LISA Pathfinder: Acquisition of Signal Analysis after Launch Injection and Apogee Raising Manoeuvres

The delivery of the S/C by the launcher into a Low Earth Orbit (LEO), followed by a series of long duration manoeuvres to guide it to its final operational orbit around L1 is a particular challenge for the signal acquisition. The complexity is explained by the fact that the dispersion of a launcher such as VEGA or Rockot greatly exceeds the 3dB (half-power) beam width of a 15m ESA network antenna in X-band. Furthermore, the S/C is flying over the short duration ground station passes equipped with a Low Gain Antenna of 9 dB gain variation, transmitting telemetry in low power mode and with an altitude varying from 200 to 1600km. The solution found to cope with a large orbit dispersion was to develop an Xband Acquisition Aid (XAA), small diameter antenna (1.2m) covering the full 3σ orbit dispersion area within its half-power beam width. However, such a small antenna is limited in its capability in acquiring a downlink signal. Given the specified LPF 200x1600km LEOP orbit and the S/C launch configuration (low power mode and high data rate), the signal strength received by the XAA is not sufficient to guarantee carrier recovery when the S/C altitude exceeds 630km. It should be noted that the orbit dispersion increases not only as a function of the antenna elevation, but also as a function of flight time. This underlines the necessity to accomplish the spacecraft initial acquisition as soon as possible using the available short station passes. The other issue is that once the 1 st acquisition has succeeded and the orbit is known with enough accuracy, the S/C is brought to its escape orbit to L1 via a sequence of large main engine burns. If the full range of main engine performance 0% to 110%, is considered, this leads to significant time as well as azimuth shift of the acquisition of signal (AOS) location. This paper explains the mitigation measures found to optimize the acquisition of signal under the unfavorable conditions of a tight link budget and high orbit dispersion both after launch injection and each main engine burn.