MANHATTAN CAN GO BY GPS --------------------- John pazmino NYSkies Astronomy Inc www.nyskies.org nyskies@nyskies.org 1994 December 1
Bernard Klempner and John Pazmino tested two GPS schemes under consideration by the NYC Police Dept and Dept of Transportation. In this test a car fitted with both a Rockwell and a Magellan unit cruised various Manhattan districts. The traverses were made on Sunday 30 October 1994, a textbook Indian summer day. The general concern was that the terrain of Manhattan will block the GPS signals from the units to prevent good geographic fixes. GPS already works, spectacularly so!, in just about any other city because of the lower skyline, wider streets, and more open sky exposure. This Manhattan project is, in a vivid sense, an acid test for GPS service. Manhattan streets are typicly flanked by close-packed stockades of towers forming a gorge a hundred and more meters deep and hundreds of meters long. The sky exposure from the floor of these gorges is but a vertical slice within which, it was feared, too few satellites would be in sight. On the other hand, if the signals could penetrate to the streets at the bottom of the gorges, Manhattan will be a massive and hogwild giddy market for GPS services. The units were powered from the car's electric socket and they collected their signals from rooftop antennae. They fed the signals into a laptop computer. The computer displayed, in addition to the lat-lon and system technical data, status reports on the available satellites and on those actually employed in the instant position fix. To the astronomers's surprise, firm fixes were obtained everywhere along their itinerary! At virtually all times, while standing still or on the go, good lat-lons were punched out continuously. Readings were written down only at stop lights and midblock pullovers. While the car was in gear all attention was applied to the chaotic traffic and to keeping the equipment from bouncing around on the bumpy roads. At only two specific spots, only a few meters across, did the units grab only two satellites and declare the resulting fix invalid. One spot was on a narrow street in East Village pressed in by tall tenements to expose a sliver of sky. The other was a side street by the World Trade Center hemmed in by the twin 400-meter towers. Even in these instances the car passed within seconds out of the deadspots and the GPS units resumed their punchout of valid positions. A good fix, by any GPS unit, requires three satellites for a surface position in latitude and longitude: this is called a '2- dimensional' fix. If four satellites are in sight a '3-0' fix is generated with the elevation above a selected datum being the extra dimension. The Klempner-Pazmino run ignored the elevation, altho many readings did employ four satellites. Back at Klempner's house the astronomers compared the recorded positions with those generated by a computer geographic system with block-level streetmaps of the United States. In every instance but one the observed and calculated position were within 20 meters of each other. In many. cases the congruence was within 10 meters. -- two car lengths! The one exception was where the observed place was on Park Row east of City Hall and the calculated was on Broadway west of City Hall. This may have been a miscopying of the display to the logsheet. A plausible correction of this entry yielded a 10-meter congruence. As outfreakinq as this experiment is, there are several caveats in applying GPS in Manhattan. The system works only outdoors under the sky. GPS can not work within a building or under roofs, tunnels, canopies, dense trees, &c. Other external information must be in hand about a feature tagged by GPS to positively identify it amidst the densely packed detail in the cityscape. A separate block-level geographic system is needed for processing the lat-lons into place descriptions. Elevation and velocity-direction data wander too much for absolute reliability, altho they still are extremely helpful. Further tests are planned to meet the special demands of the police and transportation departments, but the results of this run point to an immense and fabulous GPS industry coming to Manhattan.