Observations to date

6.1 Observations to date

Table 3 lists spacecraft Doppler GW observations to date.

Table 3:

Spacecraft Doppler Gravitational Wave Observations. MO: Mars Observer; GLL: Galileo; ULS: Ulysses; MGS: Mars Global Surveyor; AMC: Advanced Media Calibration system (tropospheric calibration). A “pass” is a tracking pass over a given DSN antenna, i.e. about 8 hours long.



Year	Spacecraft	Comment	Reference


1980	Voyager	several passes; S-band uplink, S/X downlink; burst search	[57 ]
1981	Pioneer 10	3 passes; long T₂; GW background limit; observationally excluded GW from Geminga	[4 , 6 ]
1983	Pioneer 11	3 days; long T₂; broadband periodic search	[18 ]
1988	Pioneer 10	10 days; long T₂; chirp wave search	[5 ]
1990	Ulysses	3.5 days; short T₂	[26 , 25 ]
1992	Ulysses	14 days; long T₂; periodic and chirp search; S-band uplink, S/X band downlink	[26 , 25 ]
1993	MO/GLL/ULS	19 days; X/X-band on MO; coincidence experiment; search for all waveforms	[60 , 61 , 13 ]
1994-5	Galileo	40/40 days (two oppositions); long T₂; S/S-band	[3 , 13 ]
1997	MGS	21 days; short T₂; X/X-band	[9 ]
2001-3	Cassini	40/40/20 days (three oppositions); long T₂; Ka-band; AMC; multi-link plasma calibration	[19 , 74 ]

The observations in Table 3 reflect increasing sensitivity between $≃$ 1980 and the present. These sensitivity improvements were due both to engineering improvements (in spacecraft and in the DSN) and to programmatic decisions allowing use of planetary spacecraft for these observations. Voyager sensitivity was limited by a combination of plasma noise in the S-band uplink (see Figure 10 ) and spacecraft buffeting noise from its thrusters. Although the data volume was small, those observations were used in the first formal search for low-frequency burst waves [57 ]. The Pioneer spacecraft were spin-stabilized, resulting in lower spacecraft buffeting noise, but were again sensitivity-limited by plasma noise in the S-band radio links. Despite this, Pioneer data were able to observationally exclude putative sinusoidal GW emission from Geminga [4 ] and place the then-best limit on a low-frequency GW background [6 ]. Ulysses observations in 1992 were the longest to date, motivated innovations in signal processing, and resulted in the then-best sensitivity to periodic and chirp waveforms [26 , 25 ]. Mars Observer was the first spacecraft to have X-band on both the up- and downlinks, resulting in much-reduced plasma noise. Mars Observer, Galileo, and Ulysses did the first (and so far only) coincidence experiment [60 , 13 ] which was used successfully to disqualify an event which was formally significant in one time series. Galileo observations in 1994 – 1995 had a long two-way light time and thus better GW response at lower Fourier frequencies. Unfortunately, the failure of the high-gain antenna required S-band only observations (thus high plasma noise) [9 ]. Mars Global Surveyor observations in 1997 were done with X-band links (but off of solar opposition) and were the only observations where spacecraft engineering telemetry was used to correct the Doppler data for the (slow) systematic spacecraft motion. Those data also showed strong correlation at the two-way light time, indicating the importance of tropospheric calibration and placing upper limits on antenna mechanical noise [9 , 22 ].

Most of the sensitivity discussion in this paper, however, relates to Cassini observations. Cassini was launched on a mission to Saturn in 1997 [68 ]. After earth, Venus, and Jupiter gravity-assists, it continued on a free interplanetary cruise trajectory toward orbit insertion at Saturn. The Cassini gravitational wave observations consisted of two 40-day data-taking campaigns, centered on the spacecraft’s solar oppositions during 2001 – 2002 and 2002 – 2003, and one 20 day observation taken somewhat off opposition during late 2003. This data set is distinguished by its very sophisticated multi-link radio system (allowing essentially perfect plasma correction [64 , 114 , 59 , 30 , 113 ]) and by the Advanced Media Calibration system (allowing excellent tropospheric scintillation removal) [88 , 90 , 19 , 72 ].