AF wrote:
My humble opinion would be that velocity is provided in 2D for reasons of navigation (think ETE and so on).
Also a few years ago, when GPS wasn’t that accurate, there was always 1 dimension sacrificed for the benefit of the accuracy of the other two.
Naturally altitude was sacrificed so that time, lat & long would be as accurate as possible.
Perhaps that has changed, but it would seem to lead to the conclusion that a lot of receivers would use a 2D position to interpolate velocity.
Peter wrote:
It is not calculated by the device, by taking consecutive position points and the time from one to the next. That would produce an inaccurate result with a lot of jitter.
That’s not true. General GPS receivers output new coordinates every second. All you need to do is take a few-second-average of the calculated speed (distance over time), and you eliminate the jitter. Remember too that time is incredibly accurate in GPS. We’re talking nanoseconds.
AF wrote:
Where are you getting your information? Your matter-of-fact presentation of it is a bit concerning, given the error in your statements.
Maybe are talking about different things? From my point of view, the discussion began with your claim that receivers could correct for ionospheric delays if they had access to the P code. I replied that access to the P code is not what makes that possible — it is the availability of signals on different frequencies. That is really my only claim. Then I guess that both of us have started to argue about other things.
In any case, I was not aware of the civilian L2C signal. According to Wikipedia, the first satellite with that capability was launched in 2005, so before that there was no possibility for civilian receivers to do ionospheric corrections. Do civilian receivers (including IFR navigators) typically use the L2C signal today?
Airborne_Again wrote:
No, they’re not because the L2 signal isn’t available to civilians
Where are you getting your information? Your matter-of-fact presentation of it is a bit concerning, given the error in your statements.
There are CA (coarse/acquisition) and P (precise) codes for both L1 and L2. The CA has been made publicly available (for all frequencies).
The P codes, which are far more precise, and therefore useful for ionospheric correction, are not publicly available.
I use it regularly and have a need to understand these key points and it is great to have clear and accurate information about the technology so readily available.
Wikipedia will give the basics
AF wrote:
If that were the case, WAAS would have been redundant. The CA codes aren’t fine enough to employ for that purpose. The P(Y) codes are.
No, they’re not because the L2 signal isn’t available to civilians. The major source of inaccuracy for receivers using the L1 signal only is uncertainty about ionospheric delays. Having the P(Y) codes wouldn’t help – you do need the L2 signal as well.
0.5m is surely comparable to WAAS/EGNOS in reality.
In particular on altitude I see negligible difference when the Aera 660 claims to be receiving EGNOS, versus when not.
Airborne_Again wrote:
It is
If that were the case, WAAS would have been redundant. The CA codes aren’t fine enough to employ for that purpose. The P(Y) codes are.
Peter wrote:
Is that method as good as WAAS?
Not quite, but it is incredibly good (<0.5m absolute accuracy)
Ibra wrote:
I bet that will be tough to isolate/measure,
By design, yes
Peter wrote:
Is that method as good as WAAS?
I think correction would have been good but it surely did not satisfy FAA on PBN/RNP specs in the civilian world
Not sure about military GPS costs? but my bet it is more pricey than our cheap GPS+W, no?
Is that method as good as WAAS?
Even trying to preserve the military aspect, I would have thought that doing a civilian version of this dual-frequency scheme would be cheaper than the whole WAAS/EGNOS business.
Therefore, I do not think the end result is as accurate as WAAS/EGNOS.
AF wrote:
Detail here
https://www.e-education.psu.edu/geog862/node/14075 second version here
https://www.trimble.com/gps_tutorial/sub_pseudo.aspx
Your links (the second one in particular) only support what I wrote. It is not the P code that permits ionospheric correction, it is the combination of the L1 and L2 signals. I quote from the second link: “Because [the P] code is modulated on two carriers, sophisticated games can be played with the frequencies to help eliminate errors caused by the atmosphere.”
AF wrote:
Detail here
https://www.e-education.psu.edu/geog862/node/1407
Thanks for the references, I checked the binary signal in the L1 GPS signal is far more complicated than delta-modulations (binary signal modulated in an analog signal amplitude), the P code is a modulation that happens in the phase of L1 signal, I bet that will be tough to isolate/measure, the phase of a signal never appear in power-frequency filters that are regularly used in AM/FM applications…
