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I am by no means a scientist and definitely not the smartest guy in the room, but I am curious about how the tech works that I rely on.
I've discovered a few interesting things and thought I'd share in case it helps someone else.
Altitude measurement is not reliable, and why you can safely fly off a cliff
Apparently it is not yet possible to get accurate altitude (above sonar range) using a GPS or Barometer.
The Barometer measures air pressure, not height above the ground. It turns out air pressure is an ever changing lumpy custard that gives even the best aircraft equipment grief. It gives 'sort-of' results. Flying in fog over mountains using just instruments is a crazy idea, even for a pro aircraft pilot. You will often see strange altitudes (like -90m) in your logs because it is quite hard to calculate air pressure from something that generates its own wind system, plus the flux of air as the sun changes position, a gust of wind, or a butterfly flapping its wings in Brazil.
The bad news is that your drone doesn't know how high you are above the ground (sonar excepted), only your rough height above sea level. The good news is that the drone doesn't know how high you are above the ground, so you can fly off the edge of mountains without worrying that height limits will kick in and you will have to hike down to get your craft.
The height limit is for the height above the takeoff point, so if you set your limit to 300ft then you can fly 300ft above the takeoff point regardless of the terrain.
A LIDAR (LASER range finder) would be super accurate in clear weather, but I haven't seen one on a DJI drone yet. (DJI, you there?) Although it is creeping in to higher-end drones.
Those 3D Models of every tree and house you now see on Google Earth were generated using LIDAR scanning.
There is a better way we could use today
With GSM, a drone *can* figure out a precise altitude. Because there is now a vast library of LIDAR altitudes available from e.g. Google Earth API, it is theoretically possible for a mobile phone to download the altitude for the GPS position of the craft and give a more accurate reading, even uploading a chunk of altitude data to the craft before flying.
GPS is only accurate to 5m, sometimes 15m
You just can't do precision landing without optical assistance. Changing atmosphere, numeric precision, processor speed, all play a role in calculating position at light speed. Sometimes there are weak spots in the GPS signals and calculations go out slightly, which is why you need so many (like 10) satellites to get a reliable reading.
GPS is from the US Satellites, GLONASS is from the Russian satellites. DJI Drones use both (hooray for post cold-war collab).
Altitude from GPS is not reliable at all, most craft won't use it unless the Barometer fails.
The GPS altitude is also not related to ground level, but to geometric altitude above an idealised earth shape, making calculations more complex.
WIFI, BlueTooth, and Signal loss
Most countries have 2 unregulated 'free for all' frequencies, namely 2.4Ghz and 5.8Ghz.
Free for All means you can usually do anything in these frequencies without a license (below 1 watt), which is why so many devices use it.
The bad news is that Drone controllers use it too, and are subject to interference by all the other naughty devices spewing out tons of static into the atmosphere. Fly near to a WIFI router and you will have a bad time.
WIFI and Bluetooth are just protocols that operate on these frequencies, so turn off your phone's WIFI and BlueTooth for a cleaner flight.
A lot of long range systems use 900Mhz, 433Mhz or lower, because longer frequencies penetrate better, and because fewer devices cause interference. (Cordless phones used to use these so they are often still free to use). The downside is less information per burst.
5.8Ghz can carry a lot of data, like 1080p video, but fly behind a tree and you lose most of your signal.
The magnetic field is a liquid?
Another flowing lumpy custard is the Earth's magnetic field. Most people remember the infamous toilet bowl incidents from earlier drones, now corrected automatically by software. Still, north changes from year to year, so it is wise to keep updating firmware and software to keep the compass and GPS in sync. They say the North South may even reverse one day, although it will take 7000 years so we have time to recalibrate
I've discovered a few interesting things and thought I'd share in case it helps someone else.
Altitude measurement is not reliable, and why you can safely fly off a cliff
Apparently it is not yet possible to get accurate altitude (above sonar range) using a GPS or Barometer.
The Barometer measures air pressure, not height above the ground. It turns out air pressure is an ever changing lumpy custard that gives even the best aircraft equipment grief. It gives 'sort-of' results. Flying in fog over mountains using just instruments is a crazy idea, even for a pro aircraft pilot. You will often see strange altitudes (like -90m) in your logs because it is quite hard to calculate air pressure from something that generates its own wind system, plus the flux of air as the sun changes position, a gust of wind, or a butterfly flapping its wings in Brazil.
The bad news is that your drone doesn't know how high you are above the ground (sonar excepted), only your rough height above sea level. The good news is that the drone doesn't know how high you are above the ground, so you can fly off the edge of mountains without worrying that height limits will kick in and you will have to hike down to get your craft.
The height limit is for the height above the takeoff point, so if you set your limit to 300ft then you can fly 300ft above the takeoff point regardless of the terrain.
A LIDAR (LASER range finder) would be super accurate in clear weather, but I haven't seen one on a DJI drone yet. (DJI, you there?) Although it is creeping in to higher-end drones.
Those 3D Models of every tree and house you now see on Google Earth were generated using LIDAR scanning.
There is a better way we could use today
With GSM, a drone *can* figure out a precise altitude. Because there is now a vast library of LIDAR altitudes available from e.g. Google Earth API, it is theoretically possible for a mobile phone to download the altitude for the GPS position of the craft and give a more accurate reading, even uploading a chunk of altitude data to the craft before flying.
GPS is only accurate to 5m, sometimes 15m
You just can't do precision landing without optical assistance. Changing atmosphere, numeric precision, processor speed, all play a role in calculating position at light speed. Sometimes there are weak spots in the GPS signals and calculations go out slightly, which is why you need so many (like 10) satellites to get a reliable reading.
GPS is from the US Satellites, GLONASS is from the Russian satellites. DJI Drones use both (hooray for post cold-war collab).
Altitude from GPS is not reliable at all, most craft won't use it unless the Barometer fails.
The GPS altitude is also not related to ground level, but to geometric altitude above an idealised earth shape, making calculations more complex.
WIFI, BlueTooth, and Signal loss
Most countries have 2 unregulated 'free for all' frequencies, namely 2.4Ghz and 5.8Ghz.
Free for All means you can usually do anything in these frequencies without a license (below 1 watt), which is why so many devices use it.
The bad news is that Drone controllers use it too, and are subject to interference by all the other naughty devices spewing out tons of static into the atmosphere. Fly near to a WIFI router and you will have a bad time.
WIFI and Bluetooth are just protocols that operate on these frequencies, so turn off your phone's WIFI and BlueTooth for a cleaner flight.
A lot of long range systems use 900Mhz, 433Mhz or lower, because longer frequencies penetrate better, and because fewer devices cause interference. (Cordless phones used to use these so they are often still free to use). The downside is less information per burst.
5.8Ghz can carry a lot of data, like 1080p video, but fly behind a tree and you lose most of your signal.
The magnetic field is a liquid?
Another flowing lumpy custard is the Earth's magnetic field. Most people remember the infamous toilet bowl incidents from earlier drones, now corrected automatically by software. Still, north changes from year to year, so it is wise to keep updating firmware and software to keep the compass and GPS in sync. They say the North South may even reverse one day, although it will take 7000 years so we have time to recalibrate
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