Some explanation about Total Station and Theodolite

TheodoliteElectronics has allowed to automate and angular measurements. The electronic theodolite represents the device which will transform the angular sizes which have been written down in the form of system of opaque strokes or code paths on a glass disk to electric signals. The disk is appeared through by a light beam; at turn of a theodolite on a photodetector the signal in a binary code which after decoding is deduced on a board in a digital kind is created.

Association of an electronic theodolite, small-sized phase tool and a microcomputer in a uniform design has allowed creating the electronic tacheometer-device, allowing carrying out both angular, and linear measurements with their processing in field conditions. In the foreign literature such devices have received name Total Station (universal station). Their accuracy reaches 0,5 angular seconds and 2 millimeters + 2 mm/km, and range of action - up to 5 kilometers.

Introduction of laser techniques in a geodesy has led, in particular, to development of a witty method of leveling by "laser plane " (systems Laserplane). The bright red beam of vertically located laser falls on the rotating prism creating development of a beam in a horizontal plane. It allows to sight on a light spot on lath, put in any direction of the laser. Such way does not give high accuracy, but differs on speed and ensures the functioning on unlimited number lath that is convenient for many works on high-altitude shooting. For exact measurements the digital level working on coded is designed. The code bears the information on height of any place concerning its "zero". The image will be transformed to an electric signal, and at work on two lath excess between points of their installation is automatically defined.

The laser beam represents and it is almost ideal a direct basic line in space concerning which it is possible to make measurements at exact installation of equipment, civil work and so forth.

For last twenty years there was a new quantum leap which can be named the second revolution in a geodesy. There were the global satellite systems which have cardinally changed a situation in a geodesy and navigation. They allow at once, without any preliminary measurements, to define coordinates of any points on a surface of the Earth and to find distance between them with high accuracy.

Similar systems now two: system GPS developed in the USA (Global Positioning System - global system of definition of a site) and domestic system GLONASS (GLOBAL NAVIGATING Satellite System). Both GPS, and GLONASS are constructed, in general, by the same principle though differ in some details.
The space complex represents system from twenty four satellites placed: in GPS - in six orbital planes developed through 60 on a longitude; in system GLONASS - in three planes through 120 at height of the order of 20 thousand kilometers. It allows observing constantly in any point of globe not less than four satellites of each system. On all satellites there are standards of frequency with long-term stability of the order 10-12 - 10-13. Satellites radiate radiowaves on two frequencies (with lengths of waves of the order of 20 centimeters) which "bear” the complex coded signals.

The ground complex of system defines coordinates of satellites and transfers them aboard where they are pawned in a signal sent to the Earth, synchronizes satellite "hours" and verifies them with a ground basic time scale. For this purpose at the central station there is a hydrogen standard of frequency with stability 10-14 that corresponds to leaving for 0,3 seconds for one million years.

Signals from satellites accept and the equipment in item of measurement processes. Receivers can work in two modes which have received the name of code and phase measurements. Code measurements name also absolute as at once define coordinates of item in geocentrically to system of coordinates. It is done as follows. The radiowaves radiated from the satellite, are modulated on a phase so-called rangemeter by a code, and the same code is developed in the receiver. (two codes - "rough", accessible to everything, and "exact" are stipulated, access to which should be authorized). By comparison of these two code signals define time of distribution of a signal from the satellite up to the receiver in view of a difference of indications of their hours concerning basic time. If simultaneously to measure distances up to four satellites, the system from four equations with four unknown - three coordinates and a difference in time which decision find required coordinates will turn out.

The mode of code measurements gives "navigating" accuracy - the order of several tens meters. That it to rise, use two receivers. One establish on item with known coordinates, define in it differences of the measured and calculated ("reference") sizes and transfer them on mobile the receiver for correction of measurements. Such way reduces a mistake to size up to one meter.

To the geodetic purposes apply much more exact mode of phase measurements at which define not time of distribution of a signal from the satellite up to the receiver, and shift of a phase of the bearing frequency radiated by the satellite. Carry out them with two carried receivers and define differences of their coordinates on which it is possible to calculate distance between receivers to within millimeters. And if one of them to place in a point with known coordinates that is usual and it is done, it is possible to receive easily and absolute coordinates of the second receiver at a centrimetric level of accuracy.
The primary goal here, as well as in ground phase range finders, is exact definition of an integer of lengths of the waves which have "kept within" on a line the satellite - the receiver or GPS Total Station. This that number about which it was spoken above, but in this case it is much more and to define of it much more difficultly. As the distance up to the satellite equally approximately to 20 thousand kilometers, and length of a wave - about 20 centimeters, number of lengths of waves N turns out the order of one million; to measure this it is necessary absolutely precisely: the mistake on unit will give a deviation on range on 20 centimeters. Some ways of the decision of this problem are now developed, but with it failures in work of system more often are connected.

By present time in the different countries many types of GPS Total Station differing by the opportunities are developed. As a matter of fact, the majority of geodetic problems can be solved at use of two basic measuring means: global satellite system and Total Station.

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