Mpt + gps?

N2DLX · #1 (**permalink**) 09-03-2009, 10:52 PM

Does MPT-1327 have GPS/AVL capability? I have some GPS and/or messaging data which I'd like to decode. I know that the data is MSK, but the format is a mystery to me. After doing some online research, it looks like a lot of third-party GPS modems use MPT-1327 or a variation to send the text. Is this a standard feature of the system or are they just using the MPT-1327 protocol so the engineering work is done for them already?

Looks like 1200 and 1800 Hz are the mark/space tones, but it's definitely not MDC1200 or FleetSync data. Sounds like a long cyclic preamble followed by a short databurst, repeated maybe 5-8 times in a row. The system I'm listening to uses GPS but they use external MSK modems of an unknown origin.

Any suggestions on where to begin?

N_Jay · #2 (**permalink**) 09-03-2009, 10:57 PM

Might help to know what system?
Are you sure it is MPT?

N2DLX · #3 (**permalink**) 09-03-2009, 11:19 PM

The issue is that the system itself is Passport. But the GPS data (nothing to do with trunking directly) is possibly using the MPT protocol to transfer the text data over the air instead of using a custom-designed proprietary system which would take a lot of resources to develop. Pyramid makes the Merlin GPS modem and they say it uses MPT as the transmission protocol. Obviously it's just an add-on to a conventional or trunked radio, passing the audio to the radio. How can I begin to decode the GPS data?

If GPS is built-in to the MPT protocol, maybe a program designed to trunktrack those systems can display this textual data. It's a long shot, but worth asking before I try to decode it manually which isn't the easiest task in the world.

Comint · #4 (**permalink**) 09-04-2009, 05:55 AM

Quote:

Originally Posted by N2DLX

If GPS is built-in to the MPT protocol, maybe a program designed to trunktrack those systems can display this textual data. It's a long shot, but worth asking before I try to decode it manually which isn't the easiest task in the world.

GPS is not built-in to MPT1327.

From my limited knowledge of MPT1327 systems, MDT and AVL is generally done via the MAP 27 protocol.

As far as MPT1327 Trunk Tracking programs go, you might care to try TrunkView, which is fairly easy to set up and use, and has a limited capability to decode SDMs (Short Data Messages). TrunkView does NOT require a Discriminator Tap.

--
Comint

Squad10 · #5 (**permalink**) 09-04-2009, 08:46 AM

The tone set (you mention) 1200/1800 indicates 1200 baud MSK. According to Pyramid, the Merlin can be configured for 1200/2400 baud MSK.

If you don't want to build a 1200 baud MSK modem circuit yourself, You may want to look at the
Tigertronics.com Model RTX-24 Radio Modem. The RTX-24 decodes 2400 baud MSK. Tigertronics may be able to configure it for 1200 baud MSK if you ask them.

HyperTerminal could be used to see what the Tigertronics modem is demodulating.

N2DLX · #6 (**permalink**) 09-04-2009, 08:18 PM

I tried TrunkView but it wasn't decoding those data bursts.

I was hoping there was a software solution -- I tried common Ham software like Digipan and MixW, but can't get anything decoded. While getting binary data out of the MSK signal isn't much of an issue, what data am I getting? 7-bit ASCII characters, or something different?

I'm actually amazed that a generic software MSK decoder doesn't exist. I saw something come up on Google but it didn't look too promising.

Squad10 · #7 (**permalink**) 09-04-2009, 09:02 PM

Quote:

Originally Posted by N2DLX

I tried TrunkView but it wasn't decoding those data bursts.

I was hoping there was a software solution -- I tried common Ham software like Digipan and MixW, but can't get anything decoded. While getting binary data out of the MSK signal isn't much of an issue, what data am I getting? 7-bit ASCII characters, or something different?

I'm actually amazed that a generic software MSK decoder doesn't exist. I saw something come up on Google but it didn't look too promising.

I doubt it will work for what you you are trying to do, but check out antistatic.org - winmdcd. It decodes MDC1200 ANI.

I'm not a ham, but it appears to me that Digipan operates PSK and MixW operates various modulations that ham use, and MSK is not one of them.

If you post an audio clip of what you want decoded, I will pass it through a 1200/2400 baud MSK bit-in bit- out modem I built years ago and see what it outputs.

N2DLX · #8 (**permalink**) 09-04-2009, 11:08 PM

I've been using SkySweeper Pro to try to squeeze some text out of this data, but still nothing. I get repeating ASCII patterns which makes sense considering the repeating audio pattern but not much more than that.

Here's a link to the audio, see what you can make of it:

http://nxdn.mygmrs.com/downloads/audio/msk.wav

citylink_uk · #9 (**permalink**) 09-09-2009, 06:37 PM

I don't recognise that data on your clip, it certainly doesn't sound like any kind of MPT1327 mode to me.

jsharp2316 · #10 (**permalink**) 11-05-2009, 02:56 PM

MPT-1327 AVL typically uses the built-in MPT modem and may signal using a SDM2 or a LDM (that is MPT protocol 22 byte message on the control channel or a freeform message on a voice channel (actually controlled by the radio using a MAP-27 port on the radio). A Passport system may use ANY modem for the add-on of AVL. Could be a a Bell 202 modem (1200 bps), Bell 212 2400 Bps or v.29 (FAX chip set) or yes----anything maybe even a MPT modem.