analysis of the in-flight injection of the...

ANALYSIS OF THE IN-FLIGHT INJECTION OF THE LISA PATHFINDER TEST-MASS

INTO A GEODESIC

Daniele Bortoluzzi (1,2) on behalf of the LISA Pathfinder Collaboration(*), Davide Vignotto (1), Andrea Zambotti (1),

Ingo Köker(3), Hans Rozemeijer(4), Jose Mendes(4), Paolo Sarra(5), Andrea Moroni(5), Paolo Lorenzi(5)

(1) University of Trento, Department of Industrial Engineering, via Sommarive, 9 - 38123 Trento, Email:

[email protected], [email protected], [email protected]

(2) Trento Institute for Fundamental Physics and Application / INFN, Italy.

(3) AIRBUS DS GmbH, Willy-Messerschmitt-Strasse 1 Ottobrunn, 85521, Germany, Email:

[email protected]

(4) European Space Operations Centre, European Space Agency, 64293 Darmstadt, Germany, Email:

[email protected], [email protected]

(5) OHB Italia S.p.A., via Gallarate, 150 – 20151 Milano, Italy, Email: [email protected], [email protected],

[email protected]

(*) Full list and affiliations attached in the end of the document

ABSTRACT

LISA Pathfinder is a mission that demonstrates some key

technologies for the measurement of gravitational waves.

The mission goal is to set two test masses (TMs) into

purely geodesic trajectories. The grabbing positioning

and release mechanism (GPRM) grabs and releases each

TM from any position inside its housing. The injection

phase is critical, because the free-floating TM can be

electrostatically controlled only if its linear and angular

velocities are less than 5μm/s and 100 μrad/s

respectively. Since the first injections showed significant

deviations from the expectation, a dedicated release test

campaign was performed at the end of the extended

mission phase, in order to explore different injection

strategies. The analysis of the releases shows that a

mechanism configuration out of the nominal one is

compatible with the in-flight test results. The paper

presents the findings of the in-orbit operation of the

GPRM and a possible interpretation of the underlying

dynamics.

1. INTRODUCTION

Many space missions, especially the ones dealing with

gravitational phenomena, use a free-floating mass as a

reference for their measurements ([1]). Missions based

on this principle where launched since the seventies

(TRIAD satellite, [2]). The main advantage of having a

free floating body as a reference is the fact that the

instrument resolution is improved if compared to

experiments where a mechanical contact is present

between the proof mass and the satellite ([3]).

During the launch phase and the early orbit phase, the

sensing mass is accelerated at the point that, if it is let free

to move, it can damage the inner part of the housing

surrounding it. In some missions locking the sensing

mass is not mandatory, since the mass itself is small, as

well as the gaps with respect to its housing. On the

contrary, other missions require a lock mechanism that

secures the sensing body and prevents impacts with the

surroundings housing.

The sensing body, from the locked condition, has then to

be released into free-fall to start the in-flight operations;

as a consequence, the release into free-fall of the proof

mass is a critical aspect for these missions, since it is a

necessary step to start the science phase. Different

technologies are nowadays available to perform a release

into freefall, for example electric motors, shape memory

actuators, solenoids, paraffin actuators, thermal cutters,

piezoelectric etc. In designing the release mechanism,

which is in contact with the proof mass before releasing

it, unavoidable forces arising at the contacting surfaces

must be taken into account.

Among many space missions, LISA Pathfinder (LPF , [4]

and [5]) represents a very interesting and challenging

case of study, since the requirements imposed to the

release operations are very tiny in terms of the residual

momentum of the test mass (TM).

Figure 1 The LISA Pathfinder scientific payload. The

free-floating test masses are visible, hosted inside their

housings.

_____________________________________________________________________________________________ Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019

mailto:[email protected]









LPF mission, launched in 2015 and ended in 2017, was

the precursor of a gravitational-waves space observer,

called LISA (launched scheduled for 2034). The goal of

LPF was to demonstrate some key technologies to be

implemented in LISA, showing that a LISA-like proof

mass can be injected into a pure geodesic trajectory to a

level of force noise below 10 fN in the measurement

bandwidth 1–30 mHz ([6]).

