APRS – The Best Balloon Tracking Solution
There are many ways to track balloons. There is the Radiosonde, Mobile Phone (3G), HAM radio APRS and many more. Since I both work in Radio Telecommunications and I am a HAM radio operator (VK2URB), then it is an easy choice. The amateur radio APRS system is ideal.
So what are these systems in brief:
Radiosonde: Wikipedia says: “A radiosonde (Sonde is French for probe) is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required.” This sounds more like a license is required and special Radiosonde equipment is needed.
GPS enabled mobile / cellular smart phones: We all know what these are, but do they work?. Firstly you had better hope that your payload drops in a coverage area. These work by sending an SMS to the phone on the balloon and it then relays its position back to you via another SMS. Mobile telephone coverage in rural areas might not allow you to get a fix on the balloon as it parachutes back to earth. There is also the issue of the GPS receiver. Most do not work at heights over 60,000 feet (20kms) and thus you do not know how high it got or when it is descending. Many people on a tight budget try to use cellular phones and many have great success.
UHF Tracking: Similar to Radiosonde, yet it operates on a low power UHF channel, often used for garage door openers, etc. It transmits the co-ordinates for the GPS location and must be tracked by radios especially set up to receive the transmissions. The data is often ported to the internet for display on a web page. Handheld yagi antennas are directional and look like UHF yagi TV antennas seen on rooftops and are used to track the payload when it is on the ground or in the air.
HAM Radio APRS:This is the choice that I feel best suits the situation and given that I already have a HAM license, then I do not have to ask others to help. What is APRS?: Wikipedia says: Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time tactical digital communications of information of immediate value in the local area. In addition, all such data is ingested into the APRS Internet system (APRS-IS) and distributed globally for ubiquitous and immediate access. Along with messages, alerts, announcements and bulletins, the most visible aspect of APRS is its map display. Anyone may place any object or information on his or her map, and it is distributed to all maps of all users in the local RF network or monitoring the area via the Internet. Any station, radio or object that has an attached GPS is automatically tracked. Other prominent map features are weather stations, alerts and objects and other map-related amateur radio volunteer activities including Search and Rescue and signal direction finding. APRS has been developed since the late 1980s by Bob Bruninga, call sign WB4APR, currently a senior research engineer at the United States Naval Academy. He still maintains the main APRS website. The acronym “APRS” was derived from his callsign.
Note the unusual off-white unit connected with wires in the top picture – it is the special high altitude GPS receiver. It will work up to very high altitudes but sacrifices some accuracy.
The picture above is the APRS Test Track around a street block on a hill near my home. Not precise, but very close. I was shaking the thing as I walked to make it hard for the system. I walked counter / anti clockwise from near the top without shaking and then where it goes a funny in the last quarter of the short walk I was really shaking it wildly. The unit reports on many details. These are:
VK2URB-11 is the balloon call sign
2011-09-10 02:59:41z is the date and time in GMT/Zulu
7 km/h was my walking speed
248 degrees was my bearing
alt 80m was my height above sea level
05.8v was the tracker battery voltage
20C was the temperature – about 70F
The other data is pressure, HHMMSS, and number of GPS satellites, the digipeater used (if used) and the iGate used.
Pluses and Minuses
APRS is could always be better and there are not too many iGates (APRS gateways into the Internet) in rural areas, so you must check first. In fact I have chosen to have my balloon drop near Parkes for that very reason. There is an iGate in Parkes and the Digipeater (digital repeater) at nearby Mt Canobolas will also pick up the transmissions from my balloon. I have also chosen an area for good 3Gcellular coverage to assist with tracking and maps. Just to be sure, I will have a digipeater in my car so that if I am not too far away the position will be relayed by my car to the Internet for easy tracking. The unit I have chosen is specifically bought for ballooning. It is from Argent Data in the US. The unit weighs only 160 grams (5.6 ounces). It transmits half a watt (500mW). It is pictured top right and is a pre-release model.
The next issue is finding it when on the ground. Radiosonde and APRS are well suited to this task, but the APRS has a few tricks up it’s sleeve. Fist it might be able to radio its GPS co-ordinates to the Internet tracking system. As I get close with the digipeater, it will also do that job if no other iGates are in range. Secondly it may be picked up directly by my handheld radio, nice, but since it only transmits for 1-2 seconds, it will be hard to get a fix on the unit. Finally I can decode the data with my iPhone and simple read its exact co-ordinates. Nice! That is the directly decoded packets on the right. I did the test inside my house so the GPS coordinates will not be seen.
On the minus side, there is the need for an amateur radio license and access to the expense and homemade equipment that is either out of reach of some people financially or technically.
I also replaced the long general purpose whip antenna that you can see on the top image with a highly tuned light weight dipole. It is made of hollow brass and this also makes it easy to slide some stiff wire inside the antenna for tuning. The wire was then soldered in place to get the tuning very precise. This maximizes the antenna’s radiation ability at the precise frequency of the APRS system. We are using VHF at 145.175MHz. The pictures below show the modification. The work was done by my good friend Bruce who I have worked with on and off for over 40 years. He is also an amateur radio operator (VK2ZZM) and I am very appreciative of his advice and help on the APRS side of this project.
The white Styrofoam under the unit is the lid of the UpLift-1 capsule. The antenna is mounted on a small printed circuit board, The copper wire is used to add strength to the copper on the board in case of mechanical failure that may make the copper peel from the board.
The rear side above showing the bolts that pass through to the battery mounts on the tracker unit. A small amount of “locktight” was placed on the nuts to make sure that mechanical vibration did not make them fall off.
This is a state of the art network analyzer. It is measuring Return Loss. Send a signal to the antenna and what is not radiated comes back. The dip means that it is tuned to the frequency and radiating well. It is right on the tracker frequency. The Marker frequency. It is perfectly tuned and radiating the signal – not much is being reflected back into the cable. It is best practice as far as radio is concerned.
I will post a link to the tracker website that I will be using just before the day, but this link will let you see the few test drives that I have done in Sydney: http://aprs.fi/