GPS vs Glonass

[Porthandbuoy]

I bought one of these u-blox usb GPS
and also have a much older BlueNext BN-905GR Bluetooth GPS.

I've got them both talking to OPenCPN running under WinXP on an old Toshiba N110 Netbook. I'm using UKHO raster charts.

I'm finding the newer u-blox usb GPS is the more accurate of the two, with the older Bluetooth device consistently 48 metres out in a NE direction.

Can anyone give a reason for this offset, and is there a way to eliminate it?

[bcn]

If you can configurate the old system then check the Datum used for position outputs. Shall be WGS84.

[Porthandbuoy]

Datum checked: both using WGS84.

I think I may have found the answer. The u-blox nmea sentences all begin $GN***, while the BN-905GR has fewer sentences, all beginning $GP***.

So, the u-blox, which is configurable, is using the Russian Glonass satellite constellation, while the older BN905GR, not configurable, is using the USA GPS satellites.

Further on-line digging threw up the snippet of information that the Glonass system is “more accurate at high latitudes”. I couldn’t find anything to say by how much more accurate, or what constitutes high latitude, but as I live at 66N that may be the reason for the older, GPS, unit being consistently out.

[Pelagic]

Good find[emoji106]

[s/v Jedi]

Not sure about what you have but my sensor uses both GPS and Glonass and I don’t think the sentence id determines which constellation is used (I can change it in whatever I like). Also, I would expect differences within 10’ of each other

[Porthandbuoy]

Lots of information here and here about the different satellite constellations and NMEA sentences.

Using OpenCPN's you can read the NMEA sentences being received. Two bits of information tell you which constellation you're using. The $GN, $GL or $GP prefix and the satellite number. 1 to 32 is GPS, while 65 to 88 (possibly to 96) is Glonass.

For my next trick I'm going to reconfigure the u-blox to use the GPS constellation and see if its accuracy degrades.

[Porthandbuoy]

Update:
I've bought a u-blox 7 usb GPS puck and added that to my collection.
The other u-blox usb puck, mentioned above, is the u-blox 8, which is a GNSS puck garnering data from both GPS and Glonass satellites.

I've configured OpnCPN to ignore GPGSV sentences from the u-blox 8 so that only the Glonass sentences get accepted.
The u-blox 7 only picks up GP*** sentences.

With both pucks connected and with the same priority the position flits from one to another once per second. The difference between the two is variable and, as it changes slowly, I put it down to shifting satellite angles. The difference has been as close as 2 metres and as far apart as 45 metres over a 24 hour period.

[seandepagnier]

With both pucks connected and with the same priority the position flits from one to another once per second. The difference between the two is variable and, as it changes slowly, I put it down to shifting satellite angles. The difference has been as close as 2 metres and as far apart as 45 metres over a 24 hour period.

I noticed this as well. Have you plotted the differences? Are you able to determine which is more accurate?

[B25Matt]

My guess is that if you look carefully at the sentences, the ublox will have more decimal places on the lat/lon than the older GPS.

[yachtvalhalla]

A first cut of an 'eyeball' evaluation of three USB GPS is available here.

Units: BU-353, BS-708 and BN-808

Terry

[evatanen]

Greetings from Finland!

I recently bought BN-808 usb GNSS receiver, for use with a PC and OpenCPN -software. https://www.ebay.com/itm/173344596161

Apparently BN-808 supports Galileo and Egnos, but I'm unable to get it working.
This device uses a knockoff version of U-blox M8030-KT GNSS chip. Manufacturer (Beitian) even supplies U-blox's software to configure these devices!

When I use the U-center (v.22.02) I can choose Galileo in the GNSS config page and actually see $GA*** (Galileo) messages in NMEA text console. But I can't get a fix, or see any galileo satellites in the U-center software. As soon as I choose even one another GNSS source like GPS, SBASB or Glonass, the $GA*** NMEA-messages disappear and I only see $GN and $GP/$GL messages in text view.

I have tried reverting back to the default settings in the Configuration -page, I just can't get the Galileo & Egnos working.

HELP!

GPS, Galileo & SBAS


Only Galileo

[evatanen]

Originally Posted by evatanen

When I use the U-center (v.22.02) I can choose Galileo in the GNSS config page and actually see $GA*** (Galileo) messages in NMEA text console. But I can't get a fix, or see any galileo satellites in the U-center software. As soon as I choose even one another GNSS source like GPS, SBASB or Glonass, the $GA*** NMEA-messages disappear and I only see $GN and $GP/$GL messages in text view.

SOLUTION!

Switch to NMEA protocol version 4.10 in your protocol config!