Doppler Tuning Algorithm for Gpredict
- 1 Objectives
- 2 Requirements
- 3 Configurations
- 4 Operating Modes
- 5 The Controller Algorithm
- 6 Implementation
- 7 Tests
- 8 Conclusion
The first beta version of Gpredict 1.0 included a simple Doppler tuning algorithm that could adjust the receiver frequency and even take external frequency changes (e.g. tuning dial) into account. While it was a very good first cut at automatic Doppler tuning, the shortcomings were quickly identified. For example, it didn't take the PTT status for simplex radios into account and so Gpredict kept on sending RX correction commands to the radio even when the user was trying to transmit.
Clearly, a more sophisticated algorithm is needed and this page describes the design and prototyping of that algorithm.
- Gpredict shall be able to perform Doppler tuning for both uplink and downlink.
- Gpredict shall be able to take external frequency changes into account, e.g. when the user turns the dial.
- Gpredict shall be able to perform Doppler tuning using a receiver, a transmitter, a simplex transceiver, a duplex transceiver — or any combination of these (i.e. separate device for uplink and downlink).
- Gpredict shall take into account that there might be transverters in the loop.
In order to configure a radio device for use, the user has to specify:
- The host name where rigctld is running
- The port number of the rigctld server
- The radio type, i.e. whether it is RIG_TYPE_RX, RIG_TYPE_TX, RIG_TYPE_TRX or RIG_TYPE_DUPLEX
- The conversion frequencies for the transverters, if any.
The operating mode for the controller depends on what kind of radio is used. For now, the following options are considered:
- Receiver Mode
- The radio is only a receiver or a transceiver where tuning via CAT is only possible in receive mode. In this case, Gpredict shall perform Doppler tuning on the downlink only.
- Transmitter Mode
- The radio is only used as transmitter only. Tuning via CAT must be possible in transmit mode. Gpredict shall perform Doppler tuning on the uplink only.
- Simplex Mode
- The radio can be tuned via CAT both in receive and transmit modes. Gpredict will perform Doppler tuning on the downlink and uplink depending on the PTT setting.
- Duplex Mode
- The radio is a full duplex transceiver like the IC-910H or FT-847. Gpredict will perform continuous Doppler tuning using VFO_MAIN for uplink and VFO_SUB on the downlink regardless of the PTT setting.
- Dual RIG Mode
- This mode has been introduced to allow using two different radios at the same time: One for downlink (primary device) and one for uplink (secondary device). This is kind of full duplex mode
The Controller Algorithm
The control algorithm will need the following parameters:
- The current Doppler shift at 100 MHz.
- The downlink frequency at the satellite. This is set by the user on the Gpredict control panel or automatically if the uplink and downlink frequencies are locked, or if RadioRX is changed externally (e.g. via tuning dial).
- The uplink frequency at the satellite. This is set by the user on the Gpredict control panel or automatically if the uplink and downlink frequencies are locked, or if RadioTX is changed externally (e.g. via tuning dial).
- The downlink frequency of the radio.
- The uplink frequency of the radio.
- The PTT status of the radio. When available, this is used to switch between RadioRX and RadioTX tuning on simplex radios.
- Flag indicating whether the PTT status on the radio should be polled and taken into acount (i.e. don't set RadioRX when RadioPTT = true).
- The RadioRX frequency set during the previous cycle. This can be different than RadioRX in that the user may have turned the tuning dial since the last cycle.
- The RadioTX frequency set during the previous cycle. This can be different than RadioTX in that the user may have turned the tuning dial since the last cycle.
There are also other user interface parameters like Engage and Track that influence the overall functionality, but these parameters are outside the scope of the controller algorithm.
In this mode the radio is used for reception only even if it is a transceiver. The reason could be that frequency setting via CAT is not possible in transmit mode as would be the case for the FT-817.
/* Receiver chain */ read RadioRX; if (RadioRX != PrevRX) update SatRX; calculate SatTX if locked to SatRX; PrevRX = RadioRX; else /* update Doppler correction */ RadioRX += SatRX * Doppler / 100 MHz; PrevRX = RadioRX; write RadioRX; endif;
In this mode the radio is used for uplink only even if it is a transceiver. A prerequisite for this mode is the the frequency can be adjusted via CAT in transmit mode.
/* Transmitter chain */ read RadioTX; if (RadioTX != PrevTX) update SatTX; calculate SatRX if locked to SatTX; PrevTX = RadioTX; else /* update Doppler correction */ RadioTX += SatTX * Doppler / 100 MHz; PrevTX = RadioTX; write RadioTX; endif;
In this situation the user uses the same radio for uplink and downlink but in simplex mode, i.e. can not receive and transmit at the same time. The prerequisites for this mode are:
- Radio is set to split mode. Gpredict will not care about VFO A and B — it will simply send uplink or downlink correction to the active VFO.
- RadioPTT can be read
- Radio can receive SET_FREQ commands via CAT in both receive and transmit modes.
/* Are we in RX mode? */ if (RadioPTT = false) read RadioRX; /* did user change RX frequency on dial? */ if (RadioRX != PrevRX) update SatRX; calculate SatTX if locked to SatRX; PrevRX = RadioRX; else /* update Doppler correction */ RadioRX += SatRX * Doppler / 100 MHz; PrevRX = RadioRX; write RadioRX; endif; else /* we are in TX mode */ read RadioTX; /* did user change TX frequency on dial? */ if (RadioTX != PrevTX) update SatTX; calculate SatRX if locked to SatTX; PrevTX = RadioTX; else /* update Doppler correction */ RadioTX += SatTX * Doppler / 100 MHz; PrevTX = RadioTX; write RadioTX; endif; endif;
Note that it is left to the user to set the split mode up on the radio.
In this mode, gpredict updates both uplink and downlink frequencies continuously and at the same time regardless of the RadioPTT setting — RadioPTT is still used though to decide which VFO to track.
To be written
Dual RIG Mode
This is a full duplex mode using two radios. The controller algorithm is very simple:
Execute RX mode controller using primary device Execute TX mode controller using the secondary device
Tests on the RX, TX, and Simplex modes were carried out using an FT-817 (RX only), as well as an IC-765 (RX, TX, Simplex). The FT-817 supports get_ptt via CAT, while the IC-765 does not. However, the IC-765 was connected using a microham USB II interface, which support PTT via the RTS line. The hamlib setup for this was:
rigctl -m 329 -r /dev/ttyUSB0 --set-conf=dtr_state=OFF,rts_state=OFF --ptt-file=/dev/ttyUSB0 --ptt-type=RTS
Unfortunately, this allows PTT to be set and read, but it does not read correctly when PTT is set using the radio front panel. So this option was abandoned.
RX Only Test
This option was tested using:
- FT-817 with PTT feedback ON
- IC-765 with PTT feedback OFF
During the test and error was discovered that caused the controller to be very confused and go into a mode when it was continuously updating the downlink frequency. This was only observed on the FT-817; both the Dummy backend and the IC-765 worked fine. Investigations showed that this was because the smallest tuning step the FT-817 can take is 10 Hz, thus the read frequency was almost always different from the one that has been set in the previous cycle.
The problem was solved by adding an extra
get_freq_simplex() right after setting the new frequency, and using the read back value as
The updated version of the code is available in r238.
TX Only Test
This mode was tested using the IC-765 configured as TX only device, no PTT feedback.
After a minor correction was made (ptt = TRUE by default), the controller worked just fine as it should!
To be done
Duplex TRX Test
To be done
Dual RIG Test