High Altitude Balloons and Ham Radio: Tracking a Flight with APRS

Few projects in amateur radio combine electronics, weather, geography and a good chase across country quite like a high altitude balloon flight. A latex balloon lifts a small payload through the weather to the edge of space, a radio beacon tells the world where it is, and a team on the ground follows the signal to recover the payload after it comes down. The same technology, in the form of foil pico balloons, can circle the globe for weeks. This guide explains how a flight works, which payload and radio you need, how to set up the tracker and APRS so that the signal is useful and polite, what the regulations require, how to plan a recovery and what to expect from a pico balloon.

How a Balloon Flight Works

A typical flight uses a large latex weather balloon filled with helium or hydrogen. As it rises, the pressure falls and the balloon expands until it bursts, usually at an altitude of around 25 to 35 kilometres. A parachute then slows the payload for the descent. The whole flight lasts a few hours. The ascent rate is a few metres per second, and the horizontal drift depends on the winds, which can carry the balloon a hundred kilometres or more from the launch site. The overview of high-altitude balloons and the page on weather balloons give the physics and history.

The conditions are harsh. The temperature in the upper troposphere falls to about minus 50 degrees Celsius or lower, the air is thin and the payload spins and swings. The batteries must keep working in the cold, the electronics must survive vibration and the antenna must remain connected. A flight is also an excellent test of radio range: at 30 kilometres the radio horizon extends some 600 to 700 kilometres in every direction, so a low-power signal can be heard by many stations at once.

Two Kinds of Balloon

The latex flight is the better first project, because the payload comes back and can be reused. Pico balloons are a more specialised hobby: the payload is tiny, the radio is a weak-signal beacon and the reward is a long flight that you follow online. The page on pico balloons describes the idea.

The Payload

A basic payload for a latex flight has five parts: a GPS receiver, a radio tracker, a battery, a parachute and an insulated box. Many people add a camera, a temperature and pressure sensor and a spare tracker in case the first one fails. Keep the total mass low, because mass affects regulations, the balloon size and the risk of injury or damage if the payload falls.

• GPS receiver. It must be set to a flight mode that allows operation at high altitude. Many consumer receivers limit altitude by default, and in that case they stop reporting at about 18 kilometres. A receiver with a high-altitude or airborne mode keeps working. Test the setting on the ground before you launch.
• Tracker. A small transmitter that sends the position at regular intervals. For amateur flights, an APRS tracker on a VHF channel is the common choice, since many receiving stations and gateways already exist.
• Battery. Primary lithium cells handle the cold better than alkaline ones and are lighter. Fit enough capacity for the whole flight, with extra for the time on the ground. Our article on battery options for portable operation compares chemistries.
• Antenna. A simple quarter-wave wire or a small dipole hanging below the payload is typical. Keep it away from the battery and the foam walls, and mount it so that it points down, where the receiving stations are.
• Insulation and recovery aids. A foam box protects the electronics from the cold. Add a label with contact details, and consider a small buzzer or light to help find the payload in a field.

Cameras and Extra Sensors

Most teams add a small camera, because the pictures from the edge of space are the reward that makes the project popular with schools and clubs. A compact action camera or a single-board computer with a camera module will do. Check that the memory card is large enough for the whole flight and that the battery lasts in the cold, and point the lens through a hole in the box so that frost does not form on a window. A temperature and pressure sensor adds useful data and helps you to see when the balloon bursts. Extra sensors should never delay the tracker or add weight that pushes the payload over a legal limit. If a feature is not essential for tracking and recovery, leave it for the second flight.

Setting Up APRS for a Balloon

Balloons use the same system as vehicles and fixed stations, described in our guide to APRS position reporting, igates and digipeaters, but they need special care because of their altitude. A balloon at 30 kilometres can be heard by dozens of digipeaters and gateways at once, and each of them may repeat the packet. If you use a long digipeater path, the result is a flood of duplicate packets on a busy channel. The common advice is to use a short path or none at all, and to send a beacon at a modest interval, such as every minute or two, rather than every few seconds.

1. Use the correct frequency for your region, which is 144.390 MHz in North America and 144.800 MHz in Europe.
2. Set the position beacon interval to about one to two minutes, and avoid more frequent reports.
3. Use a short or empty digipeater path, such as a single hop at most.
4. Send the altitude in the packet, and add telemetry for temperature, battery voltage and pressure if the tracker supports it.
5. Choose a call sign with a suitable suffix, and include a short comment with the project name.
6. Keep the output power low. A few hundred milliwatts is enough from altitude.

On the ground, your own receiver can decode the packets with a sound card and software such as the program described in our guide to Direwolf and packet radio TNCs. Public tracking sites collect the packets received by volunteer gateways and show the balloon on a map, which makes it easy for the team and for followers to watch the flight.

