Unlocking the power of Mid-Wave Infrared

Our unique mid-wave infrared payload can observe the natural and built environment, multiple times both day and night, looking to detect the impact of what’s hot and what’s not across city spaces, industrial facilities, waterways and forest landscapes. Here we explain what infrared is, how it works and how we are harnessing its unique capabilities to deliver unprecedented levels of insight to our customers and resellers.

What is infrared and how does it work?

Infrared (IR) is part of the electromagnetic spectrum along with gamma, x-ray, ultraviolet, visible light, microwave and radio. See Figure 1.

Figure 1: Electromagnetic Spectrum. Credits: NASA and J. Olmsted (STScI)

All parts of the electromagnetic spectrum are the same thing—radiation. Radiation is made up of a stream of photons—particles without mass that move in a wave pattern all at the same speed, the speed of light. Each photon contains a certain amount of energy. In infrared terms, this means that the hotter something is, the more energy it radiates. Therefore, the hotter something is, the brighter it appears in an image. While we can see or feel some emissions of heat - such as a fire - the human eye is unable to detect infrared waves, despite them being all around us. That is why you can burn yourself by picking up a cup of tea when it is too hot. Infrared cameras allow us to see ‘underneath the bonnet’ of things, observing infrared waves in action. For example, industrial activity from factories and buildings emit heat radiation which can be captured by an infrared camera, as shown in Figure 2. Swipe left and right to compare aerial imagery with Satellite Vu’s.

Figure 2: Comparison of Stanlow oil refinery Map data ©2022 Google (left) and Satellite Vu (right).

As this figure shows, infrared reveals an additional layer of insight. It shows us what is ‘on’ or ‘off’, what’s working or not. It also clearly captures the thermal water pollution being distributed into the river as a by-product of industrial processes. With rising global and ocean temperatures, there are questions to be asked as to whether we can continue emitting the same amount of pollution or if companies should be responsible for cooling the water further before emitting back into public waterways.

Infrared is a broad spectrum

Satellite Vu will be measuring in mid-wave infrared (MWIR), between 3-5.5 µm. However, infrared as a range of wavelengths on the electromagnetic spectrum, also includes far- or long-wave (LWIR) and near- or short-wave (SWIR) infrared, as shown in Figure 3.

Figure 3 Credit: Science Mission Directorate. "Infrared Waves" NASA Science. 2010. National Aeronautics and Space Administration.

Why did Satellite Vu choose a MWIR sensor?

For a couple of reasons:

First, MWIR requires less extreme cooling and smaller optical apertures, which has a significant impact on the satellite weight. In other words, it makes it more economical for us to build and launch a satellite constellation.

More importantly, the availability of larger detector formats in a MWIR camera gives us the possibility of wider swath widths and/or smaller ground sample distance (GSD). That means we can get much higher resolution thermal images using MWIR compared to LWIR.

MWIR has been used in a variety of scientific satellite missions, including NASA sensors like MODIS, VIIRS, and ESA sensors like SLSTR. However, most of them can only provide images with resolution up to ~300 m resolution. LWIR sensors onboard ASTER or Landsat 8/9 are able to provide 90-100m resolution imagery, with the sacrifice of revisiting time down to 16 days. See Figure 4 below for comparison.

Figure 4: Comparison of thermal image of Liverpool in the UK taken by Landsat 8 (left) and Satellite Vu (right).

As shown by Figure 4, the improved resolution that our sensor can provide is a marked improvement on existing available data. Our resolution of 3.5m and multiple revisits of 10-20 times per day and night, depending on latitude, is a game-changer in the field of infrared remote sensing. This upgraded resolution enables us to locate buildings with poor thermal insulation, and fixing them will get us to Net Zero carbon quicker and cheaper. It allows us to differentiate unique properties of factories and man-made infrastructure such as hot pipes and storage facilities.

Remote sensing with infrared

Absolute and relative temperature - explanation of our product and its different levels

Correcting for emissivity

When measuring temperature there are three components one must account for: transmitted, reflected, and emitted energy.

When sensing the earth using our payload, we are seeking to determine the emitted energy from an object, which is the heat that the object itself generates. However, the image also captures transmitted and reflected energy which alters the temperature value of the object.

When taking an image, we also include transmitted energy and reflected energy for that same object which will alter the final temperature value.

Emissivity is a measure of a body’s ability to radiate heat. It is the ratio of the heat radiated by the material to the heat radiated by a perfect radiator (a black body) at the same temperature.

Emissivity formula:

1 – (Transmission + Reflection) = Emitted

When surfaces are very shiny and reflective (like certain industrial roofs made of aluminium or other polished metals) the emissivity can be very low (sometimes around 10% of the signal). This will ultimately show a much lower measured temperature value than it is in reality.

Let’s see a practical example below:

At Satellite Vu we are currently working on a proprietary algorithm to adjust the imagery for emissivity, allowing more accurate temperature comparisons between features in a scene.