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February 5, 2023



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Controlling time II...and GPaSsing by


Space and time are intimately related. Having the right time is essential for the Gran Telescopio CANARIAS. With it we will know the position of the stars at any particular moment, so that the telescope can be pointed in the right direction at the right time.

The earth rotates both on its axis and around the sun. This means that we need to combine information about space and time. On the other hand, if we want to observe a star and we do not point the instrument in the right direction at the right time we may find that it is no longer in our line of vision. That is why the GTC has GPS to provide it with the information it needs.


Its full name is "NAVSTAR Satellite GPS". It is a system developed by the United States Department of Defence, which currently manages it, although other similar systems like the Russian GLONASS and European Galileo are in development.

The Global Satellite Navigation System operates using a group of satellites in orbit around the earth (those of the GPS are around 20,200 kilometres from earth whilst the GLONASS satellites are 19,100 kilometres away) which together with base stations give us information about the position of an object, vehicle or person located anywhere on the planet - on the ground, at sea or in the air.

It is accurate to within a few metres (or a few centimetres for some systems like differential GPS). The Russian and American systems each have 24 satellites in orbit (21 plus three spares), powered by solar energy.

The GPS devices that we can buy are receivers.

When the system is asked to locate an object, it uses at least four of the satellites in the network. These send a position and time - each one is fitted with an atomic clock which gives the time accurate to within a few microseconds (UTC, Universal Time Coordinated).

Distance is calculated by measuring the time data takes to arrive from a satellite. Using triangulation, the position of the satellites relative to each other and to the object or location requiring identification can be worked out.

This information is vital for the Gran Telescopio CANARIAS (GTC) this information is vital. Why?

It is important to have the right time as with it we will know the position of the stars at any given moment. The GTC has a GPS receiver and an antenna, which also functions as a "time server", an instrument that synchronises all of the clocks on the system. The antenna receives a signal from the satellites, which is processed by the time server to give the time and then relayed throughout the control system using a protocol called NTP (Network Time Protocol). This process keeps all of the computers in the control system, and especially those used to point the telescope, synchronised to within 1 millisecond. If greater accuracy is needed for particular applications, a special network can be created to transmit the GPS signal much more quickly to the computers that need to be synchronised. Nothing escapes us: we are controlling time.

- Galileo

Key words:

TRIANGULATION: By calculating the angle between each of three points (in this case signals from satellites) and the location from which we are measuring, the position of the object in relation to the three satellites can be determined.

Natalia R. Zelman

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