Demonstration of why Altitude Limits should be removed

[lmmavrick]

Just to be fair and honest and this mostly just for me but I would be fine with a 400 ft. max limit and a max distance of 1/2 mi for my P3S.Now,if you have a P3A or P3P or greater then the distance should be as advertised instead if updating the software and limiting the range !.
If you are on vacation or in the mountains and turn on your quad copter then it should be smart enough to set at 0 ft. even tho you might be 900 ft elevation.That would still give you 400 ft. to fly at that location.

if you're in the mountains then 400" is a huge limiting factor , I can fly the 1640 max and still not be over 400' AGL and wising I could go 2500 or more .

 

[Squirrel!]

Started at ground level in the valley and flew to 2400 feet altitude yet was never more than ~400 off the ground.

PILOTS should be responsible for their actions, NOT DJI. DJI is missing a huge hole in their safety program. Buy maintaining strict control over the hardware, DJI is opening itself up to lawsuits. Why would one sue or charge only the pilot when DJI is publicly pushing flight restrictions in the name of public safety? I'd go after DJI for the failure of it's advertised safety features, no matter what mods are made to the craft. Coupled with the fact other drone manufactures are now selling drones with 'no geofencing' as a feature is proving DJI has done more harm than good to the industry.

So then...what are you actually complaining about? 400' limit means anything can be the floor...a tree, a building, a mountain. Sooo, what are you complaining about?

 

[Bakersfield Quad]

Can you explain what the regulation intends us to do? Is it to stay 400 feet above ground? Why? Does planes fly above 400 feet? Sorry I'm not a pilot I'm not really sure what I am supposed to follow even if I want to.

Yes, planes stay above 500 feet, in most cases.

 

[nomnomnom]

Yes, planes stay above 500 feet, in most cases.

ok but what if I am at the top of a building's roof? Then I have to subtract the building's height from 400? How am I suppose to know that?

 

[Meta4]

DJI's programmatic altitude restrictions are fundamentally flawed because they fail to take into consideration changing ground elevation contours

Your argument is fundamentally flawed because of a number of your assumptions including this.
DJI are well aware that the earth is not pancake flat.
DJI's altitude measuring and limits are what they are because it's a simple solution that works and doesn't require expensive or weighty additional technology.

If you fly a Cessna or other light aircraft, chances are that you still don't have the sort of technology you think DJI should supply. It's up to you the pilot to do the mental arithmetic to deal work out AGL heights where necessary.

DJI altitude restriction algorithms assume and operate on the erroneous assumption that maximum altitude should be limited to a fixed launch-point ground-elevation (relative to sea level). Thus they erroneously cap our altitude at 400ft above launch point. That is not what the FAA guidelines say nor was it the FAA's intent.

Umm ...no. DJI don't limit you to anything relative to sea level, and they don't cap your altitude to 400 feet above launch point.
It's 1640 feet and that's relative to where ever you launch.

The reason DJI did what they did is because it's a lot easier and cheaper to implement restrictive algorithms based on gps elevations (and/or relative barometric pressures) than maintaining and referencing an on-board USGS vector map of ground elevations for every point a bird flies "normal" (meaning perpendicular) to changing ground elevations.

DJI don't use GPS at all for anything to do with altitude (if they did we'd all have crashed long ago as GPS is woefully inaccurate for altitude).
As above DJI's system is cost-effective and lightweight.
No maker has anything like what you are describing. I haven't heard of it in real planes either.
Perhaps it's not so easy to implement.
Either that or there's no real need to advance past the elegantly simple system we already use.

DJI's failure to engineer an ability to follow variable ground contours is a huge oversight, and frankly, much worse than fixing a flight ceiling based on launch elevations. Anyone who has performed aerial land surveys knows this. It reflects a laziness inconsistent with the generally good engineering we've all come to expect from DJI.

DJI's failure to provide the kind of technology that's used in expensive jet fighter aircraft but not general aviation is not an engineering failure.
You are hoping for much too much from a cheap ready-to-fly drone.
I doubt you'd be willing to pay for what you desire.

 

[Bakersfield Quad]

ok but what if I am at the top of a building's roof? Then I have to subtract the building's height from 400? How am I suppose to know that?

AGL Above Ground Level, look, there are a lot of regs, just trying to answer you question, why 400 AGL for you.

 

[Meta4]

DJI (as do other manufacturers) compute MSL altitude by computing the Doppler shifts received from the numerous satellites it's GPS systems uses to fix geographic location. You may not know where or how high your bird is, but it is a sure bet that the onboard electronics knows.

If or when GPS systems fail, comparing the starting barometric pressure and temperature at launch against barometric pressure and temperature in-flight provides a fall-back means to calculate approximate AGL altitude with similar precision as you'll find in any aircraft. Calibration is always desirable but only relevant to determining MSL altitude. Here we're concerned about AGL altitude.

This is just plain wrong.
The only thing a Phantom uses to measure altitude is its barometer.
It knows the air pressure at launch and compares that to calculate it's relative altitude.
And that's reasonably accurate over the time span of a typical Phantom flight.
Once out of VPS range, the Phantom has no idea how high it is above the ground.
There is no reference to MSL at all, simple altitude relative to launch point.

This is not all that different from what the pilot of a Cessna uses except he calibrates his barometric altimeter to give a reading above sea level so he can work with chart heights and flight levels.
But he's still using a barometer to calculate his height relative to where he launched +/- a factor to express that as MSL.

Many commercial UAVs employ small LIDAR units to measure and map altitudes "normal" to ground positions. There are more solutions out there than you realize. Adaptation to consumer UAVs means high volume low cost production that will hardly impact weight/lift/power consumption ratios.

Can you point to some of these machines?
How much do they cost?
Perhaps they would be more suitable for your purposes.

Putting vector based USGS terrain map DATA on a tiny micro SD or surface mounted memory chip is easily doable and would provide the necessary baseline data for computing an AGL altitude that conforms to the Guidelines. It's better than what we are currently buttonholed with and better than doing nothing at all.

If it was really easy and doable ......perhaps we'd see such solutions on the market.
Actually what we have is elegantly simple, reliable and cost-effective.
Those are good attributes in a ready to fly drone and quite adequate for most users most of the time.

 

[sar104]

This is just plain wrong.
The only thing a Phantom uses to measure altitude is its barometer.
It knows the air pressure at launch and compares that to calculate it's relative altitude.
And that's reasonably accurate over the time span of a typical Phantom flight.
Once out of VPS range, the Phantom has no idea how high it is above the ground.
There is no reference to MSL at all, simple altitude relative to launch point.

This is not all that different from what the pilot of a Cessna uses except he calibrates his barometric altimeter to give a reading above sea level so he can work with chart heights and flight levels.
But he's still using a barometer to calculate his height relative to where he launched +/- a factor to express that as MSL.

Can you point to some of these machines?
How much do they cost?
Perhaps they would be more suitable for your purposes.
If it was really easy and doable ......perhaps we'd see such solutions on the market.
Actually what we have is elegantly simple, reliable and cost-effective.
Those are good attributes in a ready to fly drone and quite adequate for most users most of the time.

Minor correction to prevent any confusion - they do measure altitude MSL from the GPS data - they just don't use it for flight control.

 

[DefiantChild]

You are correct that altitude MSL is computed by the FC from GPS data (although that is not a Doppler measurement - it's time of flight), but that is not the primary altitude