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Antenna

FIGURE 10-35: Schematic showing multi-path errors

Other Uses for GPS Data

What you can do with GPS data really depends on the software you’re using. While you were at the website from which you downloaded NMEATime (www.visualgps.net), you might have also come across software called VisualGPS. VisualGPS comes in two flavors:

VisualGPS: A standard free version (see Figure 10-36).

VisualGPSXP: A full-featured version of VisualGPS (see Figure 10-37). This software is not free (currently a license costs $25 and a 30-day trial is available).

FIGURE 10-36: VisualGPS

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FIGURE 10-37: VisualGPSXP

There is also a version for Pocket PC 2002 devices (only for those using the ARM processor). This is called VisualGPSce (see Figure 10-38) and is currently considered a beta/experimental program; it is currently free. I’ve used this program and have found it to be useful and stable.

VisualGPS is very useful software because it enables you to work with the data that the GPS outputs at a very low level. The following sections outline some of the features of VisualGPS.

FIGURE 10-38: VisualGPSce

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Azimuth and Elevation Graphs

This window (see Figure 10-39) enables you to view any satellites that are in view from your current location (or, more accurately, from the location of the GPS receiver).

FIGURE 10-39: Satellites currently visible

Each satellite is identified by its pseudo-random number (PRN), its azimuth (denoted by the letter A), and its elevation (denoted by the letter B). This window also enables you to do something else that is interesting: plot and print the physical mask angle. When you are using your GPS in a stationary or static application (such as to get the signal for synchronizing your

PC clock), it is important that you place the antenna where it can get a clear view of the sky. However, sometimes obstructions block the signal from the GPS to the antenna. Knowing the effect of these obstructions on the tracking of satellites can be essential because there may be times when the obstruction is adversely affecting reception.

The Azimuth and Elevation window can graphically demonstrate the elevation mask angle. The mask angle is represented by a blue, jagged line that is built by using satellite information. The mask angle is, in effect, the “horizon” for the GPS antenna at that point. Using the information from the SNR (signal to noise ratio) and satellite status, a graphical representation of the mask angle is created (as shown in Figure 10-40).

It can take up to 24 hours of continuous tracking of satellites to gather all the information needed to build an accurate mask angle.

If you want more details about satellite availability, along with details about how obstructions will affect your reception, I suggest you visit www.trimble.com/planningsoftware.html. Here you can download free software that enables you to calculate satellite availability, create sky plots of availability, and get information on visible satellites.

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FIGURE 10-40: Mask angle displayed for the current location

Surveying

The Survey window displays both position and xDOP (x Dilution of Precision). HDOP represents horizontal dilution of precision, or inaccuracy in the position displayed, while VDOP represents vertical dilution of precision, or height/altitude inaccuracies (see Figure 10-41).

FIGURE 10-41: The Survey window

If you are using a stationary antenna, this window can provide very accurate measurements of position and elevation.

You can also print this information.