How many seconds/minutes to have a ‘fixing’ gps state?

Q&A forumCategory: QuestionsHow many seconds/minutes to have a ‘fixing’ gps state?
amorosik asked 3 months ago

System consisting of Simplertk2b base + Simplertk2b choke ring and rover antenna + Survey multiband antenna

Gps + Glonass + Galileo are activated on both base and rover (no Sbas, no Beidou)
The base system sends corrections 1005, 1077, 1087, 1097, 1230 every second on uart2, the rover reads the corrections on the uarte2 port, connection between the base receiver and the rover via hardware (the base’s tx2 is connected to the rover’s rx2 )
Corrections are correctly seen on the rover (Ubx / Rxm / Rtcm)
The satellite signal amplitude for at least twenty, both on the base and on the rover, is between 40 and 50 dBm
Well, a base-rover system of this type, with optimal satellite signal, with frequent corrections, how long should it go into fixing (from the moment the rover is switched on)?
I ask this because I am experiencing very variable times, from a couple of minutes to about ten minutes and at certain times the fixing is not achieved at all
So, for an optimal base-rover configuration, what is the expected average time (and what you actually found) to arrive at the fixing starting from the rover off?

2 Answers
clive1 answered 3 months ago

From a 3D FIX to RTK FLOAT/FIXED using L1/L2 data should take in the order of 8-10 seconds.
Cold starting dual rovers here I’d say they are totally up and functional within 30-45 seconds, based on my level or patience, I haven’t sat with a stopwatch.
Where the rover antenna is significantly obstructed, by a building for instance, a receiver can stay in RTK FLOAT indefinitely.
L1 only, time frame to RTK FIXED was perhap 7-15 minutes

amorosik answered 3 months ago

A time of 30-45 seconds to get the fixing, starting from a coldstart, is in my experience, an exceptionally low time
Can I know, starting from the default configuration of the GPS receiver, what settings do you use on the rover and on the base?
What firmware do your gps receivers use (I currently have 1.13 installed)?

clive1 replied 3 months ago

Currently using 1.13 on both Base and Rover units, no special settings, although I wrote the software in the radio units, Base can see 24-25 satellites, in TIME mode, with a fixed location programmed in from a post-processed session. The Base is not cycled, resetting would be atypical as the device runs 24-7 and supports other equipment/tests, and the time I quoted was a Rover one.

With a stopwatch, as USB power is applied to system, from disconnected state.
3D Within 30 Seconds
Float Within 40 Seconds
Fixed Within 47 Seconds (8 Satellites, presumably GPS)
Within 85 Seconds 20 Satellites.

TTFF on GPS is between 18-36 seconds based on where you enter the navigation data cycle. With ephemeris provided I think this could be much lower. uBlox doesn’t support RTCM3 Ephemeris messages, but I’m looking at alternate methods.

One of the real tricks here is to have good/effective antennas, purposely placed and surveyed properly.