Satellite attitude determination

Abstract: The attitude (yaw, pitch and roll) of a home satellite is determined using position signals, derived from a GPS receiver on the home satellite and GPS receiver on other satellites, that are transmitted between the home satellite and the other satellites by an intersatellite communications link using gimbaled transceivers on the satellites. The home satellite determines its attitude from the gimbal elevation and azimuth and the position vectors between the satellites.

Description: TECHNICAL FIELD This invention relates to satellite attitude control, in particular, satellite attitude determination using GPS and intersatellite line of sight communications. BACKGROUND OF THE INVENTION The attitude (roll, pitch and yaw) of an orbiting satellite is controlled to maintain a correct orientation of devices, such as solar panels to the sun and antenna to the earth or space craft. The common way to sense attitude is with object sensors, most commonly star sensors. Star trackers are expensive complicated devices. U.S. Patent No. 5,831,572 describes an apparatus that measures attitude and includes one or more star sensors. The apparatus is used on a satellite and has nonaligned antennas for receiving radio signals from a plurality of satellites in a satellite global positioning system (GPS). The apparatus processes the radio signals received by the various antennas, measures their phase differences, and computes attitude on the basis of these phase differences. These computations may be repeatedly calibrated on the basis of measurements performed by the star sensor during periods when it provides usable data. Using GPS makes it possible to know the position of the satellite, its velocity vector, and the time. DISCLOSURE OF THE INVENTION The present invention provides a method as defined in Claim 1. The method may include the features of any one or more of dependent Claims 2 to 14. The present invention also provides a satellite as defined in Claim 15. The satellite may include the features of any one or more of dependent Claims 16 to 30. An object of the present invention is to determine the attitude of a satellite without on board dedicated attitude determining devices, such as star trackers. According to the invention, the position of two or more satellites in an orbit is transmitted to one satellite. The one satellite determines the azimuth and elevation to the satellites transmitting their position. The one satellite determines the difference in position in the orbit between it and each of the other satellites using its own position and the transmitted position. It then uses that difference and the azimuth and elevation to each of the other satellites to determine its yaw, pitch and roll. According to the present invention, satellites determine their position using an on-board GPS receiver and transmit the position to the one satellite by a line of sight (LOS) intersatellite communications link, such as a laser communications system. The one satellite has receiver for each satellite that is oriented in two axis to establish the link and the orientation of the receiver provides the azimuth and elevation to the satellite providing its own position information. A feature of the present invention, is that it can be deployed in satellite systems that already use an on board GPS and that transmit and receive data, telephony, over a line sight communications link to provide uniform satellite to earth links. Another feature is that the invention eliminates the need for star trackers and considerably lowers the cost of each satellite by using information and equipment already on the satellite for other purpose, i.e., the GPS and communications link hardware. Other objects, benefits and features will become more apparent in light of the following discussion of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a function diagram of a system embodying the present invention. Fig. 2 shows a typical satellite and the two dimensional range of motion of an intersatellite line of sight communciations link to other satellites. BEST MODE FOR CARRYING OUT THE INVENTION Fig. 1 shows a plurality of satellites S n , assumed to be in an orbit. Each satellite contains its own on board GPS receiver and an intersatellite communication link ISL, such as a laser transceiver system, for relaying data and other information between the satellites that is transmitted from one location to another location on the earth. Each ISL comprises a plurality of gimbaled communication link input/output devices 10, such as a mirror or telescope with a laser based system. The devices 10 are located on the surface of the satellite, as depicted in Fig. 2. The input/output devices 10 can be pointed at a satellite, i.e., oriented in two dimensions (elevation and azimuth) relative to the satellite to establish communications with another satellite. It is assumed that the orientation can be resolved with a resolver to produce signals El n and Az n . In practice each satellite S n has a plurality of these lens or directional antennas 10 around its surface, as shown only for satellite S 0 enabling communication with satellites three dimensionally simply by pointing the input/output or lens to make the communications connection without rotating the satellite, enabling the satellites attitude relative to the earth to be controlled independently for communications and orientation of solar panels. Each satellite comprises what is shown for satellite 0, which for this discussion is sometimes called the "home" satellite, the other satellites being the "reference" satellites. As stressed previously, in actuality each satellite is a home satellite in determining its own attitude using the GPS position of the other or reference satellites and the line of sight bearing to the reference satellite, which is provided by the orientation of the lens 10. Each satellite transmits its GPS position, line of sight (LOS), by sighting another satellite with a lens 10. Using satellite S0 as example, it receives, signals Pos 1-n , by an associated lens 10 for each satellite and likewise sends its GPS position back to the other satellites using the same lens. Each lens 10 produces an elevation and azimuth signal Az n , El n , that represents the LOS to