Garmin's "SV-Number": How to find out the corresponding "real" satellite?

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Reinhard Zwirner

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Sep 21, 2021, 4:05:51 PM9/21/21
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Hi,

can anyone of you experts here answer this question?

Thanks in advance

Reinhard

Alan Browne

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Oct 1, 2021, 4:03:16 PM10/1/21
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On 2021-09-21 16:05, Reinhard Zwirner wrote:
> Hi,
>
> can anyone of you experts here answer this question?

Look at the table in Wikipedia?
https://en.wikipedia.org/wiki/List_of_GPS_satellites


--
"...there are many humorous things in this world; among them the white
man's notion that he is less savage than the other savages."
-Samuel Clemens

Reinhard Zwirner

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Oct 2, 2021, 7:41:54 PM10/2/21
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Alan Browne schrieb:
> On 2021-09-21 16:05, Reinhard Zwirner wrote:
>> Hi,
>>
>> can anyone of you experts here answer this question?
>
> Look at the table in Wikipedia?
> https://en.wikipedia.org/wiki/List_of_GPS_satellites

Hi Alan,

Many thanks! Unfortunately, IMHO that Wiki table does not really help.

My Garmin 64sx is able to manage GPS, GALILEO, and WAAS/EGNOS
signals, my 64s GLONASS instead of GALILEO. The numbers of the
currently received satellites are displayed on the respective
satellite "page":

<https://www.bilder-hochladen.net/i/12b4-12a-7e3f.jpg>

As you will see there are satellites with identical numbers, e. g. in
this case 04, 05, and 09. So, which is which? The signals of
satellites marked by "D" are being corrected using WAAS/EGNOS data.
But why not all? And are the corrected ones GPS or GALILEO satellites?

I have asked this question in the Garmin forum but so far also
without success ...

Best regards

Reinhard

Bernd Rose

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Oct 3, 2021, 6:10:42 AM10/3/21
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On Sun, 3rd Oct 2021 01:41:52 +0200, Reinhard Zwirner wrote:

> My Garmin 64sx is able to manage GPS, GALILEO, and WAAS/EGNOS
> signals, my 64s GLONASS instead of GALILEO. The numbers of the
> currently received satellites are displayed on the respective
> satellite "page":
>
> <https://www.bilder-hochladen.net/i/12b4-12a-7e3f.jpg>
>
> As you will see there are satellites with identical numbers, e. g. in
> this case 04, 05, and 09. So, which is which? The signals of
> satellites marked by "D" are being corrected using WAAS/EGNOS data.
> But why not all? And are the corrected ones GPS or GALILEO satellites?

Garmin devices use NMEA satellite numbers, but only display the last two
digits of that number. With NMEA, each satellite system has a dedicated
numbering space:

GPS: 1 .. 32 (equal to the PRN number of the satellite)
GLONASS: 65 .. 96 (equal to slot number plus 64)
GALILEO: 301 .. 336 (equal to PRN without leading E plus 300; the added 300
is not displayed, though, because of the 2-digit limit)
SBAS: 33 .. 54 (equal to PRN minus 87)

Look here for a more complete reference:
https://gpsd.gitlab.io/gpsd/NMEA.html

From the Garmin satellite sky view plots, it would therefore only be
possible to correctly differentiate between GPS and GALILEO by comparing
their relative positions to a current position almanach.

Your central question is easier to answer, though: Differential correction
is applied or not applied "by system" and not "by satellite". From your
images, the differential correction data is derived from satellite 49.
This is SES-5, which provides an EGNOS stream. EGNOS currently only sends
GPS L1 corrections. Multi-constellation dual-frequency support will first
be introduced with EGNOS v3, which will be established between 2023 and
2025. (If everything goes according to current plans...)

Btw., if devices determine a position with differential correction enabled,
they usually only use the corrected satellites. This means, you have to
decide, whether GPS+GLONASS (or GPS+GALILEO) or GPS+EGNOS will provide you
the best position. If many GPS satellites are available in a suitable
geometric pattern, GPS+EGNOS usually is the best way to go. If there's a
phase with high solar activity, the same is usually the case. In areas with
low satellite coverage, it may be better, OTOH, to forgo the correction and
stick with multi-constellation data, though.

HTH.
Bernd

Alan Browne

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Oct 3, 2021, 5:43:42 PM10/3/21
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On 2021-10-02 19:41, Reinhard Zwirner wrote:
> Alan Browne schrieb:
>> On 2021-09-21 16:05, Reinhard Zwirner wrote:
>>> Hi,
>>>
>>> can anyone of you experts here answer this question?
>>
>> Look at the table in Wikipedia?
>> https://en.wikipedia.org/wiki/List_of_GPS_satellites
>
> Hi Alan,
>
> Many thanks! Unfortunately, IMHO that Wiki table does not really help.
>
> My Garmin 64sx is able to manage GPS, GALILEO, and WAAS/EGNOS
> signals, my 64s GLONASS instead of GALILEO. The numbers of the
> currently received satellites are displayed on the respective
> satellite "page":
>
> <https://www.bilder-hochladen.net/i/12b4-12a-7e3f.jpg>
>
> As you will see there are satellites with identical numbers, e. g. in
> this case 04, 05, and 09. So, which is which? The signals of
> satellites marked by "D" are being corrected using WAAS/EGNOS data.
> But why not all? And are the corrected ones GPS or GALILEO satellites?


Well Bernd has answered it quite well. I have embedded code on a
Raspberry Pi using a ublox sensor. Because of the way the sats are
reported I have to add offsets to report the GLONASS sats correctly as
Bernd pointed out. (No Galileo on this unit).

if GNSSID=6 then SV_ID := SV_ID+64;
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