Introduction of typical sensors

Remote sensing sensor is the key equipment to obtain remote sensing data. Due to the different characteristics of design and data acquisition, there are many kinds of sensors. At present, the types of sensors used in remote sensing can be roughly divided into: ① photographic sensors; ② scanning imaging sensor; ③ Radar imaging sensor; ④ Non-imaging sensor. The following will introduce the first three types of typical sensors.

(1) optical photographic sensor

The basic working principle of this kind of sensor is that according to the imaging principle of geometric optics, the image is focused by lens (group), and the visible light reflected by the target and the electromagnetic radiation energy in photographic infrared band are directly sensed and recorded by using photosensitive materials through photochemical reaction, thus forming a solidified image of the target on film or image paper. Its advantages are high spatial resolution, low cost, simple operation and large information capacity. The disadvantage is that it is limited to the spectrum of 0. 3 ~ 1.3 micron, and the geometric distortion of the image is serious, and the imaging is limited by climate, lighting conditions and atmospheric effects.

Typical optical photographic sensors are various cameras, which can be divided into frame camera, slit camera, multi-spectral camera and panorama camera according to the structure and film exposure mode.

1. framing camera

This is the most familiar sensor. It is mainly composed of collector, objective lens, detector and photographic film, in addition to cassette, shutter, grating and mechanical transmission device. There is only one latent image on the exposed negative, and the image can only be displayed after photographic processing. The imaging principle of this sensor is to obtain a complete photo (18cm × 18cm or 23cm ×23cm) at a certain shooting moment, and all the image points in a photo share a shooting center and the same image plane.

Figure 3-5 Slit Camera

2. Slit camera

Slit camera is also called air belt camera. On an airplane or a satellite, the instantaneous image obtained by photography is a ground image perpendicular to the course and equal to the gap width. This is because a slit baffle is placed in front of the focal plane of the camera to block all images outside the slit (Figure 3-5). When the plane or satellite flies forward, the image in the slit perpendicular to the flight direction on the focal plane of the camera changes constantly. If the film in the camera is also continuously wound, and its speed is the same as the moving speed of the image on the ground in the gap, a continuous strip aerial negative can be obtained. When the aircraft speed does not match the film winding speed, the image will have affine distortion. The projection characteristic of slit camera is still the central projection of the image with slit width obtained instantly. However, for strip images, the projection properties of strip images are different from those of frame images because they are obtained continuously with the camera moving with the plane. The image of trajectory line is orthographic projection, while the image points of other parts are the central projection relative to the photography center in their respective gaps, which is called polycentric projection. In addition, the displacement and attitude changes of the aircraft equipped with this sensor will cause complex geometric distortion of the image.

3. Multispectral camera

It is designed to take multi-spectral photos of the same target in different bands. Its structure is similar to ordinary aerial camera, but it has the characteristics of multi-lens and multi-channel. Common multispectral cameras can be divided into three types, namely, multi-camera type, multi-lens type and single-lens spectral type.

Multi-lens mode is to put several lenses with the same optical characteristics on an aerial camera and shoot the same area in different bands. Multi-camera model is to install multiple aerial cameras on the same plane and combine them into a multi-camera model. Between cameras, the optical axes are parallel to each other. By pressing a shutter button, several shutters can work at the same time, thus taking multi-spectral photography of ground objects. A single-lens spectral camera is characterized by using a prism to divide the light beam into several bands and then take pictures, or using a multi-sensitive film responding to different bands to take multi-spectral photos. After taking a photo, the film gets a combined multi-spectral photo, such as color photography and infrared color photography.

Figure 3-6 Panoramic camera

4. Panoramic camera

Panorama camera is also called a scanning camera. The structure of the panoramic camera is shown in Figure 3-6. It sets a slit on the focal plane of the objective lens parallel to the flight direction, and scans with the objective lens in the direction perpendicular to the flight trajectory to obtain a scanned image. Therefore, it is called scanning camera, and it is also called panorama camera because the objective lens swings greatly and can absorb the images in the horizon on both sides of the flight path.

