Does GPS not adjust height during flight?

[Jr. Stafford]

I may be clueless with this question, but not sure I understand it. Does the Mavic not adjust the altitude in-flight with the GPS Positioning?

Example: Do I understand that my 'height' on the screen is height above the take off point, and not the actual height above the ground at any given moment? I had taken off at my Dad's house and flown to his friend's house a half mile or so away, and his friend's house is down in a bit of a hole. We called the friend and he came outside and watched the Mavic. I tried to get as low as possible, it told me I was at 12 feet off the ground at one point, but he said I was WAY HIGHER than that. I assumed that GPS positioning would give me a reading of where it is at any point instead of the height since takeoff? I had flown to another friend's house, and when it told me 200 feet above the ground I was suddenly getting a warning for an obstacle. Seems like it would be very hard to get close to the ground as the terrain raises and falls so much if GPS don't give an actual reading of how close the ground is?

 

[F6Rider]

No, you are correct in that the GPS altitude is referenced from the take off point, and you can get negitive numbers if you fly lower than that. The mapping system used does not have altitude information for the gps to reference. The gps and barometer give altitude from the take off point, but would need the altitude information for the ground your flying over to give you accurate HAG for any given area. If you have all of your sensors on you will get altitude readings from that once you get low enough, not sure what it is without looking it up, about 30' I think.

 

[Jr. Stafford]

Good to know, I could've slammed the ground or the top of a tree thinking I was 200 feet off the ground!! I assume the sensors in the bottom would warn me before that happens?

 

[F6Rider]

Good to know, I could've slammed the ground or the top of a tree thinking I was 200 feet off the ground!! I assume the sensors in the bottom would warn me before that happens?

Yes they would, at about 30' or so you will start getting a distance reading. In situations like this I make sure to point the camera down. Also, be aware that depending on how far away you are once you start dropping in altitude, especially into a lower area you can lose signal, so pay attention to your RTH height setting.

 

[Jr. Stafford]

Good tips, thank you. I actually set the RTH pretty high just to make sure I clear EVERYTHING on the way home lol.

 

[halley]

Just some notes on altitude coordinates.

• The GPS concept measures altitude relative to the Earth's center; the math involved won't change with tides, weather, erosion, mountains, valleys, temperatures, magnets, construction, or anything else.

• The GPS concept in fact has no idea where the ground or the sea level might be at any given location on the surface of the Earth. It's completely ignorant of geography or the tides. The only geography it knows is a theoretical radius from the center of the Earth that it calls 0 MSL (0 units above mean sea level).

• Google Maps, or your automobile's navigation system, may have a database that has some sort of altitude data for geography. They gather this by a number of means, such as government survey data and aerial photography measurements. This is what Litchi's "Mission Hub" mapping system will show at each waypoint. This is very helpful, but also can be very misleading. Be super careful to study the differences between the map database's actual data resolution, and the current situation around you. Buildings come, hillsides get bulldozed away, rivers dry up, etc. Tides also ebb and flow, wax and wane, so the morning's sea level isn't even the same as the afternoon sea level.

• The Mavic concept measures altitude from GPS at the takeoff point, and we call that 0 ATO (zero units above takeoff). From then on, everything in the software of the controller and the aircraft is measured relative to that 0 ATO reference point.

• Full size aircraft calculate their altitudes in a number of ways.

• Near the runway they often use a ground-oriented radio reflector scheme to determine ALT AGL (altitude above ground level). This is noisy, easily confused, and only usefully accurate to a few dozen meters. The Mavic also has a similar system with the ultrasonic clickers, but that's only good for about a meter or two.

• Higher up, they use a barometric air pressure scheme to determine ALT (air data altitude). Once you're higher than the treetops, this scheme is surprisingly accurate, as gravity compresses the atmosphere very consistently even with changes in relative humidity. None of the Mavic IMU sensors appear to use barometric altitude (correct me if I missed something), but some wristwatches and exercise devices do.

 

[Ender]

Just some notes on altitude coordinates.

