Decades have passed since the first artificial satellite was launched into orbit in 1957. And through the span of 92 days spent there, it made it possible to study the upper layers of the ionosphere and to obtain information crucial for further launches, including about the conditions of instruments in space. Today, the number of operational satellites in orbit amounts to thousands.
Just three years after the first ever satellite launch, NASA successfully sent the first successful meteorological satellite Tiros 1 into orbit with the goal of detecting and mapping hurricanes. After that, in 1962, the organization launched the first telecommunications satellite Telstar 1, which enabled the first live broadcast of television images between the United States and Europe.
But the beginning of modern remote sensing (with digital images, instead of film images, and with high spatial resolution, as opposed to meteorological satellites) was laid in 1972, when NASA launched the first Landsat mission satellite. Another prominent satellite mission was marked with the launch of Sentinel 1 satellite, years after the start of the Landsat program. Let’s get into more detail on what these missions are and why they are so significant for remote Earth exploration.
Landsat Program
The first of the satellites under the program was launched in 1972, and the latest to date Landsat 9 was launched on September 27, 2021. Images acquired by the program’s satellites are a unique resource for many scientific studies in agriculture, geology, forestry, national security, etc. From crop monitoring to forest management, Landsat has made enormous contribution in satisfying the constantly growing demand for the data necessary to understand changes occurring on Earth.
Following the undeniable success of 8 already operating satellites, NASA and the USGS have decided to launch Landsat 9 in September 2021 as the ninth satellite of the program. Comparing in terms of band designation, Landsat 9 is pretty similar to Landsat 8.
But, when it comes to radiometric resolution, the ninth satellite is way ahead. If Landsat 8 possessed a 12-bit resolution and could differentiate between 4,096 shades of a given wavelength, the 9’s satellite of the program is able to differentiate 16,384 shades thanks to its 14-bit resolution.
Another improvement Landsat 9 has undergone concerns the thermal infrared sensor (TIRS-2), which is able to reduce the amount of stray light, improve atmospheric correction, and therefore, produce better readings for surface temperature.
Overall,it’s hard to overestimate the impact Landsat program had on expanding Earth exploration possibilities. More so, one of the biggest benefits of the program is that its imagery is open to the public to download for further processing depending on the object of research.
Sentinel Missions
Sentinel is a common name for several satellite missions. Each Sentinel constellation is aimed at fulfilling revisit and coverage requirements to provide data for Copernicus services. Each mission is equipped with radar and multi-spectral imaging instruments for the monitoring of land, ocean, and atmosphere.
Sentinel 1 is a mission focused on land and ocean monitoring that consists of polar-orbiting, day-and-night, all-weather radar imaging satellites. Sentinel 1 mission consists of two satellites Sentinel 1A launched in 2014 and Sentinel 1B launched in 2016. The mission operates in 4 acquisition modes.
The list of Sentinel 1 modes includes: Stripmap (SM), Interferometric Wide swath (IW), Extra-Wide swath (EW), Wave (WV). EW is used for wide area coastal monitoring and enables detection and tracking of ships, oil spills, and sea ice cover, while SM is for small islands monitoring and emergency management if requested.
As for Sentinel 1 bands, the mission carries a single C-band. Compared to the other bands, systems using the C-band have some advantage in signal stability, since losses due to adverse climatic conditions (in the event of thunderstorms, for example) in this band are very low. In higher frequency bands these losses can reach very high values, which makes it difficult to use them in equatorial latitudes, where rainfall is particularly strong.
Sentinel 6 is the newest Sentinel mission, carrying a radar altimeter to measure global sea-surface height, focusing on climate studies and oceanography particularly. The first satellite of this mission was launched in 2020.
Earth Observation Satellites Today
Over the past decade, the satellite market has experienced a real revolution in reducing the volume and mass of satellites. The result of this innovation was the creation of compact nanosatellites (weighing less than 10 kg) and cubesats (ultra-small satellites for space exploration weighing less than 2 kg).
The extremely low mass of most nano-satellites allows them to operate only in low earth orbit (LEO), resulting in a significant reduction of financial costs for their launch into orbit. The low weight of the satellites also significantly increases the allowed number of satellites that can be put into orbit in one launch. That means it has become much easier to launch satellite constellations.
More so, multi-spectral and hyper-spectral technologies, microwave radiometer, lasers on EO satellites, and synthetic aperture radar (SAR) are now able to satisfy the demand for precise, reliable, and relevant data across a variety of industries. Modern agriculture, forestry, environment monitoring, and other spheres are already hard to imagine without the use of EO satellites data.
