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How does RTK (Real Time Kinetic) Work?

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Normal GPS looks for changes in the signal codes sent by each satellite. This is done by measuring the times of signal edges, and then calculating the time for the signal to arrive.

When comparing two signals, we are measuring a time delay of about ~0.07s. This is the time delay from satellite to receiver, which is about 20,000km. 

 

To obtain accuracy up to 1cm, we need to measure the time delay between these signals down to 0.00000000003 (1/30th of a nanosecond). The picture below shows a single rise between 0 and 1 (highlighted in the image above). 

 

 

It is difficult to compare this with the reference to 1/30th of a nanosecond, as we do not know at which exact point along this rise, the change from 0 to 1 happened.

This means that the positional accuracy when using corrected GPS signals (called code-phase signals) alone is around 0.5 meters.

To obtain a higher level of accuracy, up to 1cm, another technique needs to be used in conjunction. 

 

We use a GPS ‘carrier wave’ signal, transmitted at a very high frequency (1575 MHz), to pinpoint the exact moment the signal rises from 0 to 1.

You can see from the image above that the phase of the carrier signal flips when the data signal changes. 

 

 

Because this signal is being transmitted at such a high frequency, it means that we can measure with far greater precision, exactly when the signal change occurred.

This provides a much higher timing resolution, so, when we are lining up our real time and corrected signals, we know with a much greater accuracy, when that signal changed. Therefore, we know to a much greater accuracy what the true distance is between the satellite and the receiver.

 

The difference this makes is shown on a 24 hour plot below. The un-aided GPS is shown in red, while the RTK is shown in green. 

 

RTK versus un-aided GPS

Network RTK

Using a single base station RTK Solution from Racelogic is a convenient and cost effective option to achieve higher positional accuracy. However, there are situations where setting up a base station is not possible, for example if the roving vehicle must travel tens of kilometres away from the base station. This can allow atmospheric errors to effect the positional accuracy of the roving vehicle.  In these scenarios, another RTK Solution is possible called ‘Network RTK’.

 

The most common method of Network RTK involves a two-way communication link which allows the roving vehicle to send its position to a service provider that is monitoring satellite observations from several reference stations around a country. The service provider will then calculate the appropriate corrections for the rovers location based on the data from nearby reference stations. This correction information is then returned to the rover in the same RTCM format as is used for a single fixed base station.

 

Most network RTK systems are internet based and utilise a protocol called NTRIP (Networked Transport of RTCM via Internet Protocol), this requires that the user maintains a constant internet connection in order to receive corrections, normally this is done via a cellular modem or smart phone.

 

The current VBOX firmware does not support NTRIP directly however cellular modems with built in support are available as are smartphone and desktop applications.

 

To learn how to connect these devices to use an NTRIP service with a VB3iSLRTK please click here.

 

There are several Network RTK providers around the world, popular companies in the UK for example are Topnet Live and SmartNet. These providers can offer a subscription service to Racelogic VB3iSLRTK customers who can then use these corrections directly into their units with the necessary intermediary hardware mentioned above.

 

For more information, See also:

How does GPS work?

How does DGPS (Differential GPS) Work?

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Last modified
16:05, 9 Feb 2017

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