Rotator Guide

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Overview
The purpose of this three part series is to give an overview of how SATnogs communicates with
rotators and how to build a CANopen controlled rotator. The European Space Agency has released a
standard for the use of CANopen in satellites. The purpose of using CANopen in our rotator is to gain
familiarity with the protocol.
Part 1 – How SATnogs talks to rotators
Part 2 – How CANopen works and how to use CANopen with SATnogs
Part 3 – How to interface a motor controller to the CANopen bus

Part 1: How SATnogs talks to rotators
SATnogs talks to rotators thru the hamlib library (https://github.com/Hamlib/Hamlib/wiki).
Hamlib allows control of radios and rotators thru a unified API. We are mainly interested in rotctld
which is used for rotator control. Rigctld is used to command gnuradio settings in SATnogs.
Rotctld is a daemon that handles the communication between rotators and applications that control
rotators. Rotctld has support for different rotators. Each rotator type has an id associated with it. The
list can be found here https://github.com/Hamlib/Hamlib/wiki/Supported-Rotators
SATnogs-client communicates with hamlib using rotctl protocol. The commands used by rotctl are
documented here http://hamlib.sourceforge.net/manuals/hamlib.html#rotctld-commands
SATnogs-client first reads the current position of the rotator and after that sends a new az/el that the
rotator should move to. https://gitlab.com/librespacefoundation/satnogs/satnogs-client/blob/master/
satnogsclient/observer/worker.py#L146
Here is an example of the communication to between SATnogs and a hamlib compatible rotator
** ** ** ** **
p\n
33.000000\n33.000000\n

// SATnogs requests the current position of the rotator, the
// command is ended with a newline \n
// Rotator responds with the azimuth and elevation of the rotator. Each
// value is separated by \n and response is ended with a newline \n

P 13.9709333812 1.67219498041
RPRT 0
** ** ** ** **

// SATnogs sends a command to move the rotator to a new
// position.

// Rotator responds with return code. If everything was ok, the code is 0

For commands that are not implemented the rotator should respond with RPRT -4.

The rotator type is determined when starting rotctld. SATnogs rotator uses the EASYCOMM III
protocol but for us NET rotctl might be better. It uses TCP sockets instead of a serial port.

For manual testing rotctld can be started with the NET rotctl rotator on the same computer on port
4333. The rotator server needs to be running before starting rotctld.
rotctld -m 2 -r 127.0.0.1:4333
By default rotctld uses port 4533 for listening to control applications. If you are already running
another instance of rotctld you probably need to change the listening port with -t. Also adding -vvv will
give you more information to debug the possible errors.
rotctld -m 2 -r 127.0.0.1:4333 -t 4000 -vvv
nc can be used to send commands to rotctld when manually testing
nc localhost 4000
\dump_state is a special command that rotctld sends when connecting to the rotator using the NET
rotctl backend. It queries the rotator for capabilities but all of the fields are for rigctl so it isn’t that
useful with only rotators.
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