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How RTK Works PDF Print

How Does RTK Work?

RTK is similar to a differential correction but it utilises a local, ground based reference station, that transmits high-accuracy corrections to a vehicle receiver (rover) via RTK radios.  Due to the proximity of the vehicle, nearly all GPS drift is eliminated and the position is highly repeatable.  In order for the base station signal to reach the vehicle, a direct line-of-sight is required between the two.

There are two different types of base; a mobile base is one which stands freely on a tripod and needs to be moved around with the vehicle to make sure the line of sight is maintained.  The only disadvantage with this unless the tripod is placed back in exactly the same place the next time you return to the field then the repeatability will not be maintained.  A fixed base is one which is mounted up high and generally fixed to the apex of a shed or mast to get the most height to help the line of sight be maintained, so because of this the higher the better.  

Both bases use the same components an RTK ready receiver and RTK radio.  A mobile base does not require a licence from Ofcom because it is a local base and transmits on a licence exempt frequency.  A fixed base requires a licence from Ofcom, who will issue the frequency which can be transmitted in the area concerned.

There is one golden rule though if the rover is working far away from the base and is on the extreme range of the base then there will be some loss of accuracy simply because the base is correcting for different satellites to what the vehicle is seeing, so not all the satellites will be corrected for hence the degradation in accuracy.

Once the base is ready to work, it has to be surveyed in so it knows the exact coordinates of its locations, this enables it to correct for the errors it sees in the GPS signal, that the base and the vehicle are seeing and send the error to the vehicle via the radio link, correcting the vehics error.

If there are obstructions such as hills or buildings in the way blocking the RTK radio signal then repeaters can be used to basically repeat the error correction message around the obstacle.  Repeaters only need a radio and an antenna to transmit from so they are cheaper than a base.  This  is where RTK Survey comes to help locate dead spots and also locate good locations for the base and any repeaters if they are needed.