r signal radar than space-based satellites. "Because they are relatively inexpensive, can be steered, are independently powered, and can carry a large payload, they will probably be the most cost- effective way of bridging the last mile in telecommunications coverage," says Kerry Nock, Global Aerospace's president. Nock estimates that a constella- tion of 400 Stratospheric Satellites covering most of the populated areas in the INTERFERENCE Figure 6-1 Stratospheric satellites. northern hemisphere would cost less than $100 million-less than the cost of a single space-based satellite including its launch. Nock belies that operational costs will come in at less than $10 million per year. 6.11 INTERFERENCE It's impossible to have a chapter on radio technologies without including some information on interference. Whether natural or man-made, interference has been an integral-although unfortunate-part of radio since the days of crystal sets. At the dawn of the 21st century, researchers are working hard to remedy one of radio's oldest problems. 6.11.1 An Allocation Approach Penn State University engineers have developed an economical way to more efficiently manage radio spectrum use and prevent interference on wireless broadband systems for high-speed Internet access. "With this technique, service providers could offer quality service to more homes using only a limited span of the radio spectrum," says Mohsen Kavehrad, director of Penn SOMETHING IN THE AIR-RADIO AND LOCATION TECHNOLOGIES State's Center for Information and Communications Technology Research. "If providers can squeeze more customers onto the available bandwidth, it could translate into lower costs for the consumer." The approach also promises equipment cost savings since simulations show that the new scheme maintains performance at top industry standards with more economical components. Currently, high-speed Internet access capable of carrying MP3 files, video, or teleconferencing is available primarily over wired networks. However, wire- less local loops 