Use of WAAS satellites for position?

[Ibra]

Hi,

Unrelated question to Mooneys, I see +18 satellites with 5 “D” on my navigator, this seems a lot? all those worries about RAIM & FDE: make sure satellites > 5 during pre-flight 

My understanding that WAAS satellites broadcast timing and ranging signal as well as correction and integrity signal: basically acts as extra GNSS satellite to enhance NAVSTAR geometry and signal, see reference bellow

Do GNS430 (non-WAAS TSO129) receive this extra GPS-like signal? (it can’t use differential correction from WAAS but what about position from WAAS)

Do GTN750xi (WAAS TSO146) receive this extra GPS-like signal? (it internally use differential correction from WAAS but I am not sure about position from WAAS) 

What about EGNOS? (SBAS in Europe which has same specs as WAAS in US), I looked at Garmin and Avydine docs and it’s clear as mud: they advise that they can’t confirm or deny that receivers get Gallelio signal for position (I get that), they don’t document if they use EGNOS signal for position (no idea)

All I know is they uses NAVSTAR for position and EGNOS for correction 

See, “1.4 WAAS Service Definition”

https://www.gps.gov/technical/ps/2008-WAAS-performance-standard.pdf

 

Edited September 25, 2023 by Ibra

[Boilermonkey]

SBAS (WASS or EGNOS) is not like having an extra GPS/GNSS satellite.  SBAS consists of a ground and space segment.  The ground segment monitors the GPS/GNSS satellites for health and performance.  The biggest thing they capture are regional atmospheric delays (something that RTK does too, but on a 10-30km scale).   That data is then sent up to satellites in a GEO orbit that transmit the corrections over L band to a SBAS enabled receiver.  Those corrections and integrity are then used along with the standard GPS/GNSS satellite signals.   EGNOS works for GPS and/or GAL.

As for avionics using GAL, I've seen some for large aircraft.  I'm personally waiting for a multi-frequency and multi-constellation system to come out before I upgrade from our GTN750.

[Ibra]

Let’s ignore differential correction, it’s clear when and how it’s used in avionics…however, SBAS satellites do provide native PRN that is used for GEO ranging and timing (GPS-like although WAAS satellites do not move much versus azimuth or receiver), this “non-SBAS mode” or “GPS-like mode” is well documented in the specs of satellite used in WAAS and EGNOS, 

Here are EGNOS specs, GEO PNT non-SBAS mode is part of their specs,

https://gssc.esa.int/navipedia/index.php/The_EGNOS_SBAS_Message_Format_Explained

I attached WAAS specs from FAA, same is stated in Wikipedia for WAAS…

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

My question was mostly what various avionics do with that ranging signal from WAAS/EGNOS source? do they ignore them and only use correction?

 

 

Edited September 25, 2023 by Ibra

[Ibra]

45 minutes ago, Boilermonkey said:

As for avionics using GAL, I've seen some for large aircraft.  I'm personally waiting for a multi-frequency and multi-constellation system to come out before I upgrade from our GTN750.

I am asking the question as I see more satellites in GTN that I would have expected in Europe (even when I disable SBAS in internal settings), I compared against GalaxyTab that gets everything including Glonass, Gallelio, Egonos, Luch…and other stuff from far east…for sure, GLO is not used (maybe unlikely ), however, it seems that GTN as of today would receive way more than NAVSTAR here in France, 30%-40% more in average, maybe now they pick from EGNOS? and GAL?

Edited September 25, 2023 by Ibra

[EricJ]

You wouldn't expect to actually have more than 16 actual GPS satellites in view at any time, under good conditions.    If it is indicating that it has more in view it may just mean that it delays removing indication of "receiving" a satellite for a bit after they have gone out of reception.    I'm just guessing at a possible mechanism by which a receiver might "see" more than one would expect.

[Ibra]

6 hours ago, EricJ said:

If it is indicating that it has more in view it may just mean that it delays removing indication of "receiving" a satellite for a bit after they have gone out of reception

Sounds plausible that 2 or 3 did "time out" and still appear as "received", I will see if this persists

I will also check if non-W G430 sees that much 

Edited September 26, 2023 by Ibra