How was the Giotto spacecraft reactivated from hibernation mode?

Question

The Wikipedia article on Giotto states that:

Giotto was the first spacecraft to be re-activated from hibernation mode.

• How was the reactivation accomplished?
• Does the statement mean the spacecraft restarted after a shutdown, or something else?

Answer 1

Giotto woke up from hibernation after a signal from ground control. This meant that at least the radio receiver and parts of the computer still had to be active. The science instruments and most of the spacecraft management systems (e.g. attitude control) were switched off.
The wakeup took several days. After the wakeup, Giotto aimed its main antenna at Earth for new instructions.
A recent question about Rosetta shows that a different approach was used there.

Answer 2

I think "hibernation" isn't a well defined term, its simply the name ESA gave to that mode of operation.

Here are some extra points beyond those mentioned by @Hobbes

By the sound of it this depended on an omni directional antenna and thus potentially relying on battery power rather than solar array. The lowest power consumption approach would be to power only the command receiver and command decoder and sufficient relays to enable subsequent switching in of a telemetry transmitter and achieve re-orientation. Obviously the last step is critical to prevent further battery discharge but the telemetry path is also pretty important to seeing what the status is on wake-up which could affect how one goes about the attitude recovery.

There is actually no need for a conventional computer in any of these steps, lets remember it was launched in 1985. All that is required is the command decoder function that turns the bit stream into hard-wired relay activations on board.

Thus far, there is very little novel about this sequence other than the apparent lack of telemetry. That aside it is exactly the path most old satellites would have taken as part of an emergency recovery sequence. Where once could have substituted the phrase:

re-activated from hibernation mode

for "recovery from loss of attitude".