Orbitron - SDRuno Interface Setup

    Orbitron to SDRuno Interface
: Why interface Orbitron and SDRuno? I started this exercise because I could not find anything on Google about using Orbitron with SDRuno. It soon became clear why this was not a common or documented interface. The reason is that for interfacing with other applications SDRuno uses the CAT protocol (‘computer aided tuning or computer aided transceiver’), while Orbitron relies on a different method or standard for communication with other processes or devices.

    Present generation ham radio transmitters, receivers, and transceivers nearly always implement some flavor of a bi-directional CAT
interface, with feature-specific variations from rig to rig. A source of possible confusion is that the term CAT is used in different ways. In some contexts CAT may refer to hardware, such as specialized interface devices or connectors. However, as the terms meaning has evolved, it most commonly describes the structure and content of messages that pass between a computer and radio equipment. As a messaging protocol CAT does not assert a transmission layer, although in practice the latter is typically some relative of the now more than 50 year old RS232 standard. Yes, serial COM ports (fondly remembered from the back panels of 1980’s PCs) linger on in the radio world, although it today’s devices they are more often virtual than physical.


I apologize if parts of this account are ‘overkill’. I want to describe all components of the hookup, even the obvious. Otherwise, some essential part might be inadvertently omitted. First the main applications: 1. Orbitron is the satellite tracking program shown in the lower right of the big illustration at the top. It is widely used and popular for controlling satellite receivers or antennas, and I think also small telescopes. 2. SDRuno came to my attention after I had purchased an SDRplay model RSP1A (https://www.sdrplay.com/, photo above). Previously I had experimented with other SDR applications: CubicSDR, HDSDR, SDRConsole, and SDRSharp (software) along with several RTL-SDR dongles (hardware). However, the SDRuno application was specifically recommended for use with SDRplay, and an immediate appeal of SDRuno is that its software distribution includes drivers for SDRplay.1

com0com setup form    
Software components of the present demonstration are installed on a Windows 10 laptop, loaned for the project. It is a killer machine with 64 GB of RAM and a 4K screen. Actually the latter can be a nuisance, as some programs, including a few that I have written myself, do not display correctly. Text that is too small can be made larger, but not buttons and such, but this display problem had no effect on the present project.

    It is not surprising that a 2016-vintage Laptop would not have any physical COM ports. No matter, there are many ways to define virtual COM ports under Windows. The software that I used is called com0com. I do not recall whether the specific virtual port pair shown in the image on the left were program defaults, or whether I picked those numbers. Any ports that are not in use will do, although I would incline toward smaller numbers, as several Internet sources have recommended. The virtual pair is connected in a ‘null modem’ configuration (color diagram), same as a null modem cable between two physical ports.

    Another useful utility (although not needed or used in the interface) is a program called SerialMon. By the way, all software used in this demonstration is free. I was tempted to purchase some high grade analysis software, but
the main project as summarized in these paragraphs involves only free applications and utilities.

    SerialMon can be configured as a COM port sniffer, capturing everything that is sent or received. This is exceedingly helpful for understanding how communication between reference applications works. At one point in the analysis stage of this project I used a terminal emulator program (Termite) to capture COM data from one of the two ports. However, with SerialMon it is possible to capture data from both ports as they communicate bi-directionally. Just to be clear, though, neither SerialMon nor Termite are needed or used in interfacing Orbitron to SDRuno. They are simply tools that may be helpful in analysis or debugging of serial communications.

SDRuno COM settings    Pressing the ‘Sett.
button in the SDRuno RX CONTROL form (or panel) opens a tabbed dialog box (left). SDRuno connects to one of the [virtual] COM ports. Parameters are entered under the CAT tab, as shown in the illustration. I have read that the SDRuno serial port connection is fragile, or that it has to be re-enabled at program startup, and in a particular order. That may have been true of a different version or application context. However, I have not observed this behavior. Once enabled the connection is automatically re-enabled when SDRuno is started. It does not matter if the ‘Play button (in the MAIN form) has been pressed or is stopped.

    Now to the novel part, an obscure middleware utility called DDEOrbitronToSerial: From its description this utility appears to have been a ‘one-off’ that the author (Alex) decided to share (Thank you!). This application should be connected to the other COM port, the one that is not connected to SDRuno. DDEOrbitronToSerial is a DDE client (receiver of DDE messages) and a serial COM sender.
This program’s main form is shown in the lower left part of the big illustration at the top of this write-up. If DDEOrbitronToSerial only forwarded DDE strings to a serial COM port exactly as they are received, it would not be serviceable to the present project. But the program’s author also implemented the ability to extract and format DDE data before forwarding to the serial device. When checked, the box called ‘Custom Format String’ in the application GUI may define a string transform of DDE input. (Strictly speaking the output does not have to reference DDE data, but that would miss the point.)

    As was previously noted, SDRuno does not speak DDE. Like tabletop ham radio transceivers SDRuno uses CAT for communicating.
CAT commands number in the hundreds, and vary between implementations. However, only one command matters in the present demonstration’s context, namely the one that requests the transceiver (or in this case SDR) to tune to a specified frequency. (I am simplifying this description somewhat.) Current generation radios typically have two or more tuners (VFOs) or in some cases more than one receiver. This project deals only with SDRuno’s primary receiver Rx0, and its VFO-A. The CAT command to set VFO-A is simply FA + the frequency and then semicolon. (CAT commands are terminated by a semicolon.)

