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Author Topic: Amalgamated DGPS Software  (Read 9960 times)

Offline ChrisSmolinski

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Amalgamated DGPS Software
« on: September 01, 2016, 1912 UTC »
DGPS stations transmit the difference between positions indicated by GPS satellite systems and the known fixed position of the station. This allows higher accuracy. DGPS transmissions are 100 or 200 baud and are transmitted on frequencies from 285 kHz to 325 kHz in the longwave band. Hundreds of these stations are operated by the Coast Guard and other agencies around the world, and they can be interesting DX targets. Each station transmits a continuous stream of messages containing correction data for GPS. These messages also contain the station ID code, so they can be used to directly ID the station.

Since SDRs are capable of recording a chunk of the RF spectrum directly to a disk file, I realized that a decoder could be written to demodulate all of the DGPS channels at the same time, at both baud rates. They write this data as a I/Q file, storing the complex representation of a portion of the RF spectrum. A 50 kHz bandwidth is slightly more than enough to cover the entire DGPS band. I set my SDR software up to record overnight, then in the morning I can run the recordings through the software, and see what stations are present.

The software sets up 82 SSB demodulators, two for each of the DGPS channels, one is for decoding 100 baud and the other for 200 baud, that allows me to use a more narrow filter for the 100 baud case. The output of each demodulator goes to a DGPS decoder that looks for valid messages. A message is considered valid if it starts with the correct preamble byte, is of message type 6 or 9 (the most common sent), has a z-count (which is a time code offset from the hour) that is within a few seconds of what it should be, and passes the 6 bit parity word test. This eliminates the vast majority of bad message decodes, although every so often one will sneak through. This is because you can get multiple bit errors on a message that corrupt both the data and parity word in such a way that the parity check still passes. It is still necessary to visually inspect the decodes, and decide if a seemingly amazing DX catch is realistic, or more likely just a bad decode.

Below is a screenshot showing the output of approximately 24 hours of recordings of the DGPS band.



The columns containing the following information:
   Count: the number of decodes of this station.
   ID: ID number of the station, stations transmit either the ID or one of the reference IDs.
   RefID1: The first reference ID of the station.
   RefID2: The second reference ID of the station.
   kHz: Frequency.
   Baud: The baud rate, 100 or 200.
   City: Station Location.
   Country: Station Location.
   Lat: Station latitude.
   Lon: Station longitude.
   km: The distance to the station from your location.
   deg: The bearing to the station from your location.

Most likely the Wiarton and Angleton decodes are corrupted messages, as the frequencies they use are both dominated by strong semi local signals.

Another way to look at the decoded data is with this graph, that shows the times that messages were received from each station



You can see the various times stations were decoded. There are cases where a single decode was received (just a thin line), which was possibly a garbled message. But there are also cases for DX stations where several messages in a row were received (a thicker line). It is quite improbable that many messages were garbled in a row, with exactly the necessary bit errors to change the ID of the station, but also preserve the parity word check.

It is interesting to observe how two stations on a given frequency will alternate reception, as one fades out and the other fades in.

A very preliminary beta version of this program, Amalgamated DGPS, is available for download for those who wish to try it. It is only for Mac OS X, and requires I/Q recording files made in either the RF Space or Perseus format (and note that I have only tested with the former, the latter should work, but you never know). While there is no Windows version available at present, I may have one available shortly, so stay tuned!

http://www.blackcatsystems.com/software/dgps_decoding_software_sdr.html

Also another article about this software here: http://www.radiohobbyist.org/blog/?p=1907
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
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Offline ChrisSmolinski

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Re: Amalgamated DGPS Software
« Reply #1 on: September 13, 2016, 2116 UTC »
I've updated this app with a new version, 0.2.0, which adds a bunch of features, including the ability to decode from regular WAVE audio files, if you do not have an SDR. The app can decode from multiple DGPS channels in the same WAVE file, as many as fit in the bandwidth. So if, for example, your radio is tuned to 300 kHz USB with a bandwidth of 6 kHz, then 301 to 305 kHz fit inside and will be decoded. You could of course tune to say 299.5 kHz and squeeze in another channel. Or make the bandwidth wider. Or both!

The graph window now shows a red graph at the top, which indicates the total number of messages per minute being decoded. It can be handy as a rough guide as to how well band conditions are.

I have also added support for a few other formats of SDR recordings, Studio1, ELAD, and Sdr-Radio, in addition to SdrDx / RF Space and Perseus formats. Note that I do not have all of these programs, so testing was done with files provided by others. I think it is all working correctly, but you never know.

