What is this world coming to when a weather satellite that was designed for a two-year mission starts to fail 21 years after launch? I mean, really — where’s the pride these days?

All kidding aside, it seems like NOAA-15, a satellite launched in 1998 to monitor surface temperatures and other meteorologic and climatologic parameters, has recently started showing its age. This is the way of things, and generally the decommissioning of a satellite is of little note to the general public, except possibly when it deorbits in a spectacular but brief display across the sky.

But NOAA-15 and her sister satellites have a keen following among a community of enthusiasts who spend their time teasing signals from them as they whiz overhead, using homemade antennas and cheap SDR receivers. It was these hobbyists who were among the first to notice NOAA-15’s woes, and over the past weeks they’ve been busy alternately lamenting and celebrating as the satellite’s signals come and go. Their on-again, off-again romance with the satellite is worth a look, as is the what exactly is going wrong with this bird in the first place.

Eye in the Sky

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

Forty-odd years later, weather satellites had more than proven their worth, and under the auspices of the National Oceanic and Atmospheric Administration, the US government body responsible for gathering weather and climate data and turning it into useful products, an impressive constellation of weather satellites was in service. NOAA-15 was added to this constellation, along with identical sisters NOAA-16 and NOAA-17.

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 that are potentially at risk by the 5G cell phone rollout, the NOAA KLM satellites were equipped with an instrument known as the 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 rotates the mirror at 360 RPM, which scans the surface below in 1.09-km wide strips. The data from each strip is digitized and transmitted over a VHF radio link, specifically designed so that low-cost ground stations would be able to receive and decode the transmissions directly.

Easy Listening

NOAA KLM’s designers likely never envisioned that their satellite would last more than ten times longer than its design life, let alone that it would still be in service when the age of cheap software-defined radios would come along. But their decision to make the downlink open and accessible is the key to how radio enthusiasts have been listening in on the birds, in good times at first, and now in bad.

Listening to NOAA satellites is actually quite easy; we’ve covered how to do it since at least this 2012 article. The basic setup for a minimal ground station is an SDR dongle, easily had from the usual suspects for a couple of bucks; an antenna that can be as simple as a 2-meter long piece of wire jammed into the SDR’s antenna jack; and a simple toolchain of software on the host PC, which ends with WxToIMG, and app that not only decodes the data coming back from the downlink and reassembles an image, but also tells you when the satellite is over your location. Setups can grow from there, with better antennas, sensitive amps, and better quality SDRs, but the basics remain very affordable.

Up until this year, the images received from NOAA-15 were pretty much normal. Results vary based on the setup used, and there’s always some noise in the signal, but in general the images were more than usable. But images sent back to the ground in mid-April started showing artifacts — wide bands of colors that were clearly not simple RF noise or some other previously seen interference. The RTL-SDR subreddit went abuzz with threads speculating on the cause, which was eventually confirmed by NASA to be some sort of failure in the scan motor for the AVHRR.

On-Again, Off-Again

NASA initially pinned the blame on the loss of lubricant from the bearing in the scan motor. That’s not much of a reach; space is a tough place to do business. Lubricant problems nearly killed Galileo’s mission to Jupiter, and bearing failures due to arcing experienced during coronal mass ejections and other space weather phenomena have been the death of several spacecraft. And with about 3 billion more revs on the scanner motor than it was designed for, it’s not a leap to assume that it was the cause.

NOAA-15’s troubles were only temporary, though, and the scanner motor seemed to unstick itself. But the problem came back in late 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 motor to 302 mA, as well as an increase in the temperature of the scanning assembly. This pointed to a stalled motor, likely due to worn-out bearings in the scanner head. The AVHRR was no longer producing data, and it looked like the end had finally come for NOAA-15.

The AVHRR came back to life on July 25, producing data again for several days. On July 30, the motor stalled again, and this time it seemed like the AVHRR wouldn’t recover to a reliable state. Yet there were more reports on August 7th that reliable images were once more streaming down.

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