    Radio frequencies range from long wave to the microwave spectrum and are specified in Hertz. In my first attempt to get this interface working I sent FA + frequency in Hertz as a canonic value + semicolon. That did not work.  In the CAT world, frequency is an 11-digit
fixed-length field, long enough to specify any frequency below 100 GHz. For CAT it is necessary to pad frequencies lower than 10 GHz with one or more leading zeros, in order to fill out the expected 11-digit length. Thus 137.62 MHz would be formatted as 00137620000 (Hz). The CAT command to set VFO-A to this example frequency is ‘FA00137620000;’. It is possible that the ‘FA’ command is not universally implemented, although I think it probably is. In any case, SDRuno is documented as being compatible with the Kenwood TS-440S or TS-480 CAT Reference. (See SDRuno User Manual section on interfacing with Ham Radio Deluxe.)

    Ideally, one would construct the CAT frequency
command string programmatically, based on the canonic length of the source frequency in Hertz. However, DDEOrbitronToSerial does not have a programmer interface (This is not a complaint!). Therefore, it is necessary to make an assumption about the length of the frequency string. For the VHF range (NOAA satellites, plus the 2 meter and 70 centimeter amateur radio satellites, etc.) the canonic length is always 9 digits. For tracking microwave satellites it could be 10 or 11 digits, but that is of no concern in the current context. To convert a 9 digit frequency to 11 digits it is only necessary to pad the frequency (on the left) with the string constant "00", which leads directly to the ‘Custom Format String’ FA00[DN]; (where [DN] is the program-supported variable for the downlink frequency, as received via DDE.

Orbitron Rotor/Radio drop-down    According to the posted directions for using DDEOrbitronToSerial it should be possible to add this utility as a named driver, such that it appears in the Orbitron Rotor/Radio Driver drop-down list. A long time ago I added an SDRSharp driver. However, for some reason, I have not got this construct to work with the DDEOrbitronToSerial client. As a workaround I start DDEOribtronToSerial.exe manually before enabling Rotor/Radio control from Orbitron. The ‘Send to serial
box may be left unchecked until DDE data are available. If logging is enabled along with Send to serial, and if theres nothing to send, the log will record nuisance warnings. After starting the DDEOrbitronToSerial client, enable the ‘MyDDE’ driver from the Rotor/Radio drop-down list. If MyDDE has not yet been installed, the zip file download and instructions may be found on the Orbitron Downloads page. Starting MyDDE will cause Orbitron to start sending DDE messages. (The MyDDE client will also start, but its form can be minimized or ignored.) Once DDE messages are being received in DDEOrbitronToSerial, check the ‘Send to serial box if not previously checked. If I find out how to register DDEOrbitronToSerial as a Rotor/Radio driver that starts DDE messages from Orbitron without needing MyDDE, that will be an addendum to these notes.

    To summarize the above, assuming that applications and accessories described in the preceding paragraphs have been successfully installed, and that either physical or virtual COM ports exist and are connected, and finally that the serial channel is appropriately configured for communication between DDEOrbitronToSerial and SDRuno, the step-by-step procedure for implementing Orbitron-sourced Doppler frequency tracking in SDRuno is as follows:

    1) Start up Orbitron and SDRuno (order doesn’t matter).

    2) In Orbitron select the satellite that you want to track.

    3) Start DDEOrbitronToSerial.exe (this is a concession to not having this utility on Orbitron’s rotor/radio driver list).

    4) If this is the first time, enter a Custom Format String to construct the CAT command for setting downlink (Doppler corrected) frequency, as has been illustrated above. The Custom Format String and other utility settings will be saved automatically in an XML file, and only need to be entered once.

    5) When ready to start tracking, select MyDDE from the Orbitron driver drop-downs and start it (button press). The client form may be minimized, as it is not used. If not already enabled, check ‘Send to serial’ in the DDEOrbitronToSerial client window.

    6) The rest depends on the broader application context, which satellite is being tracked, its transmission mode, etc. The video demo accompanying this write-up features the WXtoImg application, which decodes a NOAA VHF satellite transmission (NOAA-15, NOAA-18, or NOAA-19), and renders a pleasing weather map image.

Bandwidth    Disclaimers: First it is not necessary to process Doppler effect frequency adjustments in order to receive and decode NOAA VHF weather images (as featured in the video demo). These satellites transmit an FM signal that is about 38 KHz wide, whereas their maximum Doppler frequency shift is only 2.something KHz. On the other hand, Doppler frequency corrections are needed for orbiting satellites when they are transmitting narrowband modes, such as SSB or telemetry or, for example, the July 2017 International Space Station Ham Radio Anniversary images. Second, alternative setups can be used for Doppler frequency tracking when it is needed. I have previously used Orbitron with SDRSharp for this purpose, and have also exercised Ham Radio Deluxe with SDRuno as well as with traditional tabletop radio equipment.2

    Demo: OrbitronToSDRuno.mp4


1 The receiving antenna that is connected to SDRplay for NOAA VHF satellite reception (video demo) is a homebrew Quadrifilar Helix. As shown in the photo below, this antenna is presently mounted on a 6 foot length of 2-inch PVC in the back yard, about 150 feet distant from the SDRplay. (In the future I may move it to the roof peak for a less obstructed view of the northern horizon.)

Quadrifilar Helix Antenna

2 What’s in a name?—should pre-SDR radios be called ‘chip and solder rigs, or maybe Hardware Defined Radios (HDRs)? Surely they do not deserve the pejorative ‘legacy’ label.

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