The VBOX NTRIP Modem (RLVBNTRIPMDM) allows an RTK enabled VBOX to receive positional correction data via Network RTK, without having to use a VBOX Base Station. Correction messages received via the RTK network can be an advantage if testing is conducted tens of kilometres away from the base station, over large changes in elevation or where topographical obstacles could get in the way.
If RTK coverage is available, the NTRIP Modem allows for vehicle testing that requires high positional accuracy – such as when validating critical safety systems like AEB/ACC - to be conducted where a VBOX Base Station cannot be deployed.
The VBOX NTRIP Modem enables the roving VBOX unit to send its position to the stations within the network – just as it would to a single VBOX Base Station. The NTRIP service provider will then calculate the appropriate corrections for the VBOX’s location based on the data from nearby reference stations. This correction information is then returned to the VBOX in the same RTCM format as is used for a single fixed base station.
In order to use this type of correction you have to register and subscribe to a cellular data plan with your local NTRIP provider, such as Topnet in the UK and Germany, Swepos in Sweden or Unavco in the USA.
How Does RTK Work
Standard GNSS 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 ~0.07 s. This is the time delay from satellite to receiver, which is about 20,000 km.
To obtain accuracy up to 1 cm, 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 GNSS signals (called code-phase signals) alone is around 0.5 m.
To obtain a higher level of accuracy, up to 1 cm, another technique needs to be used in conjunction.
We use a GNSS ‘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 GNSS is shown in red, while the RTK is shown in green at the centre.
RTK versus un-aided GNSS
In order for some NTRIP Casters to work, they require your position. This is true if the service you are using has multiple reference stations over a large area, in which case it will need to know which one is closest to you.
This is achieved by your GNSS engine sending a GGA NMEA message to the caster, which needs to be enabled on your GNSS engine. This message can be set to transmit every 10 seconds, it is not necessary to send it more often than that. Note that if the NTRIP service you are using requires a GGA message, and it is not being sent, you will not receive any data. If you are using your own Base Station, or a service with only one Base Station, then the GGA message is not needed.
NTRIP delivered RTCM v3 messages are usually sent once a second or once every two seconds. Depending on your GNSS engine, RTK can usually still be accurate even if the message is up to 60 seconds old, but for best performance, keep this as short as possible.
The best performing GNSS engines can use the following signals:
- GPS L1/L2C
- Glonass L1OF, L2OF
- Galileo E1B/C, E5b
- Beidou B1I, B2I
Therefore it is important that the NTRIP service you use transmits all of these corrections.
The transmission of these correction signals at 1 Hz uses approximately 1 kB per second depending on how many satellites are visible, so if you are intending to use a SIM card to connect to the NTRIP server, ensure that you have enough data available.
So that Racelogic can continue to provide you with notification of the latest software releases, firmware upgrades and to offer technical support, please register your NTRIP Modem.
Register your unit here.
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