3D / Dgnss / Float / Fixed – what meaning exactly?

Q&A forumCategory: Questions3D / Dgnss / Float / Fixed – what meaning exactly?
amorosik asked 2 years ago

I hope you’ll excuse the questions from the profane, but the ‘gps world’ is so exciting that I have great curiosity on many aspects still unclear (for me)
I have two SimpleRTK2B cards, the first with a U-blox ANN-MB-00 antenna, the second with Trimple Zephir II
Use for both cards a Ntrip server via internet, distance about 30 Km
I would like to understand exactly:
– why a card enters 3D / Fixed and the other in 3D / Dgnss / Fixed, what is the difference between the two modes?
– the 3D value Acc. and 2D Acc. on U-Center sometimes it is displayed other times no, what is the exact meaning and what does its visualization depend on?
– what PDOP and HDOP parameters mean exactly?
– what are the typical precisions that are reached in the various modes (3D, 3D / Float, 3D / Dgnss, 3D / Fixed, 3D / Dgnss / Fixed, … etc)
– when the receiver enters 3D / Fixed mode, what is the accuracy of the position measurement provided (horizontally and vertically)?


1 Answers
clive1 answered 2 years ago

Perhaps there’s a Wikipedia on RTK? Got to be 20-30 years of technical papers.
Fixed means it has resolved within a single carrier wave cycle, figure under 20cm for GPS L1, it has phase so it know where in that 20cm it is. Can likely resolve measurement sub-mm, but obviously noise in that, as everything in the system is in motion.
Float means it typically has phase within the cycle, but can’t quite nail down which cycle (wave-front) it is looking at. The signals are synchronous, the edges of the gold code, and data modulation, wrt carrier signal. Usually it can get it down to +/- a cycle, if the signals have a lot of noise, or inconsistencies in base vs rover observations, the receiver can stay in Float indefinitely, but still have pretty serviceable accuracy.
Rule of thumb for current generation receivers 
3D 3m CEP
3D/GNSS 2.5m CEP
uCenter depends on you enabling messages carrying the data fields of interest, NMEA carries a subset, different UBX messages carry others. You’ll have to enable the messages with the status and precision you’re looking for.