Satellites in view

[tkurki]

Extend schema to support satellites in view information as available in GSV and PGN 129540

[tkurki]

SignalK/n2k-signalk#12

[faceless2]

I'll start the ball rolling then:

"gnss": {
    "satellites": {
        "constellation": {
            "gps": {
                "1": {
                    "elevation": 85,
                    "azimuth": 24,
                    "distance": /*not sure this is useful */,
                    "snr": -76,
                    "active": true      /* whether the satellite is being used for a fix */
                },
                /* keys continue, key being the PRN of each satellite */
            },
            /* keys continue, being other constellations of satellite eg GLONASS */
        }
    },
}

Justification:

1. Keying the satellites by PRN makes updating the model easier, and no arrays are required.
2. An "active" field means you can combine the GSV and GSA NMEA0183 messages into one - these are all the satellites in view, and some of them are also in use (active).
3. I'm not familiar with the N2K PGN referenced so would need clarification on the meaning of "Range Residuals" is and what values of "Status" are other than "Used". If it's just "Used or "Not Used' then the "active" field would cover it.

[tkurki]

Is there something that should appear as a sibling of constellation? Why should it be there?

Other than that I'm good with this - can you take it further?

I have no in depth knowledge of the 129540 details either.

[faceless2]

You're correct - either "constellation" or "satellites" is redundant and could be removed. "satellites" is more immediately intuitive, all else being equal I'd suggest we keep that one.

By "take it further" you're going to make me learn to use Git and submit a pull request aren't you? Argh. I knew this day would come, but I have been putting it off :-) Will try to get up to speed on this and on the spec structure.

[sumps]

Its easier than you think, I had never used it and when I met the guys at METs I promised I would have a go. Once I sat down and started, I found the GitHub part was easy and the only thing holding me back was my knowledge (zero) of JSON.
You do not need to clone a repository locally on your PC, just start by going to one of the files in the Master branch that you want to change and once you are looking at the contents of the file just click on the Edit (pencil) icon. This allows you to immediately start editing the file and automatically creates an auto-named branch of Master with just the file you are editing. Once you have done the change, you click to create a PR, add your notes and justification for the change and post it. Then others will review and merge it for you. Go on give it a go !

[faceless2]

An update based on comments above and also some data derived from @timmathews original at http://signalk.org/api/gnss-field-guide.html which I've just seen.

"gnss": {
    "satellites": {
        "gps": {
            "1": {
                "elevation": 85, /*elevation in degrees */
                "azimuth": 24,  /* azimuth in degrees */
                "range": 123123123, /* distance in meters from centre of ellipsoid, optional */
                "snr": -76,
                "active": true      /* whether the satellite is being used for a fix */
                "differential": false   /* whether fix on this satellite is differential. default is false */
            },
            /* keys continue, key being the PRN of each satellite */
        },
        /* keys continue, being other constellations of satellite eg GLONASS */
    },
}

Changes were adding range in meters - definitely useful as with elevation/azimuth it allows us to convert between other representations of position, e.g. ECEF. And adding "differential", for a differential fix. This field plus "active" would allow us to represent all the options from PGN 129540:9..+7 - well, except for "Not tracked" which I don't really understand. There is no analogy for this status in NMEA0183.

Note this doesn't (yet) cover satellites reported by multiple sources: if we have two GPS units, each will have a different set of satellites. Do we report this? If so, how? Needs more thought.

[keesverruijt]

elevation and azimuth need to be in radians?

SNR is a ratio, but then we lose the logarithmic nature so should we maybe add a dB unit?

[tkurki]

Need to account for

• non-GPS GNSSs https://en.wikipedia.org/wiki/Satellite_navigation#Global_navigation_satellite_systems
• a single device / data source using multiple systems
• multiple devices
  The examples in this issue predate multiple sources implementation.

Today I would rather start from deltas than full, as that maps the real world incoming data better to message granularity.

Also need to consider linking a position update to the underlying fix data. Is this just by coincidental timestamps or is some other mechanism possible?

[tkurki]

A log snippet with multiple GSVs and GSAs

$GNGGA,111103.500,5054.3949,N,00107.7952,W,1,12,0.5,50.8,M,46.7,M,,0000*64
$GPGSV,3,1,09,07,16,321,37,08,29,281,33,10,29,143,35,16,75,216,35,0*6E
$GPGSV,3,2,09,18,38,057,35,20,44,105,40,21,81,117,33,26,43,164,25,0*63
$GPGSV,3,3,09,27,62,289,41,0*5B
$GLGSV,3,1,10,65,14,112,18,71,15,018,11,72,25,069,31,77,10,181,30,0*79
$GLGSV,3,2,10,78,52,221,38,79,44,310,28,80,00,342,,81,35,261,40,0*7E
$GLGSV,3,3,10,87,41,052,31,88,75,350,33,0*73
$GAGSV,2,1,07,01,37,308,33,03,09,074,35,05,07,025,,13,85,237,31,0*7F
$GAGSV,2,2,07,15,39,060,33,21,63,228,39,26,30,239,40,0*44
$GNGLL,5054.3949,N,00107.7952,W,111103.500,A,A*5F
$GNZDA,111103.500,08,07,2020,,*41
$GNRMC,111103.600,A,5054.3949,N,00107.7952,W,0.02,0.00,080720,,,A,V*1E
$GNVTG,0.00,T,,M,0.02,N,0.0,K,A*11
$GNGGA,111103.600,5054.3949,N,00107.7952,W,1,12,0.5,50.8,M,46.7,M,,0000*67
$GNGSA,A,3,18,27,16,20,21,08,26,10,07,,,,0.9,0.5,0.8,1*35
$GNGSA,A,3,78,81,87,79,65,88,71,72,77,,,,0.9,0.5,0.8,2*31
$GNGSA,A,3,03,21,26,01,15,13,,,,,,,0.9,0.5,0.8,3*34

[tkurki]

Note the talker ID to distinguish between the systems:

https://www.u-blox.com/en/docs/UBX-13003221