Re: ZED-F9P and PPP processing

[Gerry Creager - NOAA Affiliate]

We might need to talk about the geodetic mode of the Z-12 and NetRS. I used Trimble and Ashtech a lot (and I still have a Z-12, as well). L1 code was better-resolved in position and timing using L2 carrier phase. The correlation between L1C/A and L2P(Y) wasn't ever good enough to allow high precision resolution involving simple code-phase solutions. Had an L2C been implemented in the clear, with the higher chipping rate, that would have been a good thing. Even if you'd had access to P(Y), there was no benefit in looking at L1C/A after it initialized the position for L2P(Y).

gerry

On Tue, May 5, 2020 at 7:59 PM John Ackermann N8UR <address@hidden> wrote:

On 5/5/20 3:37 PM, Gary E. Miller wrote:

>> See, those ancient Z-12s and (slightly less ancient) NetRS units have
>> their value.  I still have a couple of working Z12s but mainly use the
>> NetRS as my "real" reference these days.
>
> The security clearance you need for P(Y) also helps.

No, Ashtech and then others figured out how to correlate to the
encrypted military code and extract code phase without decrypting it, so
no key needed.  The results probably aren't quite as good as working
with the raw code, but it gets down into the single-mm range for RTK and
PPP.

That's what Ashtech called its "Z" technology that gave the Z-12 its
name.  Trimble and I'm sure others have their own variation on that theme.

The Trimble NetRS can be a nice eBay catch for a few hundred bucks if
you're lucky.  It's fairly small and has ethernet (in addition to four
RS-232 ports).  It runs Linux so is hackable.  The primary interface is
via web browser so it's easy to remote.  The downside is that it's GPS
only and does not support L2C, so when the old L2 P code goes away as it
may do someday, it could be out of luck.

Another big advantage, for me, of the Ashtech and NetRS is that they
support an external 10 MHz reference signal input.  With that, your PPP
results give you a timing record you can use to compare your local clock
stability and accuracy to the GPS constellation.  So I feed my local
cesium standard into it and get a running record of local vs GPS
frequency.

[ But conversely, for a time-nut the "PPS" output on both the Ashtech
and the Trimble are horrible; they're intended for time-tagging photos
and not precise time interval.  They are worse than the PPS output on a
plain GPS. ]

> I hope you write this up nicely for others to learn with less pain.

There are a number of things in the works to document what I've been
learning.  Part is in conjunction with the "HamSci" citizen science
project to build a "Personal Space Weather Station" that will include
GPS (probably F9T) both for timing and to allow the scientists to gather
ionospheric data like Total Electron Count.

John

--

Gerry Creager

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