**4.1 Nano remote sensing satellite PRISM**

Pico-satellite for Remote-sensing and Innovative Space Missions (PRISM) is a remote sensing nano-satellite, developed at the University of Tokyo. The mission objective of PRISM is to obtain 30 m resolution earth images with a 8.5 kg nano-satellite. The satellite has an extensible boom for an optical system to reduce the total mass of the telescope and to design a compact, light-weight optics system (Fig.3.). Table 1 shows the specification of the PRISM satellite. PRISM has been launched into a Sun-Synchronous LEO as a piggy-back satellite of the GOSAT by a H-IIA rocket in 2009. The satellite attitude should be stabilized to better than 0.7 deg/s during observation. PRISM has gyro sensors and magnetometers for attitude sensors and MTQs as actuators. In the design phase of the project, the attitude was intended to be controlled passively, using only the gravity gradient torque after the boom extension. However, the satellite would have difficulties achieving the attitude requirement because of the magnetic disturbance. To meet the mission requirements, the attitude is determined by sun sensors and magnetometers and controlled actively with MTQs (Inamori et al, 2011(b)).

PRISM has been launched to LEO in January 2009. After extending the boom, PRISM calibrated a magnetometer with the method explained in section 3.1. Fig.4 shows the result


Table 1. Specification of the PRISM satellite.

96 Magnetic Sensors – Principles and Applications

where *<sup>k</sup> <sup>y</sup>* and ˆ *<sup>k</sup> <sup>y</sup>* are obtained by a magnetometer measurement and the calculation using

These calibrated magnetometer measurements are applied to magnetic moment estimation in the Nano-JASMINE mission. The result of the calibration in Nano-JASMINE will be

This section presents the attitude control system of the remote sensing nano-satellite PRISM and micro astronomy satellite Nano-JAMSINE to show examples of the application of magnetometers for nano- and micro-satellite missions. PRISM has three-axis low power consumption magneto-impedance sensor. The magnetometer measurements are used for the attitude estimation and an MTQ is used for attitude control. After completing the main mission, the satellite conducted an in-orbit experiment to demonstrate magnetic moment compensation. Nano-JASMINE has two types of magnetic sensors, a three-axis magnetoresistive magnetometer and a three-axis fluxgate magnetometer. The satellite attitude should be stabilized to within 1 arcsec during observation. In order to satisfy this requirement, the satellite should estimate the magnetic moment and compensate precisely. The fluxgate magnetometer is used for this purpose. The satellite also uses a magnetometer for attitude estimation and magnetic attitude control in initial phase, when the power consumption is limited. A low power consumption magneto-resistive magnetometer is used for this initial phase in this satellite mission. The following subsections show detailed magnetometer application for nano- and micro-satellites with in-orbit data and

Pico-satellite for Remote-sensing and Innovative Space Missions (PRISM) is a remote sensing nano-satellite, developed at the University of Tokyo. The mission objective of PRISM is to obtain 30 m resolution earth images with a 8.5 kg nano-satellite. The satellite has an extensible boom for an optical system to reduce the total mass of the telescope and to design a compact, light-weight optics system (Fig.3.). Table 1 shows the specification of the PRISM satellite. PRISM has been launched into a Sun-Synchronous LEO as a piggy-back satellite of the GOSAT by a H-IIA rocket in 2009. The satellite attitude should be stabilized to better than 0.7 deg/s during observation. PRISM has gyro sensors and magnetometers for attitude sensors and MTQs as actuators. In the design phase of the project, the attitude was intended to be controlled passively, using only the gravity gradient torque after the boom extension. However, the satellite would have difficulties achieving the attitude requirement because of the magnetic disturbance. To meet the mission requirements, the attitude is determined by sun sensors and magnetometers and controlled actively with MTQs (Inamori

PRISM has been launched to LEO in January 2009. After extending the boom, PRISM calibrated a magnetometer with the method explained in section 3.1. Fig.4 shows the result

**4. Example of magnetometer applications for small satellite missions** 

the state vector with Equation 46.

presented in section 4.2.

simulation results.

et al, 2011(b)).

