Patent Description:
Satellites in geostationary orbit (GSO) have the unique property of appearing fixed to an observer on Earth. GSO simplifies many aspects of tracking and acquisition of the satellite for the purposes of establishing communication. For example, a ground station antenna used to communicate with the satellite can be configured once, with a fixed pointing angle.

To maintain the GSO property, however, a satellite must carry out periodic station-keeping thrust maneuvers to counter orbit-disturbing effects primarily caused by the gravity of the moon and the sun. These operations slowly use up the satellite's limited on-board fuel supply.

To prolong a geostationary satellite's service life, satellite operators may opt to allow the satellite to develop an inclination whereby the satellite is allowed to drift above and below the geo-arc, and station-keeping maneuvers are limited to only those required to maintain the satellite's longitudinal slot. In doing so, however, the stationary property of the GSO is no longer maintained.

Approaches to accurately pointing at this category of non-geostationary (typically referred to as inclined-geostationary orbit) satellites range from overly complex algorithms to cumbersome look-up tables.

<CIT> discloses a system and a method for antenna pointing, in which a transmit antenna system having an adjustable boresight transmits a signal exhibiting a far-field pattern including a feature in a polarization of the signal disposed at a fixed position off a beam peak of the far-field pattern of the signal. A receive antenna system scans across the far-field pattern of the signal in the polarization to locate the feature and determine a pointing error of the adjustable boresight therefrom. The principle of the signal scan is performed identically regardless of the orbit type of the satellite, although the orbit type of the satellite will affect the details of the scanning operation as the scan must amount for the satellite position and orientation as well as the relative position of the transmit antenna system boresight on the satellite.

<CIT> discloses a tracking system for tracking an inclined satellite. An antenna tracking controller for generating signals used to position an antenna for tracking the satellite includes a first Kalman filter, which preferably is a short-term Kalman filter, for estimating parameters of a first model of the motion of the satellite and a second Kalman filter, which preferably is a long-term Kalman filter, for estimating parameters of a second model of the motion of the satellite. The second Kalman filter uses the parameters estimated by the first Kalman filter as initial values. The Kalman filters are based on the Slabinski orbital model for inclined satellites and generate predicted azimuth and elevation values for the inclined satellite at a sample time. The tracking system makes a satellite position measurement at the next sample time by peaking on the signal strength of the incoming satellite signal. The predicted azimuth and elevation values are compared to the measured azimuth and elevation values to calculate a beam radial error, and the most recent predictions and the calculated error are then fed back as inputs to the Kalman filter. New azimuth and elevation predictions are then generated , and the process is repeated.

<CIT> discloses a method of communicating via satellite, which includes locating a target satellite in an inclined geosynchronous orbit, pointing an antenna at an aiming point offset from the geostationary arc, and then transmitting a signal to the target satellite in response to the absolute value of the satellite's declination increasing above a threshold. The method further includes suspending the transmission in response to the absolute value of the declination decreasing below the threshold and later resuming the transmission in response to the absolute value of the declination again increasing above the threshold. The threshold may be based on an object which blocks communication with the target satellite when the target satellite is on the geostationary arc. Alternatively, the threshold may be based on avoiding interference with a geostationary satellite sharing a geostationary slot with the target satellite. According to another embodiment, the method of communicating via satellite includes locating a target satellite in an inclined geosynchronous orbit that shares a geosynchronous slot with a geostationary satellite, pointing an antenna at an aiming point offset from the geostationary arc, estimating a maximum boresight Equivalent Isotropic Radiated Power (EIRP) Spectral Density (SD) that complies with a predetermined EIRP SD toward the geostationary satellite, and transmitting a radio frequency signal to the target satellite at a boresight EIRP SD no greater than the maximum EIRP SD. Transmission of the signal may be suspended in response to an angular separation between the target satellite and the geostationary satellite decreasing below a threshold and may then be resumed in response to the angular separation increasing above the threshold.

