9 Concluding Comments
This paper discussed the principles of operation and status of spacecraft Doppler tracking, the
current-generation GW detector technology in the
![∼](article239x.gif)
10
–6 to 10
–1 Hz band. Doppler tracking differs from
all other currently-operating detectors in that the size of the apparatus (earth-spacecraft distance) is large
compared with the GW wavelength. As a consequence times-of-flight of GWs and radio waves through the
apparatus are important, resulting in a three-pulse signal response and various two-pulse noise responses.
The different signal and noise transfer functions suggest data analysis approaches for various waveforms;
some of these approaches were outlined here. The sensitivity of current-generation Doppler observations was
discussed as well as what would be required to improve this sensitivity by another order of magnitude (to
![∼](article240x.gif)
10
–17 for sinusoidal waves). Further large sensitivity improvements in the low-frequency band
will require dedicated multi-spacecraft arrays in space. Until such a dedicated mission flys,
spacecraft tracking will provide the best observational capability in the low-frequency GW
band.