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GNSS receiver calculation of time

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GNSS receiver calculation of time

Old 11th Jun 2020, 14:27
  #21 (permalink)  
 
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GPS satellites also talk to each other and to their controlling ground station and pass information about the health of the system and the time, synchronized by the ground station.
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Old 11th Jun 2020, 14:41
  #22 (permalink)  
 
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Originally Posted by Jetstream67 View Post
Receivers start with relative time checks for received signals aided by any still current satellite almanac info they may have from a prior session or other available location or time info (e.g. time or Mobile data/ wifi derived location data on a phone). Any receivable satellite will give a quite accurate time signal and the receiver can then start to play with the subsequent signals to try to assemble a plausible set of 3D spheres from the relative delay of each time signal and assumptions about credible altitude /depth.
They start with the assumption that the receiver clock is GPS time even though it isn't. That is the basis of psuedo-ranging.
  • The receiver unit begins by runnning the PRN code for a satellite and time shifting it until it achieves a lock-on, obtaining a psuedo-range in the process.
  • After lock-on, the satellite clock error and orbital position data are received in the nav message.
  • Psuedo-range from 3 satellite positions gives a false position due to receiver clock error, whilst 4 result in a mismatch that enables the unit to figure out the error, fix the position and update the receiver clock.

Each satellite also transmits a complete set of constellation almanac data so if a rough location is not not already solved the alamanc info becomes available after about a minute and tells the receiver where each 'visible' satellite should be in space at that moment.
With this info too the rough spheres solution becomes trivial and the receiver can now move to refining the location /altitude to a precision fix and maintain those updates in use.
I don't know where you are getting this information. It takes 12.5 minutes for the almanac to be updated because it is spread over 25 nav messages. When the unit starts, the existing almanac is good enough and the precise orbital data for the individual satellite is obtained within 30 seconds of lock-on.
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Old 11th Jun 2020, 16:07
  #23 (permalink)  
 
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The GPS system knows where the aircraft WAS...
The navigation message consists of 30-second frames 1,500 bits long, divided into five 6-second subframes of ten 30-bit words each. Each subframe has the GPS time in 6-second increments.
GPS signal is about 6 seconds long from each sat... you need at least 4 sats, and for most systems, with redundancy and error trapping you need 6.
Meanwhile, the ac is moving at 400kts ...

The system correlates all of this into where the ac WAS.

Due to all of the latencies involved, the Kalman filters estimate where the aircraft is.

GPS satellites also talk to each other and to their controlling ground station and pass information about the health of the system and the time, synchronized by the ground station.
No, the GPS signal does not include integrity. Integrity has more to do with the atmospheric conditions of the broadcast when receiving. If a SAT is off, it is shut down until repaired.
WASS does broadcasts and GBAS broadcasts do include integrity corrections.

Within the FMC, not the signal, there are integrity factors for accuracy of location...especially for waypoints associated with a procedure. There is the HIL right there on the FMS.

Last edited by turbidus; 11th Jun 2020 at 16:21.
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Old 12th Jun 2020, 11:12
  #24 (permalink)  
 
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The internal clock in a cheap handheld GPS can be hideously inaccurate because knowing the time within a few minutes of correct (and the location within a few thousand miles) is ample to quickly determine which satellites should be in view.

Consider for a moment what happens if you have a (lossless) very long antenna lead between the antenna and the GPS. Let's say the antenna is 10 kilometres from the GPS and the antenna cable is straight.
  • What position does the GPS report?
  • What time does the GPS report?
Descriptions will do, no need to do the maths for the second question.
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Old 12th Jun 2020, 11:37
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I'll guess at that one. The receiver determines position based on the differences in delay of the signal from each satellite. Since the signal from each satellite will be delayed by the same amount by the antenna lead, that delay will automatically be cancelled out of the calculation. So the GPS will accurately return the position of the antenna.

As for time, the GPS will use the calculated position to determine the time delay from each satellite. Since the calculated position is at the antenna, it will only account for the over-air delay, not the delay in the antenna lead. Since the signals actually take longer to reach the GPS than is accounted for in the calculation, the reported time will be sightly slow (i.e. earlier than the actual time). By something like 30 to 50 microseconds, if my math, Latin prefixes, and assumptions about the speed of the signals in the lead are correct.
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Old 12th Jun 2020, 13:11
  #26 (permalink)  
 
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I thought I'd give a little more explanation -- if nothing else, maybe this will make it obvious where I went wrong.

