On-Orbit Stationkeeping with Ion Thrusters Telesat Canada's BSS702 Experience

Abstract

In September of 2001 Telesat acquired control of Anik F1, the first of three Boeing Satellite Systems (BSS) 702 type spacecraft in the Telesat fleet and Telesat’s first introduction to ion propulsion. In the following two months handover of control to Telesat of two more BSS-702 spacecraft, XMRadio 1 and XMRadio 2, was completed. Telesat Canada is one of only two operators of the BSS-702 bus to date. The move to ion propulsion for on-orbit control has given rise to new control strategies and has required enhanced estimation techniques and improved monitoring processes to be implemented on the ground side of operations. The end result has been a very successful transition to this new method of controlling geostationary satellites. The BSS-702 bus uses Boeing’s Xenon Ion Propulsion System (XIPS) with four low thrust, high efficiency XIPS thrusters for full stationkeeping and momentum control. Unlike conventional bipropellant stationkeeping strategies, daily multiple XIPS firings afford relatively tight control over drift, eccentricity, inclination and momentum. Daily firings present new challenges for the orbit analyst in areas such as orbit determination, thrust and cant angle bias estimation, and performance monitoring. The use of a Kalman Filter which processes tracking data and XIPS firing telemetry to estimate the orbit and various biases in real time is key to the success Telesat has had in planning and monitoring XIPS stationkeeping cycles. The Flight Dynamics Operations (FDO) group has created a number of automated scripts and software to simplify daily checking of the plan progress. This has proven to be very important and necessary in the low thrust realm where it is no longer easy to effect quick changes to orbit parameters. This paper will highlight the operational differences between stationkeeping standard bipropellant satellites and satellites using ion thruster propulsion systems. 1. Background While ion propulsion subsystems are relative newcomers to commercial space operations, the concept is not at all new. The concept of electric propulsion was first introduced by Dr. Wernher von Braun in the 1930s, and the first ion engine was developed in 1961. Early engines used Cesium or Mercury and were unsuitable for commercial operations. In the 1980s and 90s ion engines using inert Xenon gas were developed and in 1997 the first commercial spacecraft to use Xenon ion engines was launched. [1] [2]