Data Hacking

FIGURE 8-4: Waypoint information
on a Garmin eTrex
Chapter 8 — GPS Data
How Does GPS Calculate Velocity?
Many people wonder how the GPS can accurately determine velocity from the positional
information it captures for a snapshot period of time.
This is a good question. In fact, the way that GPS calculates your speed is very clever, yet at the
same time quite simple. It remembers where you were the last time it locked your position and
uses this information to calculate your speed.
Most GPS receivers update your position information once a second. For example, if you moved
30 meters since the last update, it calculates your speed as 30 meters per second (see Figure 8-5).
y
In 1 sec.
30 meters
FIGURE 8-5: Speed calculated
from distance moved
x
Is Direction Calculated in the Same Way?
Yes and no. Most GPS receivers use only the signal from the GPS to plot your direction of
travel, which means that the only frame of reference it has with regard to your direction is
where you were the last time it looked and where you are now. So in that respect, yes, it uses
that information to calculate your direction (see Figure 8-6).
169