One major step to launching a high altitude balloon is to decide how you’re going to track the payload, this decision is important for getting approval from the relevant government authorities and will likely inform how much space (if any) you have for other items in your payload.
I generally suggest using two methods to track your payload, one of them could be experimental and real-time and the other might be a safer alternative (see below). This redundancy leaves room for failure.
To be clear, tracking your payload is done in two different but interrelated steps (A) getting the position of your payload using GPS and (B) transmitting the GPS position to your mobile ground station (we’ll discuss ground stations in another article).
Part A: GPS positioning
The real challenge with getting the GPS position of your payload is twofold:
- You need a GPS chip that works with Arduino; and
- You need a GPS chip that works at your altitude and speed
These requirements mean that you often can’t just go to your local electronics store and pick up a chip. You’ll likely need to order something more specific than that. The problem with off the shelf GPS chips are in a set of globally accepted restrictions on civilian use. Some countries classify GPS chips that work higher than 18km or that can travel faster than 515 m/s as a weapon! Presumably this is so people can’t just make their own guided missiles. Unfortunately your high altitude weather ballon is most likely going to make it higher than 18km so you need something outside of those restrictions.
The Ublox NEO-6M GPS module looked great for my own projects needs since it has a height restriction of 50km and i’d already calculated my balloon would go about 30km in the air. Unfortunately my chip did malfunction during the flight at about 13km it just kept returning the same data over and over, which caused some problems for me. That means I can’t 100% recommend the Ublox chip, despite the fact that it was one of the only chips I saw that claimed to work higher than 18km. The reason for my chip problem appeared to be a fault in the chip, specifically since I bought a cheap Chinese knock-off from eBay—which I totally don’t recommend! I’m very lucky my back up GPS worked during landing so I could still recover my payload.
I still believe Ublox to be the best GPS chip, specifically because you can buy a version that is hooked up to a small board with a GPS antenna (for receiving the GPS satellite positioning data) its all ready to plug in and start receiving data into your Arduino right away.
Part B: Transmitting the GPS position
The second part of tracking is to transmit your GPS coordinates and powerfully enough so that your ground station can pick up the signal. I ended up using a Radiometrix NTX2B-FA 434.400Mhz transmitter module from Uputronics that worked up to a 500km range and used the LIPD method below.
What follows here is a non-exhaustive list of tracking options with some of the positives and negatives of each option.
1. APRS
APRS or Automatic Packet Reporting System is an amateur radio technology for real-time communication of information in the immediate area. This means you can send GPS coordinates or any other useful information on a shared frequency from your payload to an APRS receiver which is then uploaded to the internet. The frequency you broadcast on is typically dictated by the country you live in, for example Australia is 145.175 MHz. The information broadcast is open and available to any nearby APRS receivers (often referred to as an igate), which means your GPS coordinates can be plotted on a public map on the internet such as the website http://aprs.fi
The upside to APRS is that it appeared to be relatively reliable system for real time communication. It also has a history of use with high altitude balloons and many enthusiasts can follow along.
The downside to APRS and the reason I chose not to go with it was because you will most likely need an amateur radio licence to use APRS (at least in Australia). I did end up getting my basic amateur radio licence but I still needed an even higher level to operate APRS, which was going to take both time and money that I didn’t have.
Summary: APRS almost appears to be a de facto standard in international high altitude balloon groups from the US and the UK. However the licence requirements are prohibitive for many Australians because of the time involved in getting your standard amateur radio licence, which is the second tier licence grade. I think overall thought it is a hardy and easy to implement method with many off the shelf APRS solutions from companies such as Uputronics.
2. RTTY (radio teletype)
RTTY was the primary method I ended up going with for my HAB launch. RTTY also known as radio teletype is a method of transmitting text over radio using a transmitter that shifts back and forth between two frequencies e.g. high and low signal. For programmers (like myself) it is easy to think of RTTY like a form of binary language for radio. LIPD is a special class of radio licence that allows you to operate certain low powered radio communication devices at certain frequencies without the need for a formal amateur radio licence. The various frequencies that you can use with LIPD have power limits, however these are large enough for most HAB requirements. In particular the 433.05 to 434.79 MHz frequencies can be used up to an equivalent isotropically radiated power (EIRP) of 25 mW. Basically, my reading of this is that you can transmit and receive on these frequencies.