The LISA Pathfinder scientific payload include many

mechanisms. The one on which this work focuses in is

the grabbing positioning and release mechanism

(GPRM). As its name suggests, it is responsible of

locking (grabbing) the TM from any position inside its

housing, repositioning it in the center, and then release in

into freefall ([7]).

2. SYSTEM DESCRIPTION AND NOMINAL

RELEASE PROCEDURE

Inside the LISA-pathfinder spacecraft, there are two

gravitational reference sensors (GRS 1 and 2), both

containing a TM (referenced as TM1 and TM2). Each

TM is a cubed shaped 1.96 kg gold-coated Au/Pt mass

(see Fig. 1), and is inserted in an electrode housing (EH),

with gaps between TM and surrounding walls of 3-4 mm.

The housing is covered with electrodes, that both

generate the electrostatic control force, to stabilize the

TM after the release, and provide the measurement of TM

position and attitude for the control loop.

Figure 2 Reference system of one GRS. The TM position

inside the GRS is described by three translations (x, y, z)

and three rotations (θ, η, φ).

A reference system, centred in the EH, is used to describe

TM positions and attitudes (see Fig. 2). It is defined with

three translations {x, y, z} and three rotations {θ, η, φ},

since the TM has six degrees of freedom (DOFs). Inside

each GRS there is a GRPM, that is composed by two

cylindrical pistons, called plungers, that are nominally

aligned to the z-axis. The plungers are moved

independently along z-axis by means of a piezo-walk

actuator (called NEXLINE). The half of the GPRM on

the z+ side is called top, the one on the z- side is called

bottom. Two force sensors (one for each half of the

mechanism) measure the preload applied on the TM by

each plunger.

Plungers ends are designed to fit into two squared

indentations present on the z faces of the TM to lock it

during the grabbing phase.

The plunger on the z+ side has a conical end, the plunger

on the z- side has a pyramidal end. This design is adopted

in order to not over-constrain the TM rotation around the

z axis (defined by angle φ).

Inside each plunger, there is a coaxial gold tip, with a

diameter of 0.8 mm. The tip protrudes from the plunger

end when a voltage is applied to a piezo stack actuator

attached to it. A pre-loaded spring pushes back the tip if

the voltage is reduced (Fig. 3).

As previously said, the GPRM is the mechanism that

operates the release into freefall of the proof mass. The

nominal release procedure consists of grabbing the TM

with the plungers. Then repositioning the grabbed TM in

the centre of the EH. After that, the handover manoeuvre

is performed: it consists of extruding the tips from the

plunger ends, and retracting at the same time the two

plungers, in order to maintain the desired preload force

on the TM. The nominal maximum extraction of the tips

is 18 μm, and before reaching the TM surface, they travel

4 μm (free-stroke), so the z distance between TM and

plunger at release is very tiny, nominally 14 μm.

Figure 3 Sketch of the plunger, with extended tip

touching the TM.

After the handover the TM is said to be in the pre-release

phase, held only by the two tips, that are in contact with

the TM on two specific zone called landing areas. To

release the TM, tips are simultaneously and quickly

retracted (thanks to the pre-loaded spring); this

guarantees the minimization of any asymmetric contact

force on the TM and to break the adhesion force. In the

ideal case, this procedure should produce a zero TM

momentum after release.

The requirement on the TM residual velocity (i.e. the

free-falling velocity after the release) are set to 5 μm/s

and 100 μrad/s for translations and rotations respectively.


These velocity limits are required in order for the

electrostatic control force to be able to stabilize the TM

in the centre on the EH.

Given the geometry of the GPRM, the forces acting on

the TM at the release should be directed only along the z-

axis, with very low components along x and y due to not

perfect alignment of plungers and tips. In the case of a

nominal release (or quasi-nominal, i.e. with tiny

misalignment between TM and plunger), the residual

momentum on the TM is given by two main

contributions:

- the asymmetry of the adhesion-pull force

between the two contacts.

- the asymmetry of retraction, that can convert

symmetric adhesion into momentum.

