**1. Introduction**

In 2015, one of the most remarkable events in the space industry was when SpaceX realized the reusability of its rocket for the first time. Additionally, in June 2014, Russia used 1 rocket to launch 37 satellites at the same time. At present, many countries have the capability to launch multiple satellites in one mission. For example, NASA and the US Air Force launched 29 satellites in a single mission in 2013. At that time, the mission represented the most satellites ever launched at one time [1]. In 2015 and 2016, China and India launched 20 satellites in single mission, respectively. At present, six organizations have the capability to launch multiple satellites in a single mission: Russia, USA, China, India, Japan, and ESA. This trend indicates that in the future, the cost of sending satellites to space will greatly decrease. More and more remote sensing resources are becoming available. It is of great importance to have

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a comprehensive survey of the available remote sensing technology and to utilize inter- or trans-disciplinary knowledge and technology to create new applications.

(e.g., synthetic aperture radar (SAR)). A laser rangefinder uses a laser beam to determine the distance between the sensor and the object and is typically used in airborne and ground-based laser scanning. A laser altimeter uses a laser beam to determine the altitude of an object above a fixed level and is typically utilized in satellite and aerial platforms. SAR uses microwaves to illuminate a ground target with a side-looking geometry and measures the backscatter and travel time of the transmitted waves reflected by objects on the ground. The distance that the SAR device travels over a target in the time taken for the radar pulses to return to the antenna produces the SAR image. SAR can be mounted on a moving platform, such as spaceborne and airborne platforms. According to the combination of frequency bands and polarization modes used in data acquisition, sensors can be categorized as single frequency (L-band, C-band, or X-band), multiple frequency (a combination of two or more frequency bands), single polarization (VV, HH, or HV), and multiple polarization (a combination of two or more polarization modes). Currently, there are three commercial SAR missions in space: Germany's TerraSAR-X and TanDEM-X (X-band with a ~3.5 cm wavelength), Italy's COSMO-SkyMed (X-band with ~3.5 cm wavelength), and Canada's RADARSAT-2 (C-band with ~6 cm wavelength). In addition, ESA's ERS-1, ERS-2, and Envisat also carried SAR, although these missions have ended. The latest SAR satellites from ESA include Sentinel-1A, Sentinel-1B, and Sentinel-3A. Typical SAR parameters are repeat frequency, pulse repetition frequency, bandwidth, polarization,

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As sensor technology has advanced, the integration of passive and active sensors into one system has emerged. This trend makes it unclear difficult to categorize sensors in the traditional way, into passive sensors and active sensors. In this paper, we introduce the sensors in terms of imaging or non-imaging functionality. Imaging sensors typically employ optical imaging systems, thermal imaging systems, or SAR. Optical imaging systems use the visible, near-infrared, and shortwave infrared spectrums and typically produce panchromatic, multispectral, and hyperspectral imagery. Thermal imaging systems employ mid to longwave infrared wavelengths. Non-imaging sensors include microwave radiometers, microwave altimeters, magnetic sensors, gravimeters, Fourier spectrometers, laser rangefinders, and laser altimeters [6]. It has been decades since Landsat-1, the first Earth resources technology satellite, was launched in 1972. Satellite platforms have evolved from a single satellite to multi-satellite constellations. Sensors have experienced unprecedented development over the years, from 1972 with the first multispectral satellite, Landsat-1, with four spectral bands to 1997 with the first hyperspectral satellite, Lewis, with 384 spectral bands. Spatial resolution has also significantly improved over the decades, from 80 m in Landsat-1 to 31 cm in Worldview-3. A number of studies on satellite imagery processing methods and applications have been conducted. A few papers providing sensor overviews have been published, including [7–9]. Blais [7] reviewed the range sensors developed over the past two decades. The studied range sensors include single point and laser scanners, slit scanners, pattern projections, and time-of-flight systems. In addition, commercial systems related to range sensors were reviewed. Melesse et al. [18] provided a survey of remote sensing sensors for typical environmental and natural resources mapping purposes, such as urban studies, hydrological modeling, land-cover and floodplain mapping, fractional vegetation cover and impervious surface area mapping, surface energy flux and micro-topography correlation, remotely sensed-based rainfall, and potential evapotranspiration for estimating

incidence angle, imaging mode, and orbit direction [5].

