# How is the SAT timed

## Federal Agency for Cartography and Geodesy

What used to be a dream for seafarers is now a reality: Positioning accurate to the meter all over the world. Behind this are satellite-based navigation systems such as GPS (USA), GLONASS (Russia), Galileo (Europe) and Beidou / Compass (China).

### introduction

These systems have gained outstanding importance in geodesy - especially the GPS methods. They make it possible to compact the VLBI and SLR networks very economically and to make the global reference system available right down to the practical areas of surveying. With geodetic GNSS methods it is possible to record positions better than 1 cm and movements of the points, for example caused by plate tectonics, by periodic repetitive measurements or by continuous measurements.

### Measuring principle

Measurement principle Global Navigation Satellite Systems (GNSS)

The basic principle of GPS assumes that a receiver on the earth's surface determines so-called pseudoranges to at least 4 satellites at the same time. Pseudoranges are made up of the true distance from the receiver to the satellite plus a constant value "k". If the positions of the satellites are known, the position of the receiver can be determined by a kind of "arc". The satellite positions are described by so-called ephemeris (orbit data), which are continuously obtained from observations from ground stations and have to be predicted accordingly for the future. The prerequisite for this is that both the earth's gravitational field in which the satellite moves and the earth's rotational movements are known.

The pseudo-distance measurement amounts to a measurement of the signal propagation time. The satellite sends out coded, time-clocked signals that a receiver on the ground can register, decode and temporally classify. However, since the ground receiver is not initially time-synchronized with the satellite time, the runtime around the clock status between the satellite time and the receiver time is falsified. If the signals are received from at least 4 satellites, the clock error of the receiver and thus the constant "k" can also be determined.

In geodetic applications, however, one is not content with the so-called code measurements; phase measurements are also carried out on the carrier waves (L1, L2). The phase measurements can be interpreted as exact distance measurements if one knew how many whole wave trains there would be on the distance from the receiver to the satellite. With the help of suitable evaluation techniques, the indefinite number of whole wave trains can be solved and exact distances can be determined from the phase measurements. This enables position determinations with an accuracy of better than 1 cm.

### International networks

The accuracy of GNSS observations also depends largely on the precise knowledge of the satellite orbits. Networks with permanently monitoring stations, so-called reference stations, are used for this purpose, on the basis of which precise ephemeris are calculated.

### Global networks

The first network set up worldwide was CIGNET, which had been available for scientific work since 1987. Wettzell was an essential station in the CIGNET. At the beginning of the 1990s, the IGS emerged from this and established its own global network with high demands on the observation stations. Standards for observation, data flow and data analysis have been established.

IGS station network with BKG stations in red

Today more than 350 stations are in operation within the framework of the IGS, which generate highly precise coordinate time series and thus define the international terrestrial reference framework (ITRF).

The IGS products also include:

• Satellite orbits
• Orbit forecasts
• Earth rotation parameters
• Atmospheric parameters
• Clock failure

### Regional networks

In Europe, a regional network of around 240 permanently observing stations has been set up since the early 1990s, the European reference network EUREF Permanent Network

EUREF station network with stations managed from Wettzell in red

The EPN represents the European consolidation of the IGS station network and implements the European reference framework EUREF. 15 of these permanent GNSS stations are supervised by the Geodetic Observatory Wettzell. The aim of the permanent measurements is to derive time series of the current ITRF coordinates for the reference stations in order to determine changes over time and to analyze their causes. Work is intensified on using the measurements for atmospheric investigations, such as the moisture content of the troposphere. For this purpose, evaluations that are as up-to-date as possible are required, which require near real-time observation data from selected stations.

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