At what point did Obi-Wan Kenobi know that Darth Vader was on board of Death Star?

Question

Based on an assertion in the answer to: " When Can the Dark Side Be Sensed? ":

Is there some canon (ideally, G-canon but not required) information clearly indicating whether Obi-Wan Kenobi know that Darth Vader was on board of Death Star before they actually physically bumped into each other?

If earlier, just when did he find out?

UPDATE: To clarify, "Vader knew when he discussed it with Tarkin" is NOT evidence for or against, unless there's a canon proof that Obi-Wan must have detected Vader at the same time as Vader detected Obi-Wan

Answer 1

The novelisation for Star Wars: A New Hope (listed as having been authored by George Lucas but famously ghost-written by Alan Dean Foster) places Obi-Wan's sensing of Vader's presence much later on than Vader's conversation with Tarkin:

Kenobi had avoided one patrol after another, slowly working his way back toward the docking bay holding the freighter. Just another two turns and he should be at the hangar. What he would do then would be determined by how inconspicuous his charges had been.

a few minutes later

Ben sensed something directly ahead and slowed cautiously. It had a most familiar feel to it, a half-remembered mental odor he could not quite place. Then the figure stepped out in front of him, blocking his entry to the hangar not five meters away. The outline and size of the figure completed the momentary puzzle. It was the maturity of the mind he had sensed that had temporarily confused him. His hand moved naturally to the hilt of his deactivated saber.

Although Kenobi may have subconsciously suspected something was wrong (his insistence on Luke staying behind, the fact that he repeatedly checks his lightsaber, etc) he doesn't consciously realise that it's Anakin/Vader until he practically falls over him.

Answer 2

The first time Vader felt Obi-Wan's presence was shortly after the Millennium Falcon landed. From The Star Wars: A New Hope script

VADER: Send a scanning crew on board. I want every part of this ship checked.
OFFICER: Yes, sir.
VADER: I sense something...a presence I haven't felt since...
Vader turns quickly and exits the hangar.

We could assume Obi-Wan also felt his presence at this time, but he said nothing about it. He made his plan and later, Vader sensed his intentions. Just after Luke and Han made Leia out of her Cell, Vader had this discussion with Tarkin.

Also from The Star Wars: A New Hope script

VADER: Obi-Wan is here. The Force is with him.
TARKIN: If you're right, he must not be allowed to escape.
VADER: Escape is not his plan. I must face him alone.

So, Obi-Wan must be aware of Vader's presence before that because he felt that Obi-Wan planned his duel with him. To plan this duel, Obi-Wan must be aware of the presence of his apprentice.