I'm wanting to accurately time-stamp some outdoor events. This is a stationary outdoor situation so I thought a GPS module might be a good approach for accurate time. If I could find a GPS module with a clock output rather than a 1PPS output maybe I wouldn't need to consider an additional disciplined oscillator module -- since I don't expect to lose the GPS signal lock. Any low-cost suggestions?
Thanks
Search for a GTPA010 module. Gives out (obviously) your location, and time (in UTC) at up to 10Hz rate. You should be able to find it for
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Well, it isn't the serial update rate that is the issue. It is synchronizing a counter-timer or RTC with the GPS. I would like to achieve something approaching 10 millisecond accuracy. Thanks.
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Should be fairly easy with a GPS-disciplined oscillator. If you clock your microcontroller from its 10 MHz output and use the same controller to monitor the 1-pps output, you can timestamp events to within +/- 100 ns or so.
For a one-off project you can get a surplus Trimble Thunderbolt GPSDO on eBay for a couple hundred bucks, while for anything commercial the Jackson Labs modules would be ideal.
-- john, KE5FX
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Well, for a cheaper solution what if I considered an OCXO module driven by a microcontroller DAC pin? The microcontroller runs on the OCXO and captures the 1PPS from the GPS? Would that be practical?
One thing to look out for: the GPS module will have a local clock in the few tens of MHz range. It usually has to put the 1PPS edge on one of its local clock edges, instead of exactly at the start of the second. For 10 millisec resolution, you probably don't care, but for higher precision, you have to take this into account. The GPS receivers that are optimized for timing can tell you (via the serial port) the offset between the most recent 1PPS pulse and when the second actually started, so you can apply a correction.
Trimble makes some GPS modules optimized for timing applications; the Resolution T and Resolution SMT are a couple of examples. (Trimble also did the genius move of depending on some unspecified bits in the GPS data frames to have certain values, causing most Trimble GPS receivers worldwide to glitch out when the Air Force uploaded new software to the satellites a couple of years ago. Guess who got the single-source emergency contract to fix it...)
Some newer GPS modules can give you position updates at a 5 Hz rate, instead of the 1 Hz that is currently standard. I don't know if these also have a 5PPS or other faster "clock" output available.
You will. Maybe for a second or two once a day, but you will. If your application can tolerate this, OK, but if it can't, you need some kind of local clock slaved to the GPS that will let you "ride through" brief outages.
For a good time, call (303) 494-4774, assuming you have a phone line and a modem available.
In theory, you should be able to get a 1PPS signal from a WWV receiver, but I don't know if anyone makes these as an easy-to-use module like a GPS module.
Also in theory, you should be able to get a reasonably accurate time from the cell-phone networks. I know you can get GSM modem modules but I don't know if they have an easy way to report what the network thinks the time is.
What other computing resources do you have? Can you do NTP over wi-fi to either a public or local dedicated NTP server? The public server
*might* allow you to hit 10 ms accuracy; a local dedicated NTP server, maybe with a local GPS input, should work better.This probably doesn't qualify as "low cost", but you could do all of the above on board. You need a GPS receiver, a Linux kernel, and an NTP server on something like an Atom single-board computer can probably give you this level of accuracy. (Basically, NTP listens to the 1PPS and turns the system clock into a kind of a GPSDO.)
Standard disclaimers apply; I don't get money or other consideration from any companies mentioned.
Matt Roberds
It is far less work to just buy a used GPSDO. Look into Lady Heather software.
Trutime, Symetricom, etc. They are all over ebay. I bought a couple at the flea market as NOS.
These GPSDO units like high gain active antennas. Not that hard to find used. You need about 30dB gain. Some marine antennas do this, and of course there are Trimble, Sytemtricom, etc. antennas on ebay.
I have some NTP data. I have peak time offsets of about 50ms. Most the time the error is below 15ms. I think 10ms will be hard to do with NTP.
This is sample of one, and I suppose it is a matter of the stability of your RTC.
I may have said you could do 5ms in some older post, but looking at the charts, it was more like 5PPM frequency error, not absolute time error. [My bad.]
