A few months ago, I watched YouTube videos about ham radio operators launching high altitude balloons. They carry a tracking device and a camera to take beautiful pictures of Earth. The tracking device is used to follow the airship on a map but also to retrieve the payload. Ham radio operators like to embark an APRS tracker to follow the balloon. Unfortunately, APRS can be spotty in sparsely populated areas. To the equipment can be retrieved, they also carry a Spot GPS messenger. The goal for people flying this kind of balloon is to go as high as possible and take a few pictures. At around 30,000 meters (100,000 feet), the balloon blows up. Then the payload comes back on Earth safely with the help of a little parachute. On average, the distance traveled by these high altitude balloons goes from a hundred to a few hundred kilometers (60 to 200 miles) depending on the strength of the winds.
There are also amateurs focusing on long-distance flights. These high-pressure balloons fly at 10,000 meters (30,000 feet). The balloons need to be ultra-light and able to stay at high altitudes for several days or weeks. Even though they are light-weight, they need to be tough. At this altitude, winds can be intense. They also need to work at temperatures as low as -50ºC (-58ºF). Taking pictures or filming is not an option because of weight restrictions and because the balloon might crash in the middle of the ocean. Usually, these airships carry sensors and need to be able to transmit in any condition.
Tracking the airship for long-distance flights using APRS is possible. Some amateurs use this method, but the balloon can only be spotted when it passes close to populated areas. Some countries do not authorize APRS and different parts of the world use different frequencies for APRS. Dynamically changing the frequency is not always possible when using a commercial APRS transmitter. The second option amateur ballooners use to follow an airship around the world is WSPR.
WSPR stands for Weak Signal Propagation Reporter. It is a beacon mode
and sends small packets of data. The standard message is
<4 digit locator> +
<dBm transmit power>; for example
EM87 37. The message is then highly compressed into a 50 bits message
before it is transmitted. Stations at a great distance from the
transmitter can receive these messages and upload them on a central
server on the internet. Many receivers around the world are capable of
listening to a transmission sent with a few milli-watts. The WSPR
messages are fixed and cannot be changed to add metrics such as
longitude, latitude, or altitude. Ham radio operators have found a way
to encode the metrics data into parts of the message to precisely
follow the balloon.
If you run a quick search on the internet, you will be able to find companies selling kits or ready to use trackers specially designed for balloons. The WSPR-TX Mini is an excellent little tracker. This tracker is suitable for beginners. Just configure your call sign, the frequency you want to transmit on, and it is ready to go. The source code for this tracker is available on GitHub. Anyone with some C experience can modify the program using the Arduino development tools.
QRPLabs sells the APRSLight and APRSLight-W that can transmit APRS packets as well as WSPR.
These are two examples of ready to use trackers, but there are several other options.
For my project, I have decided to build my tracker. My goal is to make a small, low budget and light-weight tracker. It will allow me to tinker with different options or configurations. With a very light-weight tracker, I can use one or two mylar party balloons. Yes, the colorful balloons with happy birthday written all over it. They are called pico-balloons. Instead of using party balloons with colors, I will use transparent balloons. The pigments used for colors deteriorate with the sun and trap the heat. Transparent ones have a longer life expectancy.
Party balloons; I am aiming at long-distance travel and if everything goes well a world circumnavigation. Using party balloons lower the cost of a launch. Low cost allows me to make several attempts with different configurations, pressure, or weather conditions, without burning a hole in my bank account.
The transmitter should be cheap. For each mission, the payload is expected to be lost. To be able to have several launches, the payload needs to be affordable and feather-light. The total weight of the tracker should be a couple of dozens of grams.
The embedded computer, sensors, and transmitter should work only using small solar cells with no battery. Batteries are heavy, and they don't work at the temperatures found at 10,000 meters (-40/50ºC). I will use super-capacitors to give the electronics the boost of power they need during transmission. Between transmissions, the avionics will be into deep sleep mode to save energy and recharge the super-capacitor.
On this blog, you will be able to follow me on that journey, from making the transmitter to the launch and tracking the balloon hopefully around the world. As of today, I have designed the board, and some friends are reviewing it before I order the PCBs.
Pictures, courtesy of the Internet. ↩