Doppler Tuning Algorithm for Gpredict

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Objectives

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.

Requirements

  1. Gpredict shall be able to perform Doppler tuning for both uplink and downlink.
  2. Gpredict shall be able to take external frequency changes into account, e.g. when the user turns the dial.
  3. Gpredict chall be able to perform Doppler tuning using receivers, transmitters, simplex and duplex transceivers, including any combinations of these
  4. Gpredict shall take into account that there might be transverters in the loop.

Operating Modes

  • Separate RX and TX — possibly only one of them, possibly both.
  • Simplex TRX
  • Duplex TRX


Algorithm

The control algorithm will need the following parameters:

Doppler
The current Doppler shift at 100 MHz.
SatRX
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).
SatTX
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).
RadioRX
The downlink frequency of the radio.
RadioTX
The uplink frequency of the radio.
RadioPTT
The PTT status of the radio. When available, this is used to switch between RadioRX and RadioTX tuning on simplex radios.
PrevRX
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.
PrevTX
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.


Separate RX and TX

Simplex TRX

Duplex TRX

Implementation

Tests

Conclusion