[time-nuts] Terrestrial Tides and Land Movement

[Bob Stewart]

Hi Bob,
Thanks for taking the time to explain the 4ns and 20ns wanders.  I have just been calling them "constellation errors" without being able to explain it better than that.  I've also wondered how much of the 20ns, if any, is attributable to the PRS-45A.

I still don't have the antenna located in a position suitable for precision timing.  The power of the HOA is not to be trifled with.  That, and the sky is full of power lines and other junk around here.

Bob

      From: Bob Camp <kb8tq at n1k.org>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Monday, May 25, 2015 6:01 PM
 Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
   
Hi

Let’s step back a bit:

Your module is accurate to maybe 2 ns over a short period of time and something in the 10 to 20 ns range
over 24 hours. The 2 ns comes from a variety of issues. The 20 ns comes mainly from the ionosphere.

One example of a part of the 2 ns - the satellite positions in orbit are only known to some level of accuracy. The
data your module gets is a broadcast data set that represents a “best guess” made yesterday to fit the fight 
path today. 

A GPS that does an on the fly position solution to 3M is not that exceptional these days. You certainly see
claims of performance at the 1M or even 0.5M level. Yes people do fudge the numbers. Yes they do use WAAS
and the like to improve things. 

A 3M error is at most a 10 ns timing error. It’s more likely to be a 3-4 ns timing error. That’s not all that big compared to the numbers above. 

Am I fudging the numbers a bit? Of course I am. There is an implicit assumption that the mobile platform has
a good view of the sky. Dropping below 4 sats would not happen in this case. I’d also upgrade the LEA-6T to 
an LEA-8T (or similar). More sats is going to give you a better position solution. 

The bottom line is that an L1 GPSDO that wanders 10 to 20 ns per 24 hours compared to a Cesium or maser is 
doing pretty well. Taking that to 8 to 18 ns  vs 12 to 22 is a worthy thing to do. *Proving* that a specific  implementation better … maybe not so easy. I’ve seen a lot of plots of well respected eBay GPSDO’s that wander
30 ns or more (peak to peak) per day one day and 20 ns p-p the next….

Bob