PBS announced today that for the first time, it has subscribed to full-time Nielsen ratings, reports the New York Times. The ratings will provide weekly numbers only. Andrew Russell, a senior v.p. for PBS Ventures, told the paper that monthly ratings were no longer satisfying marketers, who want shorter time frames for sponsorships and more information on viewership. The subscription started tracking numbers with National Parks: America's Best Idea in September. That reached an average of 5.5 million viewers over its six-night run.