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Atlas Cached

Are You Smarter Than a Garmin Engineer?

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Posted (edited)
4 hours ago, stefanwilkens said:

 

 

I did, the positions seem to correlate.

 

The website:

W7wYYwI.png

 

My eTrex 20x:

fm1HBNA.png

 

The positions of the GLONASS units seems to overlap well:

iPTLXoJ.png

 

This tells me that #745 is GLONASS #91 according to my eTrex 20x. #91 is a satelite ID in the GLONASS-M range, according to the wikipedia link I shared before.

 

Could it be that they use a different numbering scheme for GLONASS compared to GPS?

 

The positions seem to be accurate though.

 

Thank you for taking the time to put that together!

 

The eTrex x0x/x2x units are certainly displaying the GLONASS constellation configuration correctly, but they are Identifying the satellites incorrectly.

 

In your example above, using SV 745, the eTrex x0x/x2x identify this satellite as "91", which is incorrect. Here is why that is incorrect:

 

All Garmin GPSr identify satellites by their NMEA numbers to avoid conflicts between different satellite systems and to keep the numbers two digits (below 100).

 

For GPS, the NMEA = the PRN, which is a one to one correlation (GPS satellite 5 = NMEA 05, etc.) 

 

For GLONASS, satellites are identified by the 'slot number' in the constellation they occupy + 64. GLONASS constellation consists of 3 planes with 8 satellites each, or 24 slots. 

 

According to GPSrChive and these Russian GLONASS tracking websites: (Example A) (Example B) - SV 745 occupies slot 7. The correct NMEA ID for this satellite is 7+64, or "71".

 

Every other Garmin GPSr correctly identifies SV 745 as "71", which is the correct NMEA conversion.

 

 

Edited by Atlas Cached

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Posted (edited)
5 hours ago, stefanwilkens said:

the positions seem to correlate.

 

 

The positions of the GLONASS units seems to overlap well:

 

 

This tells me that #745 is GLONASS #91 according to my eTrex 20x. #91 is a satelite ID in the GLONASS-M range, according to the wikipedia link I shared before.

 

Could it be that they use a different numbering scheme for GLONASS compared to GPS?

 

The positions seem to be accurate though.

 

 

Other GPS devices show GC 745 as "71", the GLONASS Satellite in slot 7 (Slot 7 + 64 = 71).  That exactly matches the chart:

https://en.wikipedia.org/wiki/List_of_GLONASS_satellites

 

Untitled-sat1.jpg

 

It looks like Wikipedia pages contradict each other.  Surprise! :yikes:

 

But what would be the purpose of Etrex 30 scrambling all satellite numbers contrary to other Garmin GPSs?  Even if it shows the "spares", numbers 65 through 88 wouldn't be affected.  Where does "90-92" even fit in, and how were those numbers, or any of the others, calculated... it's a completely different numbering system? 

 

Is Etrex 30 in fact tracking more than 24 GLONASS satellites, and if so, why?  There are no more than 24 operational at any time.

 

Untitled-sata2.jpg

 

 

Edited by kunarion

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As an added bonus, for many of them, the N,S,E,W characters, and probably the satellites as well, are rotating around a centre point that is one pixel offset from any circle graphic.

It’s evident in this image:

https://www.gundogsupply.com/al100cosc.html

The dial markings around the South and East directions are offset right from the compass edge, while the rest are touching.

More obvious with a lo res screen like GPSMAP 64. Once you see it you can’t un-see it :D 

 

The graphics are circles with an even pixel diameter, but all of their drawing has a centre pixel, and dynamic circles have odd pixel diameter,

and you can’t overlay those over each other.

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