Global Positioning System Reference
In-Depth Information
The tricky thing about the Date class is that you can never really \see"
the actual date. If you request the current time with now=newDate() , it
internally sets the UTC time. By printing System.out.println(now) , the
PC environment does the formatting to its local time zone.
The problem with time zones is that they are actually political borders
(and potentially subject to change) and do not simply reflect the path of the
sun. Although they could be determined by lat and lon implicitly, it would
require a large look-up table, which is not provided by the Java Standard
Edition. Nevertheless, GPSpoint uses a naturalTimeZone given only by the
longitude's oset to the zero meridian, which can be quite useful as a rough
estimate.
The ROAF is designed to be a global application and should carefully
ignore the local PC time and time zone. This actually means that time
and date could be dropped completely, since the objects are supposed to
interact directly.
On the other hand, date and time will be needed to
schedule events.
During the development phase, all applications can run on one computer
and implicitly use the same clock; in a distributed application, the server
might provide a common time. And, in the long run, all participating
machines could be required to synchronize with a network time protocol
(NTP) server, while all client devices with a GPS receiver can directly
access the server's clock signal.
4.7 GPStraceextendsRoute
Note that a route can be drawn as a continuous line composed of one or
more line segments (polyline) on a map. Nevertheless, a route is only a
collection of positions, which can be fed into a navigation system to be
connected via the streets of a digital map. Different systems can navigate
the same route in different ways.
The information stored in a NMEA le dene a \trace." A trace is
more than a route enriched with times stamps|a trace describes the exact
measurement of a motion of an object. Most GPS devices are designed
to provide position and time every x seconds or every y meters. In prac-
tice, the time span and precision vary, but every point of a trace can be
interpolated to reflect the actual motion.
Traces make routes authentic and are very valuable for real-world sim-
ulations. By carefully analyzing and validating a trace, a number of
assumptions can be made:
ˆ The top speed gives a clue as to whether it was recorded on a bicycle
or a car.
ˆ A large acceleration could indicate a sports car or a motorcycle.
 
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