Sentinel-5 Precursor (Sentinel-5P) is an Earth observation satellite created by ESA as part of the Copernicus Programme. It was launched in October 2017 and is built to last for 7 years. The satellite uses the TROPOspheric Monitoring Instrument (Tropomi), which helps track methane emissions in great detail.
Overview
Sentinel-5P is the first mission of the Copernicus Programme designed to track air pollution. Its instrument, Tropomi, is a special tool that uses ultraviolet, visible, near, and short-wavelength infrared light to study the atmosphere. The satellite is built on a hexagonal frame called the Astrobus L 250, which includes S- and X-band antennas for communication, three foldable solar panels that generate 1,500 watts of power, and hydrazine thrusters to maintain its position in space.
The satellite orbits Earth at an altitude of 824 kilometers (512 miles) in a Sun-synchronous path, passing over the equator at 1:30 PM local time each day.
In July 2009, the European Space Agency and the Dutch Ministry of Economic Affairs signed a contract worth €78 million for the Tropomi instrument. In December 2011, ESA chose Astrium UK as the main builder of the satellite, signing a contract worth €45.5 million. Construction of the satellite was completed in May 2014, and it was successfully combined with its main instrument. The total cost to build Tropomi was €220 million.
Sentinel-5P was launched by Eurockot Launch Services using a Rokot rocket. The launch was originally planned for late 2014 but was delayed multiple times. It was finally launched on October 13, 2017, at 9:27 UTC from Plesetsk Cosmodrome Site 133. The satellite reached its final orbit 79 minutes after liftoff.
Tropomi instrument
Sentinel-5P carries one instrument called the TROPOspheric Monitoring Instrument (Tropomi). Tropomi is a spectrometer that detects light in ultraviolet (UV), visible (VIS), near-infrared (NIR), and short-wave infrared (SWIR) wavelengths. It measures gases like ozone, methane, formaldehyde, aerosol, carbon monoxide, NO₂, and SO₂ in Earth’s atmosphere. Tropomi improves upon the OMI instrument from the Aura satellite and the SCIAMACHY instrument from Envisat.
Tropomi takes measurements every second, covering an area about 2,600 km (1,600 mi) wide and 7 km (4.3 mi) long, with a resolution of 7 x 7 km. It weighs approximately 200 kg (440 lb), uses an average of 170 watts of power, and produces about 140 gigabits of data per orbit.
Tropomi was developed by a partnership between the Netherlands Space Office, Royal Netherlands Meteorological Institute, Netherlands Institute for Space Research, Netherlands Organisation for Applied Scientific Research, and Airbus Defence and Space Netherlands.
The instrument has four main parts: the UV, VIS, and NIR spectrometers and a calibration block; the SWIR spectrometer with its optics; the instrument control unit; and a cooling block. Light is split into different wavelengths using grating spectrometers and measured by four detectors that match the spectral bands. The UV spectrometer measures wavelengths from 270–320 nm, the visible light spectrometer from 310–500 nm, the NIR spectrometer from 675–775 nm, and the SWIR spectrometer from 2305–2385 nm.
The SWIR spectrometer was designed and built by the Optical Payloads Group of Surrey Satellites (SSTL). It uses an immersed grating design, where light hits an etched grating inside a high-index silicon substrate. This design allows for a compact and efficient structure. The SWIR grating was provided by SRON (Netherlands), who also supplied the Front-End Electronics (FEE). The SWIR spectrometer receives light from the main instrument through an intermediate pupil and directs it via a telescope to a slit that defines the instrument’s footprint on Earth. Light from the slit is re-collimated, diffracted by the immersed grating, and focused onto a two-dimensional detector using a high-aperture relay lens. The SWIR detector (from Sofradir, France) has 256 elements across the track and 1,024 elements along the spectrum, with each element spaced 30 microns apart. It operates at a cold temperature (about 140 K). The SWIR spectrometer optics are mounted on a cooled optical bench (about 200 K) and insulated with a multi-layer insulation (MLI) blanket. The SWIR instrument was tested and calibrated at the Mullard Space Science Laboratory’s thermal vacuum facility in Surrey, UK.
Application examples
The Sentinel-5P Tropomi instrument recorded large decreases in nitrogen dioxide levels over Chinese cities from late January to February 2020. These changes were connected to China’s actions to address the coronavirus pandemic, which significantly reduced industrial and other activities that pollute the air. Tropomi data also supported a connection between higher rates of COVID-19 cases and long-term exposure to air pollutants.