<CIT> discloses a low-cost, limited-scan-angle, retro-directive antenna for communicating with a geostationary satellite, which features an array feed capable of steering the antenna pattern to track orbital excursions of a geostationary satellite in an orbit inclined with respect to the equator by several degrees. The retro-directive antenna autonomously detects the direction from which a signal is received, and transmits a beam that points back along the same direction. The array feed is used to illuminate a parabolic reflector. Each feed element of the retro-directive antenna is associated with a unique pointing direction of the beam in the far field. As the transmit energy is switched to different feed elements, the far-field beam is scanned, making it possible to track a geostationary satellite in a slightly inclined orbit. The use of a toroidal reflector with multiple linear array feeds spaced in the azimuth direction enables multibeam operation, allowing multiple geostationary satellites, spaced by up to fifteen beam widths in azimuth, to be tracked simultaneously and independently.

A device and method in accordance with the invention provide a practical alternative to keeping a satellite on a geosynchronous inclined orbit viable within its network. More specifically, orbital dynamics of the inclined-GSO are utilized such that only a few orbital parameters are needed to efficiently identify a search path to acquire and track the satellite.

According to the invention, a method for pointing an antenna at a satellite on a geosynchronous inclined orbit as defined in claim <NUM> and a system for locating a satellite on a geosynchronous inclined orbit as defined in claim <NUM> are provided. The dependent claims define preferred and/or advantageous embodiments of the invention,.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

In the annexed drawings, like references indicate like parts or features.

A geostationary satellite must perform station-keeping orbital maneuvers to remain in its assigned orbital position. From a location on Earth, this results in the satellite appearing to be stationary in the sky over a sidereal day. Near the end of the satellite's service life, the restriction on the allowable latitude deviation (effectively its orbit inclination) is relaxed. As a result, from the same location on Earth, tracking an inclined-orbit satellite over its orbital period produces an apparent satellite motion in the approximate form of a figure-<NUM>, or analemma. <FIG> illustrates how three representative geostationary satellites 10a, 10b, 10c and one inclined-orbit satellite 10d would appear to an observer on Earth over the period a sidereal day.

Since fixed-pointing ground station antennas are no longer able to reliably point to such inclined orbit satellites owing to the satellite's apparent movement in the sky over time, given the smaller population of viable ground station antennas capable of maintaining a communications link with them (i.e., antennas with steerable beams) satellite operators are usually forced to reduce the rates they charge to use such satellites. For ground station antennas that have steerable beams, use of inclined satellites can result in significant cost savings, as long as the systems are able to acquire and track these slowly moving inclined satellites.

From an Earth location co-longitudinal with the satellite 10d on a geosynchronous inclined orbit, the analemma will appear to be some variant of a vertically-oriented figure-<NUM> in form. No two analemmas will be identical as other orbital parameters influence the properties. As the Earth-observer moves to any general location on the surface of the Earth (within view of the satellite), the figure-<NUM> will appear to distort. This is simply due to skew angle at which the Earth-observer is viewing the satellite and the geometric projection of this shape onto the surface of a sphere. With a complex model this trajectory can be recreated.

In pointing an antenna at a satellite, one must know the position of the satellite in the sky. The narrower the antenna's beam, the more precise the satellite's position must be known. A common method of determining a satellite's present position is to process and propagate the satellite's Two-Line Element (TLE) data. TLE data is a set of orbital parameters formatted for use specifically with Simplified General Perturbation (SGP) models to predict orbital trajectories. The orbital parameters are produced and maintained by NORAD and NASA. Processing and propagating a satellite's TLE data, however, is a computationally intensive process that requires both algorithmic and administrative solutions to maintain a sufficient level of ground antenna pointing accuracy.

Due to the various simplifications and non-linearities associated with the SGP set of models, the TLE data only accurately predict orbital trajectories for a limited time horizon. (This allowable time horizon being defined differently for each specific application's needs on accuracy). As such, NORAD updates and publishes these TLE data sets daily. TLE/SGP is a very popular model-based method and is considered by many as an industry standard. It is noted, however, that the TLE/SGP model is only one particular model set describing orbital trajectories, and those with other needs/limitations can and do generate their own models.

There are two common approaches to address the problem concerning the limited time horizon of the satellite on a geosynchronous inclined orbit. A first is a table-based approach in which the antenna system points to the satellite and records its position through its <NUM>-hour period (an inclined-geostationary satellite will repeat its trajectory in the sky over its orbital period of one sidereal day). The positional accuracy of this method is proportional to the resolution of this data table. As the resolution increases, the antenna system can perform more frequency updates to the pointing position.

