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RNAV GNSS source change

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RNAV GNSS source change

Old 20th Jun 2020, 09:37
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RNAV GNSS source change

I have a question regarding RNAV equipment. Does RNAV equipments manufactured to use only one GNSS system (e.g. GPS or Galileo) or do they have ability to change GNSS if the original one is not available. So my questions are; letís say you are using Garmin 1000 (or any other RNAV system you are familiar with) and flying from US to Europe:
  1. Does it automatically switch to EGNOS Galileo or does it continue to use GPS.
  2. In case of a problem with GPS signal does it have capability to change to EGNOS Galileo.

Iím looking for answers for different kind of systems including large jet FMSs. Please indicate the system your most familiar with. Thanks.

Last edited by derdere; 21st Jun 2020 at 06:23.
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Old 20th Jun 2020, 12:17
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I used to fly DHC 8-400.
There were two types of FMS.
If it equipped with newer UNS-1Ew, it can receive WAAS, EGNOS, MSAS and GAGAN with operators choice in addition to the conventional GPS.
If with older UNS-1E, no ability to use any of above SBAS systems.
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Old 20th Jun 2020, 18:02
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As far as I understand it, The GPS receivers use both systems all the time. No one is interested which system a particular system a satellite belongs to .... there is just twice as many boxes floating round up there, than a single system provides. Both systems have global coverage, so it’s not like you’re physically travelling from one system to other....
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Old 20th Jun 2020, 19:14
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EGNOS is an augmentation system that works with GPS and Galileo. You cannot navigate on EGNOS alone
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Old 20th Jun 2020, 19:43
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Originally Posted by IRRenewal View Post
You cannot navigate on EGNOS alone
Can you elaborate. Isnít that same as WAAS.
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Old 21st Jun 2020, 02:20
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EGNOS, MSAS, GAGAN, and WAAS are all satellite based augmentation systems (SBAS): https://en.wikipedia.org/wiki/Europe...verlay_Service
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Old 21st Jun 2020, 06:33
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Oh yeah, I meant Galileo. Edited the question.

Any other equipment (Garmin, Boeing FMSs etc) experiences?
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Old 21st Jun 2020, 06:53
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Originally Posted by derdere View Post
Can you elaborate. Isnít that same as WAAS.
I think you should really do a little bit of homework before coming to PPRuNe to ask technical questions.
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Old 21st Jun 2020, 19:32
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Originally Posted by derdere View Post
Can you elaborate. Isnít that same as WAAS.
WAAS piggybacks off the basic GNSS infrastructure. As the name suggests, it augments the basic signal. Incidentally, GPS ≠ RNAV.
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Old 21st Jun 2020, 20:16
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It actually doesn't. WAAS (as well as all other SBAS) make use of geostationary satellites to transmit the correction signal, while the GNSS satellites themselves are in medium earth orbit (MEO).
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Old 21st Jun 2020, 22:22
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I shouldíve been more clear. In my understanding (please correct me if Iím wrong) the SBAS satellites alone wonít give you a signal. You need the basic NAVSTAR satellites to give you a position, and the WAAS ones will supply the correction.
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Old 21st Jun 2020, 23:29
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Yes that is correct. Your GNSS receiver can use the GPS (formerly NAVSTAR), GLONASS, Galileo, Beidou, or any combination of those systems satellites depending on the receiver's capabilities to calculate its position and state vectors, even without SBAS. SBAS without the GNSS satellites however gives you no position. SBAS typically improves the position accuracy by an order of magnitude or better (e.g. 10m precision to <1m precision), and provides additional system integrity information.

The predominant error source for position information with GNSS is in the signal path delays due to the variability of water vapour in the atmosphere. Water vapour affects the refractive index of the atmosphere at radio wavelengths, hence causes the signals to not travel in a straight line, thus the signal travels a longer distance than the assumed straight line. This translates into what's called a "path delay". The ground monitoring stations for the SBAS know their exact locations, and can reverse engineer the approximate path delays over a larger area (not just where the reference ground station is located), which in turn can be used to model the water vapour distribution in the atmosphere and depending on the receivers position, a correction factor for each signal path can then be calculated. This is also useful for other things like radio astronomy.
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Old 22nd Jun 2020, 09:11
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Originally Posted by physicus View Post
Yes that is correct. Your GNSS receiver can use the GPS (formerly NAVSTAR), GLONASS, Galileo, Beidou, or any combination of those systems satellites depending on the receiver's capabilities to calculate its position and state vectors, even without SBAS. SBAS without the GNSS satellites however gives you no position. SBAS typically improves the position accuracy by an order of magnitude or better (e.g. 10m precision to <1m precision), and provides additional system integrity information.

The predominant error source for position information with GNSS is in the signal path delays due to the variability of water vapour in the atmosphere. Water vapour affects the refractive index of the atmosphere at radio wavelengths, hence causes the signals to not travel in a straight line, thus the signal travels a longer distance than the assumed straight line. This translates into what's called a "path delay". The ground monitoring stations for the SBAS know their exact locations, and can reverse engineer the approximate path delays over a larger area (not just where the reference ground station is located), which in turn can be used to model the water vapour distribution in the atmosphere and depending on the receivers position, a correction factor for each signal path can then be calculated. This is also useful for other things like radio astronomy.
Ha! Clever bit of mathematics to figure all that out. Most certainly a level or two above what I was taught. Appreciate the detailed explanation.
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Old 22nd Jun 2020, 18:28
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Thanks for all the answers. I learned a huge deal myself. I admit I made a couple of terminology mistakes myself in the question, which veered off the discussion a little bit.

However Iím looking for a practical aspect of the system. To re-iterate:

1. Does each RNAV system comes PRE-set to work with only one GNSS system. If not, does pilot has any control over it to select which one to use (e.g. switching from GPS to Galileo).

2. Is there a automatic switch between GNSSs in case of failure in one.

If you can specify systems youíre using that would be an awesome direction for me to make further research.

Again thanks for the answers. Cheers.
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Old 23rd Jun 2020, 12:04
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That depends on the GNSS reciever you have in your aircraft.
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Old 23rd Jun 2020, 12:49
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Originally Posted by jmmoric View Post
That depends on the GNSS reciever you have in your aircraft.
Thatís exactly what Iím asking. What do you have and how it behaves.
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