These two contributions have been deeply analysed. A

dedicated experimental setup, called transferred

momentum measurement facility (TMMF), was designed

to study the adhesion contribution in detail ([8] and [9]).

The effect of the second contribution have been

simulated taking into account possible delays in the tips

retraction ([10]).

It has been shown that the GPRM, in the nominal

working condition, is capable of releasing the TM

respecting the requirement on the residual momentum

with a success rate greater than 95% ([11],[12]).

Once the two GPRM (of TM1 and TM2) have been

assembled, their functionality have been successfully

tested on-ground in the in-flight configuration ([13]).

Unfortunately, during on-ground tests is not possible to

replicate a release, due to the influence of the Earth

gravitational field.

3. IN-FLIGHT RELEASES

The first releases were performed in-flight in February

2016, with the nominal release procedure. Release results

did not confirm the expectations, as reported in Tab. 1.

Two main problems were detected, for both GRS1 and

GRS2:

- the TMs residual velocities were higher than the

requirement, so the actuation was not able to

control and stabilize the TM avoiding impacts.

- the z component of the translational velocity was

not the main one, and high rotational velocities

where recorded.

The TM stabilization was possible only after few

minutes; the strategy was to wait for impacts to damp the

TM momentum until the controller was able to capture it.

Since this behaviour of the GPRMs was totally

unexpected, and since similar mechanisms should be

used in LISA, the mission was extended and a GPRM test

campaign was scheduled in the end of mission phase.

In the extended mission experimental campaign, several

tests were performed. Many release strategies were

tested, and an upgraded release procedure was

implemented. Last tests were performed with an

automatized improved release procedure ([14]).

Table 1 Release velocities of TM1 and TM2, from two

releases performed in February 2016. Velocities are

much higher than the requirements.

Unit μm/s μrad /s

DOF 𝑣x 𝑣y 𝑣z 𝜔𝜃 𝜔η 𝜔φ

TM1 -2.9 -

20.3

-

56.7 681.3

-

797.4 1084.6

TM2 11.6 -

27.2

-

16.2 1035.4 -30.0 -429.7

Req. 5 5 5 100 100 100

The manoeuvres that were tested during the extended

mission phase and that improved the GRPM performance

are:

- Hammering: moving plungers back and forth

using the NEXLINE, with extended tips, to

improve the plunger ends settling into the TM

indentations before the release. Hammering is

performed just after the handover.

- Slow release tip retraction: repositioning the tips

instead of quickly retracting them, from max

extension (nominal 18 μm) to 12 μm.

- Slow plunger retraction: command a low

velocity plunger retraction, few seconds after

release.

A scheme of the phases of the improved release

procedure is depicted in Fig. 4. By applying the described

strategies, the percentages of compliant releases

increased (i.e. releases respecting the requirement on the

TM residual velocity).

Our focus will be mainly on the release velocities after

the tip release (referred as tip release velocity, that is the

actual release instant, at which the TM starts to free fall).

In general, at tips retraction the TM velocities sometimes

are compliant with the requirement, and sometimes they

are not.

Figure 4 Phases of the improved release procedure. The

release instant represents the fast or slow tip retraction

instant.

Few instants after the tip release, when plungers start

being retracted, the TM momentum is often increased.

This unexpected phenomenon suggests that there is an


undesired interaction between TM and plunger. It must

be clarified that, for the control force actuation, the

velocities of interest are the one after the plungers

retraction, but this is not the aim of the study presented

in this work.

If the GRPM had worked under nominal condition at

release, without any plunger-TM interaction, the residual

velocities obtained in-flight would not be explainable.

For this reason, we made the hypothesis of non-nominal

working conditions of the GPRM. The hypothesis is

reinforced looking at the high residual velocities,

recorded on all six degrees of freedom, and observing the

TM momentum change at plunger retraction.

To confirm this hypothesis, in-flight releases data have

been analysed. Unfortunately, for many tests, it is not

easy to estimate the release velocity of the TM (at tips

retraction), since many impacts took place right after the

release instant. Another limiting factor is the relatively

low sampling time, 10 Hz, not sufficient to precisely

detect instants at which there is an impact. In this work,

due the limitations yet described, only a subset of all the

test is considered.