Remote sensing is considered a primary means of acquiring spatial data. Remote sensing measures electromagnetic radiation that interacts with the atmosphere and objects. Interactions of electromagnetic radiation with the surface of the Earth can provide information not only on the distance between the sensor and the object but also on the direction, intensity, wavelength, and polarization of the electromagnetic radiation [2]. These measurements can offer positional information about the objects and clues as to the characteristics of the surface materials.

Satellite remote sensing consists of one or multiple remote sensing instruments located on a satellite or satellite constellation collecting information about an object or phenomenon on the Earth surface without being in direct physical contact with the object or phenomenon. Compared to airborne and terrestrial platforms, spaceborne platforms are the most stable carrier. Satellites can be classified by their orbital geometry and timing. Three types of orbits are typically used in remote sensing satellites, such as geostationary, equatorial, and sunsynchronous orbits. A geostationary satellite has a period of rotation equal to that of Earth (24 hours) so the satellite always stays over the same location on Earth. Communications and weather satellites often use geostationary orbits with many of them located over the equator. In an equatorial orbit, a satellite circles the Earth at a low inclination (the angle between the orbital plane and the equatorial plane). The Space Shuttle uses an equatorial orbit with an inclination of 57°. Sun-synchronous satellites have orbits with high inclination angles, passing nearly over the poles. Orbits are timed so that the satellite always passes over the equator at the same local sun time. In this way, these satellites maintain the same relative position with the sun for all of its orbits. Many remote sensing satellites are sun synchronous, which ensures repeatable sun illumination conditions during specific seasons. Because a sun-synchronous orbit does not pass directly over the poles, it is not always possible to acquire data for the extreme polar regions. The frequency at which a satellite sensor can acquire data of the entire Earth depends on the sensor and orbital characteristics [3]. For most remote sensing satellites, the total coverage frequency ranges from twice a day to once every 16 days. Another orbital characteristic is altitude. The space shuttle has a low orbital altitude of 300 km, whereas other common remote sensing satellites typically maintain higher orbits ranging from 600 to 1000 km.

The interaction between a sensor and the surface of the Earth has two modes: active or passive. Passive sensors utilize solar radiation to illuminate the Earth's surface and detect the reflection from the surface. They typically record electromagnetic waves in the range of visible (~430–720 nm) and near-infrared (NIR) (~750–950 nm) light. Some systems, such as SPOT 5, are also designed to acquire images in middle-infrared (MIR) wavelengths (1580–1750 nm). The power measured by passive sensors is a function of the surface composition, physical temperature, surface roughness, and other physical characteristics of the Earth [4]. Examples of passive satellite sensors are those aboard the Landsat, SPOT, Pléiades, EROS, GeoEye, and WorldView satellites. Active sensors provide their own source of energy to illuminate the objects and measure the observations. These sensors use electromagnetic waves in the range of visible light and near-infrared (e.g., a laser rangefinder or a laser altimeter) and radar waves (e.g., synthetic aperture radar (SAR)). A laser rangefinder uses a laser beam to determine the distance between the sensor and the object and is typically used in airborne and ground-based laser scanning. A laser altimeter uses a laser beam to determine the altitude of an object above a fixed level and is typically utilized in satellite and aerial platforms. SAR uses microwaves to illuminate a ground target with a side-looking geometry and measures the backscatter and travel time of the transmitted waves reflected by objects on the ground. The distance that the SAR device travels over a target in the time taken for the radar pulses to return to the antenna produces the SAR image. SAR can be mounted on a moving platform, such as spaceborne and airborne platforms. According to the combination of frequency bands and polarization modes used in data acquisition, sensors can be categorized as single frequency (L-band, C-band, or X-band), multiple frequency (a combination of two or more frequency bands), single polarization (VV, HH, or HV), and multiple polarization (a combination of two or more polarization modes). Currently, there are three commercial SAR missions in space: Germany's TerraSAR-X and TanDEM-X (X-band with a ~3.5 cm wavelength), Italy's COSMO-SkyMed (X-band with ~3.5 cm wavelength), and Canada's RADARSAT-2 (C-band with ~6 cm wavelength). In addition, ESA's ERS-1, ERS-2, and Envisat also carried SAR, although these missions have ended. The latest SAR satellites from ESA include Sentinel-1A, Sentinel-1B, and Sentinel-3A. Typical SAR parameters are repeat frequency, pulse repetition frequency, bandwidth, polarization, incidence angle, imaging mode, and orbit direction [5].

a comprehensive survey of the available remote sensing technology and to utilize inter- or

Remote sensing is considered a primary means of acquiring spatial data. Remote sensing measures electromagnetic radiation that interacts with the atmosphere and objects. Interactions of electromagnetic radiation with the surface of the Earth can provide information not only on the distance between the sensor and the object but also on the direction, intensity, wavelength, and polarization of the electromagnetic radiation [2]. These measurements can offer positional information about the objects and clues as to the characteristics of the surface materials.

