Why and how long do I have to wait for a RTK FIX?

Did you know everyone could survey and perform stakeouts with GPS?

Read more about the UNI-GRx, a universal RTK GNSS Receiver. Digital surveying and performing stakeouts is now easier than ever. Available in a complete package and ready to use worldwide.

The signals

To get a first position, the UNI-GRx of course needs to receive satellite signals with a clear line of sight. Apart from these signals it needs additional data for each satellite to get more accurate. Additional data such as the ephemeris data as well as for example SBAS and RTK correction data from a local base station or network. In this article we will talk you through the signals as well as the effects on the cold and hot start times of the UNI-GRx, ending with best practices to get a good fix.

Ephemeris Data

This data gives very precise information about the orbit of each satellite, quite simply this is a table with the locations of the satellite at a specific point in time. Your GPS receiver can use this data to calculate the location of a satellite to within a meter or two. The data is updated every 2 hours and is usually valid for around 4 hours and only for a certain area. If the UNI-GRx has been off for a while, it may take up to a around 30-60 seconds to receive and process the informational data and status from each satellite before the UNI-GRx can make use of that satellite. From the moment you turn on the UNI-GRx, it starts searching for these signals and in case you have a good sky view it has the data by the time the UNI is initialized. From this moment the UNI-GRx has a location, with an accuracy of a couple of meters. 

SBAS Corrections

SBAS Corrections are Satellite Based Augmentation Systems and are used to improve the performance of GNSS systems and it’s locations. There are several systems such as EGNOS, WAAS, MSAS, SDCM and GAGAN. Originally meant for aviation, these systems consist of accurately located reference stations deployed around the continents of the systems. The GNSS errors calculated by these stations are sent to a central centre which computes the differential corrections and broadcasts them to the covered area using geostationary satellites. The UNI-GRx receives these signals on a channel and uses the data to improve the single location to an accuracy of around a meter. From the moment the UNI-GRx gets its ephemeris data and its first location, it starts using SBAS to improve this location and gets it to around meter accuracy within 10-20 seconds.

RTK Corrections

RTK Corrections are like SBAS corrections in the sense of being differential correction data, but with much higher accuracy to make sure the UNI-GRx knows how to remove any remaining GNSS errors and gets to a centimetre accurate FIX. Unlike SBAS, RTK corrections come from a local base station with a sky view as the UNI-GRx in the location it’s used. Generally speaking, the distance between the UNI-GRx and an RTK base station or a virtual RTK base station making use of a network solution is maximum 15 kilometer.

The correction data generated by these base stations or networks, is usually transmitted using the RTCM protocol over either radio links or internet using the NTRIP protocol. In the case of UNI-Complete, the UNI-GRx makes use of UNI-RTK Premium which is a network of 2000+ base stations which will generate a local virtual base station for you, providing the UNI-GRx with corrections based on its location over NTRIP. Once the UNI-GRx has received this correction data it will start “floating” until it will get a FIX with centimetre accuracy.

As you can understand from the above, the time to get this fix not only depends on the satellite signals but also on correction data broadcasted over the internet based on the UNI-GRx location. This means that before the UNI-GRx can start floating, it needs an active internet connection, it needs to send its location to the network of base stations and receive the correction data. This sounds complicated but luckily, in case your UNI-GRx is set-up right such as with the UNI-Complete package, you don’t need to do anything as the UNI-GRx will do everything for you. Just start-up the UNI-GRx in good sky-view conditions and wait for your FIX.

Cold start

If the UNI-GRx has not been used for a longer time and/or has moved several hundred kilometres it will take a bit longer to get the first fix. In this state, the receiver of the UNI-GRx does not have the ephemeris data. Older GNSS receivers may take up to an hour to search for satellites, download the informational data and obtain an initial position, though newer GNSS receivers like the UNI-GRx, may take about 3 minutes after hitting the power button with the right settings and availability of satellites.

After this, the UNI-GRx just needs to get an initial location, send it to the network which takes about 15 seconds and to calculate the FIX after going into float state which will take about another 30 seconds in good conditions.

This will get your Cold Start Time To First Fix (TTFF) with CM accuracy, in good sky view conditions to around 90 seconds.

Hot start

If the UNI-GRx has been off for less than an hour and you are in the same area with the same availability of signals, the time-to-first-fix will likely be 1-10 seconds after receiving correction data.

Best practices

  • Power on the UNI-GRx in an open area with the least interference from trees or buildings.
  • Make sure the UNI-GRx has a stable and good internet connection making use of Mobile Data or Wi-Fi.
  • Make sure the UNI-GRx receives and uses around 20 satellite signals.
  • Move around slowly while keeping the survey pole level to obtain more satellite signals in case you doubt the location.
  • Moving the UNI-GRx quickly causes the RTK algorithm to re-initiate in case of sudden spikes of signal changes, try to prevent this.
  • Make sure you are within 15-20km of a base station.

Check out more on how to set-up the UNI-GRx to receive RTK correction data here. Or check out more about the difference between fix and float.

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