An introduction to Remote Sensing

For this new Space Law article on Space Legal Issues, let’s have an introduction to Remote Sensing. Remote sensing is defined by the English Oxford Dictionary as “the scanning of the Earth by satellite or high-flying aircraft in order to obtain information about it”. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation, especially the Earth. Remote sensing makes it possible to collect data of dangerous or inaccessible areas.

The goal of this article is to provide an introduction to the basics of satellite remote sensing with a special focus on the principles, characteristics, analysis, and applications of remote sensing data acquired in different parts of the electromagnetic spectrum. Topics include the principles and characteristics of remotely sensed imagery, various remote sensing systems, and the methods used to collect remote sensing data.

The history of applications and development of remote sensing are discussed, as well as the fundamentals of electromagnetic radiation and its interactions with the Earth’s surface and atmosphere. This article will discuss sensor characteristics, satellite orbits, and various current and future missions involving a range of sensors across the visible, infrared, and microwave components of the spectrum. A variety of examples of remote sensing applications for examining and monitoring environmental questions are described.

Types of remote sensing missions and applications

There are many different space remote sensing satellites in use today, including a wide variety of weather, military, civil, commercial, and scientific systems. Satellite remote sensing systems are useful for collecting information on environmental variables such as climatic and atmospheric radiation and chemistry, ocean dynamics and productivity properties, geographic and topographic locations, and land-biosphere characteristics.

Specific information that can be derived from remote sensing imagery include: land use and land cover; land surface temperature; soil type and moisture; vegetation biomass; sea surface temperature; bathymetry; atmospheric temperature and humidity; wind speed; cloud and aerosol properties; volcanic effects; and snow, sea, and polar ice distribution and thickness. Because satellite sensors estimate most of these variables indirectly, there are significant issues with assessing accuracy and error in remote sensing-derived maps. It is important to consider all of these characteristics and traits of remote sensing systems and imagery when considering which satellite data-sets to choose for any project or application.

Passive sensors gather radiation that is emitted or reflected by the object or surrounding areas. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography, infrared, charge-coupled devices, and radiometers. Active collection, on the other hand, emits energy in order to scan objects and areas whereupon a sensor then detects and measures the radiation that is reflected or back-scattered from the target.

Electromagnetic theories in application to remote sensing

Most remote sensing devices detect variations in electromagnetic energy. The electromagnetic spectrum is the range of all types of electromagnetic energy according to frequency or wavelength, ranging from shorter wavelengths (ultraviolet) to longer wavelength (near infrared, thermal, microwave). The atmosphere itself causes significant absorption and/or scattering of the very shortest wavelengths. A spectral signature is the degree to which energy is reflected in different regions of the spectrum by different Earth surface materials, enabling them to be detected by visual or digital means from remotely sensed imagery. Finding distinctive spectral response patterns is the key to most procedures for computer-assisted interpretation and digital processing of satellite images.

It is important to understand that satellite communications works in synergy with terrestrial communications. It forms only a small part of the total communication bandwidth, and is only practical for a select number of communication tasks.

Orbits used in remote sensing

Earth observation satellite systems generally operate in two major orbits: (a) Geostationary Earth Orbits (GEO), and (b) polar or sun synchronous orbits. Geostationary Earth Orbits are located about thirty-six thousand kilometres above the Earth, resulting in an orbital period equal to the Earth’s rotational period; thus, the satellites appear motionless or stationary to ground observers. Other polar (sun synchronous) orbiting remote sensing platforms cover most of the Earth’s surface over a certain period of time in a manner covering each area of the world at a constant local time of day (called local sun time) which ensures consistent illumination conditions.

Resolution

Resolution is a specific term used in remote sensing to describe the level of precision in the gathered data. In remote sensing, we generally consider four major types of resolution: Spatial, Spectral, Temporal, and Radiometric. Spatial resolution is the size of the pixel, which is dependent on the sensor type, field of view, altitude, and viewing angle of the sensor. Spectral resolution refers to the number of wavelength regions or bands in the electromagnetic spectrum to which the sensor is sensitive. Temporal resolution is a measure of how often data are collected for the same area (revisit times). Radiometric resolution is a measure of the sensitivity of a sensor to differences in the intensity of the electromagnetic radiation measured.

Gathering information remotely

Types of remote sensors can be divided further into non-scanning and scanning systems. Scanning systems employ a sensor with a narrow field of view sweeping over the terrain to collect and produce a two-dimensional image of the Earth’s surface. Reflected or emitted energy from observed objects is captured in digital picture elements called pixels. A multispectral scanning system collects data over a range of wavelengths, acquiring imagery in two main modes or methods: across-track scanning and along-track scanning. Other types of systems in operation include radar systems, thermal sensors, and hyperspectral sensors.

RADAR and Lidar

In an introduction on Remote Sensing, RADAR and Lidar are examples of active remote sensing where the time delay between emission and return is measured, establishing the location, speed and direction of an object.

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the object(s). Radio waves (pulsed or continuous) from the transmitter reflect off the object and return to the receiver, giving information about the object’s location and speed.

Radar was developed secretly for military use by several nations in the period before and during World War II. The term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging; the term radar has since entered English and other languages as a common noun, losing all capitalisation.

Lidar, sometimes LIDAR, LiDAR, or LADAR, is a surveying method that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3D representations of the target. The name LIDAR, now used as an acronym of light detection and ranging (sometimes light imaging, detection, and ranging), was originally a portmanteau of light and radar. Lidar is sometimes called 3D laser scanning, a special combination of a 3D scanning and laser scanning. It has terrestrial, airborne, and mobile applications. Lidar is commonly used to make high-resolution maps, with applications in geodesy, geomatics, archaeology, geography, geology, geomorphology, seismology, forestry, atmospheric physics, laser guidance, airborne laser swath mapping (ALSM), and laser altimetry. The technology is also used in control and navigation for some autonomous cars. This is what can be said in an introduction on Remote Sensing.