**4.1 Nano remote sensing satellite PRISM** 

**- vv T P =P +K P K k k kk k** (51)

Fig. 3. Overview of the remote sensing nano-satellite "PRISM" (a) with boom folded, (b) with boom extended.

of the magnetometer and gyro sensor calibration in orbit. In Fig.4, the magnetometer measurements before the calibration had a bias error. In PRISM project, it was difficult to calibrate assembled magnetometer with a ground facility, thus bias and scale factor were

Application of Magnetic Sensors to Nano- and Micro-Satellite Attitude Control Systems 99

(a) (a)

Nano Japan Astrometry Satellite Mission for INfrared Exploration (Nano-JASMINE) is a nano astrometry satellite, developed at the Intelligent Space Systems Laboratory (ISSL), the University of Tokyo cooperation with National Observatory of Japan (NAOJ). The launch is scheduled in 2013. The objective of the Nano-JASMINE mission is to measure the threedimensional position of stars to an accuracy of 1.8 mas (milliarcsecond), using stellar annual parallax, which is measured by the observed positions of stars at different times of a year. Table 2 shows the specification of the Nano-JASMINE satellite. Nano-JASMINE is outfitted with a telescope with a 5 cm effective diameter and a CCD (Charge Coupled Device Image Sensor) with Time Delay Integration (TDI) which is sensitive to z-band (wavelength ~ 0.9 μm). The satellite measures the position of twenty thousands of stars with a magnitude of larger than 7.5 for all-sky with an accuracy of 1.8 mas. The measurement is used to update the threedimensional positions and proper motions of the stars in the Hipparcos catalogue. The satellite also demonstrates the space compliancy of the novel type of CCD in which TDI is embedded.

Fig. 6. Angular velocity (a) without the magnetic compensation, (b) with the magnetic

compensation (Inamori et al, 2011(d)).

**4.2 Micro astronomy satellite Nano-JASMINE** 

Fig. 7. Overview of the nano astronomy satellite Nano-JASMINE

For the Nano-JASMINE mission, there are two attitude requirements: control the orientation of the telescope and the angular velocity of the satellite. For the requirement of the orientation, the attitude must be controlled to an accuracy of 0.05 degrees. The attitude requirement can be achieved using a star tracker (STT) and a fibre optical gyro (FOG), which

estimated in orbit. After the calibration, magnetometer measurements did not correspond to the IGRF mode in 2000 nT accuracy, because bias of the magnetometer assembled in PRISM had a temperature dependency, which is not considered in the calibration method. After the magnetometer calibration, the bias and scale factor of the gyro sensor was estimated using the least squared method, which is mentioned in section 3.1. Fig.4. shows the angular velocity before and after the sensor calibration. Fig.5. shows the in-orbit result of attitude stabilization before and after the calibration. After the calibration, the satellite attitude was stabilized to an accuracy of 0.3 deg/s, which meets the attitude requirement for the PRISM mission.

Fig. 4. Result of (a) magnetometer, (b) gyro sensor calibration in-orbit. With permission from ASCE. (Inamori et al, 2011(a)).

Fig. 5. Result of attitude stabilization in orbit (a) after the sensor calibration, (b) before the sensor calibration.

After the main mission, the satellite conducted an experiment to demonstrate the method to estimate a residual magnetic moment of the satellite. In this experiment, magnetometer and gyro measurement were downloaded to a ground station, then the magnetic moment was estimated using a computer on the ground. After the estimation, the estimated parameters were uploaded to compensate for the magnetic disturbance in orbit. Fig.6. shows the history of angular velocity of a satellite with the magnetic compensation and without the magnetic compensation, respectively. The satellite attitude was stabilized more precisely with the magnetic compensation.

Fig. 6. Angular velocity (a) without the magnetic compensation, (b) with the magnetic compensation (Inamori et al, 2011(d)).