the other satellite. Those signals are applied to a signal processor 12, which is programmed, as explained below, to perform a coordinate transformation that yield the bearing B 1-n to each satellite. The GPS on satellite S 0 produces the local or home GPS position signal, Pos 0. which, in addition to being applied to the lens 10 for transmission to the other satellites, is subtracted from received positions for the other satellites. The difference between the GPS position of the home satellite and the position of the remote or what may be called "reference" satellite, is position vector, signal r n , in earth or GPS reference coordinates to the reference satellite. The difference signal is applied to the signal processor 12 to produce attitude signals (yaw, pitch and roll) that are supplied to an attitude control system 14 that uses those signals to control the satellite's attitude devices, such as momentum wheels, to regulate the satellite's attitude. The signal processor 12 is programmed to carry out the following attitude determination process using the signals generated as described above. (Well known, signal processor components such as a central processing unit, memories, clocks and such are not shown.) The angles El n and Az n specify the direction to the target or reference satellite. This Cartesian coordinate conversion relates the gimbal azimuth (Az) and elevation (El) angle deflections to the line of sight (LOS) unit vector ( b and ): The direction to target satellites is also specified in an earth referenced frame by the position vector r i , which represents the difference between the GPS measured position of satellite 1 and satellite 0. The earth referenced position vectors r i are converted to unit vectors by dividing each by its vector magnitude: r i = r i r i The direction cosine matrix A that defines the spacecraft attitude is obtained by first forming two matrices - R and B. The matrix R is formed by concatenating the n earth referenced unit vectors: R = [ r 1 r 2 r 3 ... r n ] The number of unit vectors n is determined by the number of laser communication ("com") systems resident on a satellite. A minimum of 2 are required for attitude determination. Similarly, the matrix B is formed by concatenating the body referenced unit vectors (both R and B are 3xn matrices): Next, the matrices R and B are decomposed using the singular value decomposition (SVD). The singular value decomposition is a mathematical tool often used for analyzing a mapping from one vector space (such as an earth referenced vector) into another vector space (such as a body reference vector). The SVD of matrices R and B are given by the expressions R = U 1 Σ 1 V ' / 1 and B = U 2 Σ 2 V ' / 2. where U 1 and U 2 are 3x3, Σ 1 and Σ 2 are 3xn, and V ' / 1 and V ' / 2 are nxn. The matrices U 1 , U 2 , V ' / 1, and V ' / 2 are unitary (i.e. the columns are orthogonal and have unit magnitude). The matrices Σ 1 and Σ 2 are diagonal with real non-negative elements. The elements of Σ 1 and Σ 2 are known as singular values. After the R and B matrices have been formed and decomposed using the SVD, the direction cosine matrix A can be obtained by multiplying the unitary matrix U 2 with the transpose of the unitary matrix U 1 . A = U 2 U ' 1 The attitude or direction cosine matrix A has three row and three columns as shown in equation 6. The elements of the attitude matrix can be used to derive the satellite roll, pitch, and yaw angles. The satellite pitch attitude  is calculated using equation 7. The satellite roll and yaw computations are defined by equations 8 and 9 respectively. Alternatively, attitude can be described using quaternions. Quaternions are employed to avoid computational singularites when the pitch attitude  approaches 90 degrees. An attitude quaternion q contains four parameters where s , u , v , w are defined in terms of the attitude matrix A according to equations 11-14. s = 1 2 1 + A 11 + A 22 + A 33 u = 1 4 ( A 23 - A 32 ) s v = 1 4 ( A 31 - A 13 ) s w = 1 4 ( A 12 - A 21 ) s

Claims: A method of determining attitude of a first satellite (Satellite 0) comprising: receiving at the first satellite (Satellite 0) a first GPS position signal (s 1 ) from a second satellite (Satellite 1) and a second GPS position signal (s 2 ) from a third satellite (Satellite 2), wherein the first and second GPS position signals (s 1 and s 2 ) indicate the locations (POS 1 and POS 2 ) of the second and third satellites (Satellite 1 and Satellite 2) with respect to a reference frame; determining a first elevation (EL 1 ) and a first azimuth (AZ 1 ) from the first satellite (Satellite 0) to the second satellite (Satellite 1) based on a line of sight between the first and second satellites (Satellite 0 and Satellite 1) and a second elevation (EL 2 ) and a second azimuth (AZ 2 ) from the first satellite (Satellite 0) to the third satellite (Satellite 2) based on a line of sight between the first and third satellites (Satellite 0 and Satellite 2); and, determining the attitude of the first satellite (Satellite 0) using the first and second elevations (EL 1 and EL 2 ) the first and second azimuths (AZ 1 and AZ 2 ) the first and second GPS position signals (s 1 and s 2 ), and a GPS position signal indicating the location of the first satellite (Satellite 0) with respect to the reference frame. The method of claim 1 wherein the receiving at the first satellite (Satellite 0) of a first GPS position signal (s 1 ) from a second satellite (Satellite 1) and a second GPS position signal (s 2 ) from a third satellite (Satellite 2) comprises receiving the first and second GPS position signals (s 1 and s 2 ) by the first satellite (Satellite 0) over a line of sight communications link between the first satellite (Satellite 0) and the second and third satellites (Satellite 1 and Satellite 2). The method of claim 2 wherein the line of sight communications link comprises first and second optical receivers (10) mounted in the first satellite (Satellite 0), wherein the first optical receiver (10) is pointed at the second satellite (Satellite 1), and wherein the second optical receiver (10) is pointed at the third satellite (Satellite 2). The method of claim 3 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The method of claim 3 wherein the first elevation