Experiment Design and Performance. G. Catastini TAS-I (BUOOS)

Transcription

1 Experiment Design and Performance G. Catastini TAS-I (BUOOS) 10

2 EP and the GRT Einstein s General Relativity Theory Weak Equivalence Principle: all test particles at the same space-time point in a given gravitational field will undergo the same acceleration Gravity locally equivalent to accelerated frame Free falling frame locally equivalent to inertial frame WEP + Special Relativity GRT Galileo was right!! 11

3 GG: the signal The Signal (1) Scientific objective: η = Signal optimization: choice of the test masses materials Signal optimization: choice of the orbit test masses differential acceleration due to EP violation: a EP = g(h) m/s 2 Orbit altitude h as low as possible to maximise g(h), but h as high as possible to match air drag reference acceleration (depending on s/c A/M ext and solar activity during flight) m/s 2 a NG xy GG Phase A2 study spacecraft final configuration: m s/c = kg (launch mass, no margin) A s/c = 2.9 m 2 h = 630 km ( GPB!) g(h) = m/s 2 12

4 GG: the signal The Signal (2) Signal optimization: equatorial orbit Inclination depends on the launch site latitude, Kourou I 5 Eccentricity is not zero: e 0.01 Period of longitude variation of ascending node T asc_node 48 days Spin axis initially aligned with the orbit angular velocity within δi 0 1 The angle between the spin axis and the orbit angular velocity changes also due to the gravity gradient and magnetic torque acting on the satellite: δi =δi 0 (launch) + δi orbit + δi Torques after about 1-2 years But: angle is changing very slowly, each fundamental experiment is carried out with constant driving signal 13

5 GG: the signal 14 The Signal (3) Performance check vs. simulated results Signal to Noise Ratio The science measurement is: x EP = a EP (T diff ) 2 /(4 π 2 ) T diff according to the simulator, T diff = 500 s Science measurements are affected by systematic errors d x (t) and by stochastic errors n~ (t) x ' EP (t) = x (t) + d (t) n~ (t) EP x + measurement EP violation signal, x EP m Same allocation (50%) for deterministic and stochastic errors Error allocation such that SNR 2 The deterministic effects with the EP signature must be reduced as much as possible: spin modulation takes away a lot of disturbances due to the spacecraft (DC effects in the Body Fixed reference frame) The stochastic effects define T int : thermal noise Tint 1 week Mission duration 2 years Number of Tint 100 (rich statistics!) x

6 GG: drag forces Non Gravitational Forces Acting on the Spacecraft (1) Non-gravitational forces are sensed as inertial acceleration from test masses ideally pure common mode, but: test masses mechanical suspension is not ideal a fraction χ CMRRxy of common mode acceleration is sensed as differential one ext a NG xy Orbit altitude h = 630 km in order to have m/s 2 The Drag Free Control partially compensates the non-gravitational ext forces (@ orbit frequency in IRF) a CMxy = χ DFCxy a NG xy reduced mechanical balance requirement for the capacitance bridge reduced inertial acceleration acting on PGB, i.e. smaller displacement reduced test masses common mode acceleration, i.e. smaller displacement 15

7 GG: drag forces Non Gravitational Forces Acting on the Spacecraft (2) Main non-gravitational force component along Y LVLH difference wrt EP signal, SNR = 2 ext a NG xy 0.5 χ DFCxy χ CMRRxy a EP 0.5 1/SNR 90 phase χ CMRRxy 10-5 χ DFCxy Maximum inertial acceleration sensed from PGB a CMxy = m/s 2 PGB XY plane oscillation period T PGBxy = s displacement << capacitance gap, whirl period is slow TM XY plane common mode period T CM = s TM displacement << capacitance gap, whirl period is slow Mechanical balance of capacitance bridge χ bridge (T CM = 104 s) = a-b << 3 µm over a gap a 5 mm 16

8 GG: drag forces Non Gravitational Forces Acting on the Spacecraft (3) Non-gravitational acceleration time series (frozen values for m s/c and A s/c ) Date: 2013 July 12 th GG altitude h = 630 km Atmospheric model: MSIS 86 Solar radiation included (F10=180, F10B=160, Geomagnetic Index = 8) 17

9 GG: drag forces Non Gravitational Forces Acting on the Spacecraft (4) Main non-gravitational acceleration component along YLVLH phase difference wrt EP signal, but SNR = 2 90 The EP violation signal is always along X in the LVLH Frame and almost constant during an elementary science experiment. Like EP violation!! Like EP violation, but 90 out of phase!! 18

10 GG: drag forces Non Gravitational Forces Acting on the Spacecraft (5) Main non-gravitational acceleration after DFC compensation and suspension rejection CMRR- fulfills the requirement dyang=0.217 pm dxang=0.011 pm 19