Panorama camera is characterized by its long focal length, some of which exceed 600 mm, and its frame is very large, so it can be imaged on a film with a length of about 23cm and a width of 128cm. The precision lens of this camera is small and light, and the scanning field of view is very large, sometimes reaching 180. This camera uses a slit on the focal plane parallel to the flight direction to limit the instantaneous field of view, so it obtains a very narrow image parallel to the flight trajectory on the ground at the moment of shooting. When the objective lens swings in the vertical direction, a Zhang Quanjing photograph is obtained. The negative of this camera is placed in an arc shape, and the negative rotates into one when the objective lens scans once. Because every instant image is built in the small field of view in the center of the objective lens, every part of the image is clear, and the resolution on both sides of the image frame is obviously improved. However, because the image distance of panorama camera remains unchanged, and the object distance increases with the increase of scanning angle, the scales on both sides gradually decrease, resulting in the so-called panoramic distortion of the whole image. In addition, the plane moves forward during scanning, and the nonlinear swing of the scanning mirror makes the image distortion more complicated. Figure 3-7 shows the shape of the square grid on the ground in the panoramic photo.

Figure 3-7 Distortion of Panoramic Photos

(2) Scanning imaging sensor

The sensor of scanning imaging type obtains two-dimensional images point by point and line by line in time sequence. There are two main forms: one is an imager that scans the surface of an object, which is characterized by scanning and imaging the ground directly, such as an infrared scanner, a multispectral scanner, an imaging spectrometer, a stepping imager and a multiband spectrometer. The second is to instantly form a linear image or even a two-dimensional image on the image plane, and then scan the image. Such instruments are linear CCD push-broom imager, TV camera and so on.

Figure 3-8 Structural Diagram of Airborne Infrared Scanner

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The structure of a typical airborne infrared scanner is shown in Figure 3-8. Its specific structural elements include rotating scanning mirror, mirror system, detector, refrigeration equipment, electronic processing device and output device.

The scanning imaging process of infrared scanner is that when the rotating prism rotates, the first mirror scans the ground once across the route direction, and the ground radiation energy in the scanning field of view enters the sensor from one side to the other side in turn. The video signal output by the detector is amplified and modulated by an electronic amplifier, and the image line corresponding to the scene in the ground scanning field of view is displayed on the cathode ray tube. This image line is recorded on the negative after exposure, and then the second scanning mirror scans the ground. When the plane moves forward, the movies are synchronized. Going down in turn, you get a two-dimensional strip image corresponding to the ground range.

Because the ground resolution changes with the scanning angle, the infrared scanned image will be distorted, which is usually called panoramic distortion, and its formation reason is similar to that of panorama camera.

The infrared scanner also has a problem of temperature resolution, which is directly related to the responsivity R of the detector and the noise N of the sensor system. In order to obtain better temperature resolution, the noise equivalent temperature of infrared system is limited to 0. 1 ~ 0.5 K, and the temperature resolution of the system is generally 2 ~ 6 times of the equivalent noise temperature.

2.TM thematic mapper

TM thematic imager is an advanced multi-band scanning instrument, including seven spectral bands, the first to fifth spectral bands and the seventh spectral band are visible light, near infrared and short-wave infrared spectral bands, and the sixth spectral band is thermal infrared spectral band. The instantaneous field of view of visible light, near infrared and short-wave infrared spectra is 30m (track height is 705km), and the instantaneous field of view resolution of thermal infrared spectra is 120m. Due to the improvement of spatial resolution and the expansion of spectral coverage, it can be used to classify earth resources and draw a variety of thematic maps.

Figure 3-9 Optical System of Thematic Imager

The structure of TM thematic imager is shown in Figure 3-9. Its main mirror is located in the middle and lower part of the instrument, with an optical baffle and a secondary mirror in front. The second reflector is installed on the telescope structure bracket through the pillar. Behind the main mirror is a scanning line corrector, an internal calibrator and a main focal plane. The internal calibrator adopts incandescent lamp, the optical fiber bundle is used as the light source of the first to fifth and seventh spectral bands, and the temperature controllable blackbody is used in the sixth spectral band. The scanning line corrector is a small, motor-driven double-mirror scanning system, and its rotation speed is the same as that of the satellite orbit, but in the opposite direction. Through the active scanning of the mirror, the motion of the image is directly corrected. The radiation cooler, subsequent optical system and infrared detector array are located at the end of the instrument. The electronic circuit is installed in a wedge-shaped box and fixed above the telescope. The main performance parameters of thematic imager are shown in Table 3-3.

