This is a beautiful combination of interactive visuals and thorough explanation
GPS
201–210 of 296 posts
Re: GPS
#202There's a lot of really great info in here. One random things I learned from this: > As that angle increases, the signal from a satellite travels more sideways and its larger portion gets affected by the atmosphere. To account for this, GPS receivers ignore ranges measured from satellites at very low elevation angles. ... atmospheric effects are primary source of GPS inaccuracies. (I know GPS has inaccuracies, but I…
Ionospheric distortions are the largest source of errors in single-frequency solutions! And it's why the WAAS birds transmit a correction model, which all modern (post-2004 or so) receivers can apply.
Multi-frequency receivers can derive the corrections directly because the distortions affect the different frequencies in predictable ways, and they can work back to "ionosphere-free" pseudoranges, and base the rest of the solution on those.
To your quoted comment, nicer receivers also tend to have a configurable "horizon mask" aka "elevation mask", so you can tune this rejection behavior. I could swear I've heard of some that let you configure the mask height _per azimuth_ but I can't find an explicit reference right now.
Elevation masking is tricky because if you crank it up too high, you force yourself into poor-DOP geometries. But if you relax it too low, not only do you get heaping piles of ionospheric distortion, you also invite ground-clutter multipath. I think it's primarily used by stationary timing receivers, because they know their position is fixed, they're less susceptible to GDOP.
Re: GPS
#203There's a lot of really great info in here. One random things I learned from this: > As that angle increases, the signal from a satellite travels more sideways and its larger portion gets affected by the atmosphere. To account for this, GPS receivers ignore ranges measured from satellites at very low elevation angles. ... atmospheric effects are primary source of GPS inaccuracies. (I know GPS has inaccuracies, but I…
Something not mentioned: the "new" L5 signal at 1176Mhz, combined with the existing L1 signal at 1575Mhz, allows the receiver to estimate the atmospheric effects and reduce the uncertainty, allowing for a much better position fix. Think centimeters instead of meters. One more thing I've wondered: the system depends on the sattelites knowing and broadcasting their exact position, but how do you determine this position…
You're spot-on, the bootstrapping is exactly why the outage took so long to recover: https://berthub.eu/articles/posts/galileo-accident/
Re: GPS
#204No post body was provided.
Re: GPS
#205Why not bookmark* it?
* where, "bookmark" may mean a variety of things that saves a URL so that it can easily be recalled.
Re: GPS
#206This is very nice. Very clear, and the 3D interactives are excellent at guiding the explanation. One thing that I thought was a little confusing was right at the beginning, when we were estimating the position of the figurine and there was an area of uncertainty shown by the yellow circle. It isn't clear how you're estimating position, and why we have an area of uncertainty. At first I figured it was going to explain…
After reading all the replies, I still have no idea what that part means.
Re: GPS
#207On this topic, maybe someone can share the status of Galileo and the Russian and Chinese systems?
They are operational and very compatible with GPS.
Modern devices have them all, the iPhone 13 for examples combines satellite data from GPS, BeiDou, GLONASS & Galileo.
In practice, you get a "super-GPS", with way more satellites (120 vs GPS' 35) and great coverage. Potential accuracy can be in the cm's, it is all about software optimization.
Re: GPS
#208Amazing work. I work on the Android location team at Google, and I sent this article out to my team. GPS/GNSS is critical for accurate location/context, and there's still plenty of innovations happening in this field. One of our directors is Frank Van Diggelen - none other than the professor who taught the Stanford GPS MOOC referenced by the article :) I'm sure he is going to appreciate seeing the course called out t…
There are certain streets in downtown SF where Android (and iOS) location always suddenly jumps to a block away. Navigation apps then direct drivers to make incorrect turns or even dangerous turns like turning the wrong way onto a one-way street.
It seems to me that your software could prevent this error.
Re: GPS
#209Dumb question, but how does this deal with security? Can't anyone broadcast valid but malicious data on 1575.42 MHz? (e.g. to crash planes/missiles etc) EDIT: found wiki from some quick googling https://en.wikipedia.org/wiki/Spoofing_attack#Global_navigat...
Military aviation systems often have their GPS antenna designed to receive signals only from above.
Re: GPS
#210Dumb question, but how does this deal with security? Can't anyone broadcast valid but malicious data on 1575.42 MHz? (e.g. to crash planes/missiles etc) EDIT: found wiki from some quick googling https://en.wikipedia.org/wiki/Spoofing_attack#Global_navigat...
It's illegal in most countries to jam or transmit on that frequency.
But common in military activity.