and the first azimuth are determined from an orientation of the first optical receiver, and wherein the second elevation and the second azimuth are determined from an orientation of the second optical receiver. The method of claim 5 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The method of claim 1 wherein the determining of the attitude of the first satellite comprises determining the attitude of the first satellite based on differences between the first and second GPS position signals and the GPS position signal indicating the location of the first satellite. The method of claim 7 wherein the receiving at the first satellite of a first GPS position signal from a second satellite and a second GPS position signal from a third satellite comprises receiving the first and second GPS position signals by the first satellite over a line of sight communications link between the first satellite and the second and third satellites. The method of claim 8 wherein the line of sight communications link comprises first and second optical receivers mounted in the first satellite, wherein the first optical receiver is pointed at the second satellite, and wherein the second optical receiver is pointed at the third satellite. The method of claim 9 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The method of claim 9 wherein the first elevation and the first azimuth are determined from an orientation of the first optical receiver, and wherein the second elevation and the second azimuth are determined from an orientation of the second optical receiver. The method of claim 11 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The method of claim 1 wherein the determining of attitude of the first satellite comprises determining yaw, pitch and roll of the first satellite. The method of claim 13 further comprising controlling the attitude of the first satellite based on the determined yaw, pitch and roll. A satellite (Satellite 0) comprising: a surface; a first receiver (10) mounted on the surface that receives a first GPS position signal (s 1 ) over a first line of sight communication link from a first remote satellite (Satellite 1), wherein the first GPS position signal (s 1 ) indicates the location of the first remote satellite (Satellite 1) with respect to a reference frame, and wherein the first light of sight produces a first elevation EL 1 and a first azimuth AZ 1 from the surface to the first remote satellite (Satellite 1); a second receiver (10) mounted on the surface that receives a second GPS position signal (s 2 ) over a second line of sight communication link from a second remote satellite (Satellite 2), wherein the second GPS position signal (s 2 ) indicates the location of the second remote satellite (Satellite 2) with respect to the reference frame, and wherein the second light of sight produces a second elevation (EL 2 ) and a second azimuth (AZ 2 ) from the surface to the second remote satellite (Satellite 2); and a signal processor (12), GPS unit, and summers mounted on the surface, wherein the signal processor (12), and summers determine the attitude of the satellite using the first and second elevations (EL 1 and EL 2 ), the first and second azimuths (AZ 1 and AZ 2 ), the first and second GPS position signals (s 1 and s 2 ), and a GPS position signal provided by the GPS unit indicating the location of the satellite with respect to the reference frame. The satellite of claim 15 wherein the first and second receivers (10) comprise corresponding first and second optical receivers mounted in the surface, wherein the first optical receiver is pointed at the first remote satellite (Satellite 1), and wherein the second optical receiver is pointed at the second remote satellite (Satellite 2). The satellite of claim 16 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The satellite of claim 16 wherein the first and second optical receivers comprise corresponding first and second laser receivers which are orientable in two dimensions. The satellite of claim 16 wherein the first elevation and the first azimuth are determined from an orientation of the first optical receiver, and wherein the second elevation and the second azimuth are determined from an orientation of the second optical receiver. The satellite of claim 19 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The satellite of claim 19 wherein the first and second optical receivers comprise corresponding first and second laser receivers which are orientable in two dimensions. The satellite of claim 15 wherein the signal processor (12) determines the attitude of the satellite based on differences between the first and second GPS position signals and the GPS position signal indicating the location of the satellite. The satellite of claim 22 wherein the first and second receivers comprise corresponding first and second optical receivers mounted in the surface, wherein the first optical receiver is pointed at the first remote satellite, and wherein the second optical receiver is pointed at the second remote satellite. The satellite of claim 23 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The satellite of claim 23 wherein the first and second optical receivers comprise corresponding first and second laser receivers which are orientable in two dimensions. The satellite of claim 23 wherein the first elevation and the first azimuth are determined from an orientation of the first optical receiver, and wherein the second elevation and the second azimuth are determined from an orientation of the second optical receiver. The satellite of claim 26 wherein the first and second optical receivers comprise corresponding first and second optical receivers which are orientable in two dimensions. The satellite of claim 26 wherein the first and second optical receivers comprise corresponding first and second laser receivers which are orientable in two dimensions. The satellite of claim 15 wherein the signal processor (12) determines yaw, pitch and roll of the satellite. The satellite of claim 29 wherein an attitude control system (14) controls the attitude of the satellite based on the yaw, pitch and roll.

Inventors: RING JEFFREY R, RING, JEFFREY R.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1998
Main classification: G05D1
IPC codes: B64G1, G01S5