AF wrote:
No, selective availability was a modulation scheme which introduced noise into the L1 and L2 signal codes.
From the point of view of civil receivers it could have been seen as noise, but it was not random – military receivers could correct for it.
The military still has access to the P code which enables them to do ionospheric correction without any additional differential corrections (e.g. WAAS).
How exactly does an additional code enable you to do ionospheric correction?
Detail here
https://www.e-education.psu.edu/geog862/node/1407
5 second version here
https://www.trimble.com/gps_tutorial/sub_pseudo.aspx
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…
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.”
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.
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?
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
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.
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.
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