The tests having at least three aligned sampling points

(along all the six DOFs of the TM) after the release where

selected, since in free falling condition the velocity is

constant. The algorithm used to select these tests checked

a “linearity assumption”, based on the distribution of the

noise affecting the readings. These tests are defined as

aligned tests.

4. IMPULSES ANALYSIS

The analysis of the velocity of the TM after the tip release

allows to extract a subset of aligned tests among all the

test of the extended mission campaign. For each aligned

test, the release velocities along the 6 DOFs of the TM

can be considered as free fall velocities and are estimated

through a linear fit. Given the free fall velocities a

dynamical model can be applied in order to estimate the

impulses applied to the TM at the release. From now on,

we will consider only aligned tests in the analysis.

The set of the aligned tests is mainly composed of tests

with low TM velocities at the tips retraction. In

particular, the slow tip releases show translational

velocities respecting the requirements and, in general,

also compliant rotational velocities (except for few cases

with one rotational velocity slightly greater than the

limit).

Aligned tests comprehend also non-compliant tests (i.e.

tests with at least one velocity higher than the

requirement). They are composed mainly of fast tip

release tests (the nominal tip release strategy), which

show high translational and rotational velocities (also 10

times the requirement). As shown in Fig. 5, for these non-

compliant fast-tip tests the translational velocities lie

mainly in the x-z plane, with an inclination close to 45°

(i.e. x and z components of the velocity are comparable).

This fact suggests that the nominal release described in

section 2, assumed in the on-ground experiments, cannot

explain the measured velocity of the TM. It can be proved

that, based on the geometry of the system (maximum

relative inclination between tip and TM and dimension of

the contact surface of the TM), unfeasible values of

adhesive pulls or blocking forces would be required in

order to obtain the measured motion as the sum of two

opposite inclined forces between tips and TM. This is

mainly due to the high x component of the velocity (since

in a nominal release the TM should move only along the

z axis).

Figure 5 Translational velocities of the TM at release.

Non-compliant fast-tip tests from the subset of the

aligned tests are shown. Main momentum components lie

on the x-z plane.

Therefore, we can reasonably assume that a contact

between plunger and TM occurred at the tip release. This

hypothesis is supported by the inclination of the

translational velocities, which is similar to the inclination

of the grabbing indentation. This suggests that an impulse

orthogonal to the indentation surfaces near the plunger-

TM contact area was applied to the TM.

Based on this finding, we assume that, for the non-

compliant fast tip tests, the measured momentum of the

TM at the tip release is given by impulses applied to the

TM at the indentation contact zone, as depicted in Fig.

6Figure 8. For each plane (x-z and y-z) and each z face

of the TM, we assume two impulses orthogonal to the

indentation surface (whose inclination w.r.t. the z axis is

α). In general, the two impulses are not equilibrated,

therefore we can split their sum in the sum of two

balanced impulses (u and w in Fig. 6) and an unbalanced

impulse (t). The effect of the two balanced impulses will


be only a resultant along the z axis, while the unbalanced

impulse gives a contribution to z motion, transversal

motion, and rotation. It’s important to notice that each of

the impulses shown in Fig. 6 can be (in principle) the

resultant of many repeated impulses; moreover, we

assume the impulses are applied in the middle of the

grabbing contact surface.

Figure 6 Impulses given from the plunger to the TM in

the hypothesis there is a contact between them at release.

The TM indentation geometry is simplified.

Since in the considered tests the TM is not moving before

the release and starts moving linearly approximately 0.1-

0.2 seconds after the tip release, we assume that the linear

motion of the TM is created by the application of an

unknown number of impulses immediately after the tip

release.

In Fig. 7, we draw the unbalanced impulses for each

plane, x-z and y-z, at each side of the TM. Impulses x and

y components are on the top (z+) side are defined as 𝜄1x

and 𝜄1y. While on the bottom side they are defined as 𝜄2

x

and 𝜄2y.

Each of them is associated to the corresponding z impulse

through the constant tan(α), where α is the inclination of

the indentation surface w.r.t. the z axis. Balance impulses

are not represented since they do not affect rotation.

