Figure 24

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"Low-Frequency Gravitational Wave Searches
Using Spacecraft Doppler Tracking"

J. W. Armstrong

	Abstract
1	Introduction
2	Notation, Acronyms, and Conventions
3	Gravitational Wave Signal Response
4	Apparatus and Principal Noise Sources
	4.1	Frequency standard noise
	4.2	Plasma scintillation noise
	4.3	Tropospheric scintillation noise
	4.4	Antenna mechanical noise
	4.5	Ground electronics noise
	4.6	Spacecraft transponder noise
	4.7	Thermal noise in the ground and spacecraft receivers
	4.8	Spacecraft unmodeled motion
	4.9	Numerical noise in orbit removal
	4.10	Aggregate spectrum
	4.11	Summary of noise levels and transfer functions
5	Signal Processing
	5.1	Noise spectrum estimation
	5.2	Sinusoidal and quasiperiodic waves
	5.3	Bursts
	5.4	Stochastic background
	5.5	Classification of data intervals based on transfer functions
	5.6	Frequency-time representations
	5.7	Qualifying/disqualifying candidates
	5.8	Other comments
6	Detector Performance
	6.1	Observations to date
	6.2	Near-future observations
	6.3	Sensitivity to periodic and quasi-periodic waves
	6.4	Burst waves
	6.5	Sensitivity to a stochastic background
7	Improving Doppler Tracking Sensitivity
8	The LISA Low-Frequency Detector
9	Concluding Comments
	Acknowledgements
	References
	Footnotes
	Updates
	Figures
	Tables

Figure 24: Upper limits to the energy density of GWs in bandwidth equal to center frequency, relative to closure energy density. This assumes an isotropic GW background, H₀ = 75 km s^–1 Mpc^–1, and is computed from the Cassini 2001 – 2002 data [19].