Inside buildings and the urban valleys of large cities, Global Positioning System technology is often inaccurate or unusable. Matthew Rabinowitz has sharpened GPS precision by exploiting the synchronization codes embedded in broadcast television signals. These codes allow a TV receiver to compile numerous signals into a single harmonious output. Rabinowitz, who cofounded Rosum and now serves as chief technology officer, has developed a handheld device that uses sync codes to calculate how far the user is from the source of the signals and thus determine his or her location. The Rosum technology refines GPS position readings to within a meter or two, even indoors and in cities.