GNSS Accuracy

[neilwoody]

My GPSMAP 67 regularly reports a GNSS accuracy of 6 feet. I have seen some screenshots of 67's reporting a GNSS accuracy of 5 feet. How many of you get a screen accuracy of 5 feet or less with your 67?

[Gadwin]

How old are those screenshots? When 67 was released the accuracy number was really buggy and showed even 5ft sometimes. After an update it doesn't get down under 6ft anymore.

Who knows how reliable those numbers are on the screen. It is still a pretty unfinished product, which is missing some basic features.

[neilwoody]

Good points. That answers my question.

[JungleJim]

Mine get down to 1.8m (6 feet) as well, can't remember if it shows 1.6m meter sometimes but then again I don't look at the accuracy figure all the time

[henrik]

Hi all, this is my first post on this nice forum!

Who knows how reliable those numbers are on the screen.

You can try out yourself, I guess. Here is what I did:

I placed my GPSMAP 67 at a fixed point with a clear view to most of the hemisphere, with GNSS switched on.
When "Accuracy" indicated 1.8m (i.e. 6ft) I started the recording, so that every 20 seconds a track point was written.
All these point positions were then converted to UTM coordinate system (to have cartesian coordinates) and their mean value was subtracted. This way you end up with a point cloud, centered around the origin (0,0):

Coordinate Cloud

coordinateCloud.png

The coordinates are given in [meters].

Here a good specification of the accuracy indicator is given:

Modern Garmin consumer grade receivers report GNSS Accuracy as a two sigma (95th percentile) value. This means a receiver reporting an accuracy of 3.65m will be within 3.65m of the true location 95 percent of the time.

Now when you do the statistics, you find in this particular case that 78% of all point lay inside a circle of 2m radius. If some outliers are neglected this rate will improve even more. So - all in all - not bad! I for one am quite satisfied with this nice little device.

Maybe someone finds this interesting. In case I made some logical (or other) error, please do not hesitate telling me, we all want to learn!

*** formatting updated to comply with forum posting guidelines ***

[neilwoody]

The 67's Specifications page indicates that Garmin uses the Airoha AG3335M GPS chip in the GPSMAP 67 with a quad helix antenna. The Airoha product description says the following about the AG3335M as follows:

AG3335M is the first GNSS receiver that adopts advanced 12nm process, which greatly reduces power consumption and improved product endurance. It also supports GPS, Galileo, Beidou, GLONASS, QZSS five satellite system positioning, with low tracking sensitivity of the chip (<-166dBm), short initial positioning time (<25s) ), with high positioning accuracy of <1.5m (4.9 feet) with an open sky. I have never gotten better than 6 feet accuracy with my 67 but I think that reading is due to Garmin's software/firmware being coded to show no less than 6 feet accuracy.

[mimichris]

You have to know how they find 1.5m of precision, 1.8m is already completely correct. If we add up the points by recording a waypoint while leaving the 67 at a fixed position for an hour, I'm sure we arrive at 1.5m.