A follow up to my original post:
After further testing on my new computer and new C-Type USB hub, I can see that the USB configuration can have a dramatic effect on the
reception of GNSS signals. On my new system configuration I have the GNSS
antenna on one port of the C-Type USB hub. I also have a portable hard drive and a thumb drive on that same hub. Under normal operation the GNSS display will show around 14
GPS satellites in view and 12 in use for a fix. When I start additional activity on that hub (e.g., writing a large file to the thumb drive), the SNR values displayed drop dramatically and the GNSS
antenna will drop 1 or 2 of the satellites from the in use list (GSA
NMEA sentence). When the additional activity ceases the SNR values will slowly increase and within a minute or 2 the SNR values are all strong and the GSA sentence is reporting 12 satellites in use. A similar effect happens to the GLONASS satellites the antenna is
tracking.
So the USB port configuration can have a significant effect on GNSS
reception. I don't know if this is because the additional activity is causing
noise or drawing too much
power from the other hub
ports. I used a similar USB hub configuration on my old computer so a failing USB port on that computer might explain the dramatic improvement I reported in my original post when I moved to a new computer.
If I move the GNSS antenna to a separate USB port on my computer the GPS reception is not affected by I/O activity on the hub.
This testing was on my personal computer, not the computer at my nav station, The
navigation computer does not provide
power to the GPS-only receiver for ship
navigation, but I definitely would not use a USB hub for GNSS navigation. Although the effect on SNR values was dramatic, the GNSS antenna never
lost it's fix and the HDOP value never went above 5 feet, so the real effect may not be that important.
John