A second approach to address the limited time horizon problem of the satellite on a geosynchronous inclined orbit utilizes model-based (TLE data or another satellite parameter standard) approach. Under this approach, the satellite position is calculated using a mathematical model. A priori updated TLE information must be available as well as an accurate measure of UTC time (Coordinated Universal Time). The Coordinated Universal Time is simply the international standard for time-keeping and coordination. It is especially critical to any model propagation method (like TLE/SGP) since numerical propagation is relative to some agreed upon starting time as well as the agreed upon time it is "now". Providing access to this coordinated clock is a necessary and non-trivial challenge.

The table-based approach may be the simplest to implement, but still has many challenges. Before the system can use the look-up table to find the coordinates to point to, the table must be populated by the correct values. This implies that some other method of determining satellite position must be first used before the table method can be used. Once the table is filled, there must be some mechanism to correct the small variances in position as the satellite will not be following the 'table values' precisely. Storing the look-up table in memory is also not a negligible aspect of implementation. Storage for several satellites, numerical precision of the values stored, corruption of data, are some of the implementation challenges associated with a table-based approach.

On the opposite side of complexity, the model-based approach can determine the present position of the satellite in its orbital period. Accuracy of the predicted position is limited by accuracy in each of the parameters, complexity of the model, and numerical precision in the processor, among others. Processing and propagating the TLE data is a computationally resource intensive task, especially to perform in real-time on a microprocessor as opposed to offline (or ahead of time) by a larger processor without the constraints of real-time system operations. The parameters of the TLE data must also be updated frequently, without which the prediction error in position will grow non-linearly, and depending on the precision required from the antenna system, the propagated model may fall out of acceptable range in a matter of days. Upkeep of this TLE data (or any other form of math model) adds another source of complexity to implementation. In addition, the system must have access to UTC time.

In accordance with the present invention, a system and method are provided that leverage the bounded dynamics of a geostationary satellite with non-negligible inclination as a means to search for and acquire the satellite's location anywhere along its trajectory. Moreover, such search can be performed in a timely fashion using a minimum number of its identifying properties, without the need for TLE data processing and upkeep.

Referring to <FIG>, illustrated is an exemplary system <NUM> for locating and pointing an antenna at a satellite 10d on a geosynchronous inclined orbit. As shown in <FIG>, the satellite 10d is technically no longer stationary, and instead has an analemma <NUM> that, from a viewpoint on Earth, appears as a figure-<NUM> pattern over a period of twenty-four hours. Thus, the exact location of the satellite 10d is not known without additional information. As will be described in more detail below, in accordance with the invention the position of the satellite 10d is precisely located using minimal computational power and without the use of TLE data or other complex math models.

With continued reference to <FIG>, the system <NUM> includes an antenna <NUM> for communicating with the satellite 10d. The antenna <NUM> may be any conventional antenna utilized for satellite communications, such as, for example, a reflector antenna, a horn antenna, phased array antenna, and the like. A steering device or motive device <NUM> is operatively coupled to the antenna <NUM> for pointing the antenna <NUM> at various coordinates. The steering device <NUM>, for example, may be in the form of an electromechanical actuator that includes a motor 16a and corresponding drive train 16b that can change one or more of the azimuth, elevation and polarization angles of the antenna <NUM>. The steering device <NUM> may also be in the form of phase shifters, metamaterials, or other devices that enable pointing of a phased array without the need for electromechanical actuation. A controller <NUM> is communicatively coupled to the antenna <NUM> and the steering device <NUM>. The controller <NUM> provides commands to the steering device <NUM> to point the antenna <NUM> at specified coordinates, receives position data indicating the current pointing direction of the antenna <NUM>, and provides data to and receives data from the antenna <NUM>.

