One large problem with helium balloons is that their flight path is rarely simple. Whilst path and landing site prediction software systems exist, we knew that these would only give us a rough estimate – at best. In the end, we decided that two vehicles following the balloon would be the best solution, as it gave us hugely more flexibility. A school minibus was one of them; the other was the car of one of the lead project members. The downside of this was, of course, that two tracking station systems were required.
A tracking station required a laptop, radio and TNC (see Communications) but the software TNC emulator called AGWPE would more than suffice. It simply requires a soundcard with a speaker/headphone jack and a line in/microphone port. The interface cables vary slightly depending on the radio (handheld radios tend to have the PTT line included in the microphone cable), but they are extremely easy to construct and comprise simply some shielded audio cable, with a resistor and capacitor. AGWPE requires some method of keying the PTT for transmit, and this is done by dragging the RTS line of a serial port high/low. We therefore required laptops with serial ports, but most good USB-serial adapters will allow software control of hardware flow control pins (of which RTS is a member).
AGWPE outputs and inputs data from the radio on one side and from a TCP port on the other. The front end software would need to carry out three important functions. First and foremost, it must show the balloon’s current location/speed/altitude on a map. Secondly, it must display all the data from the onboard sensors as well as the battery voltage. Lastly, it needed to provide some method of talking to the balloon should the need arise. We decided that logging all position data and sensor data would also be a good idea, as we could then replay the flight later on.
The mapping solution was the fantastic piece of software called APRSPoint, which runs on top of Microsoft’s Mappoint. It has the option to place an icon on the map for each beacon coming in from the balloon, which would be perfect for our needs. Mappoint itself contains maps of the whole of Europe, and stores all data on the local hard drive, so no internet connection is required. APRSPoint can also display the current altitude, heading and velocity of the balloon. It can display the comment part of the APRS string, but it can’t decode it into a useful format.
The program AGWTermTCP, which is a front end terminal for AGWPE, would allow us to communicate with the balloon using our proprietary over-the-air command protocol (see Flight Computer). We could also monitor the raw incoming APRS text using this program.
The displaying of sensor data was more of a problem, and the eventual solution was the writing of our own program using VB, which watches the AGWPE TCP port and then displays all data from the APRS string, including sensor data. It also logs all incoming APRS beacons from the balloon to a text file, and can generate a KML file (Google Earth) from either current or imported log files. The program is called APRS Track and can be downloaded here, although some of the code is so specific to our project that it probably won’t be of any use to anyone else unless you decide to follow the same APRS comment format.
One tracking laptop was placed in both the car and the minibus, both running from 12VDC-240VAC inverters because their batteries were completely knackered. We also took two other laptops, an Asus EEE PC and a Dell Vostro 1700 for other work and in case we needed to SSH into the flight computer before launch. The Vostro also had a redundant mobile internet connection, in case we should need access to it for any reason.
The antennas at the receiving end were 446 MHz tuned eggbeaters. They have very good radiation angles and around 6dBi gain, and were attached to the radios with 50ohm RG58 coax cable. We devised simple brackets for the antennas so they could be mounted on top of the vehicles, even during travel.