The AIS Automatic Identification System have the 1PPS at its heart of the system. It was Haakan Lans that came up with the initial concept. It also form the foundation for a system named ADS-B in aviation. Those concept also apply Self-Organized Time-Division Multiple Access (STDMA) to compensate for the variable distances involved, as prox 0.3 meter makes one nanosecond 'flight-of-light', one mikrosecond makes 300 meters, one millisecond makes 300 kilometers and so on. Very good video indeed. Legacy of prof Brad Parkinson.
This is a good video and well made. There is information missing in parts that needs a little filling in. First up, the pps pulses output by the GPS receiver are not necessarily low jitter as such and it does not necessarily aim to be either. What it is, is accurate over time. And when we say accurate, we mean crazy accurate. The signal is derived from the atomic clock in the GPS satellites with errors somewhere between 1s per 30million years and 5ns in the life span of the universe. This is not an exaggeration (on my part anyway). These are the published numbers for atomic clocks, go check it out. Quite fantastic stuff. So using the pps you could have a time of day clock that is never wrong (daylight saving aside) and will never have an error of more than a nano second in your lifetime. Second area of not quite accurate information is in the discussion of the application..... To say two devices in disparate locations can use the pps and receive it at "exactly the same time" is an error. All electrical signals, be they in a wire, air or space, take time to propagate. It is that fact that means a GPS can triangulate its position based on the time signals sent from multiple satellites. The quoted application of TDM by remote transmitters requires the transmitters to know when to transmit such that their transmission arrives at the receiver at the exact time that is that transmitters TDM slot. That is a function of the distance between the receiver and each of the transmitters. A transmitter that is much further away than any others and has the last TDM slot assigned may begin transmission before any others but the distance means the transmission arrives just after the last of all the other transmitters and in alignment with the last slot position in the TDM frame. This is how analogue mobile phones worked (can't say what this scheme was called and what countries it operated in other than Australia). An over simplification leads to either confusion or a misunderstanding and neither is good. I hope I have not caused either here.
This is a simplified description of this concept, as it is only a 4 min video. Check out the link to a patent of a radio controlled locomotive system I have worked on. Have a look at Paragraph [28] under Detailed Description for a more detailed description.
@@0033mer You should do yourself and the topic justice and take as long as the subject requires in explanation. Or pick a smaller subject or less complex examples. TDM over radio is no small subject ;) There will be newbies watching this and they get the wrong idea and some of them will stick to the wrong idea like sh-t to a blanket. ;) They are the bane of my professional life.
@@0033mer Ok. I have had a look at your patent application from 2004 and I think I have found the problem. I don't think you are taking flight times into account. Even if a gps signal could be used this way it does nothing to accommodate the flight time of the signals from each transmitter to the receiver. I know your TDM frame is slow and probably has loads of wiggle room but in a rail application distances can be significant even in a rail yard context. Did your patent get approved and issued?
We have been doing remote rail yard switching here in Canada since 2004 to present. www.dropbox.com/s/46eofcxnqxouf8t/Cattron.pdf?dl=0 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-pFWlIfIZkCA.html
2:58 - I hope this actual project is not using the 1 PPS signal at every second for 24/7... a lot of problems could occur which lead to all towers not to sync. Lets say some background EMI can cause the GPS signal to be skewed and some towers would not be synced. I hope it was designed such that those towers only uses the 1 PPS radio signal maybe when it's time for synchronization it's internal clocks. So in essence, those towers should use precise internal clocks and when it's time to resync all of the internal clocks, it should use the GPS clock signal. This way those towers are not highly dependent on GPS clock signal 24/7, rather dependent on it after few days or weeks. Also, what kind of algo was used for the antenna tower transmitters to wait for it's turn to transmit data? When thinking about this project, it seems the GPS atomic clock accuracy wasn't needed. All towers can just transmit 24/7 every second of the temperature data simultaneously and continuously where each tower transmit it's own unique encrypted data or code in which the receiver tower can easily distinguish which tower is transmitting the temp data, this wouldn't need the GPS clock hardware solely used to determine which tower is transmitting data and the receiver can easily tell which tower is sending the temp data. 10 towers means it requires ten precision GPS module hardware which could cost total of $300 which could've been saved simply using software to determine which temp data is provided by which tower. Nice example explanation however for the use of 1 PPS.
This is a simplified description of this concept, as it is only a 4 min video. Check out the link to a patent of a radio controlled locomotive system I have worked on. Have a look at Paragraph [28] under Detailed Description for a more detailed description.
Some GPS units come with drivers to sync the PC clock. There is also software that will read the NMEA serial string and sync the PC clock www.visualgps.net/#nmeatime2-content
Ok but you did not explain how they delay the timing. If they all sync to the 1pps to begin sending the data, how does the delay get implemented to not all send at once?
Each transmitter is controlled by a microcontroller. When it detects the 1PPS edge the microcontroller will delay according to its time slot and then transmit, so each transmitter comes on one at a time.
@@0033mer according to its time slot and the flight time to the receiver. You have over simplified this and don't really understand it yourself (I suspect).
Find 2 or more buddies with Hackrf One SDR. Connect 1pps to hackrf trigger input -> record samples -> get time difference of arrival -> triangulate the source. That'd be a fun project.