Regulations and Safety

A balloon is an aircraft in the eyes of the law, even if it is a small one, so you need to know the rules before you launch. Most countries regulate unmanned free balloons by weight and size, and some require notification of the aviation authority, a flight plan or a permit. In the United States, the relevant rules are in the aviation regulations on unmanned free balloons, found in Part 101, Subpart D, and other countries have equivalent regulations. Read them, check the current text and, if in doubt, ask the authority. Stay well under the weight limits for light payloads, avoid airports and busy air routes, and choose a launch site that is open and clear of power lines.

The radio side requires an amateur licence for the transmitting station, with a control operator who is responsible for the emissions. Keep the power low and follow the band plan. Handle the gas safely: helium is inert, but hydrogen is flammable, so use it only if you know the safety rules and have the correct equipment. Never launch in poor weather, in thunderstorms or with strong surface winds.

Predicting and Chasing the Flight

Before launch, use a flight prediction tool to estimate the path from the weather forecast. The tool needs the balloon mass, the gas amount, the ascent rate and the burst altitude, and it returns a predicted landing point. The prediction is only an estimate, but it helps you to choose the launch site and to position the chase team. On the day, update the forecast shortly before launch, because the winds change.

The chase team follows the balloon in a car, using the live tracking page on a phone and a radio receiver as a backup. Make sure that the driver is not also the navigator. When the payload descends, the signal may fade at low altitude, since the radio horizon is short. A directional antenna and the techniques in the guide to radio direction finding and fox hunting help to find the final position. Ask permission before you enter private land, and do not climb trees or approach power lines. If the payload is out of reach, leave it and ask the owner or the authorities for help.

After the Landing

Collect the payload, check the data and share the results with the people who tracked the flight. Note the burst altitude, the flight time and the distance travelled. Thank the stations who sent reports, because the project depends on a community of listeners. The data you gather will help you to improve the next flight.

A Word on Pico Balloons

A pico balloon is a small foil balloon carrying a tracker that weighs only a few grams, powered by a tiny solar cell. It floats at a level of about 10 kilometres or higher, drifts with the wind and may circle the Earth many times. The trackers usually transmit short weak-signal messages that are collected by receivers around the world, using methods like the one described in our guide to the WSPR beacon system. Flights depend on weather, balloon quality and luck, and the designs are an exercise in saving every gram. If you enjoy a project that is both electronic and meteorological, a pico flight rewards patience.

Working With a School or Club

Balloon flights are ideal group projects. A club can divide the work into payload, tracking, launch and recovery, and each member learns a different skill. Schools often join in, with students designing the experiments and following the flight online. If you involve young people, make sure that an adult holds the licence and the responsibility for the transmitter, keep a written safety plan and brief everyone before launch. Publishing the flight path and the pictures afterwards builds interest in the hobby and brings new members to the club.

Common Mistakes

• A GPS without flight mode. The tracker stops reporting high up, and you lose the balloon.
• A long digipeater path. It floods the channel with copies of the same packet.
• Too much power and too frequent beacons. A few hundred milliwatts and a beacon each minute are enough.
• Ignoring the rules. Check weight limits and notification requirements before every launch.
• Using a poor battery in the cold. Test the battery in a freezer before the flight.
• No recovery plan. Send a team, a map and a way to ask landowners for permission.

The Bottom Line

A high altitude balloon flight turns a small amount of radio equipment into a memorable adventure. Build a light payload with a GPS in flight mode, an APRS tracker with a short path and a sensible beacon interval, a lithium battery and an insulated box. Check the regulations for your country, predict the path, launch from a clear site and chase the signal with a team and a backup receiver. Share the data with everyone who listened. Start with a simple latex flight, learn from the results and decide whether you want to try a longer project such as a pico balloon. The project teaches more about propagation, antennas and logistics than almost any other hobby activity.

Frequently Asked Questions

How high does a high altitude balloon go?

A typical latex weather balloon rises to about 25 to 35 kilometres before it bursts. A parachute brings the payload back down, and the flight lasts a few hours.

What radio do I use to track a balloon?

Most amateur flights use an APRS tracker on a VHF frequency, because there are many receiving stations and tracking sites. Other options include LoRa and weak-signal beacons.

Why does the GPS stop working at altitude?

Many GPS receivers limit altitude in their default mode, often around 18 kilometres. Use a receiver that supports an airborne or high-altitude mode and test it before launch.

Do I need permission to launch a balloon?

Often yes. Rules depend on the country and the payload weight, and some require notification of the aviation authority. Check the current regulations before you launch.

How do I find the payload after landing?

Use the live tracking page, a backup receiver in the chase car and a directional antenna for the last stretch. Ask landowners for permission before you enter private property.

What is a pico balloon?

A small foil balloon carrying a tiny solar-powered tracker. It floats at altitude for days or weeks and sends weak-signal reports that many stations can receive.