Investigation of the On-Orbit Conjunction Between the MCubed and HRBE CubeSats

John Springmann, Andrew Bertino-Reibstein, James Cutler

University of Michigan

IEEE Aerospace Conference

March 4, 2013

MCubed and HRBE overview

Michigan Multipurpose Minisat (MCubed) • Developed by the University of Michigan

• Primary mission: fly JPL’s CubeSat Onboard Processing Validation Experiment (COVE), flight testing a processing algorithm and Virtex-5QV FPGA

Hiscock Radiation Belt Explorer (HRBE) • Developed by Montana State University

• Primary mission: measure variations in the location and intensity of energetic electrons trapped in the Van Allen radiation belts

• Formerly called Explorer-1 Prime (E1P)

HRBE illustration, courtesy of SSEL

MCubed flight unit

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October 28, 2011 Delta II Launch

• Primary Payload: NASA NPP

• 3 PPOD secondary payloads – MCubed, HRBE, AubieSat-1

– DICE-F, DICE-Y

– RAX-2

Photo credit: CalPoly University Photo credit: NASA/VAFB

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Following a successful launch, JSpOC tracks 5 CubeSats from the launch…there should be 6.

TLEs from Space-Track on December 2, 2012

Additionally, Doppler characteristics show the time of closest approach of both satellites is the same.

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1.5 years after launch, the satellites have not separated.

Doppler shift during February 2013 pass

Freq

ue

ncy

(M

Hz)

Elapsed Time (seconds)

HRBE

MCubed

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The satellites must be stuck together on orbit. How?

• Exact cause unknown

• Hypothesis: conjunction is caused by permanent magnets

Each satellite contains a permanent magnet for passive attitude control. This is a common control scheme for nanosatellites.

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CubeSats are deployed with a Poly-Picosatellite Orbital Deployer (PPOD).

AubieSat-1

HRBE

MCubed

PPOD

PPOD main spring provides an expected 1.5 m/s away from launch vehicle

Photos courtesy of CalPoly University

Spring plungers on the feet of each CubeSat provide separation between the CubeSats. CubeSat deployment dynamics have not been thouroughly characterized – we vary the initial conditions in the simulation.

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Orientation of the CubeSats/magnets inside the PPOD

y-z plane (above)

x-z plane (below)

Deployment direction

Deployment direction

: Indicates Magnet north pole

North pole out of page

Drawing is to scale

MCubed HRBE AubieSat-1

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Simulations have been preformed to investigate magnetic conjunction.

• MATLAB’s ode45 used to propagate equations of motion with the following assumptions: 1. The only forces and torques acting on MCubed/HRBE are those of the

magnets.

2. The magnets are ideal dipoles.

3. Satellite collisions are not included.

4. AubieSat-1, the third CubeSat in the PPOD, is ignored.

5. The PPOD is not accelerating in inertial space.

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• The actual magnetic/mass properties and geometry of each satellite are used with three-dimensional equations of motion.

Separation distance after 30 minutes from a range of initial conditions (velocity is HRBE relative to MCubed):

(angular rate is about y-axis) (angular rate is about x-axis)

Initial angular velocity, deg/s Initial angular velocity, deg/s

Init

ial t

ran

slat

ion

al v

elo

city

, cm

/s

Init

ial t

ran

slat

ion

al v

elo

city

, cm

/s

Fin

al s

epar

atio

n d

ista

nce

, m

Fin

al s

epar

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n d

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, m

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Conjunction is possible for translational separation velocities less than 2.1 cm/s.

The MCubed and HRBE magnets are relatively strong compared to other CubeSats.

A small sample of other CubeSat magnet sizes:

CubeSat (size) Dipole Strength (A-m2)

KySat-1 (1U) 0.59

XI-IV (1U) 0.05

CCSWE (3U) 0.3

QuakeSat (3U) 2.9

MCubed (1U) 1.4

HRBE (1U) 1.9

11 References are given in the paper.

Separation distance after 30 minutes from a range of initial conditions, but with 0.5 A-m2 dipoles:

(angular rate is about y-axis) (angular rate is about x-axis)

Initial angular velocity, deg/s Initial angular velocity, deg/s

Init

ial t

ran

slat

ion

al v

elo

city

, cm

/s

Init

ial t

ran

slat

ion

al v

elo

city

, cm

/s

Fin

al s

epar

atio

n d

ista

nce

, m

Fin

al s

epar

atio

n d

ista

nce

, m

Conjunction region reduced – conjunction possible for separation velocity of less than 0.7 cm/s.

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Lessons Learned

• Magnetic conjunction is possible.

– From simulations, < 2.1 cm/s CubeSat separation results in conjunction .

– Ideal separation velocity is 15 cm/s. Actual deployment dynamics are unknown.

– Higher fidelity modeling of CubeSat deployment is needed.

• Consider limiting magnet strengths as part of the CubeSat standard.

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An aside on CubeSat deployment dynamics…

14 Photos: http://spaceref.com/news/viewsr.html?pid=42238

JAXA deployment from ISS, October 2012.

Future work

• Include AubieSat-1 in the simulations.

• Remove ideal dipole assumption – this is the “biggest” assumption made in the simulation. A near-field model is needed.

• Include disturbance forces/torques in the simulation.

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Acknowledgements

• Prof. Dave Klumpar and Ehson Mosleh at Montana State University

• Roland Coelho and the CubeSat team at Cal Poly University

• NDSEG Fellowship program

Questions?

Email: jspringm@umich.edu

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