[time-nuts] WAAS.....

Brian, WA1ZMS wa1zms at att.net
Wed Jan 8 18:47:09 EST 2014


We have a Rb for hold-over that is good for 72hrs per our needs. So we are fine in that regard.

That said, the vendor of the GPS box is a bit to fast and our equipment is also in some regards a bit too fast to report a string of alarms when both the main and hot-standby units go into hold-over.
We can manage that with the vendor and our own equipment alarm reporting.

The "jamming" events are fast and unpredictable given the ever-growing use of GPS jammers that anyone can find via a fly-by-night web page. The FCC is also staff limited to deal with fast and dynamic events. I am not sure that anybody can find a mobile and time dynamic jammer. One thing I did learn is that you will never find a truck with an active jammer at a sea port or transfer station. They WANT to be logged as being at those locations. Yet once on the road, jammer is often "on" so they can get cargo to destination ahead of schedule and get a pay bonus for doing a prompt job. Keep in mind that L1 jammers are easy to obtain that make as much as 3W of RF power on the L1 freq. Please don't ask me how I know.

On this forum I cannot go into details due to nature of my day-job and our customers, but that said my original question has been answered by the FEI  papers that claim that a WAAS directional antenna has some advantage.  As usual for me, I tried to re-invent the wheel.

CDMA or cellular timing is not a viable option if our systems need to be more robust than a cell phone network.

Sorry, I cannot go into our customer base.

-Brian, WA1ZMS/4
iPhone

On Jan 8, 2014, at 5:18 PM, Nathaniel Bezanson <myself at telcodata.us> wrote:

> 
> Brian, WA1ZMS  wrote:
>> In my case, SW masking of hold-over alarms may be a shorter fix without any HW fixes. 
> 
> If you can mask short-duration alarms while still finding out about persistent ones, then yes, that's probably the most pragmatic solution. What's your holdover tolerance?
> 
> Following from that, suppose a jammer parks nearby and doesn't leave in a timely fashion. How long does it take for the FCC to swoop in (do they swoop? in my mind they do) and find the source? Is that within your permissible holdover window?
> 
> But back to your original WAAS question, it sounds like it's time to haul out the spare hardware and do some experiments! Even with the normal antenna, you should be able to assess the validity of the configuration. Will the receiver even let you specify just those few birds? It's almost a question of whether they bothered to code an error message for such a stunt...
> 
> (This next part may deserve its own thread. Please edit the subject-line if replying to just this bit.)
> 
> You might look into a CDMA-derived time source, long term. By working one stratum away from GPS, you'll be listening to a plurality of pilots, each of which is GPS-derived, and which are geographically diverse. A single GPS jammer shouldn't knock out more than one tower at a time, and even if the tower's local holdover OCXO isn't stellar and it begins to drift, your CDMA-derived receiver is continually comparing and assessing the different signals to discard the outliers. 
> 
> Ideally, it's like having a bunch of diverse receive sites feeding back to you on a jam-resistant (very strong) channel. Pessimistically, you've got no visibility into the internal operation of those sites, and the only way to infer their status is by comparing them against each other (or a local GPS receiver, if you're not presently jammed yourself).
> 
> As far as I can tell, the CDMA receivers are less explored than GPS, so you'd be largely taking the manufacturer's word on a lot of things. 
> 
> 73 de NJ8Z
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