The Cassini May 2000 solar conjunction

Abstract

Interplanetary spacecraft, which fly in the ecliptic plane, typically encounter solar conjunctions during their main missions. The communications link between an interplanetary spacecraft and Earth is affected by the charged particles that constitute the intervening solar corona and solar wind. As the Sun-Earth-probe (SEP) angle becomes small (usually 3 for X band or 8.43 GHz), the signal suffers increased degradation. The effects on the received signal include time delay and phase fluctuations due to the fluctuating columnar electron density, which in turn cause carrier lock problems and telemetry data loss. Because of these effects, studies of solar corona charged particle effects on spacecraft signals were conducted to determine strategies for optimizing data return during these periods. The first solar conjunction of the Cassini spacecraft occurred between May 8, 2000 (2000/129) and May 18, 2000 (2000/139). During this period, the Cassini spacecraft was within 3.2 of the Sun as seen from Earth with the minimum SEP angle of 0.56 occurring on May 13 (2000/134). This solar conjunction occurred prior to the expected peak of the current solar cycle. Coherent dual-frequency X band (8.43 GHz) and Ka band (32 GHz) data were acquired from 3.2 to near the minimum SEP angle at 0.6 for both ingress and egress. The measurements of amplitude scintillation, spectral broadening and phase scintillation were examined as a function of SEP angle. As expected, these solar effects are significantly less at Ka band than at X band for the same SEP angle. This study's results will be combined with those of other spacecraft solar conjunctions in order to build a statistical database of solar effects as a function of solar elongation angle and phase of the solar cycle. Such studies are useful in the design of telecommunications systems for future spacecraft missions, which may have stringent communication requirements during their solar conjunction phases.

Spectral broadening and phase scintillation measurements using interplanetary spacecraft radio links during the peak of solar cycle 23

“…A significant increase in Doppler noise however was observed in the X band frequency residuals between 2000/133 00:00 UTC to 01:00 UTC [Morabito et al, 2003].…”

“…One method produces estimates of received frequency residuals using software PLL processing on each individual band of FSR data where long period (nonsolar) frequency trends were removed. The RMS scatter of the resulting frequency data was found to increase markedly at both bands as the SEP angle decreased [Morabito et al, 2003], where the X band scatter always exceeded the level of the Ka band scatter by the expected ratio. Examples of Allan deviation curves for selected time scales at both X band and Ka band were previously reported on [Morabito et al, 2003] for cases of SEP = 3.1°and SEP = 0.6°.…”

“…The RMS scatter of the resulting frequency data was found to increase markedly at both bands as the SEP angle decreased [Morabito et al, 2003], where the X band scatter always exceeded the level of the Ka band scatter by the expected ratio. Examples of Allan deviation curves for selected time scales at both X band and Ka band were previously reported on [Morabito et al, 2003] for cases of SEP = 3.1°and SEP = 0.6°. The phase fluctuations over all time intervals at SEP = 0.6°were significantly elevated relative to those observed at SEP = 3.1°as expected.…”

“…Significantly more detail is provided in this plot over that originally presented in Figure 12 of Morabito et al [2003], where only a small subset of selected quiet period data were shown. Here in Figure 5, the full series of B measurements are plotted for each of the smallest SEP angle passes (jSEP anglej < 2°) of the Cassini 2000 solar conjunction.…”

“…The measurements of spectral broadening are useful in assessing optimum carrier tracking loop parameters for given situations such as loop bandwidth and update time, as well as aiding in the design of future FSK or frequency semaphore systems (such as determining the temporal durations and placement of symbols in frequency space). Closed loop receiver tracking performance was previously evaluated on these data while exercising several combinations of phased locked loop (PLL) tracking loop parameters at very small SEP angles [Morabito et al, 2003]. Several recommendations for telecom strategies for the case of maintaining a communication link between Earth and Mars at small SEP angles were provided in an earlier study [Morabito and Hastrup, 2002].…”