Re: [time-nuts] Mini-time lab cost and maintenance
Adam: It depends. What are you trying to do? How much accuracy is needed? Do you mean just time, or some other metrics, too? --- Graham == On Mon, Apr 6, 2015 at 11:30 AM, Adam Blakney akblak...@gmail.com wrote: I was wondering how expensive it would be to have even a small and lower level time lab. What are some less expensive models of machinery i would need, and how much maintenance is required? Thanks, Adam ___ 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] Need advice for multilateration setup
Attila, On 04/06/2015 11:14 AM, Attila Kinali wrote: Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick roc...@watzlavick.com wrote: On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. I think that the Kaplan GPS book is better than the MisraEnge [2] in many regards. It is better at explaining the workings of a GPS receiver. [3] helps to cover some of the weaknesses of the Kaplan book. However, the MisraEnge is better at some of the more advanced topics and more thorough on details than Kaplan. So, Kaplan is better at teach how to build a normal GPS receiver, and the MisraEnge is better at teaching how to build one with advanced features. The combination kills. The Bore et. al helps to cover some details about getting that initial guess. Implementing FFT based cross-correlation phase-guessing was trivial after reading that and another book. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. I would recommend going with Kaplan first, to get the first overview. Then, as the refreshment coarse do the MisraEnge. To head off a bunch of replies - I think I stumbled upon what is being suggested. To extract the pseudorange, you have to figure out the offset of the locally generated PN code against the one that is received. In this reverse GPS case, I assume each ground station would have to start their local PN codes at the same time? Then you would be able to get the pseudoranges at each ground station and use those values for the multilateration equations. You still would have an uncertainty of one clock cycle since the phases of the local clocks at the stations wouldn't be aligned but several folks have suggested ways around that. There are multiple things here: * PRN generation: The locally generated PRN has to be time synchronous with the one received from the rocket transmitter. If you are more than one clock period off, you will only get noise out of the demodulator. What you measure is the time difference of the locally generated PRN to your ground station system time. You might want to consider the more advanced variants of loop filters as shown in Kaplan. Works great with simple dimensioning formulas to aid the setup. * Uncertainty: The autocorrelation function of a PRN sequence has a quite steep peak at \tau=0 with width of the clock period. Yes, this does mean that you get a one clock period uncertainty, if you do a hit/miss correlation. But as the correlation function is actually triangle shaped, you can get quite a bit better than that. The limit is afaik around your sampling clock period for naive approaches, which you can further improve with some statistics (you have multiple edges to work with, ie can average over those). * Synchronisation of ground stations: There are easy and diffuclt ways to do that. Probably the easiest is to use to use an additional transmitter at the launch point on the same frequency, but with a different PRN than the rocket. This way you can do a difference of the two PRN codes in your receiver, which gets away with a lot of nasty
Re: [time-nuts] Mini-time lab cost and maintenance
Hi What kind of gear do you already have? I’d guess a computer. Can it be / is it part of the time lab? What sort of background do you have (is equipment repair something you do)? How about building stuff from scratch? Do you have soldering gear etc? What sort of budget do you have? Is $100,000 “low level” or is $100 more in the right range? How patient are you? Is there a time line associated with this? In many cases there is a “if you shop for 2 years” price and a “you can get it tomorrow price”. There also are automated gizmos that do work for you. Spend less money on gear / spend more of your time doing stuff. What’s your time worth? Is your end objective timing laser pulses / setting up watches / running a telescope? Each end objective has it’s own priorities. The problem is that I have a great set of ideas for exactly what you need. In reality, since I know nothing about what you need, it’s a bunch of ideas for *my* needs. Before we head off in entirely the wrong direction - what are you after? What are your constraints? Bob On Apr 6, 2015, at 12:30 PM, Adam Blakney akblak...@gmail.com wrote: I was wondering how expensive it would be to have even a small and lower level time lab. What are some less expensive models of machinery i would need, and how much maintenance is required? Thanks, Adam ___ 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.
[time-nuts] Modern HW replacement for ATOM based NTP server?
