Overview

From the MultiPSK (v 4.31) help:

"begin quote"

The VDL2 (for "VHF Data Link mode 2") mode permits automatic transmissions between planes and ground stations, with an exchange of different pieces of information (and especially positions) through AVLC frames (I, UI, RR, XID...) very close to Packet frames. It is a short distance (400 km maximum) service proposed through a network of VHF ground stations (on 136.975 MHz mainly but also 136.875 MHz in Europe). It gradually replaces the ACARS mode.

• The simplest way to decode this mode is to directly demodulate 8PSK frames by MultiPSK, with a SDR receiver (FUNcube Dongle, for example) connected to MultiPSK and the SDR interface started (on the Configuration screen). Adjust the SDR frequency on 136963 kHz and the waterfall frequency on 12 kHz (making it 136.975 MHz).

Or adjust the SDR frequency on 136863 kHz and the waterfall frequency on 12 kHz (making it 136.875 MHz).
The I/Q PSK demodulation will be done by Multipsk, so as the decoding.

• However, a standard USB receiver can be used (both solutions are equivalent from a performance point of view). In this case, the reception central frequency must be shifted by 12 KHz, i.e the frequency on the USB receiver must be adjusted to 136.963 MHz (136963 KHz instead 136975 KHz). Of course, the SDR interface on Multipsk must not be started.

The frequency must be very precisely adjusted (tolerance: +/50 Hz).

"end quote"

As of July 2016 the following 4 frequencies have been found to be active in Europe:

136.725 MHz     dedicated to ARINC

136.775 MHz     dedicated to SITA

136.875 MHz     dedicated to SITA

136.975 MHz     ARINC shared with SITA

Also see: https://ext.eurocontrol.int/WikiLink/index.php/ARINC

Plane Plotter can decode ACARS transmissions from an audio source (scanner).  To show VDL2 data in Plane Plotter you need MultiPSK version 4.26 which will do the decoding of the VDL2 transmissions and forward all of it to Plane Plotter using TCP/IP.  The data MultiPSK will show for a VDL2 transmission (and forward to PP) depends on the options you have set in MultiPSK.  The data Plane Plotter receives from MultiPSK can be seen in the PP Message view.  Plane Plotter will interpret this data and show it in the Aircraft view and if it contains a position will show it on the chart.  Plane Plotter version 6.4.4.5 shows VDL2 messages as Report type: -1 "ACARS no position report" even for those that do have a position.  An interesting change with ACARS is that the Route will show the destination airport as it is mentioned in the VDL2 message.  This might prove to be more reliable then deriving the route from the flight-number.

So getting VDL2 in Plane Plotter all depends on configuring MultiPSK to decode the VDL2 signals and pass them on to Plane Plotter.

Assuming you have downloaded and installed MultiPSK from http://f6cte.free.fr.
If this is your first encounter with MultiPSK you might get overwhelmed by its screens and options.  This all has to do with the fact that MultiPSK can decode many, many modes and you can control a lot of options for each mode.  If you are only going to use it for VDL2 then just follow these instructions and don't start exploring :-)  VDL2 is considered a "Professional" mode which means you need a MultiPSK license to decode it for longer then 5 minutes.  A registration key (about €30) can be obtained by sending an email to Patrick  "f6cte at free.fr".  So first get the dongle connected to MultiPSK and get it "working" using any "Amateur" mode.

Connect the dongle to MultiPSK

It is possible to use a FUNcube Pro as well as an RTL dongle.  This description assumes you are using an RTL dongle.  It also assumes you want MultiPSK to do all the work and you do not use an additional SDR program.  Here is what you do...

Select the menu choice Configuration which brings you here:

and set:

• Sound card (Input) not important as it is not used.
• Sound card (Output) your "speakers".
• Under "I/Q interfaces for SDR transceivers"

If you have more than one RTL dongle:

• select: "RTL selection"

• In this new window check OK for the one you want to use.
• Close (X) the window.

Next:

• Select: "RTL/SDR key".  "Direct via the sound card" will be selected automatically.
• Set TCP/IP server "on".

If you have only one RTL dongle:

• Select: "RTL/SDR key".  "Direct via the sound card" will be selected automatically.
• Set TCP/IP server "on".

Notice the message: "TNT receiver detected and connection done, via RTL DLL"

• Press RX/TX screen and you will return to the RX screen.

RTL dongle specific settings

First it is best to set some RTL dongle specific stuff.  Select the "Transceiver" button in the RX/TX screen and a new window "Transceiver control through the Commander (or HRD) software" will appear.  Note that you do not need to install either the Commander or the HRD software, it's just the window title....

Here you can set Auto gain and AGC.

• For me Auto=off, AGC=off and a manual gain of 28 works best.
• You can also set the frequency correction in either ppp (parts per milion).
  In above screenshot the Gain is set to 28 and the Frequency correction to -50ppm.
  I have not found an easy way (Amateur mode) to determine the required value, if you don't have a license for MultiPSK.  The ACARS mode is probably the best one to use.  You can also use SDR# or any other SDR program to determine the frequency offset of your dongle.  But for now you can also leave it empty.
• If you don't want to get confused and only want to use VDL2 leave the rest alone.
• You can leave this window open or just close it (X).

