On December 13, 2022, a payload carrying a European weather satellite was sent into orbit from its launch site in Kourou, French Guiana. Known as the Meteosat Third Generation Imager 1 (MTG-I1), it was the first launch of the next generation of satellites from the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT).
Based in Darmstadt, Germany, and comprising 30 European countries as its members, EUMETSAT is the European operational satellite agency for monitoring weather and climate from space. EUMETSAT’s MTG-I1 satellite, developed by the European Space Agency (ESA), carries two instruments on board, both of which are imagers.
The first is the Flexible Combined Imager (FCI), which Gary Fowler, EUMETSAT’s MTG instrument systems manager, describes as “the workhorse of the geostationary fleet”. The second instrument is a Lightning Imager (LI). Described by Fowler as “an experimental secondary payload”, the LI is “effectively a very high-speed camera” that can be used to measure lightning flashes, which in turn can help in the prediction of thunderstorms. “People are getting quite excited about this one,” he says.
The launch of this imager satellite will be followed, in the second half of 2024, by the launch of MTG-S1, EUMETSAT’s third-generation sounder satellite. Carrying on board two sounding instruments – an infrared sounder (IRS) and an ultraviolet visible and near-infrared (UVN) sounder – the launch will be notable as the first time Europe has put sounding tech into a geostationary orbit. The central role of imagers and sounders in EUMETSAT’s latest satellites reflects the essential part both technologies play in weather and climate monitoring.
Sight or sound
According to Mo Belal, program director for microwave technologies at the satellite company Spire Global, imagers and sounders can best be thought of as the “eyes and ears” of satellite-based meteorology. Imagers, as the name suggests, rely on various kinds of imaging technology to provide a visual picture of Earth’s atmosphere. According to Dr David Smith, radiometry group leader at the UK government-funded Science and Technology Facilities Council (STFC), this visual information can be used, for example, to “map surface and cloud-top temperatures and to track the progress of weather fronts at higher spatial resolutions”. However, the use of imagers has limitations, adds Belal. “With optical imaging you can’t see through clouds,” he says. Infrared imagers also cannot penetrate cloud.
This is where the use of sounder technology comes in. The name is derived from the echo soundings that are carried out using sonar to build up a 3D picture of the sea floor. In a similar way, satellite-based sounders are used to provide a vertical profile of how atmospheric temperature, water vapor cloud properties and trace gases – including ozone, carbon monoxide, methane and carbon dioxide – change with altitude.
“Combining this data with measurements from the imager provides us with an almost 3D perspective of the atmosphere, so we get a better understanding of how certain compounds are being transported,” says Smith. For example, smoke from wildfires “tends to remain in the troposphere, whereas volcanic ash will reach the stratosphere and be transported globally”.
Sounder instruments scan for this data in different ways. The two sounding instruments on board the MTG-S1 satellite, for example, will rely on UV to near-infrared and infrared scanners to penetrate the atmosphere. On the other hand, sounding technology Belal is developing for Spire uses microwaves.
Another difference between the two technologies is in the spatial resolutions they are able to provide, according to Smith. For weather satellites, “imagers can track the progress of weather fronts at higher spatial resolution, whereas sounders measure the vertical profile of the atmosphere, but at a lower spatial resolution,” in part because “the measurement time of sounding instruments tends to be slower than imagers.”
Flash photography
For its latest generation of weather satellites, EUMETSAT has chosen to isolate the sounder and imager technology on separate satellites. “This was done partially to reduce the risk of putting all your eggs in one basket, as it were,” explains Fowler. “If you lose one satellite with four instruments on it, you’ve lost your complete capability.”
Another reason for the division was the different development times for the technologies. The sounder instruments that will be deployed on MTG-S1 “were new at the time they were being developed”, Fowler says, and as a result it was assumed they would take longer to build. Hence they were scheduled to go up on a later satellite launch. Of the two imagers on the MTG-I1 satellite, the FCI has been designed as a direct replacement to the imagers currently in operation on the second-generation Meteosats.
