Exclusion zone guidance method for spacecraft

• US 7,795,566 B2
• Filed: 03/27/2008
• Issued: 09/14/2010
• Est. Priority Date: 03/29/2007
• Status: Active Grant

• Current Assignee: Sierra Space Corporation
• Original Assignee: Spacedev Inc. (Sierra Nevada Co. LLC)
• Inventors: Koenig, Jesse D.
• Primary Examiner(s): Gregory; Bernarr E
• Application Number:
  US12/079,928
• Publication Number:
  US 20090008495A1
• Time in Patent Office: 901 Days
• Field of Search: 244 31- 33, 2441581-1584, 244/158.8, 244/172.4, 244/172.5, 244/75.1, 244/76.R, 244/175, 244/189, 244/190, 2441585-1587, 701 1- 28, 701200-226, 701/50, 701/300, 701/301, 89/11.1, 340/945, 340/963
• US Class Current: 244/3.15
• CPC Class Codes: B64G 1/244 Spacecraft control systems
• CPC Class Codes: F41G 7/2213 maintaining the axis of an orientable seeking head pointed at the target, e.g. target seeking gyro
• CPC Class Codes: F41G 7/2253 Passive homing systems, i.e. comprising a receiver and do not requiring an active illumination of the target
• CPC Class Codes: F41G 7/2293 using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 3/781 Details
• CPC Class Codes: G01S 3/786 the desired condition being maintained automatically

Abstract

Systems and methods for are adapted for automatic implementation of exclusion zone avoidance for target-tracking vehicles, such as spacecraft. The systems and methods are configured to monitor pointing commands (commanded attitude and angular rates) generated for target tracking, and modify these commands as necessary to avoid pointing a boresight into an exclusion zone.

Claims

1. A method for guiding the attitude of a body that is performing a function involving a rotation of the body, the method comprising:

identifying angular dynamic limits of the body, wherein the angular dynamic limits include maximum angular rate and maximum angular acceleration of the body;

identifying a particular body axis of concern defined by a physical element of the body;

determining that, if the body follows an originally desired attitude trajectory, the projection of the body axis of concern will intersect an angular exclusion zone comprised of an angular region within which it is prohibited to point the body axis of concern, wherein the angular region is defined by intervals in two angular dimensions, including azimuth and elevation, as measured from a reference point about which the body rotates; and

modifying one or more attitude commands that correspond to the originally desired attitude trajectory to create an actually commanded attitude trajectory for the body such that the body axis of concern is steered around the exclusion zone, so as to avoid any incursion of the body axis of concern with the exclusion zone, wherein the attitude commands are modified in such a way that associated angular rates and angular accelerations are within the dynamic limits of the body, and wherein the attitude commands are modified in such a way as to minimize the amount of time for which the actually commanded attitude trajectory deviates from the originally desired attitude trajectory.

2. A method as in claim 1, wherein the body is a spacecraft or a component of a spacecraft.

3. A method as in claim 1, wherein the function being performed by the body is to track a separate target object by maintaining the body axis of concern pointing at the target object so as to maintain an intersection of the projection of the body axis of concern with the target object.

4. A method as in claim 1, wherein the body axis of concern is coaxial with the boresight of a sensing instrument that is fixed to the body.

5. A method as in claim 1, wherein the exclusion zone is an angular region surrounding a vector, wherein the vector projects from the point about which the body rotates to a center of the sun.

6. A method as in claim 1, wherein the attitude commands are modified to steer the projection of the body axis of concern around a predetermined angular pad surrounding the exclusion zone.

7. A method as in claim 1, wherein the attitude commands are modified based on a axis {circumflex over (ω

)}_c and a magnitude ω

_c of a originally desired cross-boresight rate of the body, calculated as;

$\begin{array}{c}\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}=<br><br>\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}-\hat{b}*\left(\hat{b}·<br><br>\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}\right);<br><br>\\ {\mathrm{\omega <br><br>}}_c=<br><br><br><br>{\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}}_c<br><br>;<br><br>\\ {\widehat{\mathrm{\omega <br><br>}}}_c=<br><br>\frac{{\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}}_c}{{\mathrm{\omega <br><br>}}_c},\end{array}$

where {right arrow over (ω

)} is the body'"'"'s originally desired angular rate command, and {circumflex over (b)} is the unit vector describing the originally desired pointing of the body axis of concern.

