Barker was sacked by League One Crawley a fortnight ago and now finds himself in the bottom half of League Two after taking over from Guy Whittingham at Fratton Park.
"It's very different," he said."I've been managing in League One for two and a half years and obviously I've dropped to League Two but with the size of this club, I've gone up massively.
"It will be a very different job, equally difficult. Keeping Crawley Town and Bury in League One was difficult but this one can go a long way."
Barker's dismissal came as something of a surprise with Crawley lying in a respectable 12th spot, although they had not won in seven league games.
But the 38-year-old admits the shock decision could prove to be a blessing in disguise.
"We were four games unbeaten, in the top half and through to second round of FA Cup, but it is what it is and without that I probably wouldn't have got this opportunity," he added.
"I thought keeping Crawley in the top half of League One, within touching distance of the play-offs, with 19 players was quite a big achievement.
"Have I got a point to prove? Yes, a little bit. But it's behind me, I've moved on and been given a fantastic opportunity. It will be a point to prove if I can take this club where it needs and deserves to be."