As the main page already explains, this image gallery aims to present samples of visualized satellite data that has been received by amateurs directly from the satellite's radio broadcast. This includes satellites with dedicated "user" downlinks, such as the familiar APT, LRPT and HRPT, as well as data "intercepted" from the satellite's main mission data downlinks, like the ones encountered within the S-band or X-band. The original purpose of the gallery was to demonstrate the capabilites of the LeanHRPT software, but it has since been expanded and moved to a different domain in order to include all kinds of data, not just HRPT.
Below is an example of an image and its annotations taken from the gallery;
The top-left corner shows the name of the satellite and instrument that captured the image. In this case it was the FengYun-3C weather satellite using its VIRR instrument. The top-right corner shows the date on which the image was captured as well as the downlink that the data was received from, including the frequency used for reception. This image was received from the 1701.4 MHz HRPT downlink on the 23rd of September, 2022.
The text at the bottom-left of the image shows the software or method that was used to render/produce it, while the bottom-right corner is used to give credit to whoever submitted it. In this case the data for this image was submitted by Marquis (@meteosatellites on Twitter), while the image itself was decoded in LeanHRPT. If no credit is displayed it most likely means it is one of the images created before external submissions were included in the gallery, in which case "dereksgc" can be assumed.
Finally, the text shown in the center-top of the image gives a brief description of the actual imaged content and its location on the first line, followed by the "composite" on the second line.As most imaging sensors on satellites have multiple bands, most of which are nowhere near standard visible light, a "false color" composite has to be created in order to actually present an image. Note that the term "false color" - at least in our case - doesn't mean "artificial" or "colorized", but simply refers to the fact that light from bands other than normal visible red, green, or blue has been used. The simplest form of a composite is taking three bands imaged by the satellite and mapping them to the RGB channels of the image file. In many cases, a better, more pleasant result can be achieved using various combinations of more than three bands at a time. As this mixing process may greatly vary between different satellites and their sensors, a simplified name for the given composite will usually be presented. You can read about the most common examples below.
Visible Color
A "Visible Color" composite uses bands similar to normal human vision (visible red, green, and blue). Can be considered "true color", but does not use that name in order to not be confused with greatly processed true color imagery distributed by satellite operators such as NOAA, who sometimes use upwards of a dozen different light channels and employ advanced methods of image correction. A Visible Color composite in this gallery can be as simple as just three bands closest to visible red, green and blue mapped as RGB. Note that this particular color composite is only available from some satellites that are equipped with the appropriate imaging sensors, as the visible green and blue bands that are required to create this composite have not been included on older weather satellites, and still lack on some modern ones (depending on their mission).
False Color Near-Infrared
Usually only uses two bands, namely a visible red one and a near-infrared (NIR) one. The NIR band will normally be mapped to the red channel of the image, while the actual visible red band is used for the green and blue channels. This results in vegetated areas showing up red in the image and water bodies getting a vibrant cyan/blue-green tint.
Natural Color Near-Infrared
The most common type of color composite in weather satellite reception (except APT). Uses the same two bands as False Color Near-Infrared (visible red and near-infrared), but mixes them in a way that produces natural-looking green vegetation.
False Color Short-Wave Infrared
Uses the visible red and near-infrared (NIR) bands similar to the False Color Near-Infrared composite, but also introduces a shortwave infrared (SWIR) band. Usually the SWIR band would be mapped to the red channel of an image, with the NIR and visible red bands being mapped as green and blue respectively. This addition greatly changes the usual color appearance of False Color Short-Wave Infrared images, with vegetated land gaining a more familiar green appearance and water getting a much darker shade of blue. Thanks to its wavelength being longer than red or NIR, the SWIR band reacts differently particularly to different types of clouds, sea, ice and water. This composite has great meteorological utility, but is often avoided when aesthetics and a visually pleasant image output are desired.
Natural Color Short-Wave Infrared
Takes the concept of False Color Short-Wave Infrared (see above) but uses more complex band mixing to remove most of the blue tint introduced by the SWIR band while trying to preserve the extra land and vegetation colors. Usually produces the most visually appealing and naturally looking color composites from a given satellite (unless bands for true/visible color are available, and even then can look good alongside it).
False Color Long-Wave Infrared
Very similar composition to False Color Short-Wave Infrared, but the SWIR band is replaced by a long-wave infrared (LWIR) one. This is also known as a "thermal infrared" band, due to its ability to "see temperature" instead of color. While this is extremely useful for weather satellites, it is generally not ideal to mix visible, NIR and SWIR bands with LWIR ones as they tend to behave quite differently. This "unpredictability" can however in some cases produce visually interesting data worth including in the gallery, and can sometimes be used as an alternative three-channel composite when the SWIR band is not available.
False Color Thermal Infrared
Composes one or two long-wave infrared (LWIR) bands with a medium-wave infrared (MWIR) band. The MWIR band is at the border between "visible-like" infrared light that behaves in a way we're used to and thermal infrared where the tempreture of the imaged subject translates to its perceived brightness. This kind of composite is especially useful for images captured during night time, where the visible, NIR and SWIR bands are completely blacked out and MWIR and LWIR are the only ones available. Due to the non-conventional properties of MWIR and LWIR light, the resulting images can vary widely in color and detail, being influenced by both the physical appearance of the landscape below the spacecraft as well as its temperature.
NDVI Color
Calculates the Normalized Difference Vegetation Index (NDVI) using a visible red and near-infrared band which "brightens" vegetation, and then mixes its result into a false color composite similar to Natural Color Near-Infrared. The resulting image has a strong emphasis on vegetated areas in the form of very saturated, almost "glowing" green color. Can greatly increase the contrast between vegetated and non-vegetated areas. The NDVI result can also be displayed alone in grayscale (see "Single band images" below).
Single band images
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Can't be considered "composites" as they only utilize a single channel, rendering them black and white as a result. This kind of image is usually not particularly desirable, and may be utilized when there are no more bands available to create a color composite, or if some particular details are better visible when one band is isolated. The caption of the image will usually mention which band is used, with the exception of "Thermal Infrared". This name is a leftover from the previous use of this gallery, and refers to a long-wave infrared (LWIR) band that "sees temperature" instead of color or brightness. A "black-hot" configuration for Thermal Infrared will be used throughout the gallery as that results in clouds being familiar white (due to them being cold relative to the ground and ocean).
The standard size of the actual image frame is 1080 by 1080 pixels, while the overall image size with its border included is 1120 by 1180 pixels. The image file type is lossy webp, as it provides the best compression while still maintaining a relatively good quality. Some images, depending on the type of data and the kind of spacecraft capturing it may not work well with a 1:1 aspect ratio. An example of such image is shown below. Also note that there is no composite description present as this satellite in particular has an instrument that only captures a single "channel", and therefore the image shown contains all of the data available already.
