Monitoring the ionosphere altitude variation with a sound card
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Title 
Monitoring the ionosphere altitude variation with a sound card

Subtitle 
software defined radio processing of DCF77 signals

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CC Attribution 2.0 Belgium:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor. 
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Release Date 
2018

Language 
English

Production Year 
2017

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Subject Area  
Abstract 
[DCF77] is a German very low frequency (VLF) emitter locked on the Cs clocks of PTBused for synchronizing radiofrequencydisciplined clocks found, for example,in lowcost weather stations. VLF signals propagate through the waveguidewhose boundary conditions are defined on the one hand by ground, and on theother hand by the ionosphere (charged) layer altitude. In addition to theamplitude modulation of the Cslocked DCF77 frequency standard, a [phasemodulation] is imprinted on the carrierfor time of flight measurement, allowing for "precise" time of flightmeasurements. The topic of the presentation is 1. reception of the DCF77 VLF signal using a coil antenna, with enough power to feed a lockin amplifier and extract phase and magnitude information. When the reference signal of the lockin is referred to a local Cs primary standard, day/night ionosphere altitude variations are readily observed  here using the carrier phase analysis, 2. extraction of the time of flight through CDMA processing of the phase output from the lockin amplifier, recorded on a (low frequency) oscilloscope, emphasizing the time resolution improvement of the phase modulation with respect to the amplitude modulation gained from the increased signal bandwidth, 3. replace the lockin amplifier with softwaredefined radio processing of signals recorded with a personal computer sound card or the analog to digital converter (RTL2832U) of a DVBT receiver dongle, 4. refer the second channel of the stereo sound card/DVBT to a lowcost GPS 1PPS time reference for local oscillator drift compensation, and hence measure the ionosphere altitutde variation through time of flight variation by comparing the phaseencoded crosscorrelation peak position with the 1PPS position, yielding results consistent with the labgrade instrumentation. Although the whole processing chain is trivial, various issues making itspractical implementation challenging will be emphasized. Most significantly,the processing chain is very similar to those applied to [GPSsignals], yet easier tograsp with the strong VLF signal. We aim at analyzing the time stability ofthe phase modulated signal  allowing for sub100 us timing resolution  andcorrelated observed phase fluctuations with [ionospherealtitude]

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last talk so Martin decided to put this in
00:07
academia and actually this is more of a hobby of of an actual work so what we wanted to work with with my colleague and Rico rebuild out is using very high stability clock source which is the DCF 77 low frequency a very low frequency emitter in Germany for measuring ionosphere altitude variations and that was actually decided during one of our barbecue party in the lab when enrico was discussing with me about his PhD on measuring Lawrence II signals and being able to separate the ionosphere ik wave from the surface ground propagating wave
00:48
so basically what we wanted to say is okay he did this like 25 30 years ago as a PhD can we do it nowaday with a sound card so that was the original insight into this topic and maybe some of you remember that a couple of years ago I discussed GPS reception now GPS is a bit challenging because it's below terminal noise you cannot see it on a spectrum analyzer but the basic insight was that if you could collect the raw carrier signals from GPS well GPS is basically a collection of atomic clocks in the space and you could do some nice physics so a bit later now a lonely sake here from from tests has published in one of his reviews what physics you can do on GPS usage and this is all based on GNSS SDR so basically what the insight in this work is if you have a very high stability clock and because frequency is a physical quantity that you can measure with the highest resolution then maybe you can do something fun and so in this context what we wanted to look at is looking at DC f77 as a signal source that is locked or disciplined on the PCBs of a German national metrology Center atomic clocks season clocks and actually that was all started during the EFTA a European frequency and time similar that is organized in bazan song every summer when Andrea's Bower actually the acknowledgement is is here below showed us one of these slides where he was presenting the code system that was implemented in DCF 77 so you might know that DCF 77 is one of these a radio frequency emitter that allows you to synchronize some of his lowcost weather station clocks and this is just on by amplitude modulation I'll show you a bit later but what andreas showed during his presentation was that if you just look at the carrier with his amplitude modulation you have a narrowband signal which is basically just the timing signal once every second and you're not going to do good positioning with this because you have a very highly