RF mute events for Lagrange missions: Lessons learned from Herschel and Planck

摘要

This paper will discuss operations of space data links to spacecraft operating in Sun-Earth Lagrange points L1 and L2. These orbits are relatively stable and place an object on either the daylight side of the Earth (in the case of L1) or the night side of Earth (in the case of L2) at a distance of about 1.5 million kilometers. When viewed from the Earth, motion of the spacecraft is driven primarily by the rotation of the Earth. Visibility periods for a Lagrange spacecraft are on the order of 12-15 hours each day. When designing the space link, International Telecommunications Union (ITU) regulations stipulate that spacecraft closer than 2 million kilometers are treated as near Earth objects (Class-A) and spacecraft beyond 2 million kilometers are treated as deep space objects (Class-B). Lagrange missions being of class-A, the admissible minimum elevation angle of the ground station antenna used for uplink is as low as 5 degrees. In order to optimize the utilization of the ground station resources, scheduled passes have to start (or to stop) at the limit of the 5 degrees elevation as a constraint to visibility windows because radiating an uplink signal below 5 degrees is prohibited. The ITU also limits the Effective Isotropic Radiated Power (EIRP) of the uplink at low elevation above 5 degrees. For ESA ground stations, the Front End Controller is programmed to automatically mute the RF output when the ground station might violate this ITU regulation. In order to avoid an automatic switch-off of the uplink power during scheduled ground station passes, a solution was investigated and implemented for the Herschel and Planck projects. In this paper, a number of mitigation strategies are discussed to avoid these RF mute events or to integrate them in a proper way into the scheduling of the ground stations. Lessons learned from this experience could be applied to future missions being planned for L1 and L2 orbits.