Table 3-3 TM thematic imager parameters

Fig. 3-3- 10 HRV scanner structural diagram

3.HRV linear push-broom scanner

HRV is a linear push-broom scanner, and its simple structure is shown in Figure 3- 10. There is a plane mirror in the instrument, which reflects the electromagnetic wave radiated from the ground to the mirror group, and then focuses it on the CCD linear array element, and the output end of CCD outputs a time series video signal. Because the linear CCD element is used as the detector, the image line perpendicular to the flight trajectory can be obtained at the same time in an instant, without using the swinging scanning mirror like a slit camera, and the continuous image belt along the track can be obtained in a "push-sweep" way. CCD, also known as charge coupled device, is a solid device made of semiconductor materials such as silicon. The charge generated by light or electric excitation is carried by electrons or holes and moves in the solid to realize sequential output signals.

4. Imaging spectrometer

Imaging spectrometer is a new generation sensor that was formally developed in the early 1980s. The main purpose of developing this instrument is to obtain a large number of narrow-band continuous spectral images of ground objects and almost continuous spectral data of each pixel at the same time, so it is called imaging spectrometer. At present, the existing imaging spectrometer can be divided into hundreds of narrow bands in the visible-infrared band and has high spectral resolution. From its nearly continuous spectral curve, we can distinguish the subtle differences of spectral characteristics of different objects, which is beneficial to identify more targets. Therefore, the imaging spectrometer is mainly used for hyperspectral remote sensing.

The principle and structure of imaging spectrometer can be divided into two types, one can be called linear array detector CCD plus optical scanning (Figure 3- 1 1), and the other is area array CCD detector plus spatial push scanning (Figure 3- 12).

Figure 3- Working mode of linear imaging spectrometer +0 1

Figure 3- 12 Working Mode of Area Array Imaging Spectrometer

The former is actually the development of multi-spectral scanners MSS and TM to more spectral bands, so it has the same line center projection relationship as linear CCD and slit photography imaging and similar technical characteristics as multi-spectral scanners: ① spatial scanning is completed by swinging the scanning mirror, thus obtaining a large field of view (up to 90); (2) pixels are well registered, and different bands can stare at the same pixel at any time; (3) The spectrum covers a wide range, from visible light to thermal infrared band; ④ It is difficult to further improve spectral resolution, spatial resolution and radiation sensitivity.

The second kind of imaging spectrometer is actually a further development of point push-broom scanner, so it has the same central projection relationship and HRV-like characteristics as area CCD and push-broom photography: ① the pixel gaze time is long, which can obtain higher system sensitivity and spatial resolution; (2) In the visible light band, because the device is very mature, it can achieve quite high spectral resolution. However, the registration between spectral channels is difficult and the optical design is not easy, so the total field of view can only reach about 30. ③ Mid-infrared spectrum, especially thermal infrared spectrum, is greatly limited by devices, and there is no substantial progress at present, so it is difficult to cover this spectrum.

As the main sensor of Terra (EOS-AM- 1), the imaging spectrometer MODIS was launched in 1999. With its huge application prospect and free receiving policy, MODIS receiving and processing stations have sprung up all over the world, and MODIS will become an important information source for macro-resources and environmental remote sensing. MO-DIS is divided into 36 bands from visible light to infrared, and adopts linear CCD detector structure combining optical and mechanical scanning. The ground resolution of each point below the satellite is 250m, 500m and 1000m respectively, and the satellite orbit is synchronized with the sun. 10: 30 am transit, with a width of 2330km, covering the whole world basically once a day. In the photoelectric conversion of MODIS, the double-sided scanning mirror rotates to scan the ground, and the spectral signals of ground objects are collected with the width of 10km at a time, and then focused on the detector on the satellite through the lens. Because different wave bands need different detectors, a beam splitter is set in front of the objective lens. After the beam splitting, it is sent to four objective lenses and focal plane components, namely visible light (VIS), near infrared (NIR), short-wave infrared (SWIR), medium-wave infrared (MWIR) and long-wave infrared (LWIR). Detectors and A/D converters responding to different wave bands are set on the focal plane, and the analog signals of ground objects are converted into digital signals, which are output through formatters and buffers, and the products are provided after system correction.