Figure 7 Unbalanced impulses scheme, in both planes x-

z (on the left) and y-z (on the right). Distances a and b

are estimated from CAD model.

From the scheme of Fig. 7, we can write the relations

between the impulses applied to the TM and the

measured momentum:

𝜄1𝑥 + 𝜄2

𝑥 = 𝑀𝑣𝑥 (1)

ι1y

+ ι2𝑦

= 𝑀𝑣𝑦 (2)

(𝜄2

𝑦− 𝜄1

𝑦)𝑎 − (𝜄2𝑦

− 𝜄1𝑦

) tan(𝛼) 𝑏 = 𝐼𝑥𝑥 𝜔𝜃 (3)

(ι1

x − ι2x)𝑎 − (ι1

x − ι2x) 𝑡𝑎𝑛(α) 𝑏 = 𝐼yy ωη (4)

−(|𝜄1x| + |𝜄1

y|) 𝑡𝑎𝑛(𝛼) + (|𝜄2

x| + |𝜄2y

|) 𝑡𝑎𝑛(𝛼) + 𝜄𝑟𝑒𝑠z = 𝑀𝑣𝑧 (5)

where a and b are the arms of the rotations (estimated

from the system geometry) and 𝜄resz is the residual

impulse along z, i.e. the quote of z momentum that cannot

be attributed to the unbalanced impulses. Equations 1-4

can be solved independently, leading to a unique solution

for the unbalanced impulses; once the unbalanced

impulses are computed, the residual impulse can be

calculated from Eq. 5. It’s important to remark that the

nature of the residual impulse cannot be determined: it

can be the sum (or the difference) of many effects that do

not affect the TM rotation, like the balanced impulses of

Fig. 6 or any force applied from the tip to the TM (which

we assume directed mainly along the z axis).

In Fig. 8, as an example, the impulses for one of the

considered tests from TM1 are plotted. One can see that,

according to the solution of the system of equations 1-5,

the main unbalanced impulse is applied by the pyramidal

(bottom) plunger; the z component of the pyramidal

impulse constitutes a high percentage of the total z

momentum. The results are similar for all the non-

compliant fast tip tests of TM1.

Figure 8 Typical impulses on the TM in the fast tip

release tests. The residual impulse along z is small,

compared to the total z impulse.

The computation has been applied to all the non-

compliant fast tip tests; in Fig. 9 we compare the z

momentum with residual impulses (with associated


uncertainty). It can be seen that (like for the case depicted

in Figure 8. 8) for all the tests the residual impulse is

significantly lower than the z momentum, thus

suggesting that the orthogonal impulses, computed in

order to justify the transversal (x, y) and rotational (θ, η)

motions of the TM, can also motivate a high percentage

of the z momentum.

Figure 9 Total impulses along z for the considered tests.

Only a fraction of the total momentum (blue) is due to the

residual momentum along z (orange), that respects the

requirement of 10 kg μm/s in many cases. The red bars

are the expected values. The small rectangles on top of

each columns represent the 1-σ uncertainty around the

expected value.

As previously commented, the residual impulse cannot

be attributed to a unique effect since its nature is

undetermined; it could be due multiple effects on one

side of the TM (like adhesion, or balanced push of the

plunger), as well as to opposite (subtractive) effects on

the two sides. In general the residual impulse (considered

with its uncertainty) is compliant with the requirement

for z momentum (10 kg μm/s); for some tests it is higher

than the requirement, but still generating a momentum

that the capacitive actuation can control, as happened in-

flight ([14]). This means that critical z momentum is

essentially due to the (non-nominal) plunger-TM contact

at the indentations.

Summarizing, the transversal and rotational velocities of

the TM can be motivated only through a TM-plunger

contact at the indentations; the resulting impulses are

necessarily associated to a z impulse which is detrimental

for the final TM momentum, independently of other

effects (like adhesion) whose contribution cannot be

determined.

5. BI-STABILE BEHAVIOUR OF THE

MECHANISM

After analysing the effect of the lateral push of the

plungers on the z component of the impulses, the

attention was focused on one of the possible causes of

this phenomenon.

