The death of a weather satellite seen by SDR

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Where is this world when a weather satellite designed for a two-year mission begins to fail 21 years after its launch? I mean, really – where’s the pride these days?

Jokes aside, it seems NOAA-15, a satellite launched in 1998 to monitor surface temperatures and other weather and climate parameters, has recently begun to show its age. This is how things go, and usually the takedown of a satellite is little known to the general public, except perhaps when it de-orbits in a dramatic but brief display across the sky.

But NOAA-15 and its sister satellites have an enthusiastic following among a community of enthusiasts who spend their time teasing them for signals as they pass overhead, using homemade antennas and cheap SDR receivers. It was these amateurs who were among the first to notice the woes of NOAA-15, and for the past few weeks they have been busy lamenting and celebrating the satellite’s signals alternately coming and going. Their recurring romance with the satellite is worth watching, as is what exactly went wrong with this bird in the first place.

eye in the sky

NOAA-15. Source: Gunter’s Space Page

NOAA-15’s legacy dates back to the early 1960s, when meteorologists realized that satellites equipped with infrared imagers would be an excellent way to collect data on surface temperatures, water vapor distribution and other parameters that go into a decent weather forecast. The first Television Infrared Observation Satellites, or TIROS, were extremely simple satellites, carrying little more than a television camera and transmitter. But these satellites returned the first images of Earth from space and provided the first glimpse of what the future of remote sensing held.

Some 40 years later, weather satellites had more than proved their worth and, under the auspices of the The National Oceanic and Atmospheric Administration, the US government agency responsible for collecting weather and climate data and turning it into useful products, an impressive constellation of weather satellites was in operation. NOAA-15 was added to this constellation, along with identical sisters NOAA-16 and NOAA-17.

Exploded view of AVHRR – click to enlarge. Scanner assembly lower left. Source: NOAA KLM User Guide

Known collectively as NOAA KLM, each satellite had a slightly different sun-synchronous orbit, and each was equipped with the latest in remote sensing technology. In addition to microwave sounding units of the type potentially threatened by the rollout of 5G cellphones, the NOAA KLM satellites were equipped with an instrument known as AVHRR, or Advanced Very-High Resolution Radiometer. This instrument was to be the workhorse of the satellites, producing images of the Earth below in six different wavelengths in the visible and infrared.

To produce images, a scanning motor rotates a flat beryllium mirror mounted at an angle in front of an 8″ (20 cm) reflecting telescope. The motor spins the mirror at 360 rpm, which scans the surface below in 1.09 km wide strips. Data for each band is digitized and transmitted over a VHF radio link, specially designed so that low-cost ground stations can receive and decode the transmissions directly.

easy listening

NOAA KLM designers probably never imagined that their satellite would last more than ten times longer than its expected life, let alone still be in service when the era of cheap software-defined radios arrived. . But their decision to make the downlink open and accessible is key to how radio enthusiasts have listened to the birds, in the good times at first, and now in the bad.

Listening to NOAA satellites is actually quite easy; we’ve explained how to do it since at least this 2012 article. The basic setup for a minimal ground station is an SDR dongle, readily available to the usual suspects for a few bucks; an antenna which can be as simple as a 2 meter long piece of wire stuck in the antenna socket of the SDR; and a simple chain of software tools on the host PC, which ends with WxToIMG, and an app that not only decodes data from the downlink and reassembles an image, but also tells you when the satellite is over your location. Setups can scale from there, with better antennas, sensitive amps, and higher quality SDRs, but the basics are still very affordable.

Until this year, images received from NOAA-15 were about normal. Results vary depending on the configuration used, and there is always some noise in the signal, but overall the images were more than usable. But images returned to the ground in mid-April began to show artifacts – broad swaths of color that were clearly not just RF noise or other previously observed interference. The RTL-SDR subreddit ignited with discussions speculating on the cause, which was eventually confirmed by NASA to be some sort of AVHRR scan engine failure.

On-Again, Off-Again

What the AVHRR could do until July 28. Source: r/RTLSDR user [notipa]

NASA initially blamed the loss of lubricant from the scan motor bearing. It’s not much of a litter; space is a tough place to do business. Lube Problems Almost Killed of Galilee mission to Jupiter, and bearing failures due to arcing experienced during coronal mass ejections and other space weather phenomena have caused the death of several spacecraft. And with about 3 billion more revolutions on the scanner motor than it was designed for, it’s not hard to guess that was the cause.

NOAA-15’s problems were only temporary, however, and the scanner motor seemed to come unstuck. But the problem returned at the end of July, with even more artifacts in the images. NASA released its analysis of the telemetry data, which indicated both a sudden spike in the current drawn by the scan engine at 302 mA, as well as an increase in the temperature of the scan assembly. This indicated a stuck motor, possibly due to worn bearings in the scanner head. AVHRR was no longer producing data, and it looked like the end had finally come for NOAA-15.

Portrait of a dying satellite. Source: Karsey Renfert

AVHRR came back to life on July 25, again producing data for several days. On July 30, the engine stalled again, and this time it looked like the AVHRR would not return to a reliable state. Yet there was more reports on August 7 that reliable images were being released again.

NOAA-15 has been a backup satellite since the launch of NOAA-18 in 2005 and NOAA-19 in 2009, so weather data will continue to flow in its absence. It’s likely that the doomed satellite will soon be decommissioned, and when it is, enthusiasts will no doubt tune their SDRs to listen once it’s rocketed into a graveyard orbit.

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