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Sensor technologies supporting remote sensing
What is “Earth observation,” one of the primary missions of satellites?
Among artificial satellites, there are those classified as Earth observation satellites, such as DAICHI and ASNARO. These satellites observe the Earth using a technique called remote sensing, which enables measurement of the shape and characteristics of a target without direct contact, from the distant vantage point of space. This is possible because the technique makes use of the properties of electromagnetic waves—such as light and radio waves—that are emitted or reflected by the target. To capture these electromagnetic signals, Earth observation satellites are equipped with sensors.
The type of information obtained about the Earth’s condition varies depending on the wavelength of the electromagnetic waves used. Since developing Japan’s first spaceborne optical observation sensor, NEC has gone on to develop various satellite-mounted sensors capable of capturing a wide range of wavelengths, significantly contributing to the execution of satellite missions.
Here, we provide an overview of the technologies behind two representative types of sensors developed by NEC: optical sensors and active microwave sensors.
Optical sensors: observing reflected and emitted sunlight from Earth
In this context, optical sensors refer to passive sensors that use optical systems—such as lenses and mirrors—to collect light and acquire information on the shape and spectral characteristics of observed targets.
When mounted on artificial satellites, these optical sensors are primarily tasked with conducting continuous, global observations of the Earth to fulfill purposes such as "high-resolution observation of the Earth’s surface" or "measurement of physical quantities*."
- *Physical quantities include a wide range of observation targets, such as geology and vegetation, as well as elements invisible to the human eye, like greenhouse gases and temperature.
Optical sensors are composed of the following components:
- Optical system
A telescope collects a sufficient amount of incoming light and focuses it onto a detector. In some cases, interference filters are used to isolate specific wavelengths, or spectrometers are added to separate the light into individual spectral components for measurement. - Detector and signal processing unit
This unit converts the received light into electrical signals and performs the necessary processing to pass the data to the satellite bus. For certain observation wavelengths, detectors may include cooling functions to suppress thermal noise generated by the detector itself. - Optical support structure
This structure provides stable support to prevent image blurring due to vibrations and shocks during satellite launch, as well as from the extreme thermal conditions in space.
Active microwave sensors: observing reflected signals using radio wave emissions from the sensor
An active microwave sensor emits its own microwaves (electromagnetic waves with frequencies between 300 MHz and 300 GHz) toward a target and measures the microwaves reflected back from the object. When observing rain or clouds, optical sensors can typically capture only the surface distribution of clouds, whereas radio wave sensors have the advantage of being able to measure the three-dimensional structure of rain and cloud formations. Another key advantage of microwave observation is that it can be performed both day and night, regardless of sunlight.
Active microwave sensors are composed of the following components:
- Transmitter
Converts digital signals into analog signals and then shifts them to microwave frequencies. The signal is further amplified by a power amplifier to reach the required power level for observing the target. - Antenna
Directs the microwaves generated by the transmitter toward the target and also receives the microwaves that are reflected back from the target. - Receiver
Converts the received signal from the antenna into a frequency and power level suitable for signal processing. Power conversion is regulated based on the detected strength of the received signal. Since part of the transmitted signal can also enter the receiver internally within the sensor, the timing for power detection is carefully controlled to avoid interference during transmission. - Signal processing unit
Converts the received signal into digital form and arranges the data into the required file format based on the intended application. To avoid processing signals during transmission, conversion to digital signals is performed only at the designated reception timing.
(Left: Vertical radar reflectivity of clouds; Right: Vertical cloud motion observed via Doppler measurements)
The importance of Earth observation from space, which enables global-scale environmental monitoring and disaster assessment, is growing in response to the worldwide push to achieve the Sustainable Development Goals (SDGs), a common set of targets for addressing global challenges. NEC will continue to develop and provide satellite-mounted sensors, contributing to the realization of social infrastructure that ensures safety, security, fairness, and efficiency, and ultimately supporting the achievement of the SDGs.