In accordance with the invention, an acquisition command triggers the acquisition of the satellite's location. More particularly, a search grid is determined that includes a plurality of discrete points that are uniformly spaced apart, the search grid based on known constraints of the system. The system constraints include, for example, the longitudinal coordinate in which the satellite 10d resides (the longitudinal coordinate is known and does not significantly vary over the satellite's useful life), the beam width of the antenna <NUM> that is searching for the satellite 10d (the beam width is also known), and the analemma <NUM> of the satellite 10d of interest. The analemma <NUM> of the satellite 10d for its full orbital period can be determined, for example, using the minimum critical orbital parameters for the satellite of interest (e.g., the longitudinal coordinate and the maximum orbit inclination). For example, the analemma can be modeled using a parametric pair of equations that require only the longitudinal coordinate and maximum orbit inclination of the satellite. Other models with different precision, accuracy, and/or phasing (time dependent) requirements exist, but add unneeded complexity. In the idealized figure-<NUM> form, the satellite's analemma <NUM> is symmetric about the Clarke belt <NUM>.

In addition to the satellite parameters referenced above, parameters of the interrogating earth terminal, such as the terminal longitude, latitude, altitude, roll, pitch and heading, may be needed in order to point the antenna in the correct location. These parameters are known or can be readily determined using conventional means.

The search grid can be determined, for example, by following a centerline <NUM> of the analemma <NUM> that extends from one end 13a of the analemma to the other end 13b, as can be best seen in <FIG>. From a general location on Earth, this centerline path will not necessarily resemble a straight line perpendicular to the Clarke belt <NUM>. For pointing purposes, warping due to the spherical projection and skew angle should be taken into account.

The antenna <NUM> then is moved to each point on the search grid and a scan is performed for the satellite of interest. If a response to the scan is received, the power level of the RF response signal is recorded for that point. The power level of the received signal can be determined using conventional methods known to the person skilled in the art. The coordinates for the point(s) on the search grid corresponding to peak RF power are identified and the antenna <NUM> is moved to the location corresponding to such peak power, which corresponds to the location of the satellite 10d on a geosynchronous inclined orbit. Thus, the acquisition process of the satellite on a geosynchronous inclined orbit does not require computationally expensive propagation routine.

As part of an integrated system, a modem or other system controller can pass the relevant satellite parameters (satellite nominal longitude and max inclination angle) to the controller <NUM>. Some or all subsequent calculations and acquisition commands in accordance with the method can be performed on the controller <NUM>.

The method in accordance with the invention is advantageous in that it can acquire the present position of the inclined satellite quickly (on the order of a few seconds) with a minimum number of satellite-identifying parameters. An expansive look-up table, complex calculations/model propagation, and knowledge of UTC time are not required. In other words, the complexity of predicting the instantaneous position of an inclined satellite 10d can be greatly reduced. More particularly, in understanding that the analemma is bounded by physics and station-keeping regulations, the proposed method achieves the desired acquisition in a more straightforward and efficient method using the Earth-station location and the satellite's nominal longitude and maximum orbit inclination. Moreover, the method in accordance with the invention enables vehicle-based in-flight entertainment/connectivity companies to expand their satellite fleet to also include lower cost satellites on a geosynchronous inclined orbit.

Referring now to <FIG>, illustrated is a flow chart depicting steps of an exemplary method of acquiring a location of a satellite on a geosynchronous inclined orbit in accordance with the present invention. Variations to the illustrated method is possible and, therefore, the illustrated embodiment should not be considered the only manner of carrying out the techniques that are disclosed herein. Also, while <FIG> shows a specific order of executing functional logic blocks, the order of executing the blocks may be changed relative to the order shown and/or may be implemented in an object-oriented manner or a state-oriented manner. In addition, two or more blocks shown in succession may be executed concurrently or with partial concurrence. Certain blocks also may be omitted. The exemplary method may be carried out by executing code stored by an electronic device, for example. The code may be embodied as a set of logical instructions that may be executed by a processor. Therefore, the methods may be embodied as software in the form of a computer program that is stored on a computer readable medium, such as a memory.