An interesting behaviour of the mechanism was

discovered from an in-flight test on the repositioning

capability of the GPRM. The test consisted in

repositioning the TM while it was grabbed by the

plungers with a preload of 2 N. The TM was repositioned

along the z axis actuating the NEXLINE, moving both

plunger simultaneously to maintain the force as constant

as possible.

In this experiment the z position of the TM is

commanded, while the other five DOFs depend on the

kinematic of the system. In the nominal case, the

correlation between the five dependent DOFs and the z

position of the TM should be zero. In other words, the

TM rotations and x, y translations should not depend on

its z position.

In the in-flight experiments, the real systems showed a

correlation of the five DOFs with z postion. In particular,

the orientation of the TM around η is clearly correlated

with the direction of motion of the plungers.

In Fig. 10, the TMs z position is plotted as function of

time, along with the attitude around η. When the direction

of motion is reversed, the TMs suddenly rotate of

approximately 60 μrad. So, there is a kind of bi-stable

equlibrium of the plungers, that orient themselves

accordingly to their direction of motion.

Figure 10 Correlation between η rotation and z position

of the TMs during in-flight repositioning test.

This behaviour is taking place on the x-z plane (since η

is the rotation around y axis), that is the same plane where

the main components of the impulses lie. This fact

suggests a correlation between the bi-stable configuration

of the system and the high x, z components of the release

velocity.

6. CONCLUSIONS

The mechanism responsible for the release of the proof

masses in the LISA Pathfinder Space mission presents

some criticalities.

It was designed to perform a dynamic release into

freefall, that fulfilled a very strict requirement in terms of

residual velocities of the test masses (5 μm/s and 100

μm/s for translations and rotations respectively).

Nominal release is performed with a quick and

simultaneous retraction of two tips, in contact with the

TM on a small area on two opposite faces.

An extensive set of on-ground tests demonstrate that the

forces present at release (adhesion and pull force due to


retraction delay) are small enough that the requirement

should be fulfilled more than 95% of the cases.

In-flight releases showed unexpected velocities, that lead

to undesired impact on the TM, and prevent its

controllability and stabilisation.

A dedicated in-flight campaign of tests was carried out,

to explore the performance of the GPRM under different

working conditions. The release procedure was updated,

adding some phases to the standard manoeuvre.

Analysing in-flight data, focusing on the impulses

received from the TM, it has been proved that the GPRM

has not worked in nominal conditions.

The TM received impulses at release from unexpected

impacts with the plungers. The impulse along the z

direction has been subdivided into two contributions:

- The contribution dependent on the net lateral

push of the plunger, that produces also a z

impulse due to the inclination of the contact

zone.

- The residual impulse generated by adhesion,

delay asymmetry and probably other effects

(that cannot be predicted).

The first contribution is the main one and produced the

higher z velocity on the TM. The second contribution is

lower, and always produced controllable z velocity on the

TM.

A possible explanation of the lateral push of the plungers

on the TM comes from the analysis of other in-flight data,

that showed some kind of bi-stable equilibrium of the

plungers on the x-z plane (the one where most release

impulses lie).

To summarise, high TM residual velocities are due to the

fact that the forces that arise at release are not the ones

tested on-ground.

Further developments of this work will concentrate on

establishing what are all the possible causes that can lead

to unexpected contacts between TM and plungers.

Since the GPRM mechanism will be used in the future

coming mission LISA, the presented findings can be used

to improve its design and enhancing its performance.

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Enda & Freschi, M & Zweifel, Peter. (2018).

