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The standard VHF APRS RF network uses a 1200 baud over the air data rate which provides a limited amount of data carrying capacity. This means that stations operating on air need to be very aware of the impact that they are creating on that very network.

Finding a balance between sending position packets fast enough to keep the distance between the last known position, and the actual location of the station from being too far apart, and not overloading the RF network can be a tricky proposition.

The process of Dead Reckoning involves knowing not only the last known location of a particular station, but also knowing the course and speed of that station. Given an initial location, with a reported course and speed, one only needs to do a little bit of math where:

Distance = Velocity * Time

Then extrapolating that distance along the vector defined by the course, plotting the station location at that point.

This technique is used quite often in situations such as where an aircraft has gone missing. The search teams will base their search areas using a pattern based on Dead Reckoning the last known course and speed from the last reported position of the lost aircraft.

As the time elapsed from the last reported location/course/speed increases, the ambiguity of the possible location of the Dead Reckoned station also increases. The trick to making Dead Reckoning keep the projected location of station accurate is to have the tracked stations send position/course/speed reports at significant points along their track. This can be accomplished by using routines such as SmartBeaconing™ or Genius™ that watch for changes in velocity, course, or projected positions to determine when an updated position report needs to be sent.

The combination of an intelligent position reporting decision making routine, along with an observation platform that extrapolates positions between reports means that the observer can have a real time location plot on their local display that depicts a very accurate location for the station being tracked without overloading the local RF network with unnecessary position packets.

[edit] APRS Applications Supporting Dead Reckoning



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