Ever since I first learned about radiosondes as a kid, I’ve been fascinated by them. To my young mind, the idea that weather bureaus around the world would routinely loft instrument-laden packages high into the atmosphere to measure temperature, pressure, and winds aloft seemed extravagant. And the idea that this telemetry package, having traveled halfway or more to space, could crash land in a field near my house so that I could recover it and take it apart, was an intoxicating thought.
I’ve spent a lot of time in the woods over the intervening years, but I’ve never seen a radiosonde in the wild. The closest I ever came was finding a balloon with a note saying it had been released by a bunch of schoolkids in Indiana. I was in Connecticut at the time, so that was pretty cool, but those shortsighted kids hadn’t put any electronics on their balloon, and they kind of left me hanging. So here’s a look at what radiosondes are, how they work, and what you can do to increase your chances of finding one.
Meteorologists have long known that getting data about the upper atmosphere is critical to accurate forecasts. As early as the 19th century, kites and tethered balloons were used to put recording thermometers and barometers aloft. Limited by the instrumentation at the time, not to mention the length of the tether used to recover the payload, early sounding data was crude at best, but valuable all the same. Free flying instrument packages were also used, with the instrument package returned gently to the surface by a parachute. Unfortunately, with no way to track the package, meteorologists relied on people returning the package before they could process the data.
For the data to make a difference to forecasters, it had to be available immediately. That would have to wait until the 1920s, when electronics technology had advanced sufficiently to allow true aerial telemetry. With a simple package of weather instruments and a radio transmitter, the radiosonde was born.
In the age of the vacuum tube, radiosondes were obviously somewhat chunky, with sensors being mainly electromechanical. Data encoding was often done by an automatic Morse keyer. Battery life was limited, but still, the data produced by these early radiosondes quickly became vital to forecasters, and several companies began mass producing the devices.
Designs have obviously evolved radically over the last 80 years, but then as today, radiosondes are primarily used to measure three critical parameters: air temperature, dew point, and barometric pressure. In the days before GPS satellites, radiosondes relied either on an altimeter or a paddlewheel a bit like an anemometer mounted on a horizontal axis to detect the altitude of the package, critical to the sounding data. Modern radiosondes incorporate a GPS receiver for both altitude data and latitude and longitude, all of which is transmitted back to the ground station. GPS location data not only provides data from a vertical slice of the atmosphere, but also provides data on wind direction and velocity. A radiosonde that provides this information is technically known as a rawindsonde, but that’s such an awkward formulation that the devices are universally referred to as radiosondes no matter what data they return.
Most radiosondes these days are made by one of two companies. Vaisala, a Finnish company whose founder, Vilho Väisälä, pioneered early radiosondes in the 1930s, has a huge share of the international radiosonde market with their RS line. The US National Weather Service had their own, older standard radiosonde that was phased out by 2014 or so by the Radiosonde Replacement System (RRS), and they now favor the Lockheed-Martin Corporation LMS6 for their soundings, although some offices use the Vaisala units.
Twice a Day, Every Day
Twice a day, at approximately 00:00 UTC and again at 12:00 UTC, meteorologists around the world prepare a radiosonde mission. The ritual is pretty much the same everywhere, from inflating the balloon with either hydrogen or helium to prepping and attaching the radiosonde. Once released, the balloon quickly carries the radiosonde aloft at 300 m/min on a mission that can last up to two hours and cover hundreds of kilometers. A mission needs to reach an altitude of 7 km to be considered a success, but most missions make it to around 25 km before the balloon expands and eventually bursts as the atmospheric pressure decreases. Altitudes over 35 km are not uncommon, though. The radiosonde continues to transmit data during its parachute descent.
Radiosondes typically transmit either at 403-MHz or 1680-MHz nominal frequencies. Modulation schemes and data rates vary by model, with Gaussian frequency-shift keying (GFSK) being the mode used for most Vaisala devices. Transmitter output power is generally pretty low, in the range of 50 to 100 mW, but getting a line-of-sight signal is not generally a problem after the mission gets a few hundred meters off the ground.
Receiving and decoding live radiosonde data is possible with the right gear. First, check to see if you live somewhere near a weather service office that releases radiosondes. Next, know what kind of radiosonde the office deploys. If it’s a Vaisala, you’re in luck — there are tons of programs out there that will let you use an ordinary UHF FM receiver or SDR dongle to receive and decode data. There’s a complete tutorial on getting started with radiosonde monitoring over at RTL-SDR.com that includes everything from antenna designs to plotting the GPS data on a map.
As for recovering a radiosonde in the wild, it’s certainly possible. Google around a bit and you’ll find plenty of social media posts with pictures of used radiosondes recovered from farmer’s fields or dangling from trees in the woods. Personally, I may be out of luck; while I live downwind of the Spokane, Washington NWS office, it’s only about 40 miles away. I suspect that most missions will go right past me and land somewhere in the vast forests stretching from Idaho into Montana. Chances are slim that I’ll be able to recover one, but that won’t stop me from trying to listen in.
[Featured image source: WNEP.com]