The Cassini May 2000 solar conjunction

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 <3degrees 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 3.2degrees of the Sun as seen from Earth with the minimum SEP angle of 0.56degrees 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 3.2degrees to near the minimum SEP angle at 0.6degrees 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.