NTP monitoring software is available for free here:
Sure. It wouldn't need to be anything fancy, or even an OCXO at all, to achieve 10-millisecond timing.
As Matt R. says, you do need to plan for "holdover" timing in the event of temporary signal loss. Holdover performance is mandated by telecom standards, so it's a solved (or at least specified) problem if you go with a commercial GPSDO.
-- john, KE5FX
On a sunny day (Sat, 30 Mar 2013 00:55:19 +0000 (UTC)) it happened snipped-for-privacy@att.net wrote in :
I have my EM-411 GPS module working on Raspberry Pi. The module was something like 25 $, and the Pi maybe 35? It runs linux and you can do NTP too if yo uwre connected to that thing called 'internet'.
The EM-411 datasheet says: General Chipset SiRF Star Frequency L1, 1575.42 MHz C/A code 1.023 MHz chip rate Channels 20 channel all-in-view tracking Sensitivity -159 dBm Accuracy Position 10 meters, 2D RMS 5 meters, 2D RMS, WAAS enabled Velocity 0.1 m/s Time 1us synchronized to GPS time
The 10ms isn't any high accuracy. Just set your MCU timer at every PPS pulse. Any GPS module would do.
Vladimir Vassilevsky DSP and Mixed Signal Designs
time.
A time stamp in NMEA string corresponds to the time instant at PPS pulse directly preceding this string.
There isn't.
Vladimir Vassilevsky DSP and Mixed Signal Designs
On a sunny day (Sat, 30 Mar 2013 11:33:32 -0500) it happened Vladimir Vassilevsky wrote in :
time.
Ok thanks, that answers my questiion.
Oh yes, the parsing time remains. Interrupt latency for example. But it could be a constant, and calculated out.
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If the uP has a free-running counter, you can latch a copy of the counter at each 1 PPS rising edge, and nab other latched copies at each external event. That timestamps everything happening, to the granuarity of the counter. Do that with software or some counter capture registers. If all you need is 10 ms accuracy, the uP clock only needs to be 1% accurate... no OCXO needed.
It would be fun to servo a uP clock cal factor based on sequential timestamps of the 1 PPS from GPS. That can even be done after the fact, by analyzing the captured time stamps later. That will get you close to 1 LSB accuracy of whatever frequency your timestamp counter runs at... 10s of ns, potentially. [1]
Dimensionless measurements (in this case, time ratios) often turn out to be easy.
[1] using a crystal to clock the uP, of cource. The uPs that have internal clocks will have a lot of jitter and frequency wobble, not to mention terrible TCs. Still, 10 ms would be easy.-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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[1]Our systems have synching networks of 1pps driven from just such a purpose specific 1U gps device and 10Mhz. That gets distributed throughout all the racks involved in the gateway, or other such system where event timestamps are part of the SOP.
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[1]We sold a bunch of our fiberoptic logic links to the US Naval Observatory, to pump 1 PPS signals from various atomic clocks around for comparison. So I guess the entire GPS system runs through my fiber gadgets.
Fiber time delays are a lot better than electrical cable, numbers like 15 PPM per degree C delay change for standard stuff. Maybe there's some better stuff available.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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[1]terrible
Yes: under 1 PPM.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Fiber modules are $1000 a pop now, and are running at 10Gb/s.
Things are being synched up pretty tight these days. I'd bet they are on some symetricom gear or some such now.
'internet'.
time.
the pulse per second output presumably indicates when the NMEA time is most exact.
it's probably also the start bit on the dollar sign.
-- ?? 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
called 'internet'.
time.
One needs to be careful when using NMEA sentences. The devil, as they say, is in the details. In an RMC sentence, the UTC timestamp is the "time of fix," the time at which the position and other parameters in the sentence are valid, and has no inherent constraints as to when that instant is with regards to any element of the transmission.
NMEA 0183 itself is silent with respect to a 1 PPS signal. Manufacturers often use ZDA to report UTC time as of the leading edge of the pulse (if a pulse is even provided), but are not required to do so. There is also no guarantee (in 0183) that the 1PPS pulse occurs at the UTC rollover. That may be the case but, again, it's up to the manufacturer to so state.
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