Setting the VDL2 Mode

As the RX/TX screen is dynamic it will sometimes hide the Mode choices you have.
If that is the case just maximise the window and they both will appear.

There are two panels, one with the "Amateur modes" and one with the "Professional modes".
Select VDL2 in the "Professional modes" panel and it will be displayed as "RX: VDL2" on the left side just above the waterfall.

Setting the frequency

Go to the "I/Q direct interface" screen:  (it should be visible because you selected RTL/SDR or FUNcube in the Configuration screen)

• Here you set the "HF frequency at 0" in the top right corner field to 136965000 Hz.
• Press "Forward".
• Set the "AF frequency" to 10000, either by using the buttons or by clicking in the waterfall.  The 3 blue lines in the waterfall should sit around the +10 mark.
• Now the "HF frequency (kHz)" will show "136 965.000 kHz"  (being "HF frequency at 0" + "AF frequency"). 

Coarse frequency adjustment

Look at the waterfall in the "I/Q direct interface" screen and try to identify a VDL2 transmission.

They appear as horizontal bars about "10" points (10 kHz) wide.  Make sure that these signals are within the most outer blue markers.

Fine frequency adjustment

The easiest way is to let MultiPSK do this for you by selecting AFC in the RX screen just above the right top corner of the waterfall:

If you want to fine tune manually you can select "Go" which sits just right of the 'Shift' which is just above the right top corner of the waterfall.

MultiPSK will show you how much you are off the desired frequency.  This of course only works if MPSK detects VDL2 signals!  Try to get as close as possible to +/- 50 Hz measured over more then 1000 tests by using the "AF frequency" control.  If "Shift:" shows a negative value you have to increase the "AF frequency".

Connecting MultiPSK to Plane Plotter

On the RX-TX screen use the TCP/IP button in the top left corner.  This will bring you here:

Set the Port number to any value that both MultiPSK and Plane Plotter support (in this example I have choosen to use 3125).  If you had to change it then press "Disconnection" followed by "Link opening".  This will close and re-open the TCP/IP connection so that MultiPSK will use the new setting.  Close the window (X).

This is all you have to do on the MultiPSK side.  If you stop and start the program these settings will remain, so you have to do this only once.

What is left is to start PP and

• set the IP-address and portnumber PP should connect to.
  Go to: Options / Mode-S Receiver / MultiPSK VDL2 server / Setup TCP client
  and enter 127.0.0.1:3125 if MultiPSK is on the same PC as PP.
  If MultiPSK is on a different PC change 127.0.0.1 to the IP-address of that PC.
  
• select the VDL2-MultiPSK option in the Plane Plotter I/O settings.

If all is well the messages you see in the MultiPSK window will also appear in the Plane Plotter Message view and Plane Plotter will do its thing.

And with an 'Omit aircraft after" set to 10 minutes, this is my result:

Note on TCP/IP

As of MultiPSK version 4.26.1 two clients can connect to the TCP/IP port (3125) that you have configured. This makes it possible to use Plane Plotter as well as VDL2-Display (see below) at the same time.
As of Plane Plotter version 6.4.1.6 it is possible to connect to MultiPSK on a different PC.

See: "Connecting MultiPSK to Plane Plotter" above.

VDL2 related utilities

VDL2-Display

Although this workspace is primarily dedicated to Plane Plotter you can also use VDL2-Display. It is one of the programs out of Mike's excellent set of utilities called Display-Launcher which can be found on his website http://www.rstools.info/ .
It does not give you an integrated view of Mode-S, ACARS, HFDL and VDL2 messages like Plane Plotter does but it surely is worth having a look at.

Based on information from Mike (the author) it does connect to MultiPSK over the network.
Besides the "Source address (hexa)" which is the ICAO of the aircraft it also shows the "Destination address (hexa)" of which we currently don't have a translation list.
VDL2-Display maintains a Allheard.csv file, a "not in Database" list and you can save the grid for later analysis.

As of MultiPSK version 4.26.1 it is possible to use both Plane Plotter and VDL2-Display at the same time.
For that MultiPSK offers a second TCP/IP port which you can enable by selecting TCP/IP in the 'mode specific' part of the RX/TX screen.

This enables port 3121 and when selected it will display "Waiting".
Currently VDL2-Display only allows the use of portnumbers in the range 3122-3132. As Plane Plotter has no limitation you have to change PP so that it will use port 3121 and configure VDL2-Display to use 3125 (as configured above).
Once a client (VDL2-Display or Plane Plotter) is connected the "Waiting" changes to "Connected".

VDL2 FAQs

• Q:  What if I want to use a FunCubePro dongle instead of the RTL dongle?
• A:  In that case you have to:
• select the FunCube sounddevice as Sound card (Input).
• select FUNcube instead of RTL/SDR in the Configuration screen.
  It sits under "I/Q interfaces for SDR transceivers" just left of RTL/SDR.

