Real Time Kinetics (RTK)

For half a year now I’m walking, biking and generally moving accompanied by an uBlock F9P unit, which is a precision multichannel GPS receiver board with an external antenna and bluetooth connection to my mobile phone. The phone receives correction stream (NTRIP) by mobile net and uploads it to the unit, which in turn uses the correctional data to fix up atmospheric noise in the GPS signal.

The result is a GPS position way more precise than a phone.

There are 3 usual states:

• RTK - or “Real Time Kinetics”, when there is both satellite and internet data is available, and the base station of the stream is near. It means precision between 50 centimeters up to 5 centimeters! Imagine it like not only it’s visible when I go to the other side of the path but the track even show when I turn around, since my backpack makes a 30 cm circle around. :-)
• FloatRTK - is when the precision “floats” from a known-good position, which in practice mean 1.5 meters up to about 90 cm (on average 1 m). When I am within 100-300 km of a base station and I’m moving in a forest this is the most usual state.
• DGPS - when there is no RTK fix (usually due to no mobile network or blocked satellite receiving) the “normal” differential receiving mode is active, which means 3m - 1m precision, closer to 1.5 m on average, thanks to the external antenna and multichannel receiver.

When I am on FloatRTK or RTK it’s amazing: it can get back to the same position everytime, later on the day, or on a different day, which basically mean that the position is exact. I can draw a track with 50 cm precision and try not to draw that much detail on OSM (not to make anyone mad seeing fixes of centimeters along :-)).

Unfortunately there are no miracles: on the bottom of a gorge (say, recent beautiful Slovensky Raj) or with no mobile network the float become DGPS fast, and precision starts to wander. HDOP is not a reliable measure: while the “guesstimate” is usually a few meters there are places where my track diverges from satellite imagery too much to be trusted. (So, for a specific example, I wasn’t able to precision-fix up Slovak gorges since there was no network coverage in the middle of nowhere.)

When you see accuracy=* on my tracks it usually mean I had FRTK or RTK there and the position is pretty accurate, way more than a mobile phone can reproduce. Still, if you have an industrial receiver you may be better than me, and feel free to fix up points and tag accuracy=* accordingly. :-)

Hardware and software:

• uBlox F9P + 10Ah powerbank
• Bluetooth GNSS (Android) [logging NMEA, sending position to android and uploading NTRIP to F9P]
• Free NTRIP casters from euref-ip.net or euref-ip.be
• OsmAnd
• and an android phone with mobile net and a largish battery (+powerbank backup).

Thanks

Massive thanks to the Hungarian OSM Community, and especially Kolesár to research the receivers and usage, finding software and streams, to make it all possible.