### **4.2 Micro astronomy satellite Nano-JASMINE**

98 Magnetic Sensors – Principles and Applications

estimated in orbit. After the calibration, magnetometer measurements did not correspond to the IGRF mode in 2000 nT accuracy, because bias of the magnetometer assembled in PRISM had a temperature dependency, which is not considered in the calibration method. After the magnetometer calibration, the bias and scale factor of the gyro sensor was estimated using the least squared method, which is mentioned in section 3.1. Fig.4. shows the angular velocity before and after the sensor calibration. Fig.5. shows the in-orbit result of attitude stabilization before and after the calibration. After the calibration, the satellite attitude was stabilized to an

accuracy of 0.3 deg/s, which meets the attitude requirement for the PRISM mission.

(a) (b)

(a) (b)

Fig. 5. Result of attitude stabilization in orbit (a) after the sensor calibration, (b) before the

After the main mission, the satellite conducted an experiment to demonstrate the method to estimate a residual magnetic moment of the satellite. In this experiment, magnetometer and gyro measurement were downloaded to a ground station, then the magnetic moment was estimated using a computer on the ground. After the estimation, the estimated parameters were uploaded to compensate for the magnetic disturbance in orbit. Fig.6. shows the history of angular velocity of a satellite with the magnetic compensation and without the magnetic compensation, respectively. The satellite attitude was stabilized more precisely with the

ASCE. (Inamori et al, 2011(a)).

sensor calibration.

magnetic compensation.

Fig. 4. Result of (a) magnetometer, (b) gyro sensor calibration in-orbit. With permission from

Nano Japan Astrometry Satellite Mission for INfrared Exploration (Nano-JASMINE) is a nano astrometry satellite, developed at the Intelligent Space Systems Laboratory (ISSL), the University of Tokyo cooperation with National Observatory of Japan (NAOJ). The launch is scheduled in 2013. The objective of the Nano-JASMINE mission is to measure the threedimensional position of stars to an accuracy of 1.8 mas (milliarcsecond), using stellar annual parallax, which is measured by the observed positions of stars at different times of a year. Table 2 shows the specification of the Nano-JASMINE satellite. Nano-JASMINE is outfitted with a telescope with a 5 cm effective diameter and a CCD (Charge Coupled Device Image Sensor) with Time Delay Integration (TDI) which is sensitive to z-band (wavelength ~ 0.9 μm). The satellite measures the position of twenty thousands of stars with a magnitude of larger than 7.5 for all-sky with an accuracy of 1.8 mas. The measurement is used to update the threedimensional positions and proper motions of the stars in the Hipparcos catalogue. The satellite also demonstrates the space compliancy of the novel type of CCD in which TDI is embedded.

Fig. 7. Overview of the nano astronomy satellite Nano-JASMINE

For the Nano-JASMINE mission, there are two attitude requirements: control the orientation of the telescope and the angular velocity of the satellite. For the requirement of the orientation, the attitude must be controlled to an accuracy of 0.05 degrees. The attitude requirement can be achieved using a star tracker (STT) and a fibre optical gyro (FOG), which

Application of Magnetic Sensors to Nano- and Micro-Satellite Attitude Control Systems 101

noise is assumed to change throughout an orbital period, because time-variable bias noise has a relationship to temperature and current loops of batteries, which change throughout an orbital period. The magnetic anomaly observed from a satellite is also assumed to change throughout an orbital period, because the magnetic anomaly is fixed with respect to the

In this estimation, both bias and magnetic anomaly errors are estimated with an accuracy of 50 nT. After the estimation of the bias noise, the magnetic moment of Nano-JASMINE is estimated using the extended Kalman filter which is presented in section 2.3. Fig. 9. shows a simulation result of the magnetic moment estimation. In this simulation, the magnetic moment is assumed to change in an orbital period, because the magnitude of current in batteries changes in an orbital period. Based on this estimated value, the satellite

Earth coordinate system and constant within several days.

(a) (b)

Fig. 8. Simulation results of the on-line bias estimation. (a) Estimation of the bias. (b)

Fig. 9. Simulation result of the magnetic moment estimation (Inamori et al, 2011(c)).

