21–30 of 122 posts
Earlier quoted context omitted.
The speed limit in the US is 1000mph, else it’s a munition. I believe the US used to make their GPS slightly inaccurate, but this was done on the satellite side.
I find a bit of humor in imagining NASA rocket launches and atmospheric test vehicles being viewed, in some contexts, as a munition.
Earlier quoted context omitted.
10 ns is the quoted accuracy, but it’s not easy to verify that and it is very easy to mess up something at that timescale. If something is cheap, I’d be willing to bet their PPS output is not going to be 10 ns accurate. Most applications won’t tell the difference between 10 us and 10 ns accurate, but when you need it, you’ll know it.
You can get multiple devices and compare the output. Doesn't give you absolute accuracy of course. But you can set one as "master" and diff to that.
Back in the day when "selective availability" was turned on they added "brown noise" with strong low frequency components that would defeat attempts to use temporal averaging to approve GPS accuracy.
I remember setting a receiver up outside and looking at the tracks go in circles like a spirograph.
Earlier quoted context omitted.
The speed limit in the US is 1000mph, else it’s a munition. I believe the US used to make their GPS slightly inaccurate, but this was done on the satellite side.
I find a bit of humor in imagining NASA rocket launches and atmospheric test vehicles being viewed, in some contexts, as a munition.
For a drone to be affected by GPS speed limits it would have to be a hypersonic drone.
There are two limits: 60000ft and 1000 knots. Some manufacturers use "AND" between them, others use "OR"[1]. So you can have faster drones if you manage to stay below 60000 ft. [1] https://ukhas.org.uk/guides:gps_modules
As a scientist, open hardware is the best. I'm just not sure whether or not CoreSemi might have put its foot in something bigger than they know.
Earlier quoted context omitted.
10 ns is the quoted accuracy, but it’s not easy to verify that and it is very easy to mess up something at that timescale. If something is cheap, I’d be willing to bet their PPS output is not going to be 10 ns accurate. Most applications won’t tell the difference between 10 us and 10 ns accurate, but when you need it, you’ll know it.
The jitter can be a really important parameter. for example in pdf: https://processors.wiki.ti.com/images/f/f1/TI_GPS_PPS_Timing...
Jitter can be tracked and filtered in PPS signals. The related parameter, wander, is interesting to me. Phase noise above 10 Hz is jitter. Phase noise below 10 Hz is wander.
Why?
¯\_(ツ)_/¯
Allen variation is the typical metric for wander.
Earlier quoted context omitted.
I find a bit of humor in imagining NASA rocket launches and atmospheric test vehicles being viewed, in some contexts, as a munition.
I believe "strong" encryption is still viewed as munitions in the US, or at least it used to be.
There's still some verbiage around commercial products and embargoed countries, but realistically, North Korea and Iran have access to OpenSSL, so it hardly matters.
> Well, how about deliberate dilution of precision or intentionally limiting the speed the GPS chip can be used at? Is that a legal requirement or just done on request? I sort of chuckle whenever I hear of limitations like that being put in place as if someone that can construct an ICBM is going to be constrained by the GPS module.
It was required. But that was gotten rid of by the Clinton administration. See https://www.gps.gov/systems/gps/modernization/sa/ .
Affordable differential GPS would be a nice application... it just requires a fast way of synchronization between two devices (CW in the ISM band would probably work, for example).