equations that describe the behavior of the lever arm over time during the time period T. In particular, the time-varying lever arm vector is expressed into its component parts (e.g. x(t),y(t), and z(t)), and each time-varying component part is curve-fit to an equation definable by combination of functions, each defined by one or more functional coefficients. This is shown in block 824.
For example, each time-varying component can be fit to an nth order polynomial




  
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The appropriate curve-fitting technique (e.g. the characteristics and order of the equation) can be selected according to the characteristics of the component part of the time varying lever arm 114 vector. For example, after z(t) for the time period T has been computed, z(t) can be evaluated to determine whether it is representable more accurately with a second, third, or fourth order polynomial. This can be accomplished, for example, by computing the means squared error for each representation, and selecting the characteristics of the curve fit to minimize mean squared error. It may also be desirable to use different curve-fitting techniques and characteristics for the different component parts of the lever arm 114 vector.
Next, the lever arm vector coefficients are sent to the navigation receivers 716 in the user segment 706 via a spot beam from the spot beam antenna 106S, as shown in block 832. The navigation receivers receiving the coefficients 834, convert those coefficients to a time-varying lever arm correction prediction in ECEF coordinates, and apply the lever arm corrections at the appropriate time, as shown in blocks 834–838.
The operations illustrated in FIGS. 8A and 8B as being performed by the space segment 704 can be illustrated by software modules executed by the SCP 202, software modules executed by special or general purpose auxiliary processors in each satellite 100, or by dedicated electronic circuitry in each satellite 100. For example, in one embodiment, the operations shown in blocks 802, 806, 814, 816, 820, 822, and 824 are performed by a lever arm correction module 801, which can be implemented in the SCP 202, an auxiliary processor, or dedicated electronic circuitry. Further, the lever arm correction module 801 can include a satellite position predictor module 804, a satellite attitude predictor module 807, and a lever arm prediction module 826, which may further comprise a coordinate transformer 828 and curve fitting module 830 to perform the associated tasks shown in FIGS. 8A and 8B. These modules may also be implemented in the SCP 202, an auxiliary processor, or dedicated electronic circuitry.
Generation of Prescribed Maneuver Profile
FIG. 10 is a diagram illustrating one technique that can be used to arrive at the prescribed maneuver profile. In block 1002, the performance of the satellite attitude control system (ACS) is determined, using specifications and/or test data of the applicable components, including, for example, the reaction or momentum wheels, ACS thrusters, and attitude sensors. In block 1004, this data is used to determine the maximum angular rate and/or acceleration that can be achieved by the spacecraft 100 under the most demanding manuevers it will be required to perform, typically the yaw maneuver at orbital noon and midnight for low Sun angles, as depicted in FIGS. 4A–4E. Since the yaw channel is typically most critical, a yaw steering profile can be examined. This can be accomplished using digital, analog, or hybrid control simulations of the satellite ACS 200. In block 1006, an ideal attitude profile is generated. This profile does not consider the limitations of the ACS, and yields zero pointing errors about all axes and zero pointing errors of the solar panels at all times. In block 1008, a candidate prescribed attitude profile is selected. The candidate profile is selected so as to attempt to match or closely approximate the ideal profile generated in block 1006. Preferably, the candidate attitude profile is computed using closed form equations with parametric variability. The candidate profile will generally deviate from the ideal profile by an error, and the goal is to minimize this error.
Using this information, the candidate prescribed attitude profile is tested to determine if the spacecraft 100 can perform the maneuver described in the profile with sufficient (and typically nearly perfect) accuracy. Again, this can be performed with digital, analog, or hybrid simulations. If the test indicates that the satellite 100 can follow the prescribed maneuver profile, perhaps even better performance can be achieved by selecting a more demanding prescribed maneuver profile (using perhaps another candidate profile that follows the ideal profile more closely) that yields smaller errors when compared to the ideal profile, processing can return to block 1006 for the generation and eventual evaluation of that profile. If the test indicates that the satellite 100 cannot follow the prescribed maneuver profile, then the candidate prescribed maneuver profile is too demanding. The profile is adjusted in block 1014 and re-evaluated in steps 1008–1010. The foregoing process is iteratively repeated until a prescribed maneuver profile is found that achieves the least error between itself and the ideal maneuver profile, while still allowing the spacecraft to follow it essentially error free.
An example candidate prescribed yaw attitude profile is given below

