Space Cameras: The "Eyes" of Satellites in Space

 Basic components of a space camera

▲Haiyang-1 4-band CCD imager

A space camera generally consists of the following parts:

1. Objective Lens: Composed of optical systems and structures. Different types of optical systems correspond to different structures, such as refractive, refracting, and reflecting types. An optical system generally consists of multiple optical components, such as a camera window, lens, mirror, filter, and prism.

2. Detection and receiving circuit system: The photodetector performs photoelectric conversion, signal amplification, processing, display, storage, and annotation devices, and is generally composed of photodetectors, circuits, and mechanical clamping components.

3. Mechanical structure: including housing, focal plane components, mechanical transmission, aperture, light shield, etc.

4. Thermal control device: Generally composed of thermal control coating, heat insulation film, phase change material, heat pipe, heat conduction cable, radiator, heater, temperature controller, etc. Includes active thermal control and passive thermal control.

5. Control and Information Processor: Controls image data acquisition, storage, processing or encoding; controls various servo systems such as film transport, focusing, calibration, exposure adjustment, and image shift compensation; inputs data to the annotation device; monitors the working status of each component; and transmits fault information to the ground control center.

 Classification of space cameras

▲FY-3 Infrared Spectrometer

There are many types of space cameras, such as reconnaissance cameras, surveying cameras (also known as mapping cameras) and multispectral cameras that use film as the information carrier; CCD cameras, multispectral scanners and imaging spectrometers that use photoelectric detectors as receivers. These space remote sensors are collectively referred to as space cameras.

• Classified by spectral segment

Cameras can be categorized into ultraviolet, visible, infrared, laser active or active cameras, as well as panchromatic and multispectral cameras and imaging spectrometers. The latter can be further classified into hyperspectral and superspectral types based on spectral resolution.

Currently, the most common imaging spectrometers are dispersive and interferometric types, while current research hotspots include filter-type and computational types.

▲FY-3 Medium Resolution Imaging Spectrometer

• According to imaging target 

Based on the imaging target, space cameras can be divided into two categories: cameras that image areas and cameras that image points. For example, ground-feature cameras (collectively referred to as reconnaissance, mapping, and resource cameras) and cameras that observe the moon, planets, and the sun are cameras that photograph areas, while star cameras or star sensors photograph stars (point light sources) on the celestial sphere other than the sun.

• According to the source of the detection signal  

Based on the source of the detected signal, space cameras are divided into two categories: active and passive. Active remote sensing uses a specific artificially generated electromagnetic radiation source to illuminate the target, and then identifies the target by receiving the characteristics of the reflected electromagnetic waves from the target. Passive remote sensing, on the other hand, does not require artificial radiation to illuminate the target, but uses natural sources (such as sunlight, cosmic radiation, and infrared radiation emitted by objects due to the absorption of solar energy) to measure the reflected or radiated electromagnetic waves of the target, thus achieving the purpose of remote sensing.

• By use 

Space cameras can be divided into two types according to their purpose. One type is a space camera that observes the Earth, also known as a ground object camera, such as for investigating the characteristics of ground targets, geology and landforms, military reconnaissance, mapping and positioning, meteorological and oceanographic observation, etc. The other type is a camera that searches for, captures, tracks and monitors space targets on a space platform, such as for celestial observation in space and for measurement and aiming on intercept satellites (or kinetic weapons).

 Applications and Practices of Space Cameras

▲ Space cameras have been widely used in the field of remote sensing.

my country's first space camera was developed by the China Academy of Space Technology starting in 1967. It was launched into space on November 26, 1975, aboard China's first recoverable satellite, "Jianbing-1," and returned to Earth three days later. This made my country the third country, after the Soviet Union and the United States, to possess an Earth observation camera.

The "Jianbing-1" space optical payload is a visible light camera used for surveys. It has a focal length of 850mm, uses 2000m long film to store information, has a film width of 225mm, and a frame size of 198mm×1780mm. The camera's ground pixel resolution can reach 10m~5m.

• Meteorological observation satellite space camera 

China's FY-1 meteorological satellite carries a multi-channel scanning radiometer, which also detects ocean color, with a resolution of 1.1 km. The VISSR carried by the geostationary meteorological satellite also has a 45° mirror on the main axis of the optical system, but it achieves east-west scanning by satellite spin, while north-south scanning is achieved by the stepping motion of the north-south scanning mirror.

The visible and infrared scanning imager of China's FY-2 meteorological satellite achieves north-south scanning by using a telescope step-through mechanism. The resolution of the visible and near-infrared channels is 1.25 km, and the resolution of the thermal infrared and water vapor channels is 5 km.

• Resource observation satellite space camera

In 1999, the China-Brazil Earth Resources Satellite (CBERS) was successfully launched, carrying a 5-band pushbroom charge-coupled device (CCD) camera, a 4-band infrared scanner, and a 2-band wide-field imager. The Ziyuan-1 01 satellite operated in orbit for over three years, with a scanning swath of 113 kilometers, and possessed high-speed real-time Earth data transmission capabilities, providing effective data covering more than 1 million square kilometers annually.

After more than 20 years of development, on December 26, 2021, the ZY-1 family welcomed its ninth member—ZY-1 06. The satellite is equipped with visible and near-infrared cameras and a hyperspectral camera, and a new long-wave infrared camera has been added, enabling all-weather Earth observation. The visible and near-infrared camera boasts a panchromatic resolution of 2.5 meters and a multispectral resolution of 10 meters, capable of identifying near-infrared wavelengths between visible and infrared light, and distinguishing the shape, size, and some colors of monitored targets.

• Ocean Color Satellite Space Camera 

The ocean color scanner carried by China's HY-1 satellite is a 45° mirror scanning space camera with 10 channels, including two thermal infrared channels, and a resolution of 1.1 km. Another space camera on the satellite is a four-band CCD camera, using a linear CCD push-broom design, with a resolution of 250 m.

With continuous technological advancements, the temporal and spatial resolutions of space cameras are constantly improving. Their optical detection performance is trending towards high spatial resolution, high spectral resolution, and high temporal resolution. The targets they can target are evolving from single to multiple, from simple to complex, and they are becoming increasingly integrated, lightweight, intelligent, and multifunctional. Their role in space reconnaissance and detection is becoming increasingly important, serving as a fundamental platform and guarantee for integrated air-space operations. It is believed that with the continuous emergence of new technologies and the constant updating of detection methods, the application prospects of space cameras will be even broader, becoming a mainstream and major direction for high-tech development in various countries.

(References: China Academy of Space Technology: Development Trends of Space Optical Payloads for the 21st Century; Wang Jintang and Chen Shiping: Introduction to Space Camera Design and Testing)