If you go back to my example of the straight road between church towers two miles apart, imagine you stand at the point where you hear one chime a second before the other. If you recorded the sound, saved it to an SD card and sent it by carrier pigeon to your friend in Timbuktu, your friend could listen to the recording and calculate exactly where you stood on the road when you recorded it. But even if he knew that the those particular notes were played only at 11:15 AM on November 11, he couldn't work out the time in Timbuktu unless he knew something about the speed of a carrier pigeon.
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Old 13th Jun 2020, 01:21
  #27 (permalink)  
 
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Originally Posted by turbidus View Post
The GPS system knows where the aircraft WAS...
The navigation message consists of 30-second frames 1,500 bits long, divided into five 6-second subframes of ten 30-bit words each. Each subframe has the GPS time in 6-second increments.
Once the GPS receiver has acquired a minimum of 4 satellites and collected a 30 second frame from each, it can monitor the carrier frequency of each GPS satellite. These are synced to the data frames, so once you know where each frame starts, you can compare the phase difference between the carrier signals continuously and not have to wait 30 seconds times 400 knots to calculate the position where you were. Admittedly an overly simplistic explanation of what the receivers do.
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Old 13th Jun 2020, 12:00
  #28 (permalink)  
 
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Originally Posted by Chu Chu View Post
I'll guess at that one. The receiver determines position based on the differences in delay of the signal from each satellite. Since the signal from each satellite will be delayed by the same amount by the antenna lead, that delay will automatically be cancelled out of the calculation. So the GPS will accurately return the position of the antenna.

As for time, the GPS will use the calculated position to determine the time delay from each satellite. Since the calculated position is at the antenna, it will only account for the over-air delay, not the delay in the antenna lead. Since the signals actually take longer to reach the GPS than is accounted for in the calculation, the reported time will be sightly slow (i.e. earlier than the actual time). By something like 30 to 50 microseconds, if my math, Latin prefixes, and assumptions about the speed of the signals in the lead are correct.
Thus the receiver does not need an accurate time to determine location, rather the accurate time, at the antenna, is part of the 4 dimensional solution of 3d location and time.
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Old 19th Jun 2020, 16:36
  #29 (permalink)  
 
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Here's my basic understanding of the GPS system. The Russian and European systems, I have no idea, but I imagine the principles must be similar.

A minimum 5 GPS satellites are visible at any location at any time but often more.

A minimum of 4 signals are required for a 3D position fix, lateral position is more accurate than vertical due to the geometry involved.

5 signals are required for RAIM. This allows for one signal to be eliminated from the navigation solution if a gross error results when using it compared to using the other signals. Important point to remember : No RAIM = No guarantee of accuracy whatsoever.

WAAS/GBAS is also used for approaches, but that's an extra subject...

Distance = speed x time so,
Distance satellite to receiver = radio speed x time from satellite to receiver
Knowing the exact transmission time and exact reception time allows determination of signal travel time and therefore calculation of distance from the satellite to the receiver at the time of reception.

Receiver location is the intersection of the 4+ spherical distance lines calculated from the signals using satellite position (see below) plus a bit of trigonometry and the WGS84 global reference co-ordinate standard.

Every satellite transmits 2 signals;
L1 carrier: This is modulated with a data package containing the satellite ID, the satellite "ephemeris" (basically it's current orbital position above the surface) and exact transmission time from the onboard atomic clock. Also a "pseudo random" binary code ( "P" code) which is used to align the clock in the receiver nav unit with the onboard satellite clock therefore allowing determination of the signal travel time. This code isn't really random, it can't be because the receiver needs to know it too in order to "shift" it's internal clock time to match the satellite. It just looks random because it takes a very long time to repeat.
L2 carrier: This transmits the "P" code only but on a significantly different frequency. This allows a correction to be computed in the receiver to compensate for atmospheric delays to the signal from each satellite, enhancing accuracy.

There is a correction for the effects of relativity too. The satellites in orbit are in a weaker gravitational field, therefore causing the onboard clocks to run a bit faster and also at the speed required for orbit the clocks run a bit slower, so to correct for this and to synchronise all the clocks exactly, which is essential for the system to work, there is an update signal from a ground "master clock" once per 24 hours.

Hope that is helpful in some way!
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Old 19th Jun 2020, 18:33
  #30 (permalink)  
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I think you need 6 satellites for the FDE, fault detection and exclusion, function in RAIM to be working, otherwise the reciever cannot see which satellite to exclude, just something is wrong, FD.

I’ve kind of come to the conclusion that we don’t need to know exact time to find our position, we need the exact time between signals with the same timestamp to be recieved.

But we’re on basis of that, getting the exact time for free.
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