8. A method as in claim 7, wherein the attitude commands are modified by a function of the originally desired pointing of the body axis of concern relative to the angular direction toward a center of the exclusion zone, the method comprising:

establishing an angular coordinate system based on coordinate axes {circumflex over (X)} and Ŷ

, with Ŷ

defined as;

$\hat{Y}=\frac{{\hat{a}}_{\mathrm{sb}}-{\widehat{\mathrm{\omega <br><br>}}}_c*\left({\hat{a}}_{\mathrm{sb}}·<br><br>{\widehat{\mathrm{\omega <br><br>}}}_c\right)}{<br><br>{\hat{a}}_{\mathrm{sb}}-{\widehat{\mathrm{\omega <br><br>}}}_c*\left({\hat{a}}_{\mathrm{sb}}·<br><br>{\widehat{\mathrm{\omega <br><br>}}}_c\right)<br><br>}$

where Ŷ

may be altered from this formulation to prevent its changing direction by more than a specified angle from one time step to the next, and where

${\hat{a}}_{\mathrm{sb}}=\frac{\hat{s}\times <br><br>\hat{b}}{<br><br>\hat{s}\times <br><br>\hat{b}<br><br>}$

where ŝ

is a unit vector describing the angular direction from the point about which the body rotates, to the center of the exclusion zone;

calculating a Ŷ

coordinate, Y₀, of the originally desired pointing of the body axis of concern as;

Y₀=cos<sup>−

1</sup>(î

·

ŝ

)

where if

({right arrow over (ω

)}_i×

î

)·

(î

×

ŝ

)<

0, Y₀₌₋

Y₀,

and where

$\hat{i}=\frac{\left({\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}}_i\times <br><br>\hat{s}\right)\times <br><br>{\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}}_i}{<br><br>\left({\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}}_i\times <br><br>\hat{s}\right)\times <br><br>{\stackrel{\to <br><br>}{\mathrm{\omega <br><br>}}}_i<br><br>},<br><br>\mathrm{where}<br><br><br><br>\mathrm{if}<br><br><br><br>\hat{i}·<br><br>\hat{s}<<br><br>0,<br><br>\hat{i}=-\hat{i},<br><br>\mathrm{and}$

where {right arrow over (ω

)}_i=Ŷ

×

{circumflex over (b)};

calculating an {circumflex over (X)} coordinate, X₀, of the originally desired pointing of the body axis of concern as;

X₀=cos<sup>−

1</sup>(î

·

{circumflex over (b)}).

9. A method as in claim 8, wherein if the originally desired projection of the body axis of concern is either inside of the exclusion zone or outside of the exclusion zone but moving angularly toward an incursion of the exclusion zone, the attitude commands are modified by applying an angular rotation of magnitude Y_m about the axis Ŷ

to the originally desired attitude, where Y_m is the result of applying a low-pass filter over time to Y_calc, where Y_calc, is calculated as;

$A=r-Y_0;<br><br>$ $n_{\mathrm{\omega <br><br>}}=\frac{{\mathrm{\omega <br><br>}}_{\max }-{\mathrm{\omega <br><br>}}_c}{{\mathrm{\omega <br><br>}}_{\max }};<br><br>\mathrm{if}<br><br><br><br>n_{\mathrm{\omega <br><br>}}<<br><br>0,n_{\mathrm{\omega <br><br>}}=0;<br><br>\mathrm{end};<br><br>$ $n_d=K_2*\left(1+n_{\mathrm{\omega <br><br>}}*\left(e^{-{\left(\frac{X_0}{K_3*r}\right)}^{K_4}}-1\right)\right);<br><br>$ $n_{Y_0}=\frac{Y_0}{r};<br><br>$ $d=n_d*\frac{A}{{\left(1-n_{Y_0}\right)}^{0.5}};<br><br>$ $Y_{\mathrm{calc}}=A*e^{-{\left(\frac{X_0}{d}\right)}^2},$

where e is the base of the natural logarithm, and where K_{i {i =}2,3,4} are constants that affect the shape of the exclusion zone avoidance pointing path, to be set for desired performance with consideration of the exclusion zone angular radius and the spacecraft'"'"'s dynamic limits.

10. A method as in claim 8, wherein if the originally desired projection of the body axis of concern is both outside of the exclusion zone and not moving angularly toward an incursion of the exclusion zone, but the most recent attitude command was modified from the originally desired attitude, the attitude commands are modified by applying an angular rotation of magnitude Y_m about the axis Ŷ

to the originally desired attitude, where Y_m is the result of applying a low-pass filter over time to Y_calc, where Y_calc is taken as the magnitude after the first time step of a 1-dimensional slew, which is formulated to bring both Y_calc and the first derivative of Y_calc to zero in the minimum time possible given the body'"'"'s dynamic limits.