It is difficult to work on satellites without using Doppler effect compensation. After all, during a flight, the reception frequency changes, especially quickly in the middle of the orbit.

There are two ways to compensate for frequency changes due to the Doppler effect - manually, using two independent transceivers (or one with Duplex function) and automatically using programs such as GPredict, SATPC32 or Orbitron.

In this article I will try to set up automatic correction of transmitting and receiving frequencies in Linux.

To work, you need a configured CAT interface (which was done in previous articles about Xastir packet radio communications and WSJT-X weak signal radio).

Then, you will need two programs:

• GPredict to calculate Doppler effect compensation. Control signals will be issued via the tcp-ip port.
• Rigctld service to receive commands from GPRedict and control the transceiver via the COM port.

Setting:

1, Install rigctld (included in the package libhamlib-utils) and gpredict

su -

apt update

apt install libhamlib-utils gpredict

2. I start the rigctld service using one of the commands in the terminal window

a) only for reception (if there is no transceiver on the table, we only want to check the change in transmission and reception frequencies)

rigctld -m 1 -t 4532&

b) for receiving and transmitting using the serial port /dev/ttyUSB0 and a real transceiver.

rigctld -m 122 -r /dev/ttyUSB0 -s 38400 -t 4532 -C stop_bits=1&

where
-m 122 - model number of the HamLib library transceiver (in this case 122 - Yaesu FT-857) from "Supported Radios" list
or -m 1 - dummy model.
-r /dev/ttyUSB0 - serial port through which the transceiver is controlled using CAT commands.
-s 38400 - port speed in bytes/s, set in the transceiver menu, at which commands will be transmitted.
-t 5432 - number of a free TCP-IP port on the local PC through which the rigctld and gpredict service will work.
-C stop_bits=1 - number of stop bits (if necessary, try specifying 2)

The "&" at the end of the line forces the service into the Linux background.

3. I include calculation of frequencies to compensate for the Doppler effect and control of the transceiver in GPredict.

Menu Edit - Options - Interfaces - Add new
(Edit - Preferences - Interfaces - Add new)

First, for the test, we will add only for the appointment

Name: RXonlySAT
host: localhost
port: 4532
Radio type: RX only
PTT status: no
VFO Up/Down: select MAIN ↑ SUB ↓
LO down: 0
LO up: 0

Where
4532- port for connecting GPredict and rigctld
VFO- indicates in which direction the frequencies will change.
If the first VFO MAIN is 433 MHz and the second VFO SUB is 144 MHz, then the frequencies should change MAIN ↑ SUB ↓
If the first VFO MAIN is 144 MHz and the second VFO SUB is 433 MHz, then the frequencies should change SUB ↑ MAIN ↓
If the first VFO A is 433 MHz and the second VFO B is 144 MHz, then A ↑ B ↓ should be indicated
If the first VFO A is 144 MHz and the second VFO B is 433 MHz, then B ↓.A ↑ should be indicated

For transmission - the same, but indicated the radio type "Radio type":FT 817/857/897 (auto),
PTT status: Read PTT or none. affects the reading of the receive/transmit status when pressing the PTT button.

4. Selecting a satellite and getting started.
   To work through a linear repeater, you need to call the context menu in GPredict under the triangle - “Radio Control”.

   • select the corresponding satellite in the left part of the window and start tracking it - “Path”. Enable the CAT transceiver - “Enable”.
   • disable frequency blocking (press the "L" button),
   • tune in to the station and press the "L" (lock) button,
   • the receiving and transmitting frequencies will change synchronously in different directions (Doppler compensation for upward transmission and downward reception).
   • Now press the PTT key for SSB and speak or answer a CQ call in CW.

5. Observation and control of satellite tracking. to view the frequencies that GPredict calculates. you need to click on the small triangle on the right side of the window and select the “Radio Control” menu item.
   

Then press the "Path", "Engage" and "L" buttons.

Note:
Path- satellite tracking (calculation)
Engage- enable radio control.
L- blocking communication between reception and transmission frequencies.

The tracking of satellite frequencies will begin - Uplink and Downlink, taking into account the Doppler effect:

6. When the rigctld service is running, the latter sends commands to the transceiver to change the receive and transmit frequencies based on the calculation of the Doppler effect in the GPredict program.

Sources:

https://github.com/Hamlib/Hamlib/wiki/Supported-Radios
https://manpages.ubuntu.com/manpages/trusty/man8/rigctld.8.html
*https://sourceforge.net/projects/gpredict/lists/gpredict-discussion

Useful links:

https://www.amsat.org/wordpress/wp-content/uploads/2014/01/Work_FM_Sats-20131010.pdf
https://www.amsat.org/wordpress/wp-content/uploads/2015/09/XW-2CAS-3-Sats.pdf
*https://ukamsat.files.wordpress.com/2017/03/camsat-cas-4a-and-cas-4b-news-release.pdf

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