What can a reconnaissance satellite see?

Reconnaissance satellites can be divided into four categories according to their uses: photographic reconnaissance satellites, electronic reconnaissance satellites, missile early warning satellites and marine surveillance satellites. Photographic reconnaissance satellites use cameras, video cameras or other imaging devices installed on satellites to photograph the ground to obtain information. The obtained information is usually recorded on film or magnetic recorder, and the information is obtained by recovering or receiving the images transmitted by radio through the recovery cabin. After processing, the nature of the target is interpreted and identified, and its geographical position is determined. Electronic reconnaissance satellites are mainly used for reconnaissance of radio signals. The satellite is equipped with radio receiving and monitoring equipment, which is mainly used to intercept the transmission signals of radar, communication and other systems, and can detect the position, frequency and other parameters of the other radar and radio station. Missile early warning satellite is a reconnaissance satellite with missile launch as its specific target. The satellite is equipped with an infrared detector, which is used to detect the infrared radiation of the engine tail flame when the enemy missile flies, and cooperate with the TV camera to judge the flight direction of the missile in time and accurately, and give an alarm quickly. Missile early warning satellites generally operate in geostationary orbit, and several satellites form an early warning network. Marine surveillance satellites are mainly used to detect, track, identify and monitor ships and submarines at sea. The satellites are equipped with reconnaissance equipment such as radar, radio receiver and infrared detector. The orbit of a satellite is generally a nearly circular orbit of about 1 kilometers, and a marine surveillance network needs to be formed by multiple satellites. What we usually call reconnaissance satellites generally refers to photographic reconnaissance satellites, which are divided into visible light (infrared) photographic reconnaissance satellites and radar photographic reconnaissance satellites. In fact, the image of a photographic reconnaissance satellite is no different from the photos we usually take with a camera. It consists of many invisible pixels, which are similar to what we usually call the pixels of a digital camera. The smaller the pixels, the smaller the size of the details that can be recognized by photography. Ground resolution is an important index to measure the technical level of photographic reconnaissance satellites. Generally speaking, ground resolution is the minimum distance between two objects that can be distinguished in a photograph. It does not represent the smallest size that can identify ground objects from photos. A target with a size of about .3 meters is just an image point in a photo with a ground resolution of .3. No matter how many times the photo is enlarged, it is just an image point. Generally speaking, the smallest size that can identify the target from photos should be equal to 5~1 times of the ground resolution, that is, 1.5~3 meters. According to the different use of satellite photos, different requirements are put forward for ground resolution, and * * * is divided into four levels. The first level is discovery, which means knowing the shape of the target roughly, and only judging the presence or absence of the target from the photos; The second level is identification, which means that the target is found to be more detailed and can identify the target, such as a person or a car, a cannon or an airplane; The third is to confirm that targets can be distinguished in detail, and their types can be pointed out from the same kind of targets, such as whether the vehicle is a truck or a bus, whether the house is a private house or an army barracks; The fourth is description, which can know the specific shape of the target in more detail and identify the characteristics and details of the target. For example, it can point out the models of planes and cars and the equipment on ships. Among these four levels, "discovery" requires the lowest ground resolution, while "description" requires the highest ground resolution. At present, the most advanced photographic reconnaissance satellite in the world is the KH-12 "advanced keyhole" visible light reconnaissance satellite of the United States, and its resolution has reached .1 ~ .15m, so it is known as the "extreme orbit platform". However, this is only its highest resolution, which is actually impossible to achieve most of the time. First of all, KH-12 satellite runs in a sun-synchronous orbit with a perigee of 322 kilometers and an apogee of 966 kilometers. To achieve the highest resolution, it needs to reach the perigee of the satellite, while in other parts of the orbit, the ground resolution will decrease. Secondly, satellites need excellent visibility in reconnaissance, and dense fog, smoke and clouds will greatly reduce their reconnaissance effect or even make them impossible to use at all.