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*LISA Pathfinder Collaboration - Dated: June 11, 2019

M Armano,1 H Audley,2 J Baird,3 P Binetruy,3, * M Born,2 D

Bortoluzzi,4 E Castelli,5 A Cavalleri,6 A Cesarini,7

A M Cruise,8 K Danzmann,2 M de Deus Silva,9 I Diepholz,2

G Dixon,8 R Dolesi,5 L Ferraioli,10 V Ferroni,5

E D Fitzsimons,11 M Freschi,9 L Gesa,12 F Gibert,5

D Giardini,10 R Giusteri,5, † C Grimani,7 J Grzymisch,1

I Harrison,13 G Heinzel,2 M Hewitson,2 D Hollington,14

D Hoyland,8 M Hueller,5 H Inchauspé,3, 15 O Jennrich,1 P

Jetzer,16 N Karnesis,3 B Kaune,2 N Korsakova,17 C J Killow,17

J A Lobo,12, * I Lloro,12 L Liu,5 J P López-Zaragoza,12

R Maarschalkerweerd,13 D Mance,10 N Meshksar,10

V Martìn,12 L Martin-Polo,9 J Martino,3 F Martin-Porqueras,9

I Mateos,12 P W McNamara,1 J Mendes,13 L Mendes,9

M Nofrarias,12 S Paczkowski,2 M Perreur-Lloyd,17

A Petiteau,3 P Pivato,5 E Plagnol,3 J Ramos-Castro,18

J Reiche,2 DI Robertson,17 F Rivas,12 G Russano,5, ‡

J Slutsky,19 C F Sopuerta,12 T Sumner,14 D Texier,9

J I Thorpe,19 D Vetrugno,5 S Vitale,5 G Wanner,2 H Ward,17

P J Wass,14, 15 W J Weber,5 L Wissel,2 A Wittchen,2 and

P Zweifel10

1. European Space Technology Centre, European

Space Agency, Keplerlaan 1, 2200 AG Noordwijk,

The Netherlands

2. Albert-Einstein-Institut, Max-Planck-Institut fur

Gravitationsphysik und Leibniz Universität

Hannover, Callinstraße 38, 30167 Hannover,

Germany

3. APC, Univ Paris Diderot, CNRS/IN2P3, CEA/lrfu,

Obs de Paris, Sorbonne Paris Cit´e, France

4. Department of Industrial Engineering, University of

Trento, via Sommarive 9, 38123 Trento, and Trento

Institute for Fundamental Physics and Application /

INFN

5. Dipartimento di Fisica, Universitàdi Trento and

Trento Institute for Fundamental Physics and

Application / INFN, 38123 Povo, Trento, Italy

6. Istituto di Fotonica e Nanotecnologie, CNR-

Fondazione Bruno Kessler, I-38123 Povo, Trento,

Italy

7. DISPEA, Universitàdi Urbino “Carlo Bo”, Via S.

Chiara, 27 61029 Urbino/INFN, Italy

8. The School of Physics and Astronomy, University of

Birmingham, Birmingham, UK

9. European Space Astronomy Centre, European

Space Agency, Villanueva de la Ca˜nada, 28692

Madrid, Spain

10. Institut fur Geophysik, ETH Zurich, Sonneggstrasse

5, CH-8092, Zurich, Switzerland

11. The UK Astronomy Technology Centre, Royal

Observatory, Edinburgh, Blackford Hill, Edinburgh,

EH9 3HJ, UK

12. Institut de Ciències de l1Espai (CSIC-IEEC),

Campus UAB, Carrer de Can Magrans s/n, 08193

Cerdanyola del Vallès, Spain

13. European Space Operations Centre, European

Space Agency, 64293 Darmstadt, Germany

14. High Energy Physics Group, Physics Department,

Imperial College London, Blackett Laboratory,

Prince Consort Road, London, SW7 2BW, UK

15. Department of Mechanical and Aerospace

Engineering, MAE-A, P.O. Box 116250, University

of Florida, Gainesville, Florida 32611, USA

16. Physik Institut, Universität Zurich,

Winterthurerstrasse 190, CH-8057 Zurich,

Switzerland

17. SUPA, Institute for Gravitational Research, School

of Physics and Astronomy, University of Glasgow,

Glasgow, G12 8QQ, UK

18. Department d1Enginyeria Electrônica, Universitat

Politècnica de Catalunya, 08034 Barcelona, Spain

19. Gravitational Astrophysics Lab, NASA Goddard

Space Flight Center, 8800 Greenbelt Road,

Greenbelt, MD 20771 USA

*Deceased. †Current address: [email protected] ‡[email protected]


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