5.1. Landsats

Landsat, launched by US in July 1972, is the longest running satellite program for acquisition of earth observation imagery. The Earth Resources Technology Satellite (ERTS) was renamed as Landsat in 1975. Since then, the Landsats have provided vast amount of images at medium resolution required to study and analyse the land use/land cover, vegetation and agricultural crops, urban, water, soils, geology and other Earth resources.

The Landsat program began in July 1972 and has continued for decades, making it the longest-running program dedicated to observing the Earth's surface from space. It was initially called the Earth Resources Technology Satellite and was renamed Landsat in 1975. The core purpose of Landsat is to capture medium-resolution images that help researchers analyze various aspects of the Earth's resources such as land use and cover, agricultural crops, urban planning, water bodies, soil conditions, and geological features.

Think of Landsat like a very detailed diary of our planet. Just like you might write down changes in your life or your surroundings over the years, Landsat keeps a record of how our land and resources change over time, allowing scientists to understand and track those changes.

The launch of Landsat-2 and Landsat-3 followed in 1975, and 1978, respectively. The first three satellites were identical and their payloads consisted of a Multispectral Scanner (MSS) and two video cameras, called Return Beam Videcons or (RBVs). One scene covered an area of 170 km x 185 km, as the satellites operated at an altitude between 907-915 km in sun-synchronous polar orbit with 103 minutes of orbital period and revisit time of 18 days.

After the initial launch of Landsat in 1972, subsequent satellites, Landsat-2 and Landsat-3, were launched in 1975 and 1978. These first three satellites were designed identically and included a Multispectral Scanner to capture data across different wavelengths and two video cameras known as Return Beam Videcons (RBVs). Each satellite could cover a scene of 170 km by 185 km from an altitude of about 907-915 km, on a sun-synchronous polar orbit, which means they were timed to pass over the same areas on Earth at the same sun angle every 18 days.

Imagine taking photographs of your neighborhood not just once, but repeatedly over the years. Each time, you make sure to use the same camera settings and zoom to keep consistency. This is similar to how the Landsat satellites were built; they all were designed to take similar kinds of images to compare changes over time.

Landsat-4 was launched in 1982. Then, Landsat-5 was launched in 1984, and it continued to deliver high quality, global data of Earth’s surfaces for more than 28 years. Landsats-4 and 5 were equipped with two multispectral sensors, i.e., a MSS and a Thematic Mapper (TM). The altitude of the orbit was 705 km with 99 minutes’ orbital period, and revisit time of 16 days.

Landsat-4 was launched in 1982, followed by Landsat-5 in 1984. Landsat-5 was particularly notable for its longevity, providing valuable observational data for more than 28 years. Both Landsat-4 and Landsat-5 featured two types of multispectral sensors: a Multispectral Scanner (MSS) and a Thematic Mapper (TM), which allowed for enhanced image processing and analysis. They orbited at a slightly higher altitude of 705 km, with a quicker revisit time of every 16 days.

Imagine updating your smartphone with better cameras every few years; Landsat-4 and 5 were like those upgrades, bringing improved imaging technology to capture even more details of the Earth's surface.

Landsat-6 was launched October 5, 1993, but was lost during launch. Thereafter, Landsat-7 was successfully launched in April 1999 which has 8 separate spectral bands with spatial resolutions ranging from 15-60 m, and a temporal resolution of 16 days. It was equipped with a multispectral sensor known as the Enhanced Thematic Mapper Plus (ETM+). The ETM+ provides data in one panchromatic band and 7 multispectral bands.

Landsat-6 was launched in 1993 but unfortunately failed to reach its intended orbit. However, Landsat-7 was successfully launched in April 1999. This satellite was equipped with an Enhanced Thematic Mapper Plus (ETM+) sensor, which had 8 different spectral bands for capturing various types of data. The spatial resolutions varied from 15 to 60 meters, once again allowing researchers to monitor changes on Earth's surface every 16 days.

Think of the Landsat-7 as a high-tech camera that can take pictures with multiple filters, each capturing different aspects of a scene—like taking one photo with sunglasses and another without, to see how colors change. This allows scientists to explore various features of the environment.

Landsat-8 was launched in 2013, which continued to provide daily global data. It consisted of two sensors. The Operational Land Imager (OLI) operates in nine spectral bands, including a pan band with resolution of 30 m in multispectral and 15 m in pan, while Thermal Infrared sensor (TIR) operates in two spectral bands: (10.6-11.19 µm) and (11.5-12.51 µm) with 100 m spatial resolution. Landsat-9 is launched on 27, September 2021.

Landsat-8 was launched in 2013 and is designed to provide comprehensive global data on Earth's surface every day. It incorporates two advanced sensors: the Operational Land Imager (OLI) which covers nine spectral bands with high resolutions of 30 m for multispectral images and 15 m for the panchromatic band, and the Thermal Infrared sensor (TIR) that captures thermal data with a 100 m resolution across two spectral bands. The latest addition, Landsat-9, was launched on September 27, 2021.

Imagine the advancements in cameras over the years—Landsat-8 is like the new smartphone that not only takes high-quality pictures but also measures temperature, allowing us to see not just the visual aspect of landscapes but also their warmth.

Landsat 9 carries the Operational Land Imager-2 (OLI–2) and the Thermal Infrared Sensor-2 (TIRS–2), with improved features like higher radiometric resolution. The OLI–2 allows sensors to detect more differences in images, increasing detail especially over darker areas such as water or dense forests.

Landsat-9 improves upon its predecessor with better technology. It includes the Operational Land Imager-2 (OLI-2) which provides a higher radiometric resolution, allowing the satellite to differentiate between more shades of color. This capability enhances its ability to analyze challenging areas, particularly darker colors like dense forests or deep water, providing clearer insights into Earth's features and conditions.

Think about how some of your favorite movies look better on a new high-definition TV. Landsat-9 enhances imagery just like that, allowing scientists to see more detail and make better observations.

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