Hey guys, so I/m currently working on a project to launch a rocket past the Karman Line and I need to be able to calculate the altitude of an arduino in a vacuum, making pressure sensor not practical. Would I be able to tackle this using an IMU with gyro's, magnetometers and accelerometer to calculate simple kinematic equations thought in Physics 1? If so, how would I start? If not, what would work better?
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Hi,
Try a TOF sensor, they use lidar type optical system
Google
arduino tof sensor
There are quite a few sensors to use with Arduino.
Tom... 
One of the new ESP32 can show distance between unit's bases on wifi TOF
Good for short distances, line of sight.
Your first post as originally posted didn't have any distances listed.
Insufficient information to know what you are actually attempting to do, by a long shot! 
Come on! 
Where is this vacuum, how big is it? How does the system move in the vacuum?
Besides, NONE of the Arduino associated devices are qualified or designed for a vacuum. Many devices will explode.
Paul
Interesting result of a quick google search. apparently someone has tested an UNO in a vacuum, although the testing seems to be more concerned with the affects of radiation:
Microcontroller Survivability in Space Conditions
I am really surprised the electrolytic caps did not bulge and pop.
Paul
We've actually already tested in a vacuum and using Dry Ice to simulate Temperatures and it works no problem but above 100,000 ft barometers aren't very accurate.
Sorry just edited OP but it's a rocket, We're trying to reach around 360,000 ft and surpass Karman Line. We are doing small scale testing though but would live to stray away from Barometers as they won't operate past 120,000 ft in the best case scenario.
How far from the ground can they operate?
You should be able to find a specialized GPS receiver that will function at that altitude, I can find references to GPS being usable in low earth orbit. High quality IMU would work - certainly you already have that for the guidance system.
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I actually looked into that for a bit but unlocked GPS that allow for that altitude (above 50 km) are hard to come by and extremely expensive
They dont want to make it easy for people to launch, and control, high altitude missiles.
There is a TOF function in the newer ESP32S2 I believe, but thats WiFi stuff and unlikley to go very far.
The TOF distance measurement function of the SX1280 LoRa has been tested, by me, to 85km;
And whilst you say a proper GPS for the job would be 'extremly expensive' how much is that actually and what is the cost of the entire project ?
Our goal is to keep it sub $3,000 excluding test articles but the GPS is not just expensive on cost, but also weight as due to the regulations, not many are willing to give us an unlock GPS we can put on our own board and use, most will actually be fully integrated computers which means we will need a second for parachutes and so forth. Would you be willing to share the settings you used for LoRa, we've been testing range on ours and have yet to get more than 30 km at one point but nothing more.
Hi,
Or use TinyGPS+++
Just a thought. Not sure on high alt readings but extensive data on Neo-6M and its many variants.
Tom...
Which type of LoRa ?
UHF LoRa would be fine for the comms for a rocket and distances covered would easily be in the 100s of km.
2.4Ghz LoRa (SX1280) does not have the same range but the distance measuring function (ranging) works at up to SF10 @ bandwidth 400kHz, see the datasheet. Those were the settings used for my 85km.
We're using the RFM95W LoRa from Adafruit.
If your comms are line of sight, then if your getting a mere 30km, your doing something wrong, what LoRa settings are you using, what are the antennas and what is the test situation ?
Even at ISM band reduced power levels, distances of 400km+ for trackers are the norm on simple antennas and 800km+ has been done, ground to high altitude balloon.
We've been using the defaults to get the 30 km range but now that weve been messing with settings trying to increase, we haven't been able to even get more than 10 these are the most recent:
rf95.setTxPower(23, false);
rf95.setCodingRate4(8);
rf95.setLowDatarate();
rf95.setPayloadCRC(true);
rf95.setSignalBandwidth(125E3);rf95.setSpreadingFactor(12);
We're also using these anteannas:
Transmitter: https://www.newark.com/lprs/ant-ss900/antenna-stub-straight-868-915mhz/dp/91T4833?gclid=CjwKCAjwp_GJBhBmEiwALWBQk3ZiwtwcT1Hg9NzElBSpFwx3EsihIU6AdXdHFp24Gwpf8SRil5uSVxoCS5AQAvD_BwE&mckv=sOPFyArIG_dc|pcrid|434136793422|plid||kword||match||slid||product|91T4833|pgrid|100464451106|ptaid|pla-902275118042|&CMP=KNC-GUSA-GEN-Shopping-NewStructure-Audio-Video
Receiver: RAK LoRa Antenna - Fiberglass, 5.8 dBi, N-TYPE to SMA – Grid Connect
Our testing is basically done by having one person stand on one of four bridges that are 70ish feet tall over water and another person on another bridge, if we get signal we move to further bridges and so forth.