This paper discusses spacecraft Doppler tracking, the current-generation detector technology used in the low-frequency (∼millihertz) gravitational wave band. In the Doppler method the earth and a distant spacecraft act as free test masses with a ground-based precision Doppler tracking system continuously monitoring the earth-spacecraft relative dimensionless velocity 2Δv/c = Δν/ν0, where Δν is the Doppler shift and ν0 is the radio link carrier frequency. A gravitational wave having strain amplitude h incident on the earth-spacecraft system causes perturbations of order h in the time series of Δν/ν0. Unlike other detectors, the ∼ 1-10 AU earth-spacecraft separation makes the detector large compared with millihertz-band gravitational wavelengths, and thus times-of-flight of signals and radio waves through the apparatus are important. A burst signal, for example, is time-resolved into a characteristic signature: three discrete events in the Doppler time series. I discuss here the principles of operation of this detector (emphasizing transfer functions of gravitational wave signals and the principal noises to the Doppler time series), some data analysis techniques, experiments to date, and illustrations of sensitivity and current detector performance. I conclude with a discussion of how gravitational wave sensitivity can be improved in the low-frequency band.

中文翻译：

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使用航天器多普勒跟踪进行低频引力波搜索。

本文讨论了航天器多普勒跟踪，这是用于低频（~毫赫兹）引力波波段的当前一代探测器技术。在多普勒方法中，地球和遥远的航天器充当自由测试质量，地面精密多普勒跟踪系统连续监测地球-航天器相对无量纲速度 2Δv/c = Δν/ν0，其中 Δν 是多普勒频移，ν0 是无线电链路载波频率。入射到地球-航天器系统上的应变幅度为 h 的引力波会在 Δν/ν0 时间序列中引起 h 阶扰动。与其他探测器不同，约 1-10 AU 的地球与航天器间隔使得探测器比毫赫兹带引力波长更大，因此信号和无线电波通过设备的飞行时间非常重要。例如，突发信号被时间分解为特征签名：多普勒时间序列中的三个离散事件。我在这里讨论该探测器的工作原理（强调引力波信号的传递函数和多普勒时间序列的主要噪声）、一些数据分析技术、迄今为止的实验以及灵敏度和电流探测器性能的说明。最后我讨论了如何提高低频段的引力波灵敏度。