In February 2007, the fourth and last satellite of the BeiDou-1 system, BeiDou-1D (sometimes called BeiDou-2A, serving as a backup satellite), was sent up into space. It was reported that the satellite had suffered from a control system malfunction but was then fully restored.
Which satellite serves as a front-end satellite? (If the question is unanswerable, say "unanswerable")
unanswerable