anyone running GPS clocks in Southeastern Georgia?
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... -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 On Fri, Jan 21, 2011 at 12:31 PM, Robert E. Seastrom <rs@seastrom.com>wrote:
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...
-r
On Fri, Jan 21, 2011 at 12:35:32PM -0500, Jack Carrozzo wrote:
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.
Nahh, that was the western WAAS sat, IIRC. This is...Something Else Entirely. --msa
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 -----Original Message----- From: Jack Carrozzo [mailto:jack@crepinc.com] Sent: Friday, January 21, 2011 10:40 AM To: Majdi S. Abbas Cc: Robert E. Seastrom; nanog@nanog.org Subject: Re: anyone running GPS clocks in Southeastern Georgia? On Fri, Jan 21, 2011 at 12:36 PM, Majdi S. Abbas <msa@latt.net> wrote:
Nahh, that was the western WAAS sat, IIRC.
This is...Something Else Entirely.
Ahh, my mistake. Sitting in the back now, -Jack Carrozzo
"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
On Fri, 21 Jan 2011, Robert E. Seastrom wrote:
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...
My first thought was testing UAVs and what they do in situations where GPS is jammed, blocked or provides false information. Doing so in an area where a total loss of control of the aircraft would result in a drop in the ocean rather than in or around a populated area is a good idea. Maybe there are already unit tests for such situations. --------------------------------------------------------------------------- Peter Beckman Internet Guy beckman@angryox.com http://www.angryox.com/ ---------------------------------------------------------------------------
On 1/21/2011 9:31 AM, Robert E. Seastrom wrote:
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)
I'm not sure how you'd get increasing radius with altitude from anything but a jammer near sea level. Matthew Kaufman
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Yo All! On Fri, 21 Jan 2011, Matthew Kaufman wrote:
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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) iD8DBQFNOc/9BmnRqz71OvMRAltbAKDW62I7B4Heg1KLAbZaYiShhro5vQCgu5fM WUWTwQhT5tTajslbe3GtuXY= =Czhk -----END PGP SIGNATURE-----
On Jan 21, 2011, at 10:27 AM, Gary E. Miller wrote:
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
Yo All!
On Fri, 21 Jan 2011, Matthew Kaufman wrote:
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.
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)
On Fri, 21 Jan 2011, Gary E. Miller wrote:
For non pilots, RAIM is an indicator that the GPS has a redundant solution that matches the barometrically measured altitude.
I know this is off topic, but I don't like to let incorrect information float around uncorrected. RAIM never uses any data outside of GPS to confirm position, it is based entirely on more than the minimum number of satellites needed for a basic position to calculate redundant solutions, which means a minimum of 5 satellites. If this were not the case, it would be impossible to get a RAIM "prediction" (using data about out of service sats) in advance of a flight. -- Brandon Ross AIM: BrandonNRoss ICQ: 2269442 Skype: brandonross Yahoo: BrandonNRoss
On Friday, January 21, 2011 04:23:52 pm Michael Holstein wrote:
Aren't CDMA BTS clocked off GPS? 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). Boo. You can't find the 3816 much anymore and the 3801 isn't as good (fine for most ntp purposes,though) (the difference is mostly in internal measurement software and how long it will hold without the gps signal). And Symmetricom bought that line from HP, still sells one comparable to
On 01/21/2011 04:29 PM, Lamar Owen wrote: the Z3801 but not like the 3816 for a decent price. Personally I'd build one up out of an LPRO, a Trimble timing receiver (current replacement for the Oncore used in the Z38xx units, last I checked it was under $100), a MSP430 (probably a fairly high-end one to get enough program space for a good PLL) and some external logic for phase comparators (I don't know if the timer capture modes in the 430 are good enough by themselves...) The most expensive single part would be a decent timing antenna (yes, timing antennas *are* different from the usual civilian positioning antennas; there is a reason why the base is larger diameter than the rest...) Actually, does anyone still do soft handoff with UMTS? That was much of the reason why old CDMA needed a GPS reference. -- Pete
On Jan 21, 2011 6:49 PM, "Pete Carah" <pete@altadena.net> wrote:
On 01/21/2011 04:29 PM, Lamar Owen wrote:
On Friday, January 21, 2011 04:23:52 pm Michael Holstein wrote:
Aren't CDMA BTS clocked off GPS? 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).
