4 ms·

Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or

Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?

jankeymeulen 5y ago
Yes. For professional setups, typically you'd have a coax going from the roof somewhere to the rack inside the data center where your time server is located.
cybrexalpha 5y ago
I guess that's the point of the highly accurate onboard clock. Even if you don't have GPS reception for some period of the day, as long as it does connect at least once every two days you'll still get accurate timekeeping.
detaro 5y ago
Yes, you need to connect an antenna that usually then gets mounted somewhere outside.
dkarp 5y ago
For non-professional setups, if you have a decent GNSS module/antenna then near a window works fine. I built a clock with a GNSS module and it gets enough of a signal by the window to keep time fine but I wouldn't rely on it.
- magicalhippo 5y ago
  Same. Mine is in a basement, and didn't get any reception with the included tiny antenna. Bought one of those active antennas shaped like a small puck, and place it near the ground-level window. Worked like a charm.
hacker_newz 5y ago
Most datacenters have the GPS antennas on the roof for this reason.
traceroute66 5y ago
> is it difficult to get reception? Do you have to put the receiver near a window or something? The answer to that is "it depends". Windows are known to work, but not all windows are made equal. Which direction your window is facing and whether you are in an urban canyon can make significant difference. Some windows also have films or coatings. If you are working in tricky situations, then the quality of your antenna and the quality of your receiver will make a difference (e.g. multi-constellation support, interference mitigation, multipath rejection etc. etc.). If you do not have access to a workable window, then your alternative options are: - Accept slightly lower accuracy and use LW radio instead (e.g. MSF, DCF etc.) - Rent a leased line to your country's national time laboratory. They will deliver you certified, traceable time with no roof access required and microsecond accuracy (backed by an SLA, i.e. accurate or your money back). - Run a long cable to the roof (if money is no object you can even get super fancy setups that run exclusively on fibre, no coax in sight) - Install an internal GNSS repeater system (still involves roof install, but gives you *much* more flexibility indoors for obvious reasons).
- throw0101a 5y ago
  > […] and whether you are in an urban canyon can make significant difference. AIUI, urban canyons was one of the reasons why Japan built their QZSS system: * https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System * https://gssc.esa.int/navipedia/index.php/QZSS_Signal_Plan https://gssc.esa.int/navipedia/index.php/QZSS_Signal_Plan
- hansor 5y ago
  Or you could just have your own atomic clock. It is not uncommon, and not complicated as you might think[1]. Other methods for obtaining times are: - GPS: Satellite receiver for the Global Positioning System - GNS: Combined GPS/GLONASS/Galileo/BeiDou satellite receiver (L1 frequency band), can also be used for mobile applications - GNS-UC: GPS and Galileo Satellite Receiver with Up Converter for Meinberg GPS Antenna/Converter - PZF: DCF77 correlation receiver for middle europe - MSF: Long wave receiver for Great Britain - WWVB: Long wave receiver for the US time signal - TCR: Time code receiver for IRIG A/B, AFNOR or IEEE1344 codes - MRS: (GPS, PPS, 10MHz, NTP): Multi Reference Source - several reference source [1]: https://www.muquans.com/ https://www.muquans.com/
dfox 5y ago
One alternative would be to place the whole GNSS receiver outside and only run serial port and PPS signal from it (cat5 has just the right amount of pairs for RS-422 UART, RS-422 PPS and power. And is significantly cheaper than coax that is good enough for GNSS).
- deleted 5y ago
  [deleted]
myself248 5y ago
It needs at least the same signal your GPS watch needs. The better signal you can get it, the better time solution it'll get, and the better holdover timing it'll keep. Here's why: Imprecision in GPS (GNSS in general) solutions comes from a number of sources: Orbital errors in the satellite ephemerides, atmospheric distortions to the signal, multipath at the receiver antenna, and a bunch of minor stuff we're not gonna worry about. Those are the big three. Orbital errors, you can't do anything about. Post-processing allows _position_ solutions to be corrected once the errors are known (a process that takes hours or days after the fact, to work back where the satellites must've actually been) in a process called Precise Point Solution, but that doesn't apply to [this kind of] timing. Atmospheric errors you can kindof do something about. Signals from satellites lower to the horizon take a longer path through the atmosphere, so they tend to have wackier pseudoranges. Most timing-grade receivers have a default "elevation mask" of 15 or 20 degrees above the horizon, to simply not even bother with these crap signals, and derive timing only from what's left. (There are of course atmospheric models and corrections to apply, but these are applied to everything. Still, start with a cleaner signal and you'll end up with a cleaner result.) This means you need clear sky view to the middle of the sky, in order to raise the chances of an adequate number of satellites passing the elevation mask. Multipath is huge. The reason survey-grade antennas have these enormous choke-ring structures around them is to eliminate multipath reflections off the surface of the ground below. Even cheap little antennas try to be more sensitive to signals from above than below, to limited success. (Especially if they're wrist-worn and thus not orientation-controlled. Ugh.) Any situation where the antenna is subject to multipath will have demonstrably worse precision as a result. So, most timing antennae are considerably elevated (gives the reflections an opportunity to fall apart), shielded (choke-ring mounts), or both. Cellular sites usually go with the elevation option since there's already a giant lightning-rod nearby, elevating the GPS antenna doesn't come with additional risk for the receiver. Worse GPS reception means the receiver has a noisier signal to start with, and can't discipline its local oscillator as tightly. Even if you don't care about nanoseconds in the final application, you probably do care about _holdover performance_. And if your LO can be disciplined with nanosecond-accurate signals in the first place, it'll hold millisecond-accurate timing for days or weeks in a GPS outage. If the disciplining process is crap, the holdover will also be crap.
- 5y ago

jankeymeulen 5y ago

Yes. For professional setups, typically you'd have a coax going from the roof somewhere to the rack inside the data center where your time server is located.

