Mr. Murder

light near the middle of the screen.
        The dot moved steadily along the emerald-green line that was Interstate 40.
        "About four miles ahead now," Oslett said.
        Karl Clocker, the driver, did not respond. Even in the best of times, Clocker was not much of a conversationalist. The average rock was more talkative.
        The square screen of the electronic map was set to a mid-range scale, displaying a hundred square miles of territory in a ten-mile-by-ten-mile grid. Oslett touched one of the buttons, and the map blinked off, replaced almost instantly by a twenty-five-square-mile block, five miles on a side, that enlarged one quadrant of the first picture to fill the screen.
        The red dot representing their car was now four times larger than before. It was no longer in the center of the picture but off to the right side.
        Near the left end of the display, less than four miles away, a blinking white X remained stationary just a fraction of an inch to the right of Interstate 40. X marked the prize.
        Oslett enjoyed working with the map because the screen was so colorful, like the board of a well-designed video game. He liked video games a lot. In fact, although he was thirty-two, some of his favorite places were arcades, where arrays of cool machines tantalized the eye with strobing light in every color and romanced the ear with incessant beeps, tweets, buzzes, hoots, whoops, waw-waws, clangs, booms, riffs of music, and oscillating electronic tones.
        Unfortunately, the map had none of the action of a game. And it lacked sound effects altogether.
        Still, it excited him because not just anyone could get his hands on the device which was called a SATU, for Satellite Assisted Tracking Unit. It wasn't sold to the public, partly because the cost was so exorbitant that potential purchasers were too few to justify marketing it broadly.
        Besides, some of the technology was encumbered by strict national-security prohibitions against dissemination. And because the map was primarily a tool for serious clandestine tracking and surveillance, most of the relatively small number of existing units were currently used by federally controlled law-enforcement and intelligence-gathering agencies or were in the hands of similar organizations in co "Three miles," he told Clocker.
        The hulking driver did not even grunt by way of reply.
        Wires trailed from the SATU and terminated in a three-inch-diameter suction cup that Oslett had fixed to the highest portion of the curved windshield. A locus of microminiature electronics in the base of the cup was the transmitter and receiver of a satellite up-link package.
        Through coded bursts of microwaves, the SATU cculd quickly interface with scores of geosynchronous communication and survey satellites owned by private industry and various military services, override their security systems, insert its program in their logic units, and enlist them in its operations without either disturbing their primary functions or alerting their ground monitors to the invasion.
        By using two satellites to search for-and get a lock on-the unique signal of a particular transponder, the SATU could triangulate a precise position for the carrier of that transponder. Usually the target transmitter was an inconspicuous package that had been planted in the undercarriage of the surveillance subject's can-sometimes in his plane or boat-so he could be followed at a distance without ever being aware that someone was tailing him.
        In this case, it was a transponder hidden in the rubber heel and sole of a shoe.
        Oslett used the SATU controls to halve the area represented on the screen, thereby dramatically enlarging the details on the map.
        Studying the new but equally colorful display, he said, "He's still not moving. Looks like maybe he's pulled off the side of the road in a rest stop."
        The SATU microchips contained detailed maps of every square mile of the continental United States, Canada, and Mexico. If Oslett had been operating in Europe, the Mideast or elsewhere, he could have installed the suitable cartographical library for that territory.
        "Two and a half miles," Oslett said.
        Driving with one hand, Clocker reached under his sportcoat and withdrew the revolver he carried in a shoulder holster. It was a Colt.357 Magnum, an eccentric choice of weaponry-and somewhat dated-for a