11 GG: gravitational forces The Gravitational Forces TM higher mass moment coupling with Earth monopole TM quadrupole mass moment couples with Earth monopole Inner TM δj i /J i = Outer TM δj o /J o = Test masses quadrupole are both small and of the same order Differential acceleration = m/s a EP Tidal effects (displacements on XY plane) Gravity gradient T jj magnitude corresponding to reference local gravity: T jj (h GG ) = m/s 2 /m T jk (h GG ) = m/s 2 /m (worst case) Signature difference with respect to EP signal During science measurement the TM whirl radius r w must not affect the EP signal detection (through T jj ): r w m 20

12 GG: error budget GG Error Budget (1) Amplitude Spectrum of EP signal and disturbing effects (affecting measurements in the XY plane) as observed in the IRF Signal and most effective deterministic disturbances (smallest frequency separation) Most effecting disturbances at ν EP, well below the signal One week duration of the elementary experiment ensures good frequency separation for all the competing lines, also the first one at 2 ν EP 21

13 GG: error budget GG Error Budget (2) Detailed error budget for the first lines of deterministic forces affecting GG. 22

14 GG: error budget GG Error Budget (3) 23 Acceleration (transverse plane) Frequency in IRF Differential Differential Frequency in BF (Hz) Phase due to: (Hz) acceleration (m/s 2 ) displacement (pm) EP signal νorb νs (wrt Earth) X LVLH External non gravitational forces νorb νs Mainly along Y LVLH External non gravitational forces after DFC compensation and νorb νs Mainly along Y LVLH CMRR Earth coupling with TMs quadrupole moments ν orb ν S X LVLH Radiometric effect along Z coupled with Earth tide 2 νorb νs ± νorb X LVLH Emitted radiation along Z coupled to Earth tide 2 νorb νs ± νorb X LVLH Tide coupled to non grav. acceleration along Z 2 νorb νs ± νorb X LVLH TM1 inner magnetic dipole coupled to B magnetized TM2 2 νorb νs ± νorb X LVLH TMs inner magnetic dipoles coupled to B 2 νorb νs ± νorb X LVLH TM1 and TM2 with B induced magnetization couple 4 νorb νs ± 3 νorb X LVLH TM1 with B induced magnetization couples with B 4 νorb νs ± 3 νorb X LVLH Whirl motion coupled to Earth tide νs ±νw ν w, ν w ± 2 ν orb (νorb << νw) X LVLH Frequencies far from Higher frequencies ν S X LVLH

15 GG: error budget GG Error Budget (4) Variation of 2ν EP due to the change of the angle in between spin axis and orbit angular rate Variation very slow in time (signal and line are almost constant during one week) 11 pm can be distinguished very well with one week of continuous data Error budget is here 24

16 GG: error budget GG Error Budget (5) Thermal noise due to TMs spring dissipation (Q TM = 20000) directly affects the science measurement (it is the main stochastic disturbance): 4K BT DM SD(n ~ ω TM th) = T int 1 week m Q TM TM Overall dx LVLH takes into account deterministic and stochastic contribution ~ 14 2 SD( n ) = m/s / Hz TM th Overall dx LVLH SD ( ~ n TM th = (dx LVLH) + (dylvlh) + 4 Tint ωdm ) 25 T int : elementary experiment duration

17 GG: Simulated Performance Simulator for GG Hz (1) Spacecraft shell, PGB, TMi, TMo, dummy body for not solving the orbit motion in a rapidly spinning reference frame: 27 DoFs Gravity and gravity gradient are on Current mass/inertia properties for all bodies (included proof masses quadrupole moment) Orbit altitude h = 630 km to match the reference non gravitational acceleration m/s 2 Stiffness are reproducing PGB modes and common and differential proof masses modes in the XY plane and along Z according to the mission requirements Mechanical quality factor is lowered for TMs in order to amplify whirl motion Environment fully modeled η = for all the science simulations (science target) Quadruple precision simulations are carried out in order to predict science performance of the mission 26

18 GG: Simulated Performance Simulator for GG Hz (2) 1 Hz is better for the simulator CPU time is reduced by a factor 2 Dynamics is changed, orbit is changed and F EP is changed too but FEP Predicted value: x LVLH = 2 GM mtm ωdm 3 (R + h) Measured value: x LVLH = m!! Time series of the TMs differential displacement in the LVLH Reference Frame 3 = m Polar plot of the TMs differential displacement in the LVLH Reference Frame After a transient the TMs relative position has almost null mean value along Y LVLH and pm mean value along X LVLH 27

19 GG mythology Conclusions It s nice weather today!! I ll spend a little time by bike, visiting the S. Piero Basilica: it is close to the INFN old laboratory, and someone told me there is a strange alien monolith inside, devoted to my Equivalence Principle. Uhm, people here are very strange: they believe God plays dice, and everyone knows He only likes to play chess 28