stability frequency but your timing capability is very poor because nothing looks more similar from one sine period than another sine period so you have no timing capability and what they did at ptb are the people working running DC f77 is that they are added absurd a random phase noise over the carrier which is amplitude modulated and when you've been working for a year on GPS and reading all the documentation of GPS but just jumps on you as as a typical spectrum spreading using phase modulation and to the random phase modulation so when we had a look at this with Enrico we said we have to try to do this and do this as easy as possible so what we want to do here is to have access to the raw signal from this year 77 and implement not the basic amplitude modulation detection that everyone has been doing maybe but the more fancy phase modulation detection and why did we do this well if you look at the literature it's been very well known and I think that people are forgetting this little bit actually I have never been introduced to this before this work is that in the 60s people were very much interested in very low frequency propagation they were looking at RNA sphere altitude because they have these waves are bouncing or valence here and if you look in the older literature you see that between daylight and nighttime sorry nighttime and daytime ionosphere changes altitude because ionization due to cosmic rays from the Sun will change the electron density in the atmosphere and basically you have a twenty kilometer altitude variation in addition to reflection capability change due to electron density so what we want to do here in this work is we have this very high stability clock source located manthang in Germany and we are locating those on some 370 kilometers away and we have this beautiful season locked oscillator that generates a carrier wave with a 50 kilowatt emitter and we wonder whether we could do something more interesting than simply synchronizing a weber station clock by looking at the time of flight of this wave and this requires of course accurate timing because carrier frequency is one story but if you want to do some timing you need to have some sort of time of flight measurement and this is where the carrier phase measurement will be involved so just to get an order of magnitude what are we looking for we're considering GCF 77 370 be 370 kilo meter away from me a ground wave propagating around hundred microsecond if you make this wave bounce of the ionosphere it will take another hundred microseconds additional to go to the atmosphere and back to me and if the ionosphere moves by our twenty kilometer you could expect some variation of day to night variation of about an additional ninety five microseconds so our question is can we time the signal generated by this year seventy seven were received in in the zone zone by better than or much better than one hundred microseconds let's wait so question with a few approximation we consider that the wave is propagating at velocity in in a vacuum and you presume that the ground wave is delay is negligible with respect to what we're looking at so that's really the
05:58
objective of this work so the first thing I would like to introduce I know this is not a topic session about antennas but again for someone who's been raised with VHF antenna microwave antenna for me an antenna is a 50 ohm or 75 ohm piece of wire which is usually resonant and when you go back to this free kilometer wavelength signal any antenna will be much much shorter than the wavelength and if you go back in the literature you know or you might find that such antennas are necessarily high quality factor antennas which for someone raised in VHF and s HF is usually a poor thing you don't like high resonance antenna because they are narrowband and usually we want wideband signals to have a lot of signal a lot of information content like Shannon told us and so on the one hand we have these narrowband but actually narrowband is good for SDR because narrowband in ser will allow you to select only one signal and get rid of all the very old the very low frequency signals that might prevent us from receiving easier 77 so we have this high quality factor antenna which is simply a coil which is made resonant the other thing that is not usual to us or at least to me is that these antennas are very high impedance if you calculate the impedance at anti resonance of the system its hundred seventy five hundred twenty five kilo ohm so you need another adaptation see circuit so just feel the effect transistor that has a very high impedance that will generate a low impedance output and which you can feed directly to a sound card of your PC now if you do this in the lab well you get a lot of signals basically because your cathode ray screen is generating signal you're switching supplies generating that was a fridge discussion this morning so you need to take antenna out door so you go through emergency exits out door that gate gets out of my lab put the antenna outside and if I put the antenna like three five kilo meter away from the lab then I get my signal from DC f77 so at least we first spec to GPS with DC f77 I can take a spectrum analyzer and just have a look at the signal and I see it so now I have a
08:06