• Higher up, they use a barometric air pressure scheme to determine ALT (air data altitude). Once you're higher than the treetops, this scheme is surprisingly accurate, as gravity compresses the atmosphere very consistently even with changes in relative humidity. None of the Mavic IMU sensors appear to use barometric altitude (correct me if I missed something), but some wristwatches and exercise devices do.

Hi !
The IMU only contains gyro (angular speed) and acc (gravity) sensors.(Inertial Measurement Unit).
Problems: Thermal drift and "overload" caused by strong vibrations i the right frequency causing the measuremens to go haywire.
The Mavic contains at least one barometer, its a seperate sensor. And YES, it can have a great precision if it is shielded against light (surrisingly influences measurements) and the crafts ohn local turbulences.
So most are wrapped in black porous foam or something
even more sophisticated ;-)

And the ultrasonics have a greater range then 2 meters its somewhere in the manual but i'd guess between 10 and 15 meters. Range depends on the quality, focus and number of the sonars as well of the smartness of the data processing.

Sorry, no "smartasssing" intended, just providing the info you asked for i hope,

Ender

 

[Logger]

Yeah Ender that was how I understand it too.. It has and uses a Barometric altimeter as its reference during flight, not GPS Alt. Baro alt (essentially flying a pressure contour) is a well proven reliable system that is not prone to dropout as is GPS alt. The baro altimeter in my wristwatch is surprisingly accurate too and is usually within a handful of feet of a very sophisticated aircraft that I compare it too. I expect the Mavics baro altimeter/s would be as accurate or better. No doubt if we check - the controller will still give an Altitude readout if the Mavic finds itself in Atti mode due to loss of GPS or flying indoors. Interesting about the light sensitivity. Never knew that.

 

[halley]

Thanks, Ender. I agree that a barometric ALT is a solid and reliable source of data even in GPS dropouts. I would be surprised if it used (or relied mainly upon) the baro ALT for the initial 0 ATO reference though, because it's not particularly accurate near the ground. Too many thermal or eddy current distortions to the air pressure down there.

 

[BudWalker]

The Mavic seems to be just like the P3 in this regard.

After batteryOn gpsAlt typically is the first to start reporting a value.

Next baroAlt starts reporting a value. At this time the gpsAlt is changed to have the value reported by baroAlt. I would have thought that baroAlt would be pegged to gpsAlt. Go figure.

At flightStart relativeHeight is set to 0.0. The Mavic .DAT file shows that the "TakeOff" altitude is set to the baroAlt. The P3 .DAT doesn't have a log entry for this but I suspect it does the same.

Also at flightStart the homePoint is set to the current GPS lat/long. And, the homePoint altitude is set to be 20 meters higher than the baroAlt. I think this is to allow for the possibility that a barometric pressure change could occur between takeOff and a subsequent RTH. This has to mean that the baroAlt is used for navigation.

You can see all this by using DatCon and setting the lower time to recordingStart since most of this stuff takes place before motorStart or gpsLock.

 

[Vlakvark]

Thanks everyone! This thread, so far, has been very instructive for me. The area over which I do most of my flying changes approx. 100 meters in altitude, so I constantly have to calculate waypoint altitudes to be safe. I do not rely on any obstacle avoidance because there are many dead trees and also power lines on 40 meter masts, which will not be seen by my Mavic.
So far, thankfully, no mishaps after 107 flights. A few anxious moments though, with two TBE events and an aerial combat type attack by plovers, locally known as "Kieviets". My Mavic also happens to be named "Kieviet".


Sent from my iPad using MavicPilots

 

[MavicKhan]

• Full size aircraft calculate their altitudes in a number of ways.

• Near the runway they often use a ground-oriented radio reflector scheme to determine ALT AGL (altitude above ground level). This is noisy, easily confused, and only usefully accurate to a few dozen meters. The Mavic also has a similar system with the ultrasonic clickers, but that's only good for about a meter or two.

Regarding RA:

It is not "noisy".

It isn't easily confused.

It is accurate to a couple of feet.

MK