Tropospheric scintillation noise

4.3 Tropospheric scintillation noise

Phase fluctuations also arise from propagation through the neutral atmosphere. Here the so-called dry component of the troposphere is large but fairly steady with the wet component (water vapor fluctuations) being smaller but much more variable [20

, 89

, 88

, 90

, 72

]. Unlike plasma phase scintillation, propagation in the troposphere is effectively non-dispersive at microwave frequencies [65 ]. Figure 10

shows the magnitude of the effect: The blue cross-hatched region is the approximate level of uncalibrated tropospheric scintillation at NASA’s Goldstone Deep Space Communications Complex [71

]. Roughly, tropospheric scintillation is worse in the summer daytime and better on winter nights. Usually its raw magnitude is large compared with, e.g., antenna mechanical noise (discussed below.)

Figure 11:

Two water-vapor-radiometer-based Advanced Media Calibration units located near DSS 25, shown here in November 2001. These are used to calibrate tropospheric phase scintillation for the Cassini Gravitational Wave Experiment and other Cassini precision Doppler tracking observations. (Mark Gatti, project manager for the Cassini Radio Science ground system upgrades, is in the foreground.)

Experiments by George Resch and colleagues [89 , 88 , 90 ] were influential in showing that suitably boresighted water vapor radiometer measurements could calibrate and remove much of the tropospheric scintillation noise in both radioastronomical and precision spacecraft Doppler tracking observations. A water-vapor-radiometer-based Advanced Media Calibration (AMC) system (Figure 11 ) was developed and installed near DSS 25 to provide tropospheric corrections for Cassini radio science observations. The AMC system [88 , 90 , 72 ] consists of two identical units placed close enough to each other and to DSS 25 that the coherence of the tropospheric signal on the time scales of interest was high in all three time series (see [10 , 11 ] for examples of the squared-coherence as a function of Fourier frequency). The AMC calibrations were used successfully in both the Cassini gravitational wave observations [19 ] and in relativity and plasma experiments taken near solar conjunction [30 , 114 , 113 , 22 ]. The transfer function of tropospheric scintillation to the two-way Doppler is $tropo y ∗ [δ(t) + δ(t − T2)]$ . Examples of the cross correlation function of Doppler and the AMC-estimated tropospheric scintillation are shown in [10 , 11 ].