Attitude determination using earth horizon sensors

Patent metadata

Field	Value
Publication number	US-10220965-B2
Application number	US-201715725478-A
Country	US
Kind code	B2
Filing date	Oct 5, 2017
Priority date	Apr 22, 2014
Publication date	Mar 5, 2019
Grant date	Mar 5, 2019

Abstract

Described herein are systems and methods for attitude determination using infrared Earth horizon sensors (EHSs) with Gaussian response characteristics. Attitude information is acquired by detecting Earth's infrared electromagnetic radiation and, subsequently, determining the region obscured by Earth in the sensors' fields of view to compute a nadir vector estimation in the spacecraft's body frame. The method can be applied when two sensors, each with known and distinct pointing directions, detect the horizon, which is defined as having their fields of view partially obscured by Earth. The method can be implemented compactly to provide high-accuracy attitude within small spacecraft, such as CubeSat-based satellites.

First claim

What is claimed is:

1. A system for attitude determination of a spacecraft, the system comprising:
a plurality of horizon sensors configured to be mounted to the spacecraft, each of the plurality of horizon sensors having a respective field-of-view (FOV), each of the plurality of horizon sensors having a sensitivity to the portion of the respective FOV obscured by Earth;
an attitude determination module coupled to the plurality of horizon sensors, the attitude determination module configured to:
obtain horizon sensor readings from the horizon sensors;
determine an Earth disk radius in a body frame of a spacecraft;
convert the horizon sensor readings to nadir angles using an approximation selected in accordance with a sensitivity characteristic across the horizon sensor FOV; and
estimate a nadir vector of the spacecraft using the nadir angles and the Earth disk radius.

2. The system of claim 1 wherein each horizon sensor has a constant sensitivity characteristic across its FOV.

3. The system of claim 1 wherein each horizon sensor has a Gaussian sensitivity characteristic across its FOV.

4. The system of claim 1 wherein each horizon sensor has at least one of:
a Gaussian sensitivity characteristic across its FOV; and
a constant sensitivity characteristic across its FOV.

5. The system of claim 1 further comprising a reference sensor coupled to provide reference sensor readings to the attitude determination module, wherein the attitude determination module is configured to:
determine a plurality of possible nadir vector solutions using the nadir angles and the Earth disk radius;
obtain a reference sensor reading; and
choose one of the plurality of possible nadir vector solutions based upon the reference sensor reading.

6. The system of claim 1 , wherein the plurality of horizon sensors comprise a first horizon sensor mounted along a first axis in a body coordinate system, a second horizon sensor mounted along a second axis in the body coordinate system, and a reference sensor mounted along a third axis in the body coordinate system.

7. The system of claim 1 further comprising an attitude control module coupled to receive estimated nadir vector information from the attitude determination module, the attitude control module configured to adjust an attitude of the spacecraft based upon received estimated nadir vector information.

8. The system of claim 1 wherein the attitude determination module is configured to:
receive spacecraft position information; and
determine an Earth disk radius in the body frame of the spacecraft based upon the spacecraft position information.

9. The system of claim 1 wherein each of the plurality of horizon sensors has a Gaussian sensitivity to the portion of a respective FOV obscured by Earth, wherein the attitude determination module uses a Gaussian approximation to convert the horizon sensor readings to nadir angles.

10. A method for determining an attitude of a spacecraft, the method comprising:
obtaining horizon sensor readings from a plurality of horizon sensors configured to be mounted to the spacecraft, each of the plurality of horizon sensors having a respective field-of-view (FOV), each of the plurality of horizon sensors having a sensitivity to the portion of the respective FOV obscured by Earth;
determining an Earth disk radius in a body frame of the spacecraft;
converting the horizon sensor readings to nadir angles using an approximation selected in accordance with a sensitivity characteristic across the horizon sensor FOV; and
estimating a nadir vector of the spacecraft using the nadir angles and the Earth disk radius.

11. The method of claim 10 further comprising:
determining a plurality of possible nadir vector solutions using the nadir angles and the Earth disk radius;
obtaining a reference sensor reading from a reference sensor; and
choosing one of the plurality of possible nadir vector solutions based upon the reference sensor reading.

12. The method of claim 11 , wherein obtaining horizon sensor readings from horizon sensors comprises obtaining a first horizon sensor reading from first horizon sensor mounted along a first axis in the body frame of the spacecraft and obtaining a second horizon sensor reading from a second horizon sensor mounted along a second axis in the body frame of the spacecraft, wherein obtaining a reference sensor reading comprises obtaining a reference sensor reading from a reference sensor mounted along a third axis in the body frame of the spacecraft.

13. The method of claim 10 wherein obtaining horizon sensor readings from horizon sensors comprises obtaining readings from at least one of: thermopiles and infrared radiation (IR) sensors.

14. The method of claim 12 wherein obtaining a reference sensor reading comprises obtaining a reading from a magnetometer or a Sun sensor.

15. The method of claim 10 further comprising adjusting an attitude of the spacecraft based upon an estimated nadir vector.

16. The method of claim 10 further comprising receiving spacecraft position information, wherein determining an Earth disk radius in the body frame of the spacecraft comprises determining an Earth disk radius based upon the spacecraft position information.

17. The method of claim 10 wherein each of the plurality of horizon sensors has a Gaussian sensitivity to the portion of a respective FOV obscured by Earth, wherein converting the horizon sensor readings to nadir angles comprises converting the horizon sensor readings to nadir angles using a Gaussian approximation.

18. A method for determining an attitude of a spacecraft, the method comprising:
obtaining horizon sensor readings from a plurality of horizon sensors configured to be mounted to the spacecraft, each of the plurality of horizon sensors having a respective field-of-view (FOV), a sensitivity to the portion of the respective FOV obscured by Earth, and a boresight vector in a body frame of the spacecraft;
determining an Earth disk radius in the body frame of the spacecraft using a geocentric latitude of the spacecraft, an equatorial radius of the Earth, and a polar radius of the Earth;
converting the horizon sensor readings to nadir angles; and
estimating a nadir vector of the spacecraft using the Earth disk radius and by finding the intersection of a plurality of cones, each cone defined by the boresight vector of a corresponding one of the plurality of horizon sensors and a corresponding one of the nadir angles.

19. The method of claim 18 wherein obtaining horizon sensor readings from a plurality of horizon sensors comprises, obtaining horizon sensor readings from a plurality of horizon sensors with each of the plurality of horizon sensors having a constant sensitivity characteristic across its FOV.

20. The method of claim 18 wherein obtaining horizon sensor readings from a plurality of horizon sensors comprises obtaining horizon sensor readings from a plurality of horizon sensors with each of the plurality of horizon sensors having a Gaussian sensitivity characteristic across its FOV.

Assignees

• Massachusetts Inst Technology

Inventors

• Nguyen Tam Nguyen Thuc
• Cahoy Kerri L
• Quadrino Meghan K

Classifications

• B64G1/366

  using magnetometers · CPC title

• B64G1/407

  Solar sailing · CPC title

• B64G1/286

  using control momentum gyroscopes (CMGs) · CPC title

• B64G1/363

  using sun sensors · CPC title

• B64G1/26

  using jets · CPC title