242 Part III — Data Hacking

FIGURE 10-30: Choosing how often to update the PC clock

With the software set up, it’s now time to set up the hardware that you will need.

Setting Up the Hardware

To set up the required hardware, you will need the following:

A GPS

A cable/wireless connection

A long-term power supply (a wall adapter is ideal — choose one with the appropriate input and output voltages, depending on your location and equipment) for the GPS (batteries aren’t going to last long if the GPS is on 24/7)

The type of GPS you use is important. I consider the long-term use of a handheld GPS such as the Garmin eTrex to be a bit wasteful; and depending on location, it might be vulnerable to loss, theft, or damage. I recommend investing in a small receiver such as the Leadtek GPS Smart Antenna GPS 9532 (www.leadtek.com/gps/gps9532/9532.htm). These are low-cost receiver-only setups and are designed for long-term exposure to the elements. However, if you only need to set up the time occasionally, your existing GPS receiver will suffice.

Power is important — for long-term use, you don’t want to change batteries regularly. The Leadtek receiver mentioned in the preceding paragraph draws power from the 5/12 volt supply and contains an internal backup battery in case of power failure.

Choosing a good position for your receiver is vital. The best position for the receiver is the tallest, least obstructed spot, away from trees and not overshadowed by other buildings. Figure 10-31 shows some good locations for the antenna, while Figure 10-32 indicates some bad locations for a receiver.

Chapter 10 — More Data Tricks 243

FIGURE 10-31: Good antenna locations

FIGURE 10-32: Poor choices for the antenna

Hooking Up Hardware to Software

Now all that is left to do is create the interface between the hardware and the software:

1.Make sure that the GPS is connected to the PC properly.

2.Switch the GPS on and make sure that it is getting an adequate signal (remember that a spot that provides you with good signal at one time might not do so later as the satellites move — check on it).

3.Run the NMEATime application.

4.From the NMEATime Properties screen, click the GPS Signal Quality tab (see Figure 10-33). Use this to check the signal quality being received by the software.

244 Part III — Data Hacking

FIGURE 10-33: GPS signal quality displayed

5.That’s it! Check the signal quality periodically to ensure that you’ve chosen a good location for the receiver. If the signal quality becomes poor, you might need to reconsider the placement of the receiver.

The accuracy you get from using GPS to synchronize your PC clock is truly amazing. Furthermore, you can access the signal 24/7 and it requires no connection to the Internet. It’s fast, reliable, and very, very geeky!

Bringing a GPS Signal Indoors

This atomic clock system relies on an antenna located outdoors, using this to bring the signal indoors. The signal is usually routed indoors via cable because wireless technologies such as Bluetooth don’t offer the range needed, especially when signals have to work through walls, roofs, and ceilings. In addition, leaving a GPS antenna outdoors 24/7 not only exposes it to the elements, but also to theft and damage.

Much better than leaving a GPS outdoors is to use only an antenna to bring the GPS signal indoors. The cheapest way to do with is to use what’s called a reradiating antenna. A typical reradiating antenna is shown in Figure 10-34.

These antennas consist of the following:

A power supply (usually 12V, DC)

An active antenna (that is, a powered antenna)

A transmitting antenna (this is, the antenna that retransmits, or reradiates, the GPS signal)

Connecting cables

As you can see, the active antenna is the antenna that receives the signals from the GPS satellites. After the signal is received, it is transmitted along a coaxial cable to the transmitting, reradiating antenna. This antenna, also powered by the 12-volt DC supply, transmits the signal received by the active antenna. The transmitted signal is then picked up normally by the GPS receiver.

Chapter 10 — More Data Tricks 245

FIGURE 10-34: A reradiating antenna

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.