signal I want to decode it so I steal a lot in amplifier from one of my colleagues I take a synthesizer and I simply generate 70 70 70 7.5 kilohertz but DC at 77 carrier frequency and surely enough on the oscilloscope I see my amplitude modulated signal these are the amplitude drops every once every second and I have the phase information which comes out of a lockin and because there's no reason for this receiver to be locked to a cesium clock the local oscillator of this which this synthesizer has no reason to being locked on the cesium of course you have a phase which depends on the thing we want to decode plus a time varying signal which is a frequency offset as we've just seen but because I'm in a time and frequency lab I'm lucky enough to have access to a cesium clock so now I take a cesium clock I lock the synthetizer on a seasoned clock and surely enough ptb has a season Club which is locked to the business observatory club of course these two clocks are used to generate time so these are synchronized or at least they have a very slow drift so basically we can get amplitude modulation phase modulation and this is just my locking output my lockin amplifier output there is not much I can do out of it so I take this on an oscilloscope I grab the oscilloscope signal and by recording the raw signal from DC at 77 can I extract amplitude phase and if I get the phase because I do a cross correlation so that I can recover my total random information so
09:40
here is the pseudo random code that has been added on top of VCF 77 carrier so it's a proto and emits a 511 bit so it's very similar to GPS GPS is 10 bit this one is 9 bits so the random means that it doesn't repeat over these 511 sequence and you have a plus or minus 13 degree phase variation as opposed to GPS which is 100 plus or minus hundred zero 180 this is just a very tiny phase variation plus or minus 13 degree so you generate this code this is stolen from DCF 77 web page from Wikipedia you generate the code you load the code your recent pull your signal to have as many ones and 0 as you have samples in each bits you cross correlate and surely enough this is one of your Eureka day where everything works nicely once every second you've got one of these Peaks coming out cross correlation peak and you might say no this is just noise well if you zoom into one of these Peaks you have a real cross correlation in signal that is significant so this is an example of getting the signal by cross correlating my pseudorandom code over the phase information that I've recorded and I can get a nice timing signal and if you wonder if you wonder why we're doing this this is the amplitude modulation I assumed into the amplitude modulation this is one of my cross correlation T if you look at this over one minute this is what you have once every second you have a drop in amplitude once every second you have a cross correlation peak if you look at them you see that obviously we're gonna have a much better climbing accuracy with such a signal than with this one if you do this over one minute you see that you have a beautiful cross correlation peak which is very narrow and here you have this broad drop of your amplitude so basically what has been published earlier is that you have at least a ten times ten fold improvement in timing capability by using the phase cross correlation as opposed to amplitude modulation okay so so far I've shown off with my friends instruments I have an experiment which is a few thousand zero and you need a cesium clock quite a few people here will not be able to Bay to to perform it so now let's go to softwaredefined radio I want to do this now I have my lucky amplifier with my cesium locked source and I want to compare the results with a sound card and again my sound card has a local oscillator which has no reason of being locked on on cesium so I need to compare my sound card recording with some sort of time reference and the obvious time reference is 1 PPS one pulse per second coming from GPS I have one of these cheap ublox receivers ublox is a switch company making very good GPS receivers this is a 4 88 Euro you can get a phase output a phase lock a GPS receiver which are one PPS signal which is never late to be plus or minus 40 nanoseconds so you have a very good time reference and so again we do the same thing we have our audio signal because I have a sound tower that can record at 192 kilohertz I am weaving Nyquist theorem of twice two samples for 70 7.50 Hertz carrier frequency I do a frequency translating filter and I just record magnitude and phase out of this signal and if I do this I will try to tune the frequency translating fear filter so that I get well i compensate 492 kilohertz offset from the one two frequency now as I just mentioned during the session panel one of the things that always baffles me with this translating few filter is so I put plus 77 or minus 77 I always do it wrong I have to try twice what is the sine of a frequency offset that you have to put in frequency filter and somehow in this case it worked whatever I put plus or minus 77 5.5 and this is obvious once you realize that you've recorded a real signal this is not an IQ output a complex output it's a real signal a signal a real signal is even so you have the same component on the positive frequency and the negative frequency so ever you put plus 77 or minus 77 you always having end up having one of the information component into the baseband and so for this once we can do whatever we want plus or minus sign and you end up having a nice signal so you do this like the first time you do this you get once every second the amplitude drop and this is your face face from the recording and you try playing with this Delta F that you cannot reach here so I originally I'm slightly off frequency with an offset of zero Hertz I try to move it closer so let's say one Hertz where so removing this phase offset we're getting better whoops I went too far and at the end you can compensate for the phase offset of due to the difference between local oscillator of sound card and this year 77 so 15 ppm is not too bad is much better than most dvbt receivers so now I can get my
