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The relativistic clock correction that Phanatic mentioned is actually a convenient attack vector when spoofing GPS with the intent of deliberately messing with timing. You can apply updates to the clock correction polynomial in near real-time and a lot of receivers will buy it, causing mad drift. Ground segment typically doesn't update this stuff more than once a day. About selective availability - this was implemented in two different ways. The first was to deliberately screw with the clock sync aboard the satellites. The second was an imprecise ephemeris library in the nav message, inducing satellite position errors. quote:With a dGPS station nearby, you can determine location to within centimeters. I looked at a spec sheet today for a portable receiver with built-in INS module and diff-GPS capability. It was spec'd at 1cm.
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Feb 26, 2021 04:47
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| # ¿ Apr 23, 2024 23:24 |
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Some receivers can get a carrier phase match (instead of just PRN phase match). Not quite enough though, so geodetic GPS typically also used a fixed base station.
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Feb 27, 2021 16:08
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There are indeed plenty of multi-constellation, multi-band receivers out there. Some of them use the different constellations simultaneously, some of them do a priority system instead. But you're right - having multiple available makes the end result more accurate. Farmer Crack-rear end posted:Why does it take older receivers a while to get a fix? I remember my old Garmin would sometimes take like a minute or more to show my location. Is it actually taking that long to solve some math or is there other stuff happening? As other posters mentioned, the receivers need to download data carried on the GPS signal. Each satellite sends its own very precise ephemeris (a description of where in space the satellite will be at a given time) and also a lower-precision ephemeris catalog of the entire constellation of satellites. The receiver must have some way of determining where each received satellite was at the time the signal was sent. Ephemeris information makes this possible and this is updated by the ground segment periodically. Most receivers will store the lower precision ephemeris information for the entire constellation even if you turn the receiver off. Now when you turn it back on, it will not have to re-download the entire thing before being able to make a fix. This is called a warm start. If too much time has passed and the ephemeris info is out of date, it must be downloaded again - cold start. This process takes about 20 minutes if the entire catalog is downloaded. Really old receivers may not have had much storage capability, if any at all.
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Mar 9, 2021 21:13
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Phanatic posted:Once we disassembled a Chinook, loaded it onto a C-17, put it back together in India and started it up. So a standard L1 C/A receiver takes ~20 minutes to download the entire catalog in fragments from various sats in sight. But that's with the C/A transmit rate of 1.023 Mb/s A Chinook presumably has a P(Y) receiver that gets the data at 10.23 Mb/s, which should make the cold start faster? Are there other factors? TotalLossBrain fucked around with this message at 05:43 on Mar 11, 2021 |
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Mar 11, 2021 05:37
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babyeatingpsychopath posted:
I'm not sure how it works with dual antenna aviation receivers like that. I work mostly with industrial GPS timing applications, like substation receiver clocks providing high-precision timing to electric grid control devices. In those receivers, the cable length parameter just introduces an internal time shift proportional to signal travel time down the cable. AFAIK, position is not affected by this - the receiver will always calculate the position of the antenna unless you configure a fixed position
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Mar 23, 2021 21:07
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PittTheElder posted:Yeah we have hundred meter long cable runs in the office building up to the roof antennas, and I don't think it introduces any shift in position at all. I'm very confused as to why that parameter needs to exist. For precise timing purposes. Some grid applications require up to 500ns precision or will soon. Each foot of antenna cable not accounted for introduces a ~1ns error. This may not be a huge deal to a local application but when you need to sync multiple sites together it becomes important.
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Mar 23, 2021 21:19
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