[time-nuts] GPS Modules Indoors
Greetings I know that the accuracy of GPS degrades indoors. However, suppose that I just want to turn a GPS module on, acquire the current time accurate to a second, and then turn it off. I can get a good deal on the U-Blox LEA-5H modules (same as used on Arduino shields I think), which have a high sensitivity, and I can use an active antenna if needed. Am I wasting my time. Sorry for not requiring the time more accurate than a second, but that's all the clients require. -- Tom Harris celephi...@gmail.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPS Modules Indoors
Greetings I know that the accuracy of GPS degrades indoors. However, suppose that I just want to turn a GPS module on, acquire the current time accurate to a second, and then turn it off. I can get a good deal on the U-Blox LEA-5H modules (same as used on Arduino shields I think), which have a high sensitivity, and I can use an active antenna if needed. Am I wasting my time. Sorry for not requiring the time more accurate than a second, but that's all the clients require. Tom Harris = Tom, If the GPS has a battery backup, then you might get re-acquisition within less than 30 seconds of powering up. That's approximate with a u-blox NEO-6M on the top floor of a two-storey building, about 1/3 the way into the room, so most of the signal is through the roof. Without battery backup, the acquisition time might be a lot longer (15-20 minutes), or even worse (hours) with a different GPS module which needs a much stronger signal to acquire than to track. The one which took hours had a 15 mm square patch antenna (AdaFruit), versus the 25 mm square antenna on the u-blox board I had. The two units I am comparing are shown on my Web page here: http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html although it's down right now. I quite accpet that other folks may have different results (or if indoors is next to a window!). Cheers, David -- SatSignal Software - Quality software written to your requirements Web: http://www.satsignal.eu Email: david-tay...@blueyonder.co.uk ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPS Modules Indoors
I don't think GPS is ever so in accurate as to have a 1 second error. If it has a solution at all it is going to be at least within a millisecond. The way everyone handles holdover (Holdover is timekeeping with GPS is not available) is to keep a local clock driven off a stable oscillator. Then when you do have GPS you adjust the clock. Then when GPS goes away you free wheel. With your very loose one second requirements you can use a rather inexpensive clock, perhaps just a TCXO. The question is how long of a hold over period must you be able to handle. If GPS will is unavailable for a month then you will need a VERY good clock but if you will only loose GPS for a few hours at most it can be cheaper. Active antenna are not a perfect solution. What the GPS needs is good signal to noise and the amplifier amplifies noise as well as signal. It can help in some cases and is worth doing if you have the space and power budget. So to design this step one is to select an oscillator that has the required stability for one second error durring the worst case holdover period thn build a clock based on that. Next design a way to re-set the clock based on GPS. On Tue, Nov 20, 2012 at 3:33 AM, Tom Harris celephi...@gmail.com wrote: Greetings I know that the accuracy of GPS degrades indoors. However, suppose that I just want to turn a GPS module on, acquire the current time accurate to a second, and then turn it off. I can get a good deal on the U-Blox LEA-5H modules (same as used on Arduino shields I think), which have a high sensitivity, and I can use an active antenna if needed. Am I wasting my time. Sorry for not requiring the time more accurate than a second, but that's all the clients require. -- Tom Harris celephi...@gmail.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPS Modules Indoors
celephi...@gmail.com said: I know that the accuracy of GPS degrades indoors. However, suppose that I just want to turn a GPS module on, acquire the current time accurate to a second, and then turn it off. I can get a good deal on the U-Blox LEA-5H modules (same as used on Arduino shields I think), which have a high sensitivity, and I can use an active antenna if needed. You haven't defined indoors. I think you will have to try it in your location(s). It won't work deep inside a machine room. We tried it once in a typical two-story silicon valley office building. It didn't work there. We also tried it in the 5th floor of a modern office building with a great view. It didn't work, probably because of some anti-heat coating on the windows. Inside my single floor house, they work most of the time. albertson.ch...@gmail.com said: I don't think GPS is ever so in accurate as to have a 1 second error. If it has a solution at all it is going to be at least within a millisecond. I've seen occasional way-off solutions from GPS modules. I don't have examples handy but I wouldn't depend on a single/quick answer in any critical application. -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPS Modules Indoors