14:34
phase information and again same story I have my one PPS one my one pulse per second from GPS I have my amplitude signals somehow I don't know why but if I do have two sources I get out of sync sometimes in inverter the graphical displayed on the Y and the green is the phase so I don't want I'm going to show you I've been running this since October so I don't want to tune manually the frequency offsets every time so what I did here is a very ugly what I'm doing here is I'm recording using new radio and when a process in MATLAB I have easier 14 in octave reverb and then we work indirectly in C++ so I find it easier to record the data and then process them so what I'm doing here is I'm recording the raw data I'm having a local oscillator 70 7.50 Hertz and I'm shifting by a discretetime my DCF 77 signal and I low pass filter Vella decimate so basically here i reproduce the translating fir filter that's easy enough and then what I do is I make a coarse frequency offset measurement by taking the magnitude of the free transform and I say okay by Fourier transform is going to tell by how much it's a very rough estimate but by how much my local oscillator is offset with respect to DCF 77 and I remove I create a local oscillator which is exponential of J 2 pi the the frequency offset times the discrete time and I remove this offset and then once I've removed this chorus frequency offset I make a linear fit of the phase the resulting phase and once again I remove the average value of this linear feet of a phase which is an accurate or precise frequency difference estimate and if I do this I have my my offset corrected phase information if I cross correlate this signal I get my beautiful cross correlation Peaks once every second so I can do the same thing we did with a lockin amplifier with a sound card and all this will be reference to the 1 PPS of GPS by recording on a serial sound card on one channel I record DC f77 on the second channel I record GPS and I can measure here the difference between GPS and one PPS at some point you sometimes have a slight offset of the face sometimes you lose some of these cross collation Peaks and your timing will go away but if you run a median filter on this thing you will find the median value and it will give you a pretty good idea of what the phase but so the time stamp is at the time the
17:02
time delay is and this is an example of a threeday measurement where I have on top the lockin amplifiers of a high quality high grade lockin amplifier and on the bottom this is the output of my sound card so you see that the results are pretty consistent you have here daytime nighttime so indeed I can measure some sort of situation of the atmosphere during night time with respect to daytime and what you cannot see here below the text is the resolution the timing resolution is about nine microseconds if you remember I was claiming that the daytime to nighttime variation is about 100 microseconds so we have ten times better time resolution by using this cross collation technique so this is what I want to say here this was done in September this was my very first measurement in September I to live for a free week trip in end of September October and unfortunately for you I will not be able to show you any better result than this because since October it's been winter in winter  fear is not stable and I have to wait for a spring to come back to have again these measurements I was hoping that yesterday maybe the Sun might be helpful but no not yet we have to wait a few more weeks before the atmosphere shows again this high stability during daytime now one of the thing here is that not everyone has access to 192 he rolled some cards so of course we all seen that we have dvbt receivers and these dvbt receivers they also allow me to assemble a 2 mega sample per second so with 2 mega sample per second I can timestamp the GPS one PPS with a much better accuracy because now I have 500 nanosecond resolution however I'm losing in quantification resolution because I'm going from a 16bit sound tower to an 8bit atod converter so will the 8bit a to D converter from the RTL 28:42 you be enough to get my signal out of so my ground wave out of my my air wave out of my ground wave and we've seen that the antenna is a high quality factor bandpass filter so will I be able to get rid of of a jammer here so we did this and actually I'm putting this as a provocation for the Osmo as your people in the room because I couldn't get the direct sampling function to work with the new radio block and so what I did is was very ugly Hawk I'm sorry but at least it works is I just put in the liberality are the fact that I'm using a four