It is still Mac only, but the changes to this version (which is close to a complete re-write) move me closer to being able to release a Windows version. It can be downloaded here http://www.blackcatsystems.com/software/dgps_decoding_software_sdr.html
« Last Edit: September 13, 2016, 2123 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline ChrisSmolinski

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Re: Amalgamated DGPS Software
« Reply #2 on: September 15, 2016, 2230 UTC »
Version 0.3.0 for Mac OS X has been released. This version adds live decoding from an audio input. It also adds a window of UNID stations (those that were received but the ID does not match any known ID/frequency/baud rate combinations). There's also the usual bug fixes.

Live decoding of audio was the last major feature that needs to be added. So now the task of making a Windows version can begin.

Version 0.3.0 can be downloaded here http://www.blackcatsystems.com/software/dgps_decoding_software_sdr.html
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

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Re: Amalgamated DGPS Software
« Reply #3 on: September 17, 2016, 0520 UTC »
So is the software smart enough to triangulate your receive location based on receive timing, et al, or do you tell it where you are?

+-RH
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WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline ChrisSmolinski

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Re: Amalgamated DGPS Software
« Reply #4 on: September 17, 2016, 1145 UTC »
So is the software smart enough to triangulate your receive location based on receive timing, et al, or do you tell it where you are?

+-RH

You have to tell it where you are located. I'm not sure that the transmissions are synchronized to UTC time, allowing for triangulation based on the time delay.
« Last Edit: September 17, 2016, 1156 UTC by ChrisSmolinski »
Chris Smolinski
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eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline ChrisSmolinski

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Re: Amalgamated DGPS Software
« Reply #5 on: September 19, 2016, 2209 UTC »
Wake the kids and phone the neighbors, Amalgamated DGPS is now out for Windows!

It seems to work OK, I've done all my testing on Windows 10. It should probably work on 7 and 8, maybe Vista and XP?  Try it and let me know.  This is the first release of a Windows version, so there are likely still some rough edges.

It decodes from both WAVE files and I/Q recording files from several popular SDR programs. I also decodes from live audio. It will decode as many DGPS channels as can fit in your audio passband (assuming your computer can keep up), I've been able to go up to 19 kHz wide.

Details and downloads here: http://www.blackcatsystems.com/software/dgps_decoding_software_sdr.html
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline KPC6NDB

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Re: Amalgamated DGPS Software
« Reply #6 on: November 06, 2016, 0752 UTC »
Hi Chris,

Kool program! This is really nice to be able to monitor the full DGPS band......

I've found a typo a few days ago: #667 Ulling Do, South Korea, 319 kHz is showing station #001 Ull every time it is logged. The RFId is correct/the one assigned.

Also #821 Portsmouth C2CEN, VA, 313 kHz has been off air for over two years. #821 shows up occasionally and is a false decode. It is/was a testing facility for the USCG.

Next, we know there is a problem with the ADGPS RFId display. Is it your intent to show the assigned RFId's or only the received RFId? I would prefer to see what ADGPS received to further verify the log and for submission of logs to Rxx.

Interestingly, I have been seeing a lot of logs from South African stations primarily #203. Building up over a week or so from one log at a time to 4, 5, 6 daily logs. Accompanied by usually another AFS station but they only log one hit. The interesting part is that the logs are happening during AFS local daylight morning/early afternoon time (+9 hours from here - nighttime here on the left coast)! A few days none are logged but seem to follow the daily ebb and flow of what I log on HF DSC from Cape Town 8 & 12 mHz.   

I listened to the Perseus SDR audio I/Q stream of several of these AFS #203 logs and it sounds like the USA station on this channel, #828 Angleton, TX fades/flutters. I ran DSCDecoder at the same time (I knew it would not log #203 as it takes DSCDecoder about 12 seconds to sync) but it skips a few seconds of solid decodes of #828 and then ADGPS logs #203 during these "skips". I checked down to the exact seconds.

Interestingly some of the ADGPS daily logs seem to show a pattern of LF propagation to some regions.

Finally, I read somewhere in the description of ADGPS that the user would have access to the station list. I did not find that function in the windoz distribution of ADGPS. Would like to see that to eliminate the "clutter" of truly impossible logs like India, Egypt and so on.

The Graphing function is also kool Chris. Once in a while it shows a gradual slope of decline in logs but mostly peaks and valleys then a decline near local daylight.

Don

Offline ChrisSmolinski

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Re: Amalgamated DGPS Software
« Reply #7 on: November 08, 2016, 1601 UTC »
Hi Don,

Thanks for the info about #667 and #821, I've corrected them in the station listing. I have noticed #821 popping up every now and then, with very few decodes, considering how close I am to Portsmouth, I assumed I would be getting more decodes if it was actually on the air, but did not know what the actual status was.

