Data Hacking

FIGURE 10-34: A reradiating antenna
Chapter 10 — More Data Tricks
The same rules of placement regarding the GPS receiver apply to the antenna. Because it is an
active antenna and has more power at its disposal, active antennas on reradiating kits are normally
better at getting a decent signal in less than ideal conditions.
Here are some tips to help you with your reradiating set up:
Site the antenna well, firmly attaching it — if possible, use either the magnetic base on
the antenna or screws.
Remember that the antenna needs power. Even though it is only drawing 12V DC, take
care to waterproof all connections and joints in the cables. Do this properly and don’t
rely on taped up connections. Short circuits can cause fires and damage your gear.
Be aware that the longer the cable you put between your receiving antenna and transmitting
antenna, the more signal you lose through attenuation. If you need to use a cable
longer than the one provided with the antenna kit, consider using cables that are low-loss
and shielded. This kind of cable can be expensive, but so is buying poorer quality cable
and finding it won’t work because of signal loss.
Finally, it’s vital to carefully consider placement of the transmitting antenna. If you place
the transmitting antenna and the receiving GPS in a place where you can still pick up a
signal directly from the GPS satellites, this can result in what is known as multi-path
error (see Figure 10-35), whereby the GPS picks up a signal from the antenna and the
satellites directly. This can cause quite a substantial error in position. The error in time is
quite small, however, so unless you require atomic clock precision, you should be okay.
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