It is unclear from this NOTAM whether this is an intentional
perturbation of the satellite signals vs. a terrestrial transmitter
(my money is on the latter), but it illustrates why one might want
geographically dispersed time sources on one's network, as well as why
the current trend towards decommissioning LORAN (and in the future,
other navaids) in favor of reliance on a single source is a Bad Plan.
I'd be curious to see what effects (if any) those who use
GPS-disciplined NTP references in Southeastern Georgia see from this
experiment.
https://www.faasafety.gov/files/notices/2011/Jan/GPS_Flight_Advisory_CSFTL11-01_Rel.pdf
-r
As I understand it, they're trying to get the WAAS sat back online and
working properly after it went on walkabout some time ago. It's currently in
a nonstandard orbit while they work on it. I suppose it's just pure
speculation that they'd only be working on the WAAS service since the NOTAM
doesn't say anything about it, but if that were the case there wouldn't be
any effect to timing.
-Jack Carrozzo
Nahh, that was the western WAAS sat, IIRC.
This is...Something Else Entirely.
--msa
I'm not sure how you'd get increasing radius with altitude from anything but a jammer near sea level.
Matthew Kaufman
Probably related to:
http://www.engadget.com/2011/01/20/faa-warns-of-ongoing-gps-issues-in-so
utheastern-us-due-to-defens/
Sounds like they're doing 'tests' on GPS near SE Georgia.
Ken Matlock
Network Analyst
Exempla Healthcare
(303) 467-4671
matlockk@exempla.org
Yo All!
I'm not sure how you'd get increasing radius with altitude from anything but a
jammer near sea level.
Agreed.
One of these tests was recently run in Utah and we saw the effects
in Central Oregon well outside the NOTAMed area. During the tests,
airplanes using GPS navigation would suddenly lose RAIM and had to
abort their approach to landing. Not sure if they lost all GPS nav
information or just RAIM.
For non pilots, RAIM is an indicator that the GPS has a redundant
solution that matches the barometrically measured altitude. GPS will
continue to report a nav solution when lacking this redundancy but
pilots are not allowed to shoot an approach when RAIM is off.
Good thing most air carriers do not use GPS yet.
No effects seen on the ground here, but we were several hundred miles
from the test.
RGDS
GARY
- ---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
gem@rellim.com Tel:+1(541)382-8588
Not true.
Most air carriers do use GPS. Most airliners are even WAAS capable
these days.
Good news is that while RAIM appears to fail under these tests, the
WAAS capable GPSs seem to not lose their ability to shoot LPV
approaches.
Not sure why that is, but, I've been on an LPV approach during one of
these tests when other pilots were calling missed on the LNAV version
of the approach due to RAIM loss and I was able to continue the approach
down to minimums and land.
Owen
(Commercial Pilot, Airplane, Single Engine Land, Instrument Airplane)
"Matlock, Kenneth L" <MatlockK@exempla.org> writes:
Probably related to:
http://www.engadget.com/2011/01/20/faa-warns-of-ongoing-gps-issues-in-so
utheastern-us-due-to-defens/
Sounds like they're doing 'tests' on GPS near SE Georgia.
Yes, very likely related considering that the map from the NOTAM is
published on the engadget site. 
The questions I implied are twofold:
Firstly (idle curiosity) - does anyone have further publicly
divulgable details on what's apparently a terrestrial jammer test or
maybe an operational exercise involving the Bermuda Triangle and
making planes and ships disappear...
Secondly (operationally motivated) - I'd be interested to hear of any
issues experienced by folks using GPS as a (terrestrial) NTP
discipline source.
-r
I'd be curious to see what effects (if any) those who use
GPS-disciplined NTP references in Southeastern Georgia see from this
experiment.
Aren't CDMA BTS clocked off GPS?
NTP isn't going to be the only "ripple".
Regards,
Michael Holstein
Cleveland State University
Yep; and many of the aftermarket GPS receivers commonly used for the disciplined clock for NTP originally came from that service (Agilent/HP Z3801 and Z3816, for instance).
NTP isn't going to be the only "ripple".
Most of the "brand name" GPS NTP solutions have a clock
with is more than stable enough to survive without GPS
lock for 45 minutes(*). Some of the more expensive units with
temperature controlled oscillators have hold times in the
many weeks. My guess is that the NTP ripples will be
limited to those NTP servers just (or recently) booted
which have not yet achieved a stable clock state.
Gary
(*) This presumes that this test results in loss of signal
lock, and not intentionally injected false information.
Possibly relevant section from
Agilent
Designing and Testing 3GPP W-CDMA
Base Transceiver Stations
(Including Femtocells)
Application Note 1355
1.15 Asynchronous cell site acquisition
One of the W-CDMA design goals was to remove the requirement for GPS synchronization.