The upside to RTTY is its flexibility and power. I had a lot of fun connecting and programming my transmitter and putting together my own mobile ground station setup. The range of the transmitter was great, I was still getting data at 100km+ from the payload and I definitely could have improved my setup to receive data at a larger distance. I was able to transmit whatever data I wanted too, I could have even transmitted information that made up an image, though that was more complicated than the scope of my project.
The downside to RTTY is that it is line of sight, which means you’ll need to have a clear day and no local obstructions to get a clear line of sight to the payload. This was OK in rural Australia where everything is flat, dry and you have great visibility. If the weather was bad I most likely would not have flown the balloon anyway. You’ll need to acquire (or build) a directional antenna for your ground station too, this can be tricky and time consuming but is a good learning experience overall.
Summary: I have a soft spot for RTTY, there’s nothing as cool as seeing text transmitted from your payload to your ground station in real time during the flight. The method is a hackers delight since you can keep improving on your setup, creating better antennas or transmitting different information.
3. Cellular
Cellular tracking is done by using of the mobile phone network in your local area to periodically SMS yourself the GPS coordinates of your payload. This can be achieved using a GSM module (also known as a shield) for Arduino. This has two problems detailed below.
The upside with cellular tracking is that it’s relatively simple to implement and does not require any licence to operate like APRS does. Nor is it directional as LIPD is.
The downside is that (a) it doesn’t work in extremely high altitudes and (b) it doesn’t work in low coverage areas. The massive problem is that even if you were flying above a phone tower you’d struggle to send and receive above a certain altitude, which is less than half of the 25-30km that your balloon is going to fly. Cellular works at best to track the location of your balloon when it lands back on the ground. This however can be a problem if you launch in a regional area (like I did) since mobile phone coverage is shaky at best, for example, in rural Queensland where I launched, if I had used cellular tracking it most likely would have failed.
Summary: I really can’t recommend the cellular tracking option, it doesn’t work at high altitude and isn’t as hardy or reliable as the personal GPS tracker. I’ve even heard conflicting reports that it might be illegal to include a cellular device in your payload (need to confirm this though).
4. Personal GPS tracker
A personal GPS tracker is a device used by a person or vehicle to track their location for example when out hiking in the wilderness or to track fleet cars. One of the more popular, rugged and affordable brands in this category is the SPOT line of personal GPS trackers. Basically you turn on the device and it transmits your location every X minutes (5 minutes was the minimum) you can receive the location information on SMS or via a web interface.
The upside is that these devices are designed to work in extreme rural areas, for example, it was perfect to find my payload in the middle of outback Queensland. Personal GPS trackers are also relatively small and lightweight with their own in-built long life power sources. They take little to set up and activate and the web interface can be shared with other people (I shared mine with Air Services Australia which helped me to get approval to fly).
The downside is twofold (a) first you need to have the GPS trackers antenna facing toward the sky. If you don’t do this it wont be able to pick up any satellites and simply wont work. This can be a massive problem if your payload lands upside down or underneath some thick foliage. (b) secondly the personal GPS only works within commercial limits, which is approximately 18km. This is well below the average high altitude balloon’s maximum expected altitude. This mean that a personal GPS tracker is better for tracking where you land than during flight, though I did get about 10 data points during flight until the balloon disappeared and reappeared during descent.
Summary: personal GPS is fantastic to use as a back-up GPS or even a primary one if you don’t need to track the payload during the flight. I recommend using it in combination with one of the other methods since there is a reasonable chance that it may fail.
Conclusion
I hope this helps in understanding the pros and cons about some of the tracking methods for launching a high altitude balloon. Tracking really is one of the most important parts of launching your balloon because you’ll never recover all that expensive gear (and the awesome footage you got) if you can’t recover your payload. Most HAB projects fail at this specific point so I think its very important to have two tracking methods, a primary and back up method. It is also very important to repeatedly test your tracking methods before launch. For me this meant going into a very large field and tracking my RTTY at meaningful distance as well as using my personal GPS device while driving around town and looking at the results.