thousand four thousand is a tuner that is not using intermediate frequency so by setting your RTR with such a setting you just tell it recall whatever sample you have and don't put any frequency offset and somehow this works so if there is a better way to do it I would like I think this is very powerful way of using this RTL 2832 you and it would be nice to ever make sure that they work or at least find some way of getting people used using them and so once you get this working you have one area input which is DC or 77th virtual input which the GPS signal and again we do the same thing I record for one minute once every once every minute I record for duration of one minute my simple GPS this year 77 I process my data and I will save them by doing the same processing I will not get into details but is exactly the same story except that you need to do well by processing a two megahertz signal with a very narrow band you you are targeting a 700 Cure's bandwidth you realize why you need to do multiple fear filter with decimation and again we go back to this whole DSP thing you can take all the courses on DSP that you want if you haven't realized that if you do directly to megahertz to 700 Hertz you have like 10,000 coefficients and then it's not going to run efficiently like then you haven't understood what DSP means and so by doing this with practical signals you will realize why you have to cascade few filters with decimation steps in between and so you do this again with your art rtlsdr and at some point you should have a chart displayed but there is a lot of points on this next chart so recording the data you extract phase magnitude again I have my once every second amplitude information like phase information my GPS time stamp the face which doesn't look very good but at least crosscorrelation Peaks come once every second here because I have a bit of leftover phase drift like I lose my cross correlation and I get here my my cross correlation Peaks so again I can do my time in experiment and I will be able to get a precise time of timing time stamping by using dvbt so even some of you who do not have 192 kilohertz sampling audio sound cards you can reproduce this with 10 euro worth of equipment with an RTL SDR don't go and again by doing this we have a collection of timing so this is a conclusion of this has been running since November this is 6th of November this was done yesterday so what you see here here I changed my algorithms so this is just a slight change in my goal even so what you see here is that phase of course we reference we are referring GPS to a clock which is used for defining UTC so these even though GPS is artificially linked to UTC to Universal Time well we we have good reasons to think the GPS will be synchronized with UTC so what you see here is over three months you do not see any observable drift which is well known from the petrology community and every one of these dips here is another day so they night day night which one of these like this and if I plot an island deviation for those of you who are more into the time frequency so this is basically the stability of my timing capability as a function of integration time so this is going from one minute up to well three months you see that you are at the ten to a minus seven down to 10 to a minus 11 which is about 1,000 times worse than what has been published by ptb but I think I've spent much less than a thousand times less than what they did it very experiment and just to give you some hint the red curve here is a tuning fork this is basically the stability of your watch I took a tuning fork from a watch and I measured the stability so what this chart tell you is that a tuning fork is very stable on the short term but on the long term it's going to drift if only due to time temperature variation when I wear although I do not wear my watch so over time the tuning fork is going to drift whereas these two clocks that we're using here the cesium lock clocked generated by this EF is going to always demonstrate better stability as the
23:46
longer you integrate and at some points you have this intersection course which tells you that below this your course watch will be better than DC f77 and above so sometime you should look at this year 77 this is basically what cesium atomic clocks are doing you have high stability from the quartz oscillator on the shortterm and you lock your quartz oscillator on the atomic transition so basically what you can do with this sound card is get acquainted to what people are doing when they try to design a tuma clock so having various time sources and these time sources will give the best ability over various integration durations so basically what we showed here is that we have the ability to receive this EF 77 to see a signal using active antenna we have been able to extract amplitude and phase information using a locking amplifier we've been able to reproduce this with ser Iver with no joke sound card or with dvbt dongle and we have consistent results I'm eagerly waiting for a spring to get again a BAE a beautiful stable signal I try to correlate fluctuations in the DCF 77 signal with xray density measured by geosynchronous satellites from NOAA and there is no obvious correlation I believe at 70 7.5 kilohertz is a bit too high and we're probing layers of atmosphere that are too deep to detect what people are usually