Quite a while ago I wrote a post on one of my hobbies, tracking Bureau of Meteorology (BOM) radio sondes. For many years, the local BOM office have been launching analog radiosondes, which need to be tracked using traditional radio direction-finding methods. Recently (I don't know long ago it started), they have switched back to flying the Vaisala RS92SGPA digital radiosondes. These are decodable using COAA's brilliant SondeMonitor software, and have a GPS receiver, allowing calculation of their location (with some caveats).

It had been quite some time since I last chased a radiosonde, and given the prediction for Sunday the 21st looked good (Landing on the northern end of the Yorke Peninsula), I decided to give it a go. At 8am, 2 prospective F-calls (Josh and York) and I headed to the Adelaide Airport to watch the 00Z sonde be launched.

We didn't have good vantage point of the launcher itself, but as soon as the balloon was in the air we caught sight of it. Surprisingly, the BOM are still launching radar reflectors, even though the local sonde tracking radar is now gone.

With York driving and me in the passenger seat manning the laptop, we headed north towards Pt Wakefield and the predicted landing site.

Some info on the chase car setup: Previously, I've always chased sondes in Adrian VK5QU's Landcruiser, which is set up with all sorts of radio paraphernalia. Unfortunately, his truck was out of service, with all the radio gear stripped out! This meant I had to configure my car for tracking instead. This consisted of a 1/4 wave monopole on a mag-mount, my Icom IC-7000, and a nice preamp and interdigital filter set we recovered from an old radiosonde tracking station. Audio from the radio was fed into my laptop via a Tigertronics Signalink.

While SondeMonitor is great at decoding the sonde telemetry, its mapping functions leave a bit to be desired. For our Project Horus launches, an interface between dl-fldigi and OziExplorer was developed by Terry Baume, which also included live updating predictions of the balloon's flight path. A bit of Python coding (tested on the previous night's launch) let me pull data from SondeMonitor's live-KML interface and emulate dl-fldigi, allowing use of Terry's software. With that, we had the sonde position and a live prediction of where it was going:

With a few losses of signal due to passing through the null of the sonde's antenna, we arrived at the expected landing site about 10km south-west of Kulpara. Parking at a crossroads, we received the sonde's telemetry until it landed.

Now, the position of the sonde is calculated within the SondeMonitor software, using raw pseudorange data transmitted in the telemetry. For SondeMonitor to make any sense of this data, it needs to be supplied with either RINEX ephemeris data from a local GNSS reference station, or with the current gps almanac. Unfortunately for me, all the GNSS reference stations in my area charge for access, so I had to use the almanac method. This produces far less accurate positions, as we found out when we drove to the the reported landing site.

As expected, the sonde had lost GPS signal on landing, and I had a yagi and Icom IC-R10 at the ready. I got out the car, and started swinging the beam towards the predicted landing site, to the south of the road we were on. Nothing. Doing a full 360 degree sweep, I instead got a strong bearing to the northern side of the road, into a wheat field.

A bit of radio direction-finding later, we spotted the radar reflector about 20m into a wheat field. Walking carefully and in single file to avoid damaging the crop, we walked in and recovered the sonde.

The actual landing site ended up being about 300-400m from the last reported location, showing the importance of having radio direction-finding capability when chasing these sondes.

All in all, it was a good day, and I hope to hunt many more of these over the coming months.

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