From memory, it's likely related to the presence of pilots / reference signals in the waveform. This helps initial acquisition in bad conditions, and also helps with multi-path handling (the problem with high rises / urban canyons). But don't quote me on this, for confirmation you'll have to look for details on the Galileo signal waveform.
Remembering the start of it and seeing the US pressuring enough to alter Galileo to be US-blockable[1] makes me feel old.
Really excited though to see it finally getting usable.
I don't think there'd be hardware reasons not to, but I'd be surprised if anyone did that instead of just selling it as a reason to upgrade to a new phone :P
It should be the same as with GPS. I can't remember the exact details since my study is quite some time ago. But in principal GPS (and most likely also Galileo) works by correlating in the incoming signal with a locally generated one. The transmitted sequences and basically look like noise in the spectrum. You can only decode them with their counterparts. For professional (and military use) other sequences are generated and transmitted in parallel (could be on other frequencies). These are longer and therefore allow decoding with a higher precision. But you have to be aware of the code to be able to use those signals.
There some info written on the web, but for details you need to be aware of lots of signal theory:
I think the assistance provided by DGPS goes down as your distance from the correction-factor transmitter increases. Likely it's an encrypted message from the satellites themselves.
I did not meant DGPS. The satellites themselfes transmit longer sequences for the "professional" users which allow for a higher precison through the correlation process.
do you know if the clocks are in sync with the other constellations?
can we use the carriers of 2 constellations at the same time to get an accurate position?
I am probably backward thinking, but GPS (sat) and DGPS (land and naval) combination for precision applications is pretty good. I pinpointed a euro coin on a closed circuit of several hundred meters with an ordinary Trimble receiver which accepted DGPS corrections, and it was about 15 years ago.
If Galileo eliminates need of DGPS around the EU, and is more cost-effective, its fantastic!
UPD: also, I read back then that GPS signal is very accurate, submeter accuracy locations by default, but signal is artificially roughened, with clear data available for military use only.
You're thinking of "Selective Availability", whereby a (random walk IIRC) was added to the signal to deliberately reduce accuracy. It was turned off in 2000 (though it could be turned back on at any time):
https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...
That's true right now, but the new GPS-III satellites do not have that feature anymore, so in the future it will not be possible to turn SA on again. The link you posted explicitly mentions that.
A large source of GPS inaccuracy is atmospheric variations that cause the signals to bend and delay. The data sent by the satellites contains information that can be used to correct for this but it is encrypted so only approved parties can use the resulting accuracy.
Using a fixed nearby station as in DGPS is another way to correct for this.
Can anymore more "in the know" explain why an accuracy increase from "several meters" to 1m would decrease the SAR time from 3 hours to 10 minutes in the open ocean? It's not like the precision difference matters here. I could see it helping in the mountains where there's a lot of signal interference so Galileo might provide better real-world accuracy, but this doesn't make sense on the open ocean. There must be some other component to the service that's not being described.
The increase in accuracy for GPS receivers is not related to the search and rescue service which works by sending a distress signal from the surface to a satellite.
That satellite might be one of the 4 that form the LEOSAR system - low altitude, in a polar orbit, capable of Doppler-based location of the signal (accuracy of 5-10 km), if exact coordinates are not transmitted by the beacon, but having to wait until they get in the range of a ground station to relay the data. The average accumulated delay is 1 hour at mid-latitudes.
Or it can be from the GEOSAR system - geostationary satellites that cannot locate the beacon by themselves because there's no Doppler effect, so they only work with beacons with an integrated and functioning GPS receiver.
MEOSAR - the new system they are introducing by installing components on new GPS, GLONASS and GALILEO medium altitude satellites is going to do proper beacon location like LEOSAR and send it much faster to ground stations (multiple satellites will pick the same distress signal and send it to multiple ground stations). Further more, they are adding a 2-way communication functionality, so beacons with the required capability can get a confirmation that help is on the way.
The SAR feature is more than just positioning, I believe it involves the beacon transmitting to the satellite and the satellite relaying to a ground station. So it is another feature of the constellation.
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[ 2.7 ms ] story [ 65.2 ms ] thread[1] https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)...
[0]: https://upload.wikimedia.org/wikipedia/commons/b/b4/Comparis...
http://www.usegalileo.eu/EN/inner.html#data=smartphone
There some info written on the web, but for details you need to be aware of lots of signal theory:
https://en.wikipedia.org/wiki/GPS_signals
http://www.navipedia.net/index.php/Correlators
UPD: also, I read back then that GPS signal is very accurate, submeter accuracy locations by default, but signal is artificially roughened, with clear data available for military use only.
That's true right now, but the new GPS-III satellites do not have that feature anymore, so in the future it will not be possible to turn SA on again. The link you posted explicitly mentions that.
Using a fixed nearby station as in DGPS is another way to correct for this.
That satellite might be one of the 4 that form the LEOSAR system - low altitude, in a polar orbit, capable of Doppler-based location of the signal (accuracy of 5-10 km), if exact coordinates are not transmitted by the beacon, but having to wait until they get in the range of a ground station to relay the data. The average accumulated delay is 1 hour at mid-latitudes.
Or it can be from the GEOSAR system - geostationary satellites that cannot locate the beacon by themselves because there's no Doppler effect, so they only work with beacons with an integrated and functioning GPS receiver.
MEOSAR - the new system they are introducing by installing components on new GPS, GLONASS and GALILEO medium altitude satellites is going to do proper beacon location like LEOSAR and send it much faster to ground stations (multiple satellites will pick the same distress signal and send it to multiple ground stations). Further more, they are adding a 2-way communication functionality, so beacons with the required capability can get a confirmation that help is on the way.
More details:
- http://www.cospas-sarsat.int/en/system-overview/cospas-sarsa... (see the bottom tabs)
- http://www.navipedia.net/index.php/Galileo_Search_and_Rescue...
- https://en.wikipedia.org/wiki/International_Cospas-Sarsat_Pr...
- http://www.egmdss.com/gmdss-courses/mod/resource/view.php?id...
A bit more here: http://www.navipedia.net/index.php/Galileo_Search_and_Rescue... .