I'll have to look and see how difficult it would be to report the actual received RFid, right now the software does display both of the assigned values. It might be possible to display the received RFid in bold?

The app will look for your own copy of the station list, named dgps_station_list.txt and if it finds it, will use it in place of the built in list.

I do find pattern(s) of stations fading in and out, and propagation in general, very interesting. Sometimes you can see correlations between stations in the same general geographic area, and other times you can't probably because what you receive/decode is a function of not only whether that station's signal is making it to your QTH, but also whether there is another, interfering, station being received on the same frequency. It seems the more I learn about propagation, the less I know.  ;D
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline Terry

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Re: Amalgamated DGPS Software
« Reply #8 on: November 08, 2016, 2218 UTC »
Impressive software and some good DX. For those that may be interested, Card Sound Fl is at the SE tip of Florida.
What antenna were you using?
QTH Florida's Treasure Coast, near Stuart 100 mi N of Miami Grid locator EL97uf
Equipment: Kenwood TS-480SAT, R-600, Yaesu FT-857D, R. S. SW portable (Sangean), R.S. Pro-106 Scanner 25-1300 MHz, HyGain 18AVQII, M2 6M 3 el beam, Misc verticals and dipoles
73,
Terry

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Re: Amalgamated DGPS Software
« Reply #9 on: November 08, 2016, 2335 UTC »
I've been using a 500 ft beverage (which really isn't a beverage down here). I have more recent logs in the Longwave forum, including some more distant catches, such as Portugal: https://www.hfunderground.com/board/index.php/board,16.0.html
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline Terry

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Re: Amalgamated DGPS Software
« Reply #10 on: November 09, 2016, 0156 UTC »
Thanks for the info. Quite a list in the LW forum. I listened a bit just now with my TS-480SAT and the HyGain vertical and was able to hear more stations than I thought I would be able to. Watching the signals on Spectran and they are pretty distinctive. I will see if my neighbor will let me run a long wire across the back of his property. :)
QTH Florida's Treasure Coast, near Stuart 100 mi N of Miami Grid locator EL97uf
Equipment: Kenwood TS-480SAT, R-600, Yaesu FT-857D, R. S. SW portable (Sangean), R.S. Pro-106 Scanner 25-1300 MHz, HyGain 18AVQII, M2 6M 3 el beam, Misc verticals and dipoles
73,
Terry

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Re: Amalgamated DGPS Software
« Reply #11 on: November 12, 2016, 0157 UTC »
Installed the software on one of my win 7 computers yesterday. Still using the Kenwood and HyGain but pretty good results so far. Cool stuff, I needed something to keep me from working DX on 40 at night. :) The bandwidth only allows receiving about 3 or 4 stations at a time though.
QTH Florida's Treasure Coast, near Stuart 100 mi N of Miami Grid locator EL97uf
Equipment: Kenwood TS-480SAT, R-600, Yaesu FT-857D, R. S. SW portable (Sangean), R.S. Pro-106 Scanner 25-1300 MHz, HyGain 18AVQII, M2 6M 3 el beam, Misc verticals and dipoles
73,
Terry

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Re: Amalgamated DGPS Software
« Reply #12 on: November 16, 2016, 2335 UTC »
Hi Chris,

I found another station that popped up the other day and should be deleted:

#877 290 kHz Pahoa, HI  (Decom 26Apr14)


Also decoding a WinRadio I/Q file is a problem: it is 32 bits, 40 kHz bw, stereo.


Cheers,

Don

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Re: Amalgamated DGPS Software
« Reply #13 on: November 17, 2016, 1325 UTC »
Thanks, I deleted #877 Pahoa, it will be reflected in the next update.

Can you save 16 or 24 bit I/Q data? I am not aware of other SDRs that record the data as 32 bits, and there's likely no meaningful data in those extra bits, so I would prefer to not add a special case for processing 32 bit files. Also, I am not sure what is meant by stereo, as these are I/Q data not audio, or is that their way of signifying that the files contain two channels of data, I and Q?
Chris Smolinski
Westminster, MD
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NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline KPC6NDB

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Re: Amalgamated DGPS Software
« Reply #14 on: November 23, 2016, 2347 UTC »
Hi Chris,

I found another one that just popped up and has been off air since Oct 2014:

#820 Alexandria, VA  It was an engineering test site.


So I guess there still will be the odd false decode. On some of the true Dx I don't think one could ever get 10 or more logs. That is why I have been keeping a log on paper to track them. Maybe as we go in to winter there will be a change.   

It's like that with NDB's too. Sometimes they are there for a few cycles then gone.   

Cheers,

Don