ψp=a tan 2└−sin(βp),−cos(βp)sin(α)┘

where ψp is the prescribed yaw attitude (angle), α is the spacecraft orbit angle (i.e. its position) atan2 is the two-argument arctangent function and





  
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                (
                
                  β
                  a
                
                )
              
            
            ⁢
            
              β
              min
            
          
          -
          
            β
            a
          
        
        ]
      
    
  




where βa is the actual current sun beta angle (i.e. the angle between the orbit plane and the sun), βmin is a minimum sun beta angle below which the satellite cannot perfectly follow the ideal profile, and the orbit angle α is normalized (“unwrapped”) so that its value always falls between −π and +π. The exponent n is the parameter of the above equations, and it would be adjusted on each pass of the blocks 1008–1012 to achieve the best performance. Note that for this example there are some constraints on n, such as it must be even and positive. While other forms of the candidate profile may yield better performance, the above technique and candidate profile yields a value of n=10.

CONCLUSION
This concludes the description of the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

## Claims

1. In a system of navigational satellites, at least one of the navigational satellites having a wide beam antenna transmitting a first navigation signal and a spot beam antenna transmitting a second navigation signal, a method of estimating a lever arm correction between the wide beam antenna and the spot beam antenna, comprising the steps of:
receiving a target position of the spot beam over the time period T in the satellite;
generating a predicted position of the at least one satellite over a time period T;
generating predicted satellite attitude over the time period T from a profile of a prescribed satellite maneuver over the time period T;
generating predicted gimbal angles of the spot beam antenna over the time period T from the predicted satellite attitude and the target position of the spot beam over the time period T;
predicting the lever arm correction between the wide beam antenna and the spot beam antenna over the time period T from the predicted gimbal angles of the spot beam antenna over the time period T and the predicted satellite attitude over the time period T; and
transmitting the predicted lever arm correction from the satellite to a navigational receiver.

2. The method of claim 1, wherein the predicted satellite attitude is generated from an angle between an orbital plane of the at least one satellite and the Sun and the predicted position of the at least one satellite within the orbital plane.

3. The method of claim 1, wherein the prescribed satellite maneuver is constrained to limit an attitude angular rate of the navigational satellite to a maximum value.

4. The method of claim 1, wherein the prescribed satellite maneuver is constrained to limit the yaw angular rate of the navigational satellite to a maximum value.

5. The method of claim 1, wherein the prescribed satellite maneuver is constrained to limit an attitude angular acceleration of the navigational satellite to a maximum value.

6. The method of claim 1, wherein the prescribed satellite maneuver is constrained to limit the yaw angular acceleration of the navigational satellite to a maximum value.

7. The method of claim 1, wherein the prescribed satellite maneuver is constrained to limit an error between the predicted satellite attitude and the actual satellite attitude to less than 0.2 degrees.

8. The method of claim 1, wherein the prescribed satellite maneuver is constrained such that when the at least one navigational satellite executes the prescribed maneuver, the at least one navigational satellite can concurrently direct a satellite solar panel to the Sun and the wide beam antenna to the Earth.

9. The method of claim 1, wherein the predicted gimbal angles of the spot beam antenna over time T are further generated from spot beam antenna geometry.

10. The method of claim 1, wherein the target position of the spot beam over the time period T is received from a command station.

11. The method of claim 1, wherein the target position of the spot beam changes during the time period T.

12. The method of claim 1, wherein the step of predicting the lever arm correction between the wide beam antenna and the spot beam antenna over the time period T from the predicted gimbal angles of the spot beam antenna over the time period T and the predicted satellite attitude over the time period T comprises the steps of:
computing a time-varying lever arm correction vector; and
curve fitting the computed time-varying ever arm correction vector.

