Taking the star as the background, the satellite direction is measured, and the sunlight reflected by the satellite or the laser beam reflected by the mirror on the satellite is used for photography. The spatial direction of the satellite at the moment of photography can be obtained through photo processing and restoration. The satellite orientation accuracy obtained by photogrammetry can reach 0.3 "under good conditions.
Directional observation is the main method used in the 1960s, and its observation data is used to establish spatial triangulation by geometric method. Because the observation accuracy is not easy to improve, and there are few satellites and observation opportunities, it is rarely used.
Satellite Laser Ranging The satellite laser rangefinder installed at the ground station emits laser pulses to the satellite, receives the pulses reflected by the satellite reflector, and measures the time between pulses, thus calculating the distance from the station to the satellite. In the early 1960s, people tried to measure the distance from the ground station to the moon with laser technology. The attempt of diffuse reflection ranging on the lunar surface failed to achieve satisfactory results. In the future, with the appearance of artificial satellites with laser mirrors and the improvement of instruments, the ranging accuracy has been continuously improved. The first generation laser rangefinder adopts visual tracking observation, and the ranging error is 2 meters; Second generation automatic tracking, error decimeter; The accuracy of the third generation rangefinder reaches centimeter level.
The working principle of satellite laser rangefinder is shown in Figure 3. The laser pulse emitted by the solid-state laser is intercepted by the sampling circuit, and after photoelectric conversion, a reference signal is formed, which is sent to the timing device as a door-opening pulse for timing. Most laser pulses are emitted to satellites by optical systems. The mirror on the satellite reflects the pulse back to the ground, which is received by the receiving system, converted into electric pulse by the photomultiplier tube, amplified and shaped, and sent to the timing device as a closing pulse for timing. The counter records the propagation time of the laser pulse between the rangefinder and the satellite, from which the distance between the rangefinder and the satellite can be calculated.
Satellite laser rangefinder is divided into fixed and mobile. The former is installed on the ground fixed station, and the latter can be installed on the vehicle, which has strong maneuverability. The accuracy of the two rangefinders is roughly the same.
In order to control the laser rangefinder by computer and make it track the satellite automatically, it is necessary to have accurate orbit prediction. According to the forecast data, it is converted into the coordinates of the satellite during observation, and then the azimuth angle, altitude angle and distance of the satellite are calculated. Input into computer for automatic control and satellite tracking.
Satellite laser ranging technology has been widely used in geodesy and geodynamics. In the 1970s, great progress was made in measuring the distance between the earth and the moon. Lunar laser ranging not only plays the same role as satellite laser ranging, but also can improve lunar ephemeris and calculate the earth gravity parameter GM. The accuracy of lunar laser ranging has reached about 10 cm.
The principle of Doppler frequency shift measurement is based on Doppler effect. The frequency of electromagnetic waves continuously emitted by the radio transmitter installed on the satellite is fs, and the frequency of electromagnetic waves received by the ground station receiver is fe. Due to the relative motion of the satellite to the ground station, according to the Doppler effect, there is the following relationship:
Where makeup is the variability of the distance from the satellite to the ground station, and c is the speed of light. By introducing the local oscillation frequency f of the receiver and the wavelength λS=c/fS of the electromagnetic wave emitted by the satellite, the above formula is written as follows: the receiver accumulates the frequency shift from time t 1 to t2, that is, the above formula is clearly integrated, where n is the frequency shift from t 1 to t2 recorded by the receiver. Accordingly, the variability of the distance or range from the satellite to the ground station can be calculated from the observed frequency offset. Fig. 4 shows the change of Doppler frequency shift.
In order to improve the accuracy, the satellite emits two coherent frequencies, and the main part of ionospheric influence can be eliminated through data processing. Doppler frequency shift measurement can work around the clock, and a large number of observation data can be obtained in a short time.
Meridian satellite system, also known as Naval Navigation Satellite System (NNSS), is a typical navigation and positioning system based on the principle of Doppler measurement. The meridian satellite of the system continuously emits electromagnetic wave signals for Doppler frequency shift measurement, and the frequencies are 150 and 400 MHz respectively. On the 400 MHz carrier, the time signal and "broadcast ephemeris" are modulated. The Doppler receiver of the ground station not only observes Doppler frequency shift, but also receives information. Using the observed Doppler frequency shift and the mathematical relationship between the instantaneous position of the satellite and the station coordinates, the geocentric coordinates of the station can be calculated. The instrument used for meridian satellite Doppler measurement is called Doppler receiver.
At the ground station, it can be observed that the meridian satellite passes once an hour. Generally observed for 40 ~ 50 times, the geocentric coordinates of the station obtained by using broadcast ephemeris and single point positioning technology have an accuracy of about 3 ~ 5 meters. In addition, joint positioning technology (two stations synchronously observe meridian satellites) and short arc positioning technology (multiple stations synchronously observe meridian satellites) can also be adopted. Both of these positioning techniques can weaken the influence of satellite ephemeris error and atmospheric refraction, but the former regards satellite broadcast ephemeris as a known value and the latter as an observed value. By using these two techniques, the relative position error between every two points can be reduced to less than 1 m. The United States also calculated the precise ephemeris of 1 ~ 2 meridian satellites afterwards. According to this ephemeris and single point positioning technology, the error of geocentric coordinates of the station is also within 1 meter.
The meridian satellite Doppler positioning method is not affected by the weather, and the instruments used are light and easy to operate. Now it has become the main method to determine the geocentric coordinates of ground points. In the astronomical geodetic network, properly setting Doppler stations can check and improve the quality of the network and transform the local geodetic coordinate system into a global unified geocentric coordinate system. Combined with satellite Doppler positioning and ground leveling, the relative elevation anomaly with accuracy better than 1 m can also be obtained.