This chapter shows the application of magnetometers in nano- and micro-satellite missions. Previously, most of satellites had magnetometers for coarse attitude estimation and attitude control. The magnetometers are also used on nano- and micro-satellite for attitude determination and control system. These days, these small satellites are applied to more sophisticated objectives such as remote-sensing and astronomy missions, which require precise attitude stabilization. For the achievement of the requirements, the satellite must control attitude precisely. Because the small satellites have a small moment of inertia, the

compensates for the magnetic disturbance.

Estimation of the magnetic anomaly.

**5. Conclusion** 

have been used in many satellite missions. For the requirement of the angular velocity, the angular rate must be controlled to the Time Delay Integration (TDI) rate in the CCD. The error of the angular velocity with respect to the TDI rate must be smaller than 4×10-7 rad/s. This requirement implies attitude sensing and actuation capabilities that are beyond the current state of the art of small satellites. To achieve the mission requirement, the satellite determines the attitude using blurred star images obtained from the mission telescope. Specifically, this method assesses the quality of star images based on how blurred it appears. To get star images using the telescope, the satellite must stabilize the attitude with attitude sensors such as the FOG before using the telescope. Nano-JASMINE stabilizes the attitude using several sensors and actuators before the observation.


Table 2. Specifications of the Nano-JASMINE satellite.

The satellite uses magneto-resistive magnetometer for attitude estimation and an MTQ for actuation in the initial phase after the launch. In this phase, the satellite estimates the attitude using the QUEST method with the magnetometer and sun sensors. After controlling attitude coarsely using the magnetic attitude control system, the satellite calibrates the magnetometer for the estimation of a magnetic moment. Fig.8(a) and Fig.8(b) are simulation results of the bias and magnetic anomaly estimation, respectively. In the simulation, the bias noise is assumed to change throughout an orbital period, because time-variable bias noise has a relationship to temperature and current loops of batteries, which change throughout an orbital period. The magnetic anomaly observed from a satellite is also assumed to change throughout an orbital period, because the magnetic anomaly is fixed with respect to the Earth coordinate system and constant within several days.

In this estimation, both bias and magnetic anomaly errors are estimated with an accuracy of 50 nT. After the estimation of the bias noise, the magnetic moment of Nano-JASMINE is estimated using the extended Kalman filter which is presented in section 2.3. Fig. 9. shows a simulation result of the magnetic moment estimation. In this simulation, the magnetic moment is assumed to change in an orbital period, because the magnitude of current in batteries changes in an orbital period. Based on this estimated value, the satellite compensates for the magnetic disturbance.

Fig. 8. Simulation results of the on-line bias estimation. (a) Estimation of the bias. (b) Estimation of the magnetic anomaly.

Fig. 9. Simulation result of the magnetic moment estimation (Inamori et al, 2011(c)).

### **5. Conclusion**

100 Magnetic Sensors – Principles and Applications

have been used in many satellite missions. For the requirement of the angular velocity, the angular rate must be controlled to the Time Delay Integration (TDI) rate in the CCD. The error of the angular velocity with respect to the TDI rate must be smaller than 4×10-7 rad/s. This requirement implies attitude sensing and actuation capabilities that are beyond the current state of the art of small satellites. To achieve the mission requirement, the satellite determines the attitude using blurred star images obtained from the mission telescope. Specifically, this method assesses the quality of star images based on how blurred it appears. To get star images using the telescope, the satellite must stabilize the attitude with attitude sensors such as the FOG before using the telescope. Nano-JASMINE stabilizes the attitude

Satellite Size 508×508×512 [mm3 ] (When the boom is folded) Total mass 35 [kg] Orbit Sun-Synchronous LEO

Mission Infrared Astrometry

Up S-band 1 [kbps]

requirement 740 mas/8.8s

requirement 0.05 [deg]

Diameter 90 [mm]