cybrexalpha 5y ago

I guess that's the point of the highly accurate onboard clock. Even if you don't have GPS reception for some period of the day, as long as it does connect at least once every two days you'll still get accurate timekeeping.

detaro 5y ago

Yes, you need to connect an antenna that usually then gets mounted somewhere outside.

dkarp 5y ago

For non-professional setups, if you have a decent GNSS module/antenna then near a window works fine. I built a clock with a GNSS module and it gets enough of a signal by the window to keep time fine but I wouldn't rely on it.

magicalhippo 5y ago
Same. Mine is in a basement, and didn't get any reception with the included tiny antenna. Bought one of those active antennas shaped like a small puck, and place it near the ground-level window. Worked like a charm.

hacker_newz 5y ago

Most datacenters have the GPS antennas on the roof for this reason.

traceroute66 5y ago

> is it difficult to get reception? Do you have to put the receiver near a window or something? The answer to that is "it depends". Windows are known to work, but not all windows are made equal. Which direction your window is facing and whether you are in an urban canyon can make significant difference. Some windows also have films or coatings. If you are working in tricky situations, then the quality of your antenna and the quality of your receiver will make a difference (e.g. multi-constellation support, interference mitigation, multipath rejection etc. etc.). If you do not have access to a workable window, then your alternative options are: - Accept slightly lower accuracy and use LW radio instead (e.g. MSF, DCF etc.) - Rent a leased line to your country's national time laboratory. They will deliver you certified, traceable time with no roof access required and microsecond accuracy (backed by an SLA, i.e. accurate or your money back). - Run a long cable to the roof (if money is no object you can even get super fancy setups that run exclusively on fibre, no coax in sight) - Install an internal GNSS repeater system (still involves roof install, but gives you *much* more flexibility indoors for obvious reasons).

throw0101a 5y ago
> […] and whether you are in an urban canyon can make significant difference. AIUI, urban canyons was one of the reasons why Japan built their QZSS system: * https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System * https://gssc.esa.int/navipedia/index.php/QZSS_Signal_Plan https://gssc.esa.int/navipedia/index.php/QZSS_Signal_Plan
hansor 5y ago
Or you could just have your own atomic clock. It is not uncommon, and not complicated as you might think[1]. Other methods for obtaining times are: - GPS: Satellite receiver for the Global Positioning System - GNS: Combined GPS/GLONASS/Galileo/BeiDou satellite receiver (L1 frequency band), can also be used for mobile applications - GNS-UC: GPS and Galileo Satellite Receiver with Up Converter for Meinberg GPS Antenna/Converter - PZF: DCF77 correlation receiver for middle europe - MSF: Long wave receiver for Great Britain - WWVB: Long wave receiver for the US time signal - TCR: Time code receiver for IRIG A/B, AFNOR or IEEE1344 codes - MRS: (GPS, PPS, 10MHz, NTP): Multi Reference Source - several reference source [1]: https://www.muquans.com/ https://www.muquans.com/

dfox 5y ago

One alternative would be to place the whole GNSS receiver outside and only run serial port and PPS signal from it (cat5 has just the right amount of pairs for RS-422 UART, RS-422 PPS and power. And is significantly cheaper than coax that is good enough for GNSS).

deleted 5y ago
[deleted]

myself248 5y ago

It needs at least the same signal your GPS watch needs. The better signal you can get it, the better time solution it'll get, and the better holdover timing it'll keep. Here's why: Imprecision in GPS (GNSS in general) solutions comes from a number of sources: Orbital errors in the satellite ephemerides, atmospheric distortions to the signal, multipath at the receiver antenna, and a bunch of minor stuff we're not gonna worry about. Those are the big three. Orbital errors, you can't do anything about. Post-processing allows _position_ solutions to be corrected once the errors are known (a process that takes hours or days after the fact, to work back where the satellites must've actually been) in a process called Precise Point Solution, but that doesn't apply to [this kind of] timing. Atmospheric errors you can kindof do something about. Signals from satellites lower to the horizon take a longer path through the atmosphere, so they tend to have wackier pseudoranges. Most timing-grade receivers have a default "elevation mask" of 15 or 20 degrees above the horizon, to simply not even bother with these crap signals, and derive timing only from what's left. (There are of course atmospheric models and corrections to apply, but these are applied to everything. Still, start with a cleaner signal and you'll end up with a cleaner result.) This means you need clear sky view to the middle of the sky, in order to raise the chances of an adequate number of satellites passing the elevation mask. Multipath is huge. The reason survey-grade antennas have these enormous choke-ring structures around them is to eliminate multipath reflections off the surface of the ground below. Even cheap little antennas try to be more sensitive to signals from above than below, to limited success. (Especially if they're wrist-worn and thus not orientation-controlled. Ugh.) Any situation where the antenna is subject to multipath will have demonstrably worse precision as a result. So, most timing antennae are considerably elevated (gives the reflections an opportunity to fall apart), shielded (choke-ring mounts), or both. Cellular sites usually go with the elevation option since there's already a giant lightning-rod nearby, elevating the GPS antenna doesn't come with additional risk for the receiver. Worse GPS reception means the receiver has a noisier signal to start with, and can't discipline its local oscillator as tightly. Even if you don't care about nanoseconds in the final application, you probably do care about _holdover performance_. And if your LO can be disciplined with nanosecond-accurate signals in the first place, it'll hold millisecond-accurate timing for days or weeks in a GPS outage. If the disciplining process is crap, the holdover will also be crap.

5y ago