Satellite remote sensing consists of one or multiple remote sensing instruments located on a satellite or satellite constellation collecting information about an object or phenomenon on the Earth surface without being in direct physical contact with the object or phenomenon. Compared to airborne and terrestrial platforms, spaceborne platforms are the most stable carrier. Satellites can be classified by their orbital geometry and timing. Three types of orbits are typically used in remote sensing satellites, such as geostationary, equatorial, and sunsynchronous orbits. A geostationary satellite has a period of rotation equal to that of Earth (24 hours) so the satellite always stays over the same location on Earth. Communications and weather satellites often use geostationary orbits with many of them located over the equator. In an equatorial orbit, a satellite circles the Earth at a low inclination (the angle between the orbital plane and the equatorial plane). The Space Shuttle uses an equatorial orbit with an inclination of 57°. Sun-synchronous satellites have orbits with high inclination angles, passing nearly over the poles. Orbits are timed so that the satellite always passes over the equator at the same local sun time. In this way, these satellites maintain the same relative position with the sun for all of its orbits. Many remote sensing satellites are sun synchronous, which ensures repeatable sun illumination conditions during specific seasons. Because a sun-synchronous orbit does not pass directly over the poles, it is not always possible to acquire data for the extreme polar regions. The frequency at which a satellite sensor can acquire data of the entire Earth depends on the sensor and orbital characteristics [3]. For most remote sensing satellites, the total coverage frequency ranges from twice a day to once every 16 days. Another orbital characteristic is altitude. The space shuttle has a low orbital altitude of 300 km, whereas other common remote sensing satellites typically maintain higher orbits

The interaction between a sensor and the surface of the Earth has two modes: active or passive. Passive sensors utilize solar radiation to illuminate the Earth's surface and detect the reflection from the surface. They typically record electromagnetic waves in the range of visible (~430–720 nm) and near-infrared (NIR) (~750–950 nm) light. Some systems, such as SPOT 5, are also designed to acquire images in middle-infrared (MIR) wavelengths (1580–1750 nm). The power measured by passive sensors is a function of the surface composition, physical temperature, surface roughness, and other physical characteristics of the Earth [4]. Examples of passive satellite sensors are those aboard the Landsat, SPOT, Pléiades, EROS, GeoEye, and WorldView satellites. Active sensors provide their own source of energy to illuminate the objects and measure the observations. These sensors use electromagnetic waves in the range of visible light and near-infrared (e.g., a laser rangefinder or a laser altimeter) and radar waves

trans-disciplinary knowledge and technology to create new applications.

20 Multi-purposeful Application of Geospatial Data

ranging from 600 to 1000 km.

As sensor technology has advanced, the integration of passive and active sensors into one system has emerged. This trend makes it unclear difficult to categorize sensors in the traditional way, into passive sensors and active sensors. In this paper, we introduce the sensors in terms of imaging or non-imaging functionality. Imaging sensors typically employ optical imaging systems, thermal imaging systems, or SAR. Optical imaging systems use the visible, near-infrared, and shortwave infrared spectrums and typically produce panchromatic, multispectral, and hyperspectral imagery. Thermal imaging systems employ mid to longwave infrared wavelengths. Non-imaging sensors include microwave radiometers, microwave altimeters, magnetic sensors, gravimeters, Fourier spectrometers, laser rangefinders, and laser altimeters [6].

It has been decades since Landsat-1, the first Earth resources technology satellite, was launched in 1972. Satellite platforms have evolved from a single satellite to multi-satellite constellations. Sensors have experienced unprecedented development over the years, from 1972 with the first multispectral satellite, Landsat-1, with four spectral bands to 1997 with the first hyperspectral satellite, Lewis, with 384 spectral bands. Spatial resolution has also significantly improved over the decades, from 80 m in Landsat-1 to 31 cm in Worldview-3. A number of studies on satellite imagery processing methods and applications have been conducted. A few papers providing sensor overviews have been published, including [7–9]. Blais [7] reviewed the range sensors developed over the past two decades. The studied range sensors include single point and laser scanners, slit scanners, pattern projections, and time-of-flight systems. In addition, commercial systems related to range sensors were reviewed. Melesse et al. [18] provided a survey of remote sensing sensors for typical environmental and natural resources mapping purposes, such as urban studies, hydrological modeling, land-cover and floodplain mapping, fractional vegetation cover and impervious surface area mapping, surface energy flux and micro-topography correlation, remotely sensed-based rainfall, and potential evapotranspiration for estimating crop water requirement satisfaction indexes. Recently, a survey on remote sensing platforms and sensors was provided by Toth and Jóźków [9]. The authors gave a general review in current remote sensing platforms, including satellites, airborne platforms, UAVs, ground-based mobile and static platforms, sensor georeferencing and supporting navigation infrastructure, and provided a short summary of imaging sensors.

