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Topic: Yay Space (Read 705 times) previous topic - next topic

srnet

I'm curious - would LoRa cope with the Doppler shift of orbital velocities?
Interesting you should mention that.

Back in 2014, LoRa was very new, there were no libraires and it was a case of reading the data sheet to work out what was going on.

I had started looking at LoRa, following the $50SAT project, because it showed great potential for simple very long distance comminications and thus very promising for very small satellites. The RFM22B used in $50SAT had been made to work for two way digital comms, but it needed some decent antennas and expensive low noise amplifiers to make it work.

I noticed during my endevours that at low LoRa data rates, 100bps or so, I was getting packet CRC errors where the rate of errors went up the longer the packets were and the more power was used to transmit them. I theorised that this was due to thermal heating of the RF chip causing drift in the reference oscillator.

LoRa receivers have a capture range in that the receivers frequency has to be within 25% of the transmitters for packet reception to start. At a bandwith of 61.5kHz, the capture range would be +/- 15kHz or so, within the  range of the normal satellite doppler shift of +/-10kHz.

However it appeared that once a packet reception had started the amount of frequncy drift of receiver or transmitter was very small at around 1.5% of the bandwidth. There was no mention of this issue in the datasheet at the time, but it has since been added by Semtech. There is a low data rate optimisation flag that can be set which virtually eliminates the problem.

Potentially there could then be a problem with the rapid rate of change of doppler, 100s/hz a second, that occurs as a satellite moves overhead. However when a satellite is very distant, 1000km+, the doppler is fairly static at +/- 10kHz so it seemed likely that LoRa would only have a problem close in on overhead passes.

I mentioned all this in a report in early 2015, pointing out that LoRa could have a issue with satellite doppler.

Many years later LoRa was actually tested from space by borrowing some time on a 169Mhz maratime satellite that had a decent SDR on it that could generate LoRa. This was the 'LoRa from Space' experiment during the Things Network conference 2018 and it was a success.

The LoRa was transmitted on 169Mhz at 26dBm (400mW) into an approximate 6dB gain steerable yagi which was pointed to the horizon and at Amsterdam during the pass, data rate was 292bps. The LoRa was received on the ground with a simple omni as soon as the satellite cleared the horizon, a range of 2763km.

I was there at the conference and despite only having a 434Mhz LoRa receiver with rubber duck antenna, I was able to receive the 'LoRa from Space'

The experiment is documented here;

LoRa From Space

I get a mention at 25:40


The current LoRa distance record, ground to high altitude balloon is 702km. That was at 868Mhz, 14dBm, 300bps, simple omni antennas. So moving down to the 437Mhz amateur satellite band would reduce free space losses by 5dBm and you could increase power by 6dBm to 20dBm, this is an increase in link budget of 11dBm or equvalent to a range of 2,490km, should be plenty.   

$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

Fivevolts

Hello all :D

this is my first post ever on arduino.

i heard about the satellite week  rollover on April 6th 2019. anyone know if sim808 modules will be affected by this?



thanks!

srnet

Hello all :D

this is my first post ever on arduino.

i heard about the satellite week  rollover on April 6th 2019. anyone know if sim808 modules will be affected by this?


thanks!
Its a GPS issue, there is a forum for GPS stuff, better to ask in there, not many people use GPSs in space.

Networking, Protocols, and Devices - I2C, SPI, XBee, GPS, etc.

$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

Fivevolts

i see, sorry about that , thanks :) ill ask in the other forum

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