Due to the high spectral resolution and high spatial resolution, the amount of data is growing rapidly, so it is necessary to consider the real-time compression method of massive data. One of the methods is to select effective bands in real time and flexibly change the band width and spatial resolution as needed. In this way, the sensor system of imaging spectrometer in the future needs intelligent real-time control and processing ability. In addition, like other remote sensing data, the imaging spectral data are also affected by the atmosphere, the attitude and terrain of the remote sensing platform, resulting in geometric distortions such as horizontal, vertical and distortion and edge radiation effects, so it is necessary to preprocess the data before providing it to users. The content of preprocessing mainly includes platform attitude correction, geometric correction along flight direction and scanning direction, and image edge radiation correction.

(3) Radar imaging sensor

Radar is an active microwave remote sensing sensor, which has two forms: side-looking radar and panoramic radar, among which side-looking radar is mainly used in geoscience. The side-looking radar emits microwave to one or both sides of the remote sensing platform in the vertical direction, and then receives the microwave reflected or scattered by the target. By observing the amplitude, phase, polarization and round-trip time of these microwave signals, the distance and characteristics of the target can be determined.

Figure 3- 13 General Structure of Pulse Radar

Side-looking radar imaging is different from aerial photography, which uses sunlight as illumination source, while side-looking radar uses emitted electromagnetic waves as illumination source, which is basically similar to the structure of ordinary pulse radar. Figure 3- 13 shows the general composition format of pulse radar, which consists of transmitter, receiver, switch and antenna. The transmitter generates a pulse signal, which is controlled by the changeover switch and transmitted to the observation area through the antenna. The pulse signal reflected by the ground object is also controlled by the transfer switch to enter the receiver. The received signal is displayed on a display or recorded on a magnetic tape.

When the radar is working, the transmitter on it emits powerful pulse waves in a short microsecond time through the antenna. When it meets a ground object, the signal reflected back to the instrument is received by the antenna. Because the distance between the system and the ground object is different, the receiving time of the transmitted pulse is also different (Figure 3- 14).

The echo received by radar contains all kinds of information. Such as the distance and orientation from the radar to the target, the relative velocity between the radar and the target (that is, the Doppler frequency shift caused by relative motion), the reflection characteristics of the target, etc. Distance information can be expressed by the following formula:

Where: r is the distance from the radar to the target; V electromagnetic wave propagation speed; T is the pulse round-trip time between radar and target.

The echo intensity received by radar is a complex function of system parameters and ground target parameters. System parameters include wavelength, emission power, irradiation area and direction, polarization, etc. The parameters of ground targets are related to the complex dielectric constant and ground roughness of ground objects.

Figure 3- 14 working principle of radar propagation

According to different antenna structures, side-looking radar can be divided into real aperture side-looking radar (RAR) and synthetic aperture side-looking radar (SAR).

1. Real aperture side-looking radar

The working principle of real aperture side-looking radar is shown in Figure 3- 15. The antenna is installed on the side of the plane, and the transmitter emits narrow pulses to the side plane. Microwave pulses reflected by ground objects are collected by antennas and received by receivers. Because the distance between the grounding point and the plane is different, the receiver receives many signals, which are recorded in sequence according to their distance from the plane. The strength of the signal is related to the characteristics, shapes and slope directions of various ground objects in the irradiation area. As shown in figure 3- 15, the ground objects are at a, b, c, d, e, etc. Due to the uplift of the ground object in A, the reflecting surface faces the antenna, resulting in strong reflection; B is a shadow, no reflection; C is grass, which is a moderate reflex; D is a metal structure with high conductivity and the strongest reflection; E is a smooth surface with specular reflection and weak echo. The echo signal is processed by the electronic processor, and image lines corresponding to the reflection characteristics of various objects in the irradiation area are formed on the cathode ray tube and recorded on the film. When the plane flies forward, it continuously scans one irradiation area after another, and the film at the cathode ray tube rotates synchronously with the speed of the plane, thus obtaining a strip image along the plane route, which is represented by the strength of the echo signal.

