Earth Observation satellites imagery: Types, Application, and Future Trends5th Mar 2022
Space exploration has not only been exciting but has also given way to many useful things, including Earth Observation satellites. People now have a chance to see the Earth from hundreds of kilometres above and discover previously incomprehensible things. Today, thanks to satellite imagery, the military, scientists, geologists, forecasters, and businesses can study environmental conditions, events, and activities that were not obvious before. The significance of Earth Observation for the security and sustainable development of mankind is enormous, and the scope of satellite imagery is very extensive. This article will tell all about Earth Observation technologies, how they work, and offer some satellite comparison of captured images.
Seeing the invisible: how do Earth Observation satellites work
Simply put, Earth Observation satellites are equipped with special sensors that photograph, measure, track, and explore everything that happens on the surface of the Earth and even below it.
Most of this information is converted into a collection of images that make up a single geographic information system of the Earth (GIS). By the way, not only are satellites equipped with sensors, but also aircraft, drones, and robots, however, it is Earth Observation satellites that give us the most complete picture with a large spatial coverage. This is possible because EO satellites are placed in orbits with an altitude from 200 (SSO) to 36,000 km (GSO)! There are currently over 1,000 Earth Observation satellites in space from NASA, European Space Agency, Roscosmos, CNSA, and others, and this number continues to grow. Below, you can see a figure of the main groupings, along with their spatial resolution, revisit time, and satellite comparison of captured images.
EO satellite comparison
We have already talked about Earth Observation history and areas of application; now let’s find out how EO satellites get such important data for all of us.
Types of Earth Observation satellite sensors
The classification of remote Earth sensing methods is based on the type of signal source for studying the object, which can be either active or passive.
Passive sensors register a signal emitted or reflected by an object or an adjacent territory. Usually, it is reflected sunlight or heat emanating from objects on the earth’s surface.
Active sensors emit a signal on their own in order to scan an object and space, after which they detect and measure the radiation reflected or formed by backscattering.
Passive sensors usually take pictures in the ultraviolet (UV), visible and near-infrared, thermal infrared, and microwave spectral ranges. Active sensors use radar satellite imagery and lidar imaging. Now, let’s talk about all types of sensors in order, and take a look at satellite comparison of captured images.
Optical sensors in Earth Observation satellites
They conduct photographic and scanner imaging in the visible and infrared ranges with a resolution from sub-meter to 2 meters. These sensors include passive panchromatic systems, multispectral systems, and hyperspectral systems and are quite common, despite their obvious shortcomings. Since these sensors lack their own radiation source, they can capture an image only during daylight hours and in cloudless weather. On the other hand, such sensors are easier to manufacture, deploy, and their data are easier to process and analyze, which is why they are widely used. One application example is to visualize multiple targets: image acquisition of linear features such as coastlines, pipelines, roads, and borders. The most illustrative example of optical sensor imagery, familiar to almost everyone, is Google Maps. Examples of optical EO satellite systems include LandSat, SPOT, and IKONOS.
This is what the image of Hong Kong, made by high-precision IKONOS optics, looks like.
Thermal sensors in satellites
The operation of thermal sensors (infrared radiometers, infrared cameras, and spectroradiometers) is based on the principle of measuring thermal energy. Any object with a temperature above zero is capable of radiating thermal energy and creating a thermal image. This makes thermal sensors indispensable for tracking living things, including animals and humans, as well as for detecting volcanic and hydrothermal activity, wildfires, climate research, ice thickness, thunderstorm intensity, and more. Thermal sensors are classified as passive devices. And even though the image in this range does not depend on lighting and can be done at night, clouds are still an obstacle.
Radar satellite imagery sensors can be both passive and active. Passive (radio wave) sounding in the microwave range is based on registering the surface’s own radiation via mechanical scanning. The disadvantages of this method are the low resolution of radar satellite images, limited to several kilometers, low fluctuation sensitivity of microwave radiometers; and a strong dependence on the state of the surface (primarily on the degree of roughness).
Active (radar) sounding is based on radiation from a satellite and the reception of a radio signal reflected by the surface. Side-scan radars with a real antenna (RLSAR) and a synthetic aperture antenna (SAR) are used here. The latter allows replacing a large antenna with a more compact and efficient receiver to receive higher resolution radar satellite images (up to several meters). In other words, SAR capabilities are wider than those of conventional radar satellite imagery. Not only can it capture data from multiple angles, but can also compile these data to create stacked or multiple feature images. At the same time, the main advantage of such a survey is its all-weather capability because radio waves can penetrate under the surface and reflect, for example, groundwater lenses.
This is what radar satellite images of the Noatak National Wildlife Refuge, Alaska from the ICEYE-X1 SAR satellite look like.
Lidar survey (Light Identification Detection and Ranging) is active and is based on the continuous reception of a response from a reflective surface illuminated by laser monochromatic radiation with a fixed wavelength. Essentially, Lidar works exactly like SAR, but it uses a laser instead of radio waves and can operate in infrared, visible, or UV wavelengths. Lidars are used to accurately measure topographic features, monitor the growth or melting of glaciers, profile clouds, measure wind, study aerosols, and quantify various atmospheric components. Take a look at the Lunar topography measurements taken by LIDAR of US automatic interplanetary station Clementine.
Earth Observation technologies do not stand still. The sector is gradually moving from using large, complex, and expensive EO satellites to building constellations of many small, affordable satellites. The focus is shifting from increasing spatial resolution to finding better coverage and revisit time to provide real-time Earth observation monitoring for everyone. This means that, soon enough, we can expect the emergence of new types of high-precision sensors and a more flexible reconfigurable payload for Earth Observation satellites.