The method according to <FIG> can be initiated upon an acquisition command being issued. The acquisition command may be issued by any device that seeks to locate the satellite of interest. Beginning at step <NUM>, the controller <NUM> obtains the analemma <NUM> of the satellite on a geosynchronous inclined orbit. In one embodiment, the analemma <NUM> of the satellite 10d on a geosynchronous inclined orbit may be determined in advance and stored in memory of the controller <NUM>. The controller <NUM> then can simply retrieve the analemma <NUM> from memory upon receiving an acquisition command. In another embodiment, the analemma <NUM> of the satellite 10d on a geosynchronous inclined orbit may be determined in real time upon receiving the acquisition command. In obtaining the analemma <NUM>, the controller <NUM> may use the minimum critical orbital parameters for the satellite 10d, e.g., based on a nominal longitude of the satellite 10d on a geosynchronous inclined orbit and a maximum orbit inclination of the satellite <NUM> on a geosynchronous inclined orbit, the analemma <NUM> can be inferred. For example, the analemma for inclined geo-stationary satellites can be represented by a model which is simplified by the orbital properties of these specific satellites. First, the orbital inclination, which is known by the satellite operator, can be used to determine the "height" of the analemma. The satellite's longitude is also used to know what part of the sky the analemma must be superimposed on, in relation to the Earth observer's position. Lastly, the width of the analemma is bounded by physics, and thus forms a parametric set of equations that describes the X-Y coordinates of the analemma over the sidereal day, however this invention obviates the need for using the width of the analemma in acquiring the desired inclined GEO satellite.

Next at step <NUM>, the analemma <NUM> is analyzed to determine its characteristics. In this regard, the characteristics may include a centerline of the analemma <NUM> and/or warping in the analemma <NUM> due to spherical projection. Warping may be detected, for example, when the analemma <NUM> does not exhibit a symmetrical pattern with respect to the Clarke belt <NUM>. If warping is present, it may be compensated for by performing coordinate transformations.

Next at step <NUM> a search grid is constructed, the search grid including a plurality of discrete points defining a line <NUM> that intersects the analemma <NUM>. Preferably, the line <NUM> defined by the plurality of discrete points bisects the analemma <NUM> along a longitudinal axis of the analemma <NUM> (e.g., along a centerline of the analemma). In constructing the search grid, the beam width of the antenna <NUM> should be taken into account such that there are no dead zones in the scanned space. More specifically, the spacing of a plurality of points that form the grid can be based on the beam width of the antenna <NUM> such that each point is spaced apart from an immediately adjacent point by no more than the beam width of the antenna. In one embodiment, illustrated in <FIG>, the spacing of points <NUM> is selected such that when the antenna <NUM> is pointed at a first point 22a and then pointed at an immediately adjacent point 22b, an outer scan region of the antenna beam width <NUM> for the first point 22a and an outer scan region of the antenna beam width <NUM> for the second point 22b are tangent to each other. In another embodiment, illustrated in <FIG>, the spacing of points <NUM> is selected such that when the antenna <NUM> is pointed at a first point 22a and then pointed at an immediately adjacent second point 22b, a scan region of the beams corresponding to each point at last partially overlap with each other.

Moving to step <NUM>, the controller <NUM> selects one point <NUM> of the grid of points, and at step <NUM> the controller <NUM> commands the steering device <NUM> to point the antenna <NUM> at the selected point <NUM>. At step <NUM> a scan is performed while the antenna <NUM> is pointed at the selected point in space. For example, the controller <NUM> may command the antenna <NUM> to transmit a query to the satellite 10d and then listen for a response from the satellite 10d. If a response is received, the controller <NUM> records a power level of the RF signal response. In the case where a response is not received, the RF power level may be recorded as zero. Next at step <NUM> the controller <NUM> determines if every point <NUM> on the grid has been scanned. If there are more points to scan, the method moves to step <NUM> where a different point <NUM> is selected that has not yet been scanned and then the method repeats steps <NUM>-<NUM> until each point <NUM> of the grid has been scanned.

Moving back to step <NUM>, if all points have been scanned, then the method moves to step <NUM> where it is determined which point of the plurality of points produced a response to the query having the highest power level. The point <NUM> associated with the highest power level can be identified, for example, by comparing the power level associated with each point to determine which point is associated with the highest power level. In the event that two points are each associated with a response that exhibits the same power level, a midpoint between the two points can be identified as the location of the satellite 10d. Once the point corresponding to the highest power level is identified, the controller <NUM> then commands the steering device <NUM> to point the antenna <NUM> at the identified point <NUM> (or location between points), as indicated at step <NUM>.

