Chapter 7

Satellite Remote Sensing in Precision Agriculture

Satellite Characteristics

Satellites share fundamental characteristics, yet artificial satellites, specifically engineered for space, exhibit a more detailed set of features related to their design, function, and operation.

Swath Width

Swath width refers to the horizontal distance covered by a satellite sensor as it captures images of the Earth's surface. The width of the swath can vary depending on the sensor and the orbit of the satellite, but it is typically several kilometers (km) wide. The swath width is important because it determines the area that can be imaged by the satellite in a single pass. For example, a satellite with a swath width of 10 km can capture an area of 10 km x 100 km (assuming a 10:1 length-to-width ratio) in a single pass.

Altitude

Satellites orbit the Earth at a wide range of altitudes, categorized into different orbit types like Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary Orbit (GEO). LEO satellites are typically between 160 and 2,000 km (100 and 1,243 miles) above the surface. MEO satellites orbit at altitudes between 2,000 and 35,786 km (1,243 and 22,236 miles), while GEO satellites are positioned at a fixed altitude of 35,786 km (22,236 miles).

Orbits Used for Remote Sensing Satellites

The path followed by a satellite is referred to as its orbit. Satellite orbits are matched to the capability and objective of the sensor(s) they carry. Orbit selection can vary in terms of altitude (their height above the Earth’s surface) and their orientation and rotation relative to the Earth. Remote sensing satellites are typically located in one of three popular orbital regimes: low Earth orbit (LEO), medium Earth orbit (MEO), and geosynchronous orbit (GEO).

Low Earth Orbit (LEO)

The majority of satellites orbiting the Earth do so at altitudes between 160 and 2,000 km, which is low compared to other orbits but still very far above Earth’s surface. This orbital regime is called low Earth orbit, or LEO, due to the satellites’ relative closeness to the Earth. Unlike satellites in GEO that must always orbit along Earth’s equator, LEO satellites do not always have to follow a particular path around Earth in the same way—their plane can be tilted.

Medium Earth Orbit (MEO)

Although over 90 percent of all satellites are situated in LEO (below 2,000 km) and GEO (near 36,000 kilometers), the space between the two most popular orbital regimes can be an ideal environment for a smaller subset of satellite systems.

Geosynchronous Earth Orbit (GEO)

The period of a satellite, or how long it takes to orbit the Earth one time, is dependent on its orbital altitude. Satellites in LEO take about 90 minutes to orbit the Earth. Satellites in MEO take about 12 hours to do the same. Satellites in geostationary orbit (GEO) circle Earth above the equator from west to east following Earth’s rotation–taking 23 hours, 56 minutes, and 4 seconds–by traveling at exactly the same rate as Earth. Most satellites in GEO have no inclination, meaning they orbit directly above Earth’s equator.

Other Orbits

Some orbits have special properties that make them ideal for specific satellite missions. Only a small fraction of operational satellites fall into this category.

Subsystems

A remote sensing satellite also contains a number of other subsystems to support image data collection. A power supply is required, typically consisting of solar panels and batteries. Precise altitude and orbital control are needed so satellites carry navigation and positioning subsystems.

Satellite Imaging Sensors

A satellite sensor is a device or instrument mounted on a satellite that is designed to detect and measure different types of energy and signals from the Earth’s surface and atmosphere. These sensors can capture various types of data, such as imagery, temperature, and radiation, and transmit it back to Earth for further analysis. The sensor in a satellite ultimately determines the type of data that the satellite detects. It’s critical to understand which type of data you need for a given use case, as different types of sensors are used for different purposes, depending on the intended application.

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