Hi Ten seconds a month is wrist watch accuracy at constant temperature. For $3 Maxim will sell you a +/- 2 ppm real time clock. That will hold your 1 second for 500,000 sec (5.787 days). A cheap cell phone style TCXO at $1 or so should hold 1 ppm over a modest range. That will get you out to 10 days. A lot depends on how stable and repeatable your environment is. Most indoor areas cycle up and down, but they repeat the cycle. What happens this week is a lot like what happened a week ago. If you can take a time reading every few days, you likely can do some smarts to extend the holdover quite a bit. With a modern low power CPU, the smarts will pull a lot less current than your real time clock or your TCXO. Yup, this is all basement engineering. If somebody's life depends on it, maybe not such a good idea. Bob On Nov 20, 2012, at 10:46 AM, Chris Albertson albertson.ch...@gmail.com wrote: I don't think GPS is ever so in accurate as to have a 1 second error. If it has a solution at all it is going to be at least within a millisecond. The way everyone handles holdover (Holdover is timekeeping with GPS is not available) is to keep a local clock driven off a stable oscillator. Then when you do have GPS you adjust the clock. Then when GPS goes away you free wheel. With your very loose one second requirements you can use a rather inexpensive clock, perhaps just a TCXO. The question is how long of a hold over period must you be able to handle. If GPS will is unavailable for a month then you will need a VERY good clock but if you will only loose GPS for a few hours at most it can be cheaper. Active antenna are not a perfect solution. What the GPS needs is good signal to noise and the amplifier amplifies noise as well as signal. It can help in some cases and is worth doing if you have the space and power budget. So to design this step one is to select an oscillator that has the required stability for one second error durring the worst case holdover period thn build a clock based on that. Next design a way to re-set the clock based on GPS. On Tue, Nov 20, 2012 at 3:33 AM, Tom Harris celephi...@gmail.com wrote: Greetings I know that the accuracy of GPS degrades indoors. However, suppose that I just want to turn a GPS module on, acquire the current time accurate to a second, and then turn it off. I can get a good deal on the U-Blox LEA-5H modules (same as used on Arduino shields I think), which have a high sensitivity, and I can use an active antenna if needed. Am I wasting my time. Sorry for not requiring the time more accurate than a second, but that's all the clients require. -- Tom Harris celephi...@gmail.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPS Modules Indoors
Hi If indoors always includes a view through unshielded windows to towards the equator, then you might be ok. If it includes pole only facing offices in a conventional steel structure - not so much. The issue isn't so much time, as ability to get a good enough signal to lock to. If you add a unknown location (you don't have a survey) and an unknown almanac (you have been off for a while), that just makes things harder. Consider that while I can get cell phone coverage anywhere in my home, there are buildings in the area that I can't get coverage in. Around here, (as in most urban areas), if I can't get a cell phone signal, there's no chance at all of getting GPS. Provided you have a valid almanac, a rational location fix (as in a survey), and are tracking multiple satellites, I can't think of a case where you would have a significant time error. That may just be my lack of imagination. For one second accuracy, a time tick over a cell phone should be fine. That would be far more robust than a GPS setup. Bob On Nov 20, 2012, at 6:33 AM, Tom Harris celephi...@gmail.com wrote: Greetings I know that the accuracy of GPS degrades indoors. However, suppose that I just want to turn a GPS module on, acquire the current time accurate to a second, and then turn it off. I can get a good deal on the U-Blox LEA-5H modules (same as used on Arduino shields I think), which have a high sensitivity, and I can use an active antenna if needed. Am I wasting my time. Sorry for not requiring the time more accurate than a second, but that's all the clients require. -- Tom Harris celephi...@gmail.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