Hi,Years ago this forum helped me put together my first GPSDO and NTP server. Using a then-popular INTEL ATOM board, FreeBSD w/ the NTP kernel, 1PPS input from aTB, works great. But as the years go by, HW improves/evolves and it might be time to recreate this functionin some other modern HW. Looking for a platform not needing a fan. While the ATOM and SSD seem to be OK w/o direct airflow, the Mini ITX Power Supply fan is needed. After three years, the stupid sleeve bearings are beginning to rattlewhich led me to this design review. Recently built a Caching DNS for the home LAN using a Technologic Systems TS-7250-V2, general purpose PC/104 embedded system Runs Debian, 3.14 kernel. Hardware Description:800 MHz CPU (PXA166)512 MB RAMMicro SD card socketRTC with battery and temp compensationTemp Sensor10/100 ethernet port2 USB HS Host ports3x RS-232 serial ports3x TTL serial portsRS-485 portPC/104 connectorUSB device console port (Micro B connector)LCD and DIO Headers75 TTL DIO (including PC/104 conn)5VDC or 8-28VDC power inputA/D converter, 5 channels (with 4-20 mA current loop) A more expensive alternative to the Pi, but reasonable. Wondering if a TS-7250-V2 is worth a try for the NTP function, or if anyone here has already used one of thesefor NTP? Or other non-Pi alternatives? I would very much appreciate some guidance as to the HW/SW combinationthat would best replace the existing NTP server. - 1PPS is available from both a Trimble TB and a Jackson Labs Fury, so I could build a parallel system. Or just put a ball-bearing fan in...73Frank HughesKJ4OLL ___ 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] Need advice for multilateration setup
Hi Attila, On 04/06/2015 11:21 AM, Attila Kinali wrote: On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. You want to keep your chip-rate up to make the integer ambiguity of the carrier phase simple. The carrier frequency divided by chipping rate ratio indicate how difficult problem it is to solve (GPS L1 C/A code has 1540). The 70 cm band has rather narrow allocations. The 23 cm band allow for much wide allocations. The benefit of the 70 cm band is naturally the easy of getting hardware. Another benefit of a higher chipping rate is that it can allow for a higher bandwidth, allowing for tighter tracking of the rocket dynamics. The chipping rate at some code legnth creates the maximum tracking rate, and some fraction of that is the highest bandwidth tolerable. Cheers, Magnus ___ 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] Mini-time lab cost and maintenance
You can do a lot for $200 if you can build electronics yourself. Decent GPS with PPS starts at under $20 on eBay. Same for surplus 10MHz oscillators. People have build usable counters for cheap microprocessor development boards and software.But it all depends on what you want to measure On Mon, Apr 6, 2015 at 9:30 AM, Adam Blakney akblak...@gmail.com wrote: I was wondering how expensive it would be to have even a small and lower level time lab. What are some less expensive models of machinery i would need, and how much maintenance is required? Thanks, Adam ___ 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] Striking change in iPhone time accuracy with 8.2
On Wed, 1 Apr 2015 23:20:19 -0400 David McGaw n1...@dartmouth.edu wrote: There is evidence that the ear can discern differences in timing down to the uS range. See a discussion by David Blackmer of DBX and Earthworks: http://www.earthworksaudio.com/wp-content/uploads/2012/07/The-world-beyond-20kHz.pdf I am not an expert on hearing, by far not. But this text reads a lot like half informed, pseudo scientific gibberish, made up from real facts but distorted heavily to support the point of an audiophool manufacturer. So i would be very carefull what you take out of that text. Attila Kinali -- _av500_ phd is easy _av500_ getting dsl is hard ___ 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] Need advice for multilateration setup
Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick roc...@watzlavick.com wrote: On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. To head off a bunch of replies - I think I stumbled upon what is being suggested. To extract the pseudorange, you have to figure out the offset of the locally generated PN code against the one that is received. In this reverse GPS case, I assume each ground station would have to start their local PN codes at the same time? Then you would be able to get the pseudoranges at each ground station and use those values for the multilateration equations. You still would have an uncertainty of one clock cycle since the phases of the local clocks at the stations wouldn't be aligned but several folks have suggested ways around that. There are multiple things here: * PRN generation: The locally generated PRN has to be time synchronous with the one received from the rocket transmitter. If you are more than one clock period off, you will only get noise out of the demodulator. What you measure is the time difference of the locally generated PRN to your ground station system time. * Uncertainty: The autocorrelation function of a PRN sequence has a quite steep peak at \tau=0 with width of the clock period. Yes, this does mean that you get a one clock period uncertainty, if you do a hit/miss