[HN Gopher] Improved Galileo Time to First Fix
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Improved Galileo Time to First Fix
Author : todsacerdoti
Score : 65 points
Date : 2022-09-07 13:22 UTC (9 hours ago)
(HTM) web link (berthub.eu)
(TXT) w3m dump (berthub.eu)
| tasqa wrote:
| I think these calculations are done in some specific part of a
| SoC (at least on mobile phones). So do you need new hardware to
| take advantage of these upgrades?
| jsmith45 wrote:
| That probably varies. I'm not actually even slightly familiar
| with the design of these chips. At one extreme, I know you can
| have a software defined radio (possibly with limited range) and
| do everything in software. But in practice there will be
| dedicated hardware for at some things, like acquisition
| hardware that tries to lock onto a specific satellites prng
| encoding.
|
| I'm not sure if the position calculations themselves usually
| use specialized hardware, but I would expect that processing of
| most data frames is handled by a small embedded processor,
| simply for flexibility. If the position calculations are done
| in software, then updating to use this data is probably
| possible in some designs with just a firmware update. Even if
| the position calculations are done in hardware, I'd bet the
| parameters are passed in from the software on the embedded
| processor, so it sounds like the conversion to the traditional
| parameter could allow that to be reused, as long as the design
| does not try to reuse these parameters in other ways which
| could cause trouble. If it does then the new limited Ephemeris
| data cannot be used, but the parity data might still be usable
| to improve time to first fix.
|
| But all of this relies on the SOC supporting upgradable
| firmware for the embedded processor, that the SOC maker
| actually develop and provide that new firmware, and that the
| device manufacturer decides to incorporate that firmware update
| into a device update.
| addaon wrote:
| This fault condition --- a 30 second message where the client
| regularly needs to wait for a complete retransmit due to
| temporally-local interference --- seems like the perfect case for
| fountain codes.
| ahubert wrote:
| hmm not seeing it yet - this is very low bandwidth but fixed
| rate stuff, where the messages being broadcast also change from
| time to time. But I may not be understanding fountain codes
| well enough - they sure seem interesting!
| johnklos wrote:
| I love a good technical explanation like this. I don't plan to
| implement a Galileo position calculator any time soon, but it's
| still fascinating all the same to know what's going on behind the
| scenes.
| ahubert wrote:
| Let me know if you ever plan to, I'm ready to help :-) (OP)
| netsharc wrote:
| > [...] if an important message is missed or too garbled, we have
| to wait 30 seconds for the next transmission.
|
| > For this reason, most phones attempt to download the orbit and
| clock details from the cloud.
|
| Pun intended?
| ahubert wrote:
| No :-)
| WJW wrote:
| This takes me back to when I was still in the (Dutch) navy, there
| was a project to replace the submarines and as part of the
| upgrade they were planning to replace the old periscope mast with
| an upgraded optronic system. This was basically a webcam on a
| pedestal that would quickly go above water, take a 360 degree
| photograph and then dip below the waves again. This would take
| mere seconds, minimizing the time that the submarine would be
| visible. The data could then be analyzed at length while safely
| underwater.
|
| However, the newer system was so much faster that the GPS antenna
| mounted on the periscope mast could not get a fix in time. If
| they wanted a position fix as well as a look around, they would
| need to leave the mast up much longer. (Submarine navigation is
| much more difficult than navigation for surface vessels,
| basically all INS. Getting a GPS fix every now and then is really
| helpful) By the time I left, it was still an open research
| question on how they would go about it. IIRC, one of the ideas
| was to simply collect a wide spectrum recording of whatever could
| be received and do all the analysis later. This would result in a
| position fix somewhere in the past, which would then be combined
| with the data from inertial navigation sensors to get an
| approximate current location. No idea if that system is what they
| eventually went with though.
| BuildTheRobots wrote:
| The idea of doing a spectrum recording and having something
| else process it in non-real time was actually used for Mobile
| Assisted GPS used to locate mobile phone users ringing
| emergency services.
|
| Iirc it was a stopgap measure to improve cell tower location
| without requiring handsets smart enough to process GPS or that
| didn't have enough time to get a fix. It's briefly mentioned in
| Wikipedia though details are scant:
| https://en.m.wikipedia.org/wiki/Radio_resource_location_serv...
| MayeulC wrote:
| Nice explanation, thank you. I hope this will surface in oss libs
| soon.
|
| > the Galileo satellites themselves provide properly equipped
| receivers (not your phone) with near-realtime updates that can be
| used to achieve very good accuracy.
|
| I have wondered about that. What stops a mobile phone from
| benefiting from that and RTK?
| ahubert wrote:
| Hi! So a phone could well benefit from RTK, but this is a local
| signal, so you have to pick up something that is relevant for
| your location. Meanwhile, the High Accuracy Service broadcasts
| realtime corrections, but it does so on a frequency band that
| is not supported by phone handsets. Also, the RF conditions in
| most urban environments are such that I'm not sure such
| corrections would be received well enough to matter in most
| cases. Phones are moving to dual frequency chipsets which will
| also improve accuracy without further help. Hope this helps!
| MayeulC wrote:
| Thanks, it being another frequency makes it clear :)
|
| Getting access to local RTK data isn't much of an issue with
| projects like https://centipede.fr/ and you can always build
| your own base staion. I was told multiple times that it
| couldn't work for phones, without elaborating.
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