Britney Spears' Big Concert Announcement -- Desert, Helicopter, 567 Red Bulls

Britney Spears will spill the beans about her upcoming Vegas spectacular tomorrow, TMZ has learned -- and the announcement itself will be an extravaganza in the middle of nowhere.

We're told Britney will unveil plans for her big new Planet Hollywood show from an undisclosed location in the Mojave desert at around 4AM PT, and it will be broadcast live on "GMA."

It's going to be huge -- 1,350 participants will be bused from Vegas to the desert. And get this ... all attendees must be between 5' and 6' tall ... we don't know why.

As for Brit ... she'll be arriving via helicopter.

All told, we've learned the show announcing the show will cost roughly $100,000 -- including $27,176 to bus all the people (28 buses total), $4,200 for tents, and $5,154 for craft services ... which includes 2,100 bottles of water, coffee, 918 Gatorades, and 567 Red Bulls.

We're told Britney will be performing her new single "Work Bitch," and she'll then blast specifics about her new show that debuts in December. In case you can't wait, we have a bunch of particulars, including what she'll be raking in.

4 AM, huh. Bets on lip-synching anyone?