**Phases Time series Remarks**

During the First and Second World Wars

During the Cold War (1947–1991)

Since the launch of the Terra satellite in 1999

same time as EOS

surveillance

applications

software

The use of photographs for surveying, mapping, reconnaissance and military

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Remote sensing for military use spilled over into mapping and environment

Since then, data in digital formats and the use of computer hardware and

a Thematic Mapper sensor; Landsat 7 carries an Enhanced Thematic Mapper; Landsat 8 carries the Operational Land Imager. Landsat satellites have high resolution and global coverage. Applications were initially local and have

carried a variety of earth observation instruments: a radar altimeter, ATSR-1, SAR, wind scatterometer, and microwave radiometer. A successor, ERS-2,

In the late 1950s The launch of Sputnik 1 by Russia in 1957 and Explorer 1 by US in 1958

1960~ The launch of the first meteorological satellite (TIROS-1) by the US in 1960.

1991~ The European Space Agency launched the first satellite ERS-1 in 1991, which

Terra/Aqua satellites carrying sensors, such as MODIS and taking measurements of pollution in the troposphere (MOPITT). Global coverage, frequent repeat coverage, a high level of processing, easy and mostly free

Next generation of satellites and sensors, such as Earth Observing-1,

2. A revolutionary means of data acquisition: daily coverage of any spot on

4. The launch of GeoEye-1 in 2008 for very high-resolution imagery (0.41 m)

formerly Skybox): RapidEye was launched in August, 2008, with five EOS. These are the first commercial satellites to include the Red-Edge band, which is sensitive to changes in chlorophyll content. On March 8, 2016, Skybox imaging was renamed to Terra Bella. Satellites provided the ability to capture the first-ever commercial high-resolution video of Earth from a satellite and the ability to capture high-resolution color and near-

2. For the first time, Russia carried out a single mission to launch 37 satellites

3. ESA launched the first satellite of the Sentinel constellation in April of 2014.

5. Current satellites in high revisiting period, large coverage, and high spatial

3. Google streaming technology allows rapid data access to very high-

acquiring the first spaceborne hyperspectral data

earth at a high resolution, such as Rapideye

2000~ 1. Very high-resolution data, such as IKONOS and Quickbird satellites

2008~ 1. Small satellites and satellite constellation (RapidEye and Terra Bella,

4. SpaceX reusable rocket capacity since December of 2015

Landsat 1972~ Landsat 1, 2, and 3 carrying a multispectral scanner; Landsat 4 and 5 carried

become global since then

was launched in 1995

resolution images

infrared imagery

in June of 2014

**Table 1.** Evolution and advancement in remote sensing satellites and sensors.

resolution, up to 31 cm

access to data

Airborne remote sensing

Rudimentary spaceborne satellite remote sensing

Spy satellite remote sensing

Meteorological satellite sensor remote sensing

European Space Agency's first Earth observing satellite program

Earth observing system (EOS)

Private industry/ commercial satellite systems

Microsatellite era and satellite constellations

New millennium Around the

In the literature, we found that overviews of remote sensing sensors were quite rare. One reason for this finding was that this topic is fairly broad. Usually, one can find detailed knowledge from thick books or a very simple overview from some webpages. As most readers need to obtain relevant knowledge within a reasonable time period and with a modest depth, the contribution of our paper is valuable. In this paper, we review the history of remote sensing, the interaction of the electromagnetic spectrum (EMS) and objects, imaging sensors and nonimaging sensors (e.g., laser rangefinders/altimeters), and commonly used satellites and their characteristics. In addition, future trends and potential applications are addressed. Although this paper is mainly about satellite sensors, there is no apparent boundary between satellite sensors and airborne, UAV-based, or ground-based sensors except that satellite sensors have more interaction with the atmosphere. Therefore, we use the term "remote sensing sensors" generally.
