AMIGOS workshop, October 15 2005

Noise Characterization Using Transfer Functions: Examples from Spacecraft Doppler Tracking

John Armstrong for the Cassini Radio Science GW Group*

* R. Ambrosini, J.W. Armstrong, B. Bertotti, L. Iess, P. Tortora, H.D. Wahlquist

AMIGOS workshop, October 15 2005

Noise Characterization Using Transfer Functions

• The Doppler technique & its signal and noise transfer functions

• Examples of noise identification using transfer functions– Frequency & timing system– Antenna mechanical noises– Phase noise due to propagation through irregular media

• Model example for LISA: identifying proof mass noises

AMIGOS workshop, October 15 2005

DSS25 and Cassini

AMIGOS workshop, October 15 2005

Cassini Radio System

AMIGOS workshop, October 15 2005

Three-Pulse GW Response

AMIGOS workshop, October 15 2005

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

AMIGOS workshop, October 15 2005

Frequency/Timing Glitch

AMIGOS workshop, October 15 2005

Antenna Mechanical Event

AMIGOS workshop, October 15 2005

Plasma Events

AMIGOS workshop, October 15 2005

AMIGOS workshop, October 15 2005

Isolating Proof-Mass Noises with Transfer Functions

• LISA unequal-arm geometry• Nomenclature for proof

mass disturbances vi and vi*• Idea: isolate which proof

mass glitched (Braginsky, ≈2001) -- or which proof mass is noisier -- through their transfer functions

AMIGOS workshop, October 15 2005

Isolating Proof-Mass Noises with Transfer Functions

• Temporal transfer fucntion of 6 PM’s to the three unequal-arm Michelson combinations (X, Y, Z) + symmetrical Sagnac ()

• First generation TDI

AMIGOS workshop, October 15 2005

GW Transfer Function to TDI “X”

• Source/detector geometry dependent

• “8-pulse” response in general for first-gen TDI [equal arm lengths assumed in this example so it degenerates to 7-pulse response; ApJ 527, 814 (1999)]

• Will not be confused with PM glitches

AMIGOS workshop, October 15 2005

Isolating Proof-Mass Noises with Transfer Functions (Statistically)

• Noise-only spectra of first-generation TDI combinations X and Z (expressed here as spectra of fractional Doppler fluctuations, vs. phase)

• Proof mass 1 assumed 10X noisier than nominal

• PM1 does not enter in Z, so it is OK -- spectrum of X affected, however

CQG, 20, S283 (2003)

AMIGOS workshop, October 15 2005

Concluding Comments• In spacecraft Doppler tracking, at least, the data

themselves are much more sensitive than any of the engineering housekeeping channels which monitor experiment health -- so you have to use the data themselves, in general, to understand the noises

• I would expect this to be true for LISA, too

• Transfer functions of the signal and noises to the TDI combinations offer a way to use the “science” data to understand the noises and instrument performance