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Topic: Yay Space (Read 3084 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.   

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.

http://www.50dollarsat.info/
http://www.loratracker.uk/

Fivevolts

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

Michael_Parker

I've started building a 7m tall rocket that runs on nitros oxide and solid rocket fuel. Im hoping for 85000 feet, ever hopeful.
Two years ago i bought a cnc lathe ,cnc mill and some 3d printers,i didn't have a clue how to use them.
Thanks to open source help and a chance meeting im good enough to start building.
Now ive started learning about arduino, which im very excited about.
I can edit clever peoples code and now can fill my rocket with gas from 25m away, which after the last test
is a good thing.
Im trying to build a load cell to measure thrust and time and down load it to a sd card or direct onto excell
this is for test firing so i  can decide whether angles or holes should change.
Also i have to trigger two chutes and was optimistic when i was able to program a pressure sencer to down load data to a sd card.I never expected them to be that accurate, recording a 2m variation.
However it turned out they only work up to 35000ft.

Mission:
To photograph/video the curvature of the earth
Record temp ,radiation,height, G force etc
Anything interesting that will fit and i haven't thought of.

Happy to make it a open source project.
 

TomGeorge

Nice. I'm not sure if LoRa will reach 330,000 ft for edge of space. I'm working on my FCC Amateur radio license for some short wave highpower transceivers. I'm only looking for 50,000 feet tops.
Spunik 1.
https://www.popularmechanics.com/space/satellites/news/a28496/how-sputnik-worked/

http://mentallandscape.com/S_Sputnik1.htm]http://mentallandscape.com/S_Sputnik1.htm]http://mentallandscape.com/S_Sputnik1.htm

Transmitted on 7m and 15m bands. 
The elliptical orbit of the satellite had an perigee of 228 km, an apogee of 947 km, and an orbital inclination of 65.1 degrees from the equator. It completed a revolution in 96.17 minutes.

It is possible to talk to ISS on very few watts VHF and UHF with the appropriate aerials.

https://en.wikipedia.org/wiki/AMSAT-OSCAR_7

Oscar7, 144MHz up 28MHz down, great days with IC211 and IC701 transcievers, manually compensating for doppler.
Simple aerials, crossed dipoles.

Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

wolframore

So how long do you actually have while the ISS races overhead? 
Bad boys rate our young girls but Violet goes willingly :)
- this is a mnemonic from BEC

Infraviolet

Post #21, in the UK at the northern most extent of it's orbit (ISS is in an orbit inclined to 50 something degrees) it takes several minutes from appearing in the west to setting* in the east, but can be quite hard to spot until it's overhead, at which point it is easy to track by eye until it sets. The timing shouldn't vary much when seen at other latitudes, the ISS's orbit is not particularly eccentric so doesn't get further away and slower/closer and faster at any location along the orbit.

*you'll only see it in the hours before dawn or after dusk while the sun is down from where you are standing on earth but still illuminating the ISS some 300km above your location.

Type your location into this sight for pass predictions near you:

https://www.heavens-above.com/

wolframore

I saw it just the other evening... it was pretty quick between the time it was visible to when it faded out... 17,500 mph (28,000 kph) is pretty fast.  I read that each pass is different and some can last up to 5 minutes. 
Bad boys rate our young girls but Violet goes willingly :)
- this is a mnemonic from BEC

3893toscano

#24
May 25, 2019, 02:34 am Last Edit: May 25, 2019, 02:45 am by 3893toscano
Buenas Noches A Mi me encantaría viajar a la luna saber mas del universo, de todo lo que se esconde mas aya de la luna, viajar a marte, a veces me da miedo también conocer mas aya, pero me gustaría mucho como cambiar de monotonía quisiera ir a la Nasa...

Jejejeje también tengo intriga de saber si hay extraterrestres como vemos en películas..




bay...wenas noches

juan camilo Toscano :)  :)

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