               You will now use the Funcube Gain and Correction settings in the ""Transceiver control ..." screen.

• Q:  What if I want to use my analog radio and/or SDR# setup?
• A:   The MPSK Help says this:

"A standard USB receiver can be used (both solutions are equivalent from a performance point of view). In this case, the reception central frequency must be shifted by 12 KHz, i.e the frequency on the USB receiver must be adjusted to 136.963 MHz (136963 KHz instead 136975 KHz). Of course, the SDR interface on Multipsk (Configuration screen) must not be started."

If you have a receiver that supports USB (Upper Side Band) and can select a bandwidth of 25 Khz, you should be fine.

This is what you do:

• Set your receiver to USB (not AM).
• Set the bandwidth to 25 Khz, which is about 10 x the normal USB bandwidth!
  If you are using SDR# and you can't set the bandwidth beyond 16000 have a look at one of the other FAQ.
• Roughly tune to the right frequency, which must be about 12.5 KHz below the wanted frequency.
  So for 136.975 MHz tune to 136.963.500 (see the screenshot below).
  You can fine tune from there.

I have tried this using my FunCube Dongle Pro and SDR# (and used VAC to get the audio into MultiPSK) which looks like this.

Steps taken:

• Select the FUNcube Donge Pro  *

• Set mode to USB
• Set Filter Bandwidth to 25000
• Tick "Correct IQ"  *
• Tick "Swap I & Q"  *
• Set AF gain. The value shown gave a level of 25% in MultiPSK.
  This also depends on the setting of Sound / Recording / Level.
                   
• As Input select "[MME] FUNcube Dongle V "  *
• As Output select "[Windows Direct Sound] Line 1 (Virtual Audio Cable)  **
• Tick "Filter Audio".
• Tick "Use AGC".

*  Setting depends on the hardware used. Can also be an RTL dongle, Kinetics (SBS-3), etc.
** Depends on the Virtual Audio Cable you use.

PS. I tried my AOR AR8000 using NFM and a discriminator output but could not get it to decode VDL2.

• Q:  Can I use my SBS-3?
• A:  Yes you can (with thanks to Stuart for allowing me using his equipment).
  "Kinetic have released the ability to get I & Q from your SBS-3 and a free plug-in for the amazing SDR # radio software".
  Just install the Basestation version that implements this feature and follow the instructions on how to connect it to SDR#.
  ( The Kinetics software can be found here:  http://www.kinetic.co.uk/basestationdownloads1.php  and is called "SDR# PLUG-IN".)
  ( The instructions are on the same page and are called "Firmware V118 BaseStation V180) ".)
  You ofcourse need SDR# which can be found here: http://www.sdrsharp.com/
  You also need a virtual audio cable to connect the output of SDR# to the input of MultiPSK.

The following two virtual audio cable programs have  proven to be working:

• VAC from  http://software.muzychenko.net/eng/vac.htm
• VB-Cable from http://vb-audio.pagesperso-orange.fr/Cable/index.htm

In SDR# you have to select "Kinetic" as the input and the VAC of your choice as output.
Make sure you:

• Set the mode to USB (not AM) in the 'Radio" tab on the top left side of SDR#,
• Roughly tune to the right frequency, which must be about 12.5 KHz below the wanted frequency.
  So for 136.975 MHz tune to 136.963.500 (see the screenshot below).
  You can fine-tune from there, preferably using the Kinetic "Configure Frequency correction" setting.
• Set the "Filter bandwidth" to 25000.
  If you can't set the bandwidth in SDR# beyond 16000 have a look at one of the other FAQ.

The VDL2 signals should now be visible as horizontal brusts in the SDR# waterfall and they should be in the middle of the bandwidth as indicated in the spectrum display (if you hoover your mouse over the waterfall, the bandwidth will be visible there too).

In MultiPSK select the VAC of your choice as input. Do not select the "Direct via the soundcard" option.
As the MultiPSK waterfall is not very usefull for a wide signal like VDL2, you should position the filter in SDR# in such a way that the transmissions appear in the middle of the filter.
Press the small "go" button in MultiPSK so that when messages are received MultiPSK will tell you how far "off frequency" you are.

In above example D: 179/48 means: 179 messages received, 48 decoded.
The -70 Hz (3737 tests) means that the frequency is -70 Hz off based on the average of 3736 messages.
Pressing the "Stop" and "Go" will restart the measurement.

Instead of fine-tuning the frequency using SDR# it is better to use the Kinetic "Configure Frequency correction" setting.

That way the frequency shown in SDR# is the actual/correct one. An SBS-3 user reported that a –7.0 setting was what he needed to make that he could tune SDR# to the real frequency.

• Q: What if I cannot set the bandwidth in SDR# higher than 16.000?

• A: In that case you have to change a setting in the  "SDRSharp.exe.Config" file.

  Open this file using a simple editor like Notepad. DO NOT use WORD, Wordpad or any other sophisticated editor.
  Change this line

  <add key="minOutputSampleRate" value="32000" />

  to this

  <add key="minOutputSampleRate" value="50000" />
  ..