measuring with I assume which are lower frequency devices 10 15 kilohertz where you really see the effect of of xray density variation on the ionic content of atmosphere I do have a few questions though if anyone wants to pursue this thing we're supposed to be told that we have two waves propagating on DLF a ground wave in an ionospheric wave that was initially insights that Lawrence C allows you to distinguish these two waves and if as published by PT be the ground wave is much stronger than the ionospheric wave how come that my unique signal that I'm measuring is varying from day to night this is a question for me and I wonder whether you have to use a model a waveguide model where we have varying boundary conditions or whether you have truly two ways propagating at the moment I did everything on a PC whether it's worth spending time putting all of this in a microcontroller is questionable but it's kind of fun and it's not too difficult to do so the next thing I would like to look at is basically looking at the at moment I've just been looking at the position of a cross correlation peak but I have not investigated the shape of a cross correlation peak and what I would expect is that either air wave is slightly shifted with respect to the ground wave what you should see is a broadening of a cross correlation peak because you have two ways slightly timeshifters you should remember that the phase did is 1.5 milliseconds apart it's 700 Hertz these are 100 microsecond time delay so you will not be able to receive a two cross correlation peak separates but you should be able to see some sort of cross correlation peak broadening as a function of his air wave and ground wave separation so this is still open to questions so they what I want to show you is that you can get involved in today's electrical phenomena in atmosphere involved in this topic while investigating a surface or softwaredefined radio just I've put in a food waste of course the slides on my website and on the Fordham website there is there has been a very extensive literature on this topic but I haven't seen anyone trying to attack all the issue of can anyone reproduce the experiment usually these are very fancy experiments with rather fancy equipments and I believe it this is a topic that would be worth investigating but more people especially because very low frequency communication has lost some of its appeal since everyone is looking at a tie Bend which propagate transmissions and nevertheless low frequency is a very amusing topic so with this I thank you for staying all the way to the end of a talk and if there is any question I will be happy to answer thank you I know a question just flowed your way out look around you pick up all the trash goes in the hallway and go here don't go is uh actually I heard 20 euro III buy mine for 8 euros actually this is just one of these h0 dongles and you just have to install the two capacitance and put an SMA connector instead what I it's like the previous presenter I believe it's in academia we are not careful enough to make something that other people can use I will be very happy to put on the website all my octave script it might be that you have to send me an email  I didn't tell you how it works because we are not very definitely not plugandplay but I would be very happy to send them to you we do so I have 192 kilohertz sound card I'm well below half the sampling frequency why wouldn't my sound card allow me to record this well the fact is it doesn't tell luckily enough it doesn't okay well I tried it on the lab is fitted with Dell laptops I tried it on to Dell laptops it works and on panasonic laptops it works I have a question yeah the first time I did this before I was
30:25
even sure that my pseudo random number generator was correct the first thing I did was I also created to see if I could get my piece once every second the question about using directly but because then once I got my autocorrelation I wanted a cross correlation to work lack of GPS you can only get the cross correlation if you correct for a face and then I for God's returning back to the autocorrelation I wonder whether what you can grab out of the autocorrelation it could be but at some point I want to compare this autocorrelation with the GPS tag and then I need to have my my copy my local copy that I will face a line with my with my GPS so at some point by taking external I think the autocorrelation will tell me whether I got the right signal and whether I get a peak every once every second but if I want to synchronize with an external source which is the one PPS when I need to run a local copy of my oscillator or my maps random generator so the random number generator and and facing furnaces as we do in GPS but autocorrelation is beautiful this is completely off topic but I was I've been spending the last three weeks working on our RDS because Bastion tells us that hard yes is so easy it took me three week to get it and the first thing I got with the stupid RDS is indeed the autocorrelation and once I got the autocorrelation I need another three weeks to figure out how to get the bits out how to get the vac relation things autocorrelation is really something that you should be the first thing tested whenever  we want to check whether the signal is integrity is correct so localization is very powerful tool indeed that's it [Applause]