13. A navigational satellite, comprising:
a first antenna transmitting a first navigational signal;
a steerable second antenna transmitting a second navigational signal;
a satellite receiver, for receiving data from a ground station, the data including a target position of the spot beam over a time period T;
a lever arm correction module, for predicting a lever arm correction between the first antenna and the second antenna, the lever arm correction module comprising:
a satellite position predictor module for generating a predicted satellite position over the time period T;
a satellite attitude predictor module, for generating a predicted satellite attitude over the time period T;
a lever arm prediction module, for predicting the lever arm correction between the first antenna and the second antenna over the time period T from the target position of the spot beam over the time period T and the satellite attitude over the time period T; and
a transmitter for transmitting the predicted lever arm correction to a navigational receiver.

14. The apparatus of claim 13, wherein the predicted satellite attitude is generated from an angle between an orbital plane of the at least one satellite and the Sun and the predicted position of the at least one satellite within the orbital plane.

15. The apparatus of claim 13, wherein the prescribed satellite maneuver constrained to limit an attitude angular rate of the navigational satellite to a maximum value.

16. The apparatus of claim 13, wherein the prescribed satellite maneuver is constrained to limit the yaw angular rate of the navigational satellite to a maximum value.

17. The apparatus of claim 13, wherein the prescribed satellite maneuver constrained to limit an attitude angular acceleration of the navigational satellite to a maximum value.

18. The apparatus of claim 13, wherein the prescribed satellite maneuver is constrained to limit the yaw angular acceleration of the navigational satellite to a maximum value.

19. The apparatus of claim 13, wherein the prescribed satellite maneuver is constrained to limit an error between the commanded satellite attitude and the actual satellite attitude to less than 0.2 degrees.

20. The apparatus of claim 13, wherein the prescribed maneuver is constrained such that when the at least one navigational satellite follows the prescribed maneuver, the at least one navigational satellite can concurrently direct a satellite solar panel to the Sun and the wide beam antenna to the Earth.

21. The apparatus of claim 13, wherein the predicted gimbal angles of the spot beam antenna over time T are further generated from spot beam antenna geometry.

22. The apparatus of claim 13, wherein the target spot beam position changes during the time period T.

23. The apparatus of claim 13, wherein the lever arm prediction module computes the time-varying lever arm correction vector in navigational satellite coordinates, and the lever arm prediction module further comprises a curve fitting module for curve fitting the time-varying lever arm correction vector to produce lever arm correction vector coefficients.

24. In a system of navigational satellites, at least one of the navigational satellites having a wide beam antenna transmitting a first navigation signal and a spot beam antenna transmitting a second navigation signal, an apparatus for estimating a lever arm correction between the wide beam antenna and the spot beam antenna, comprising:
means for receiving a target position of the spot beam over the time period T in the satellite;
means for generating a predicted position of the at least one satellite over a time period T;
means for generating predicted satellite attitude over the time period T from a prescribed satellite maneuver over the time period T;
means for generating predicted gimbal angles of the spot beam antenna over the time period T from the predicted satellite attitude and the target spot beam position over the time period T;
means for predicting the lever arm correction between the wide beam antenna and the spot beam antenna over the time period T from the predicted gimbal angles of the sport beam antenna over the time period T and the predicted satellite attitude over the time period T; and
means for transmitting the predicted lever arm correction from the satellite to a navigational receiver.

25. The apparatus of claim 24, wherein the prescribed satellite maneuver constrained to limit an attitude angular rate of the navigational satellite to a maximum value.

26. The apparatus of claim 24, wherein the prescribed satellite maneuver is constrained to limit the yaw angular rate of the navigational satellite to a maximum value.

27. The apparatus of claim 24, wherein the prescribed satellite maneuver constrained to limit an attitude angular acceleration of the navigational satellite to a maximum value.

28. The apparatus of claim 24, wherein the prescribed satellite maneuver is constrained to limit the yaw angular acceleration of the navigational satellite to a maximum value.

29. The apparatus of claim 24, wherein the prescribed satellite maneuver is constrained to limit an error between the predicted satellite attitude and the actual satellite attitude to less than 0.2 degrees.

30. The apparatus of claim 24, wherein the means for predicting the lever arm correction between the wide beam antenna and the spot beam antenna over the time period T from the predicted gimbal angles of the spot beam antenna over the time period T and the predicted satellite attitude over the time period T comprises:
means for computing a time-varying lever arm correction vector; and
means for curve fitting the time-varying lever arm correction vector to produce lever arm correction vector coefficients.