The speed of light is 1.8026175×10^12 furlongs per fortnight.

A few years ago I was out around sunset taking photos in a country area near the town where I live. The light was almost gone so I was about to turn around and go home when I came across this scene. I quickly got out of the car, set up the camera, got a reading off the light meter and got just two photos taken before the sun had set and the truck was covered in shadow. Pleased with what I had seen I planned to come back again and take a few more photos when I was less rushed.

Fast forward a few weeks and I decided to go out and re-shoot the photo. I drove out to where I remembered the truck had been and could not find it. Initially I thought I must have turned down the wrong lane as there are a few in the area that are similar. Having not found the truck and with the light fading once again I decided to head home and try again another day. I did this once more before concluding that the truck had been moved elsewhere.

It was then that I decided I need to splash out and buy a GPS so that I could record the location of each photo I took and know where to find them again later if need be. During my research of GPS units I learned about the process of Geotagging. Geotagging is simply the adding of geographical meta data to various media, in this case photographs. Since I do not use digital for my photography I would need to manually record the GPS data and store this information along with the other manually collected meta data (shutter speed, aperture, date, time, comments) for each photograph in a notebook. These days many professional and consumer targeted digital SLRs are coming with a built in port for connecting a GPS. These cameras can then automatically save the GPS data as part of the image file. Even mobile phones with built in cameras and GPS units (such as the Nokia 6220 and Apple iPhone) can do this.

With my background in electronics and computing I was curious to learn more about how GPS devices worked and how they could be interfaced to. As it turns out GPS devices have a rather simple method of communicating with their host through industry standard NMEA 0183 messages. Most GPS devices also have a binary format that is proprietary to the chipset that they use e.g. SiRF, Motorola, etc. Many manufacturers make GPS modules that can easily be integrated into a larger project. These OEM GPS modules usually have a TTL level RS232 port for communications to the host. This make interfacing to a micro-controller a simple task for the experienced electronics enthusiast.

With this in mind I began to design and build a simple USB based GPS receiver as my first GPS project. This device allowed me to investigate the NMEA 0183 format and was also useful in prototyping other projects that you can expect to read about in the near future.