The M8 chip can be set to anything between 1 and 18hz, so why only 5 10 and 18?
I guess analysing software likes an even number of cycles to process, and if the period isn't a factor of 1000 you can't set a whole number frequency. As is the case for 18hz, this cannot be set exactly, it needs a period of 55.55555555555ms
The factors of 1,000 Answer : 1,2,4,5,8,10, 20, -------------
So disallowing the 20, we can have 6 whole number frequencies, but as we are only interested in 2s and 10s calculations we can also have half number frequencies, as in 12.5hz this is easily set at 80ms period. I think I will give this a try next, as a test of the system.
No reason that I am aware of why we can't have any Hz rate in between. I think GPS-Logit (BT) is just set to certain options as a matter of convenience. When you connect to the Ublox BT gps, it has to send a ubx message instruction to the GPS to set the sentences and Hz rate. Manfred just choose to program it for 1, 2, 5, 10 and 18Hz, but I am sure that was just expediency.
I tried programming the USB ublox 7 chip to other update rates and it worked as expected.
If there are problems with bottlenecks and a few missed points trying to run @10Hz with lots of NAV-SAT data, I see no reason not to run @ 8Hz if that stops the missed points.
OK. Here is the ublox7 USB dongle running @8Hz with NAV-PVT and NAV-DOP and @1hz with NAV-SAT. 
What was strange is that I tried to turn off all GNSS except GPS but U-Centre insisted I have SBAS and QZSS on.????
Yeah, as far as I know you set the interval and not the frequency. I can't remember the terms as I haven't looked at it for ages.
I set the updates to 5 seconds when I was trying to parse the info using Arduinos.
> What was strange is that I tried to turn off all GNSS except GPS but U-Centre insisted I have SBAS and QZSS on.????
- Revision 3 errata : SBAS has to be disabled in CFG-SBAS.
- QZSS is mandatory when using GPS to avoid cross-correlation issues.