Figure 3- 15 Working principle of real aperture side-looking radar

The ground resolution of real aperture side-looking radar includes range resolution and azimuth resolution. The range resolution is the minimum distance that can distinguish two targets in the pulse emission direction (Figure 3- 16), which is related to the pulse width and can be expressed as:

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Where: r tau is the range resolution; C is the speed of light; τ is pulse width; φ is the radar beam depression angle. In addition, the range resolution of real aperture side-looking radar is independent of range. In order to improve the range resolution, from the above analysis, it is necessary to reduce the pulse width, but this will reduce the operating distance. At present, pulse compression technology is widely used to improve the range resolution. In addition, when φ = 50 and the pulse width is 0. 1 μs, and the range resolution is 23m. A and b in the picture are 20m apart, so they can't be distinguished. When φ = 35 and the pulse width is constant, the distance resolution is 18m, and the distance between C and D is 20m, which can be distinguished. That is to say, the greater the depression angle, the lower the range resolution; Otherwise, the range resolution is improved.

Figure 3- 16 Resolution of Radar in Range Direction

Azimuth resolution refers to the minimum distance between two adjacent pulses that can distinguish two targets, which is related to the lobe angle β (Figure 3- 17). The microwave emitted by radar radiates in petals in all directions, which is called lobe, but mainly in one direction, which is called main lobe. The radiation energy in other directions is small, forming sidelobes, in which β is called lobe angle. At this point, the azimuth resolution is

Figure 3- 17 Azimuth Resolution of Side-looking Radar

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Where: Rβ is the azimuth resolution; λ is the wavelength; D is the antenna aperture; GR is the observation distance. In order to improve the azimuth resolution, it is necessary to use electromagnetic waves with shorter wavelength, increase the antenna aperture and shorten the observation distance. These measures are limited whether they are used on airplanes or satellites. At present, synthetic aperture side-looking radar is used to improve the azimuth resolution of side-looking radar.

2. Synthetic aperture side-looking radar

Synthetic aperture side-looking radar is a kind of radar which uses the forward movement of remote sensing platform to install a small aperture antenna on the side of the platform instead of a large aperture antenna to improve the azimuth resolution (Figure 3- 18). In order to replace large-aperture radar antenna with small-aperture radar antenna, several small-aperture antennas are usually used to form an array on the ground, that is, a series of antennas connected with each other and with the same performance are arranged in a straight line at equal distance to receive narrow pulse signals (such as the phase and amplitude of backscattering of target objects). ) to obtain higher azimuth resolution. The longer the baseline of the antenna array, the better the directivity.

Figure 3- 18 Schematic Diagram of Synthetic Aperture Antenna

Fig. 3- 19 workflow of synthetic aperture side-looking radar

The working principle of synthetic aperture side-looking radar is that the remote sensing platform emits a pulse signal at a certain time interval while moving at a constant speed, and the antenna receives the echo signals at different positions and records and stores them. These signals received at different positions are synthesized and processed, and the same results are obtained as the real antenna receiving the same target echo signal. In this way, the function of small-aperture antenna is the same as that of large-aperture antenna.

Compared with the real aperture side-looking radar system, the biggest advantage of synthetic aperture radar system is that the azimuth resolution is independent of the distance r, so the system can be placed on both aircraft and spacecraft, and the resolution will not be reduced because of the distance from the ground objects. Theoretical calculation shows that the pixel size (resolution) of synthetic aperture radar along the track is

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Where: RS is the azimuth resolution; D is the length of the antenna along the track direction (not the total length of all antennas). For example, the synthetic aperture radar antenna is installed on the spacecraft with a total length of 2km. It consists of a plurality of small antennas arranged in an array. The real aperture of each small antenna is 8m, the radar wavelength is 4cm, and the azimuth resolution of the synthetic aperture is 4m when the distance between the spacecraft antenna and the target is 400km. If the true aperture of 8m small antenna is used as the side-looking radar antenna, its azimuth resolution is 2000m. If the total length of the antenna is 2km, the azimuth resolution is 8m (Figure 3- 19).