FOV 0.5 deg×0.5 deg

The satellite uses magneto-resistive magnetometer for attitude estimation and an MTQ for actuation in the initial phase after the launch. In this phase, the satellite estimates the attitude using the QUEST method with the magnetometer and sun sensors. After controlling attitude coarsely using the magnetic attitude control system, the satellite calibrates the magnetometer for the estimation of a magnetic moment. Fig.8(a) and Fig.8(b) are simulation results of the bias and magnetic anomaly estimation, respectively. In the simulation, the bias

CDH CPU MicroBraze micro processors×6

Communication Down S-band 100 [kbps]

Optics Architecture Refractive

Focal length 500 [mm]

Altitude 750 [km]

configured in Virtex-5×2

Coarse Magnetometer (HMC2003, three axis) Precise Magnetometer (AMI204, three axis) Mems gyro sensor (, three axis) Fiber optic gyro (three axis) Sun sensor (in-house, two axis)×6 Star trucker×2

Magnetic torquer (in-house, three axis) Magnetic canceller (in-house, three axis) Reaction wheel×4

using several sensors and actuators before the observation.

AOCS Attitude rate

Sensor

Actuator

Detector

Table 2. Specifications of the Nano-JASMINE satellite.

Attitude

This chapter shows the application of magnetometers in nano- and micro-satellite missions. Previously, most of satellites had magnetometers for coarse attitude estimation and attitude control. The magnetometers are also used on nano- and micro-satellite for attitude determination and control system. These days, these small satellites are applied to more sophisticated objectives such as remote-sensing and astronomy missions, which require precise attitude stabilization. For the achievement of the requirements, the satellite must control attitude precisely. Because the small satellites have a small moment of inertia, the

**6** 

*Italy* 

**Magnetic Field Sensors Based on** 

Maria Teresa Todaro1, Leonardo Sileo1,2 and Massimo De Vittorio1,2,3

*2Center for Biomolecular Nanotechnologies UNILE, Istituto Italiano di Tecnologia* 

In the last decades magnetic field sensors have been developed and realized for analyzing and controlling thousands of functions (Ripka, 2001), and they have become a widespread presence in modern lifestyle. Numerous applications in different fields of science, engineering, and industry rely on the performance, ruggedness, and reliability of magnetic field sensors.

The applications of magnetic sensors depend on magnetic field dynamic range and resolution and include position sensing, speed detection, current detection, non-contact switching, space exploration, vehicle detection, electronic compasses, geophysical prospecting, non-distructive testing, brain function mapping (Lenz & Edelstein, 2006).

Nowdays there is an increasing requirement for magnetic devices with improved sensitivity and resolution, trying to keep as low as possible their cost and power consumption. Additionally there is the need to develop compact devices with several sensors able to measure different parameters including magnetic field, pressure, temperature, acceleration. In this way a multifunctional device could be integrated on the same substrate containing transducers and electronic circuits in a compact configuration without affecting device

In this context microelectromechanical systems (MEMS) technologies play a prominent role

In general MEMS devices are miniaturized mechanical systems produced using fabrication techniques already explored in the electronics industry. The exploitation of MEMS technology for device fabrication not only makes possible the reduction of the device dimensions on the order of micrometers, but also allows the integration of the mechanical and electronic components on a single chip. In addition to the small device size this involves other important advantages such as light weight, minimum power consumption, low cost, better sensitivity and high resolution. This technology was successfully employed for the realization of portable devices such as gyroscopes (Chang et al., 2008), accelerometers (Li et

al., 2011), micromirrors (Singh et al., 2008), and pressure sensors (Mian & Law, 2010).

for the development of a new class of magnetic sensors.

**1. Introduction** 

performances.

*1National Nanotechnology Laboratory (NNL), Istituto Nanoscienze-CNR* 

**Systems (MEMS) Technology** 

**Microelectromechanical** 

*3University of Salento, Lecce* 

magnetic moment which causes magnetic disturbance should be compensated for achievement of accurate attitude control. For the compensation of the magnetic disturbance, the satellites estimate a magnetic moment using a magnetometer. This is the new requirement for magnetometers in the space applications. Magnetometers are indispensable for attitude control and determination as well as magnetic disturbance compensation to achieve the sophisticated objectives in nano- and micro-satellite missions.

### **6. References**

