Figure 9

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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 9: Doppler time series for DSS 25 Cassini track on 2003 DOY 324. Upper panel: time series of the two-way X-band, showing two discrete events at about 10:20 and 10:40 ground received time, echoed about a two way light time, T₂, later. Middle panel: time series of X-(880/3344) Ka1, which isolates the downlink plasma (and cancels nondispersive noises and signals: FTS, troposphere, antenna mechanical noise, and GWs). This shows that the events observed in the upper panel are due to plasma scintillation. Lower panel: acf of the two-way Doppler time series. The arrow marks the two-way light time. The lower right panel is a blow-up of the acf near the two-way light time (indicated by the vertical line). The acf peaks at lags slightly smaller than T₂ $≃$ 8021.5 s, indicating that the features observed in the upper panels are caused by near-earth plasma.AepubtkUpdateA2Modified figure to discuss the blowup of the acf.