From its geostationary orbit the FCI will provide scans of weather systems over Europe and Africa. According to Fowler, the FCI is better in terms of radiometry and spatial resolution than what EUMETSAT currently has in use. The LI, meanwhile, will provide an early warning system for severe weather events. “With this instrument we’re aiming to detect very faint lightning pulses, which can be indicators of severe convection cells building up,” says Fowler.
The LI works by taking continuous pictures of Earth. These pictures allow the LI instrumentation to build up what Fowler describes as “an average background”. “Then when you get a bright signal, you subtract the background from that signal and, if it’s above a certain amplitude, according to a threshold you have set, you can say that it’s lightning,” he says.
New sound
On the MTG-S1, set to launch next year, the IRS will be used mostly for temperature and humidity profiling. It does this using a Fourier-transform spectrometer to measure infrared light. “In the infrared you can measure the water vapor and temperature very well,” says Fowler.
The UVN sounder “does a similar job, but in a different area of the spectrum and is used for detecting chemical pollutants in the atmosphere,” he adds. When the MTG-S1 goes into orbit it will be the first time sounders have been used on an operational satellite in geostationary orbit.
According to Smith, it was not possible to use sounders on earlier versions of Meteosats because of the rotational velocity of those earlier satellites. Smith says, “Those satellites were spinning at 100rpm and producing 16g,” while the MTG-S1, by contrast, has “a three-axis stabilized platform,” providing the sounding instruments with more time to take their measurements. The sounder tech being developed by Belal at Spire is also a first of its kind. Known as the Hyperspectral Microwave Sounder (HMS), it was developed by Belal and his team while he was working at the Rutherford Appleton Laboratory, a UK-based research lab that falls under the auspices of the STFC. After receiving funding from the UK Space Agency to miniaturize the HMS, Belal met with Spire, who saw that the technology was a “clear fit with its goals and mission around weather and climate science”.
Channel surfing
Belal is now developing the HMS in-house at Spire with the aim of fitting the sounder to next-generation constellations of Spire’s nanosatellites. According to Belal, the HMS instrument differs from conventional microwave sounders, which work by using multiple channels.
The channels on a sounder, he says, represent “a particular frequency band that you’re trying to measure”. The more channels a sounder has on board, the greater the range of frequencies it can measure, which in turn provides a broader picture of the atmosphere.
To capture a particular frequency band, each channel requires a receiver. Therefore, to expand the range of the sounder to include more channels, Belal says, “You must build more and more receivers, and your system becomes very complex.” For this reason, current microwave sounders are limited in the number of channels they use.
The HMS gets around this problem by digitizing the signal. This means that, “Rather than physically building more and more receivers, you digitize it in the back end, so that rather than 10 or 20 channels, you can have up to 1,000 channels in our case,” says Belal. This massive increase in the number of available channels is not only useful in providing better resolution of the atmosphere. It also makes the instrument better at withstanding potential interference.
The meteorological industry is fighting an ongoing battle to protect frequency bands that are under threat from the current rollout of 5G – and also potentially from 6G when it comes online. Of particular concern, according to Belal, is the frequency band used to measure atmospheric temperature, which is the most critical for weather forecasting accuracy. The HMS is useful in this fight, he says, because “By increasing the number of channels, you build much more resilience against interference.”
As well as being an entirely new technology, the HMS will also be the first use of a microwave sounder on a Spire satellite. Spire’s nanosatellite constellations fly in low-earth orbits as that is better suited to microwave sounders, which lose resolution at higher orbits, explains Belal. While no launch date has been specified for when the HMS will go into orbit, Spire CTO and co-founder Jeroen Cappaert says it will happen “relatively soon”, with the customers for the data likely to be “major weather institutions around the world”.
This article originally appeared in the April 2023 issue of Meteorological Technology International. To view the magazine in full, click here.
