MANHATTAN CAN GO BY GPS
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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.