I have been reading this forum for a couple months as it is related to what I am doing but have never actually posted, I have a lot of questions since RF is not exactly my field of expertise but here is one.
In a short sentence, We are developing a small 5x5x5cm satellite with its communication based on an SX1278 using the LoRa protocol. We are also using FSK and Direct mode with RTTY in order to have a backup and expand the communication options for users. LoRa is the main Focus though.
This has been proven to work on TTNsat-1 using 125khz bandwidth at SF11 and 160mHz.
However, due to bandwidth limitations by the IARU and the increased range of lower bandwidth we are aiming for the lowest bandwidth possible. The only obvious problem with this is the limitation of the transmitters crystal. Another issue to take in mind is the Doppler effect which causes approximately 10khz of deviation during a pass.
We are currently using the DRF1278 Modules at 62.5khz that have a crystal accuracy of -+10PPM, based on the Lora calculator this is within the range of the required crystal. Tests show that they maintain communication with a temperature differential of approximately 60ºC. I am unsure on how much each PPM of difference will make to the frequency offset, it looks to be correct though taking in consideration Lora can tolerate a 20% frequency offset of the bandwidth.
With Doppler and the more extreme temperatures of the earth, this would most likely not give us enough headroom for proper communication unless we increase the bandwidth.
We have implemented higher bandwidth AFC tuning packets but are unsure on whether we will be allowed to use them.
The clear solution to this is the use of a TCXO LoRa module. However, we cannot seem to find a TCXO LoRa module on the market, and the best crystals we could find where ±10PPM.
We can probably manufacture our own custom module for the satellite in the future but for the moment would rather use a proven working module.
Thanks for your help!