At present, there are many satellite monitoring systems and satellite environmental products with great value in solar energy prediction and resource assessment. Satellite measurements with different resolutions mainly consider the following factors: space, spectrum, time and radiation. After weighing various factors, each classification can have a higher fidelity, and the satellite can have excellent performance in a certain environmental parameter subset of its design.
Kidder and Vonder Haar made a comprehensive summary of meteorological satellites, including the mechanical properties of different orbits, radiation transmission considerations related to measured values, and a variety of satellite applications in the research and operation stages. The satellite orbits mainly used in solar energy forecasting and resource assessment are geosynchronous (stationary satellite) and sun-synchronous (polar orbiting satellite). We briefly introduce the significant differences between the two.
Polar-orbiting satellite orbits are located about 700~850km above the surface. It is a special type of near-polar orbit. The inclination angle of the orbit (the angle between the ground track of the orbit and the equatorial plane) is about 98°. Due to the non-spherical shape of the earth, the speed of the orbital plane rotation is equal to the earth’s orbiting speed around the sun. The satellite can pass by at the same local time every day. equatorial. For example, NASA’s Terra satellite passes through the equator at 10:30 in the morning (the opposite side of the orbit at 22:30), while NASA’s Aqua satellite passes through the equator at 13:30 in the afternoon (the opposite side of the orbit at 01:30). side). Since satellites in this type of orbit can pass through a certain place at the same time, they can provide observations within a conventional assimilation time window and are beneficial to weather forecasts. At the same time, since such satellites can also provide measurements of the day and night cycles at the same location, they can also be used for climate research. In addition, the sun-synchronous orbit can observe the entire surface of the earth, but its disadvantage is that the update frequency is low (only 1 to 2 times a day, depending on the scan width and latitude). If you want to increase the update frequency, unless you place it on the same orbital plane Multiple satellites.
The altitude of the geostationary satellite orbit is 35,790km, and the angular velocity of its rotation is the same as the angular velocity of the earth’s rotation. The geosynchronous orbit is a special kind of relatively geostationary orbit, and its inclination and eccentricity are both zero. The orbit of the satellite is directly above the intersection of the equator and a certain longitude, which enables the geostationary satellite to update data at a higher frequency and collect images of cloud evolution within the satellite’s observation perspective. The field of view can provide useful images at a great arc distance of about 60° from the sub-satellite (6 geostationary satellites spaced by 60° in pairs can cover the tropical and mid-latitude regions of the world).
Usually, the pixel size (image element) is used to express the spatial resolution of the satellite imaging radiometer. Most imaging radiometers generate images by scanning, in which the aperture of the telescope determines the instantaneous geometric field of view on the ground, and the coupling of the scanning rate of the instrument and the measurement time determines the pixel resolution. The image pixels are arranged according to scan lines, and each scan line is composed of adjacent pixels. Although the spatial resolution and spectral bands of the next-generation geostationary satellite sensors have been significantly improved, since the orbit radius of the geostationary satellite is larger than that of the polar orbiting satellite, the spatial resolution of the former imager is lower (for example, visible light is 1km and infrared light are 4km) and the spectral band is narrow. By reducing the spatial average of the measured values, higher spatial resolution has better detection capabilities and the ability to describe the properties of clouds and aerosols, but the disadvantage is that it requires a higher data rate and may reduce the signal-to-noise ratio (noisier Measured value).
Generally speaking, the more spectral information, the better the ability to detect and describe the properties of clouds and aerosols. Studies have proved that liquid droplets and ice particles in meteorological clouds have complex spectral absorption and scattering when passing through most of the visible spectral range (0.4~14μm wavelength) sensed by passive imaging radiometers. By measuring this phenomenon in the atmospheric window (where the gas atmosphere is more transparent) and the absorption band (gas absorption/radiation emission), important information about cloud composition and height can be obtained. In the same way, atmospheric aerosols also exhibit spectral behaviors related to their composition. The more bands the satellite radiometer can observe, the stronger the ability to determine the “spectral fingerprint” of multiple atmospheric parameters. The operating systems currently running are the Advanced Very High Resolution Radiometer (AVHRR) installed on the Polar Orbiting Environmental Satellite (POES) and the Visible Light and Infrared Spinning Scanning Radiometer (VISSR) installed on the Geostationary Environmental Observation Satellite (GOES) ).
First, quantify the analog signal measured by the satellite detector into a numerical value or “quantity”, and then convert it into equivalent emissivity, reflectance or brightness temperature through correction. The resolution of radiometric measurement refers to the interval of the above-mentioned quantization process within the dynamic range of the effective sensor. For example, when the radiation measurement resolution of a certain reflectance is relatively coarse (for example, the nominal dynamic range is 0-100), only 1% can be used as an interval, and up to 100 different cloud reflectance values can be reported. , While a sensor with a higher radiation measurement resolution may report 1000 values at intervals of 0.1%. The radiation accuracy of the latter is 10 times that of the former, and the latter (assumed to be a well-calibrated sensor) has a stronger ability to describe the optical properties of clouds related to ground radiation estimation. The current radiometer’s radiation measurement resolution range is between 6 bits (or 26=64 levels) and 14 bits (16,384 levels).