Due to the fact that a line of points is used to approximately find the current location of the satellite (which is constantly moving along the analemma), the point on the grid with the highest power level may not be the "exact" location of the satellite. To further optimize the location at which the antenna is pointed, RSSI may be performed on the collected data to refine the determined location of the satellite and maintain accurate tracking once found.

Accordingly, the device and method in accordance with the invention can quickly identify the location of a satellite on a geosynchronous inclined orbit without requiring significant computational power and without using complex mathematical models, such as TLE data sets.

The above-described method <NUM> (referred to as an acquisition function/module) may be performed by the controller <NUM>, an example of which is illustrated in <FIG>. The controller <NUM> may be any type of electronic device, examples of which include one or more integrated circuits, discrete circuits, ASICs, processors, or combination thereof. The controller <NUM> includes the acquisition function/module configured to carry out the acquisition method <NUM> described herein.

The controller <NUM> may include a primary control circuit <NUM> that is configured to carry out overall control of the functions and operations of the system. The control circuit <NUM> may include a processing device <NUM>, such as a central processing unit (CPU), microcontroller or microprocessor. The processing device <NUM> executes code stored in a memory (not shown) within the control circuit <NUM> and/or in a separate memory, such as the memory <NUM>, in order to carry out operation of the controller <NUM>. For instance, the processing device <NUM> may execute code that implements the acquisition function <NUM>. The memory <NUM> may be, for example, one or more of a buffer, a flash memory, a hard drive, a removable media, a volatile memory, a non-volatile memory, a random access memory (RAM), or other suitable device. In a typical arrangement, the memory <NUM> may include a non-volatile memory for long term data storage and a volatile memory that functions as system memory for the control circuit <NUM>. The memory <NUM> may exchange data with the control circuit <NUM> over a data bus. Accompanying control lines and an address bus between the memory <NUM> and the control circuit <NUM> also may be present.

The controller <NUM> may further include one or more input/output (I/O) interface(s) <NUM>. The I/O interface(s) <NUM> may be in the form of typical I/O interfaces and may include one or more electrical connectors. The I/O interface(s) <NUM> may form one or more data ports for connecting the controller <NUM> to another device (e.g., a computer-controlled device) or an accessory via a cable. The I/O interface(s) <NUM> may also include one or more of analog input/output ports for receiving analog data from or providing analog data to other devices, such as the steering device <NUM>. The I/O interface(s) <NUM> may further include one or more digital input/output for controlling operation of the steering device <NUM> and/or antenna <NUM> and for receiving status data therefrom. Further, operating power may be received over the I/O interface(s) <NUM> from power supply unit (PSU) <NUM> within the controller <NUM>.

The controller <NUM> also may include various other components. For instance, a system clock <NUM> may clock components such as the control circuit <NUM> and the memory <NUM>. A local wireless interface <NUM>, such as an infrared transceiver and/or an RF transceiver may be used to establish communication with a nearby device, such as a radio terminal, a computer or other device.

Claim 1:
A method (<NUM>) for pointing an antenna (<NUM>) at a communications satellite on a geosynchronous inclined orbit (10d), comprising:
a) obtaining an analemma (<NUM>) of the satellite (10d) based on minimum critical orbit parameters;
b) determining a plurality of points (<NUM>) that are to be searched, wherein determining the plurality of points (<NUM>) comprises selecting points that follow a centerline (<NUM>) of the analemma (<NUM>);
c) selecting one point (22a) of the plurality of points (<NUM>);
d) pointing the antenna (<NUM>) at the selected point (22a);
e) while the antenna (<NUM>) is pointing at the selected point (22a), querying the satellite (10d) and recording an RF power level of a response to the query received from the satellite (10d);
f) selecting another point (22b) of the plurality of points (<NUM>);
g) repeating steps d-f for each of the plurality of points (<NUM>);
h) determining which point of the plurality of points (<NUM>) produced a response to the query having the highest power level; and
i) pointing the antenna (<NUM>) at the point having the highest power level.