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GPS

ciechanow.ski

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Re: GPS

#221

Post by butwhywhyoh »

SamBam wrote: post #41

This 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…

I actually stopped reading the guide when I got to this point, and I have a lot of experience working with GPS.

My thought was, "If the simple parts are this unclear, I don't want to spend time getting to the more complex portions".

Re: GPS

#222

Post by askvictor »

ragona wrote: post #45

I'm weirdly impressed by the "switch to metric/imperial" button that updates the article text. It's just so helpful.

I wish this was standard in recipe webpages. It really makes a difference, and shows the author is thinking about their audience.

Re: GPS

#223

Post by litoE »

jcrawfordor wrote: post #148
CaliforniaKarl wrote: post #46

One of the things I love about GPS is: Since you know your exact position, you can pick a good GPS satellite (one of the satellite's you're using to calculate your position), look at the timestamp from that satellite, and use it as a highly-accurate time source! Purpose-built GPS time servers (like those from Meinberg) give you an option to enter the length of the coax cable connecting the receiver and the antenna, s…

It's a bit more complex than this, the entire GPS fix is 4D since position depends on time and vice versa. The time reported by a GPS receiver, once fix is attained, is not just the time from one of the satellites but the time resulting from the 4D fix in space and time. This eliminates (to within a certain precision) the latency. A lot of discrete GPS receivers have some nonvolatile storage where they "cache" fixes…

I have a hand-held GPS receiver that was last used in Chicago in November. I just turned it on again in another part of the world but inside a reinforced concrete building, where is gets no satellite signals. It still thinks it's at the Chicago airport.

Re: GPS

#224

Post by skim_milk »

coryfklein wrote: post #176

It's so depressing that – at some point in the future – I'm going to want a really clear and precise explanation of how GPS works and the likelihood that an internet search directs me to this excellent, clean, and ad-free blog is essentially nil.

IMO the best place to find in-depth documentation of existing technologies is the patent office. A while ago I programmed a way to locate the source of gunfire with microphones and drew a lot of inspiration from GPS patents because I couldn't find anything in-depth on the subject on Google (finding the time and position of the source of a shockwave is basically just inverse GPS multilateration)

Re: GPS

#225

Post by zb »

skzv wrote: post #214
mleonhard wrote: post #208

Earlier quoted context omitted.

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.

Yup! This issue, which is commonly described as the "wrong-side-of-the-street" or "wrong-city-block" error, is one of the biggest ones, and is where a lot of the innovation I mentioned is happening today. This occurs because in "urban canyons", meaning streets with tall high-rises or sky scrapers, there is little or no line of sight to GNSS satellites (GNSS being the generic term for all satellite positioning systems…

Fun fact: when I was looking into patenting this concept in c. 2007, the prior art search revealed that it had already been patented by someone in Japan in c. 1997.

Amazing to think that ideas people had back when Selective Availability was on for the foreseeable future are just now becoming available to consumers. Congrats on getting it done!

Re: GPS

#226

Post by groggo »

How are receivers sensitive enough for this to work? A GPS watch can tell the difference between a few billionths of a second?

> It only takes light one billionth of a second to travel the distance of around 1 foot.

Re: GPS

#227

Post by mleonhard »

skzv wrote: post #214
mleonhard wrote: post #208

Earlier quoted context omitted.

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.

Yup! This issue, which is commonly described as the "wrong-side-of-the-street" or "wrong-city-block" error, is one of the biggest ones, and is where a lot of the innovation I mentioned is happening today. This occurs because in "urban canyons", meaning streets with tall high-rises or sky scrapers, there is little or no line of sight to GNSS satellites (GNSS being the generic term for all satellite positioning systems…

Trying to predict reflections by processing building geometry sounds very complicated and error-prone.

Wouldn't it be easier to query Google Maps & Waze telemetry for impossible position jumps? Then you could make geofences where Google Play Services ignores position jumps and falls back to Wifi-based location and integrating the accelerometer.

Re: GPS

#228

Post by zarzavat »

groggo wrote: post #226

How are receivers sensitive enough for this to work? A GPS watch can tell the difference between a few billionths of a second? > It only takes light one billionth of a second to travel the distance of around 1 foot.

1 billionth of a second is an age in the world of radio signal processing. That's 1 GHz. Wifi goes up to 6GHz. 5G gets up to ~30GHz!

Re: GPS

#229

Post by taubek »

myself248 wrote: post #189
taubek wrote: post #28

If all of the teaching materials would be so good... I've encountered first GPS devices back 1997.i remember when my coulegues were explaining them to me. At that time you wouldn't get precise measurements right away. You had to wait for correction factors or something like that. The GPS signal was scrambled at that time.

That's post-processing, and it's still done. Here's why: The satellites only know their own position to a certain precision, and there are only so many bits to express it in the data packet. More bits wouldn't make sense because the measurements aren't that good in the first place. So what you get "live" is naturally limited by both of those things. Single-frequency unassisted solutions are usually good to a few mete…

Thanks. Yes, I remember that it took about two weeks to get the final results. I always thought that the data on how much "interference" was added was released with some offset.

Re: GPS

#230

Post by taubek »

myself248 wrote: post #212
modeless wrote: post #38

Earlier quoted context omitted.

The article explains the delay. The satellites transmit their ephemeris data and other important data very slowly, 50 bits per second, so you have to listen to the signals for a long time to get all of it. Not explained in the article is that modern GPS receivers in phones download this data separately from the internet, so they can calculate positions instantly without waiting for the data to finish transmitting.

I think you're talking about receiving a full almanac and ephemeris, and the parent is talking about post-processing. See my parallel comment about PP. Even sidestepping the internet just handing you a full alm+eph dump, modern standalone receivers can perform a cold-start much faster than their predecessors, because they have huge numbers of receiver channels available. The system operators cleverly offset the alman…

Could be that we were using Trimble survey receivers. They stranded on something like camera tripods and would stay on same position for longer time.
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