Surface combatant's surface surveillance radius?

An article in the RN’s news site says HMS Bulwark currently on deployment in the Arabian Sea can “cast her net wider than most RN assets” and the area the ship can “keep an eye on movements” is “over 1,500sq miles” which is a circle with a 35-km radius.

If the 35-km radius is really _wider_ than most RN assets can cover, how large an area a Type 23 can do?

Below is the link to the original article.

http://www.navynews.co.uk/articles/2006/0602/0006021701.asp

“With eight landing craft, five smaller sea boats, plus Sea Kings, Bulwark is able to cast her net wider than most RN assets; the ship’s command team reckon they can keep an eye on movements over 1,500sq miles of ocean.”

Cheers,
Sunho