December 2005, Issue 185

Browser-Based Telemetry System

NEMA SENTENCES

Most GPS devices use NMEA-0183 standard output for location and altitude data. The standard is 4,800 bps over RS-232. Capturing sentences is straightforward, that is if you don’t mind paying $30 for the waterproof cable! Garmin is quickly moving away from the serial interface in favor of USB. For testing against the AAG altimeter, we borrowed a Garmin eTrex Vista navigation system, which is billed as a combination GPS, altimeter, and electronic compass (see Figure 4).

Parsing out these values is a trivial task when you’re using Java’s rich string class. Buffering is the only issue. Don’t generate garbage too often. The serial port class has event handlers to simplify the code (rather than trying to poll the port often enough to ensure the buffer isn’t overrun). The final protection is the NMEA checksum.

I initially had been keen to use the GPS unit to obtain altitude data, but I was informed it would most likely produce disappointing results during a climb. A GPS unit requires lines of sight to as many as 12 satellites. I was told that if I were on the side of a mountain, I’d lose as many as 50% of the available satellites. In addition, I’d ideally want satellites far off on the horizon in order to generate the largest possible triangles for resolving my location. Therefore, I was informed that when operating in fjords and urban settings (i.e., places with limited views of the sky), my best bet would be to use the pressure-derived value for obtaining altitude data. High-end GPS hand-held units include barometric sensors and auto-correction features to make the best of both technologies, both of which generate inaccuracies that you must keep in mind (see Figure 5). For more information on this topic, refer to Jeff Stefan’s article entitled “Navigating with GPS” (Circuit Cellar 123, October 2000).