Without dependence on GPS, the system could potentially be deployed in locations
where GPS is not readily available, such as in a basement of a building or in temporary
locations. W-CDMA accomplishes this asynchronous cell site operation through the
use of several techniques.
First, the scrambling codes in W-CDMA are Gold codes, so precise cell site time synchronization
is not required. There are, however, 512 unique Gold codes allocated
for cell site separation that the UE must search through. To facilitate this task, the
SSC in the S-SCH channel is used to instruct the UE to search through a given set of
64 Gold codes. Each set represents a group of eight scrambling codes (64 x 8 = 512).
The UE then tries each of the eight codes within each code group, in an attempt to
decode the BCH. The ability to recover the BCH information (system frame number)
completes the synchronization process.
James R. Cutler
james.cutler@consultant.com
Even if there actually was an "NTP ripple", a properly designed NTP solution should rely on at least three geographically diverse sources. Given the ubiquity of the internet this is not difficult to achieve, barring extreme circumstances.
James R. Cutler
james.cutler@consultant.com
Michael Holstein <michael.holstein@csuohio.edu> writes:
I'd be curious to see what effects (if any) those who use
GPS-disciplined NTP references in Southeastern Georgia see from this
experiment.
Aren't CDMA BTS clocked off GPS?
NTP isn't going to be the only "ripple".
Sure, and there are GPS-steered Rb clocks in telco-land too as well as
a ton of stuff I don't know about yet until everyone else here chimes
in; it's just that NTP is highly visible to NANOGers.
-r
Gary Buhrmaster <gary.buhrmaster@gmail.com> writes:
NTP isn't going to be the only "ripple".
Most of the "brand name" GPS NTP solutions have a clock
with is more than stable enough to survive without GPS
lock for 45 minutes(*). Some of the more expensive units with
temperature controlled oscillators have hold times in the
many weeks. My guess is that the NTP ripples will be
limited to those NTP servers just (or recently) booted
which have not yet achieved a stable clock state.
Gary
(*) This presumes that this test results in loss of signal
lock, and not intentionally injected false information.
Seeing the ripples also presumes that people are monitoring their NTP
system health more closely than just looking at the output of ntpq -p.
-r
What about Modular DOCSIS 3.0 deployments with external timing sources
between the QAM and CMTS
Michael Holstein <michael.holstein@csuohio.edu> writes:
I'd be curious to see what effects (if any) those who use
GPS-disciplined NTP references in Southeastern Georgia see from this
experiment.
Aren't CDMA BTS clocked off GPS?
NTP isn't going to be the only "ripple".
Sure, and there are GPS-steered Rb clocks in telco-land too as well as
a ton of stuff I don't know about yet until everyone else here chimes
in; it's just that NTP is highly visible to NANOGers.
if your high quality stratum one time source isn't capable of
free-running for a little while then it's not really high quality...
you can of course test this simply by disconnecting the antenna. if the
dilution of precision gets sufficiently high or the boise floor climbs
above the signal then it should fail the gps out of the mix. our
symerticoms have upgraded ocxo and backup geographically distant ntp
sources in the pool to account for localized gps failure...
I'm way to cheap to spring for the rubidum upgrade, the ocxo holdover is
supposed to be 1ms a day.
Loss of lock is a non-issue.
However, the tests they are doing do not necessarily cause loss
of lock. Sometimes, instead, they give you a wrong enough time
to insure that your navigational fix is off by, well, enough to
guarantee that you don't hit what you're aiming at.
Owen
Joel Jaeggli <joelja@bogus.com> writes:
Sure, and there are GPS-steered Rb clocks in telco-land too as well as
a ton of stuff I don't know about yet until everyone else here chimes
in; it's just that NTP is highly visible to NANOGers.
if your high quality stratum one time source isn't capable of
free-running for a little while then it's not really high quality...
<cough> no comment. 
you can of course test this simply by disconnecting the antenna. if the
dilution of precision gets sufficiently high or the boise floor climbs
above the signal then it should fail the gps out of the mix. our
symerticoms have upgraded ocxo and backup geographically distant ntp
sources in the pool to account for localized gps failure...
I'm way to cheap to spring for the rubidum upgrade, the ocxo holdover is
supposed to be 1ms a day.
Actually, if you are rolling your own (admittedly not what you're
doing when you buy Symmetricoms, but then again they are maintainable
in the field as opposed to the mad scientist stuff we have in our
basements), surplus Rb clocks are astonishingly cheap. I have seen
used Datum LPROs (admittedly they are cheapies with higher Allan
deviation over short intervals than the PRS10) for circa $100.
-r