correlation. But as the correlation function is actually triangle shaped, you can get quite a bit better than that. The limit is afaik around your sampling clock period for naive approaches, which you can further improve with some statistics (you have multiple edges to work with, ie can average over those). * Synchronisation of ground stations: There are easy and diffuclt ways to do that. Probably the easiest is to use to use an additional transmitter at the launch point on the same frequency, but with a different PRN than the rocket. This way you can do a difference of the two PRN codes in your receiver, which gets away with a lot of nasty effects that you would need to account for otherwise. Another approach would be to use a GPSDO on each ground station and run all the receivers already synchronized. This also enables you to get the position of all stations very accurately, especially if you let the GPSDO average its position for some time. But for ultimate accuracy, you'd need to calibrate the GPSDO's (including antennas) against each other, to know what the systematic offsets are (ie set them up all together at the same location and measure the time difference of the PPS). Of course, it's possible to use a combination of multiple approaches. Eg a nice one would be to GPSDO's to provide position and a precise frequency reference, but then use a central transmitter for the synchronization. HTH Attila Kinali [1] Spread Spectrum Systems with Commercial Applications, 3rd edition, by Robert C. Dixon, 1994 [2] Global positioning system signals,
Re: [time-nuts] Need advice for multilateration setup
On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. Attila Kinali -- _av500_ phd is easy _av500_ getting dsl is hard ___ 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] Need advice for multilateration setup
On 4/6/15 2:14 AM, Attila Kinali wrote: Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick roc...@watzlavick.com wrote: On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. Actually, if you're post processing, you can just record raw bits and do the correlation in software. You don't really need to track it in real time. Although, that might not be a bad way to do it. ___ 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] Need advice for multilateration setup
On 4/6/15 2:21 AM, Attila Kinali wrote: On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. One strategy for this kind of application is to do the fine measurement using carrier phase, and use the PN code to do ambiguity reduction. Then, a low chip rate is fine: you're basically using it as a check that you haven't slipped a cycle. I would think that the RTL dongles would work just fine, especially if you radiate a pilot tone from a fixed location as well as the tone from the rocket. You basically set up two PLLs in software one to track each tone, and subtract the phase of one from the other for each ground station. Attila Kinali ___ 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] Need advice for multilateration setup
Attila, Thank you very much for the references. I had come across [4] when searching on Kalman filters for GPS aiding of INS measurements. I didn't pay much attention to the GPS chapter at the time but I'll look at it again. I just downloaded [3] and it appears to have a good mix of practical vs. theoretical aspects. I appreciate the help! -Bob On 04/06/2015 04:14 AM, Attila Kinali wrote: The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. Attila Kinali [1] Spread Spectrum Systems with Commercial Applications, 3rd edition, by Robert C. Dixon, 1994 [2] Global positioning system signals, measurements, and performance, 2nd edition, by Partap Misra and Per Enge, 2012. [3] A Software-Defined GPS and Galileo Receiver, by Bore, Akos, Bertelsen, Rinder, Jensen, 2007 [4] Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems, 2nd edition, by Paul D. Groves, 2013 ___ 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] Mini-time lab cost and maintenance
I was wondering how expensive it would be to have even a small and lower level time lab. What are some less expensive models of machinery i would need, and how much maintenance is required? Thanks, Adam ___ 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] Need advice for multilateration setup
Jim, On 04/06/2015 03:13 PM, Jim Lux wrote: On 4/6/15 2:21 AM, Attila Kinali wrote: On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. One strategy for this kind of application is to do the fine measurement using carrier phase, and use the PN code to do ambiguity reduction. Then, a low chip rate is fine: you're basically using it as a check that you haven't slipped a cycle. I would think that the RTL dongles would work just fine, especially if you radiate a pilot tone from a fixed location as well as the tone from the rocket. You basically set up two PLLs in software one to track each tone, and subtract the phase of one from the other for each ground station. Indeed. Considering that in the start location you can solve integer ambiguity, especially with a pilot-tone or several radiated. Hacking in on the RTL-SDR to steer or replace the clock with a more stable clock might be considered. Cheers, Magnus ___ 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.