Boo. You can't find the 3816 much anymore and the 3801 isn't as good (fine for most ntp purposes,though) (the difference is mostly in internal measurement software and how long it will hold without the gps signal). And Symmetricom bought that line from HP, still sells one comparable to the Z3801 but not like the 3816 for a decent price. Personally I'd build one up out of an LPRO, a Trimble timing receiver (current replacement for the Oncore used in the Z38xx units, last I checked it was under $100), a MSP430 (probably a fairly high-end one to get enough program space for a good PLL) and some external logic for phase comparators (I don't know if the timer capture modes in the 430 are good enough by themselves...) The most expensive single part would be a decent timing antenna (yes, timing antennas *are* different from the usual civilian positioning antennas; there is a reason why the base is larger diameter than the rest...)
Actually, does anyone still do soft handoff with UMTS? That was much of
Yes. Providing accurate clock to a cell site is critical for 2/3/4g. This usually requires a primary (GPS) and backup (1588). Cb
the reason why old CDMA needed a GPS reference.
-- Pete
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.
On Jan 21, 2011, at 4:45 PM, Gary Buhrmaster wrote:
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.
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
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
On Jan 21, 2011, at 1:45 PM, Gary Buhrmaster wrote:
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.
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
----- Original Message -----
From: "Gary Buhrmaster" <gary.buhrmaster@gmail.com>
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.
Do such clocks reduce their advertised stratum when doing so? Or are they always considered "GPS-steered", and therefore there's no meaningful change short-term? -- jra
On Jan 21, 2011, at 4:23 PM, Michael Holstein wrote:
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
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
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
On Fri, Jan 21, 2011 at 5:26 PM, Robert E. Seastrom <rs@seastrom.com> wrote:
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.
What about Modular DOCSIS 3.0 deployments with external timing sources between the QAM and CMTS
What about Modular DOCSIS 3.0 deployments with external timing sources between the QAM and CMTS
A CMTS DS payload is formatted as an MPEG TS (it even has PIDs; however, no PCR). This in turn establishes cadence for associated downstream devices (eg. they sync to whatever is within allowable tolerances). Any digital 'baseband' output from a so-called 'modular' CMTS (one with external QAM modulation) should likewise serve as a recoverable timing source for the modulator. Inter-DS cadence is allowably async; the only important coupling here are these associated US channels available for a given DS. Unless I've missed something horribly obvious, I can't see where strict timing is important, nor can I see where inter or intra-system synchronization would be critical either, save for corner-cases (i.e. US or DS dynamic channel change, DCC/UDC, etc). Even in these cases, the modem is more than able to rapidly resynchronize with a new/differing cadence in both frequency and phase domains. I guess, in short, don't worry (about this). -Tk
On 1/21/11 2:26 PM, Robert E. Seastrom wrote:
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.
-r
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. :-P
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
On 1/21/11 2:26 PM, Robert E. Seastrom wrote:
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.
The recomndation for a UTC timescales is to be within less than 1us of UTC at any given time. -P
Sex On Jan 21, 2011 12:32 PM, "Robert E. Seastrom" <rs@seastrom.com> wrote:
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...
-r
participants (21)
-
Anton Kapela -
Brandon Ross -
Cameron Byrne -
Cutler James R -
Day Domes -
Gary Buhrmaster -
Gary E. Miller -
Jack Carrozzo -
James Brown -
Jay Ashworth -
Joel Jaeggli -
Lamar Owen -
Majdi S. Abbas -
Matlock, Kenneth L -
Matthew Kaufman -
Michael Holstein -
Owen DeLong -
Pete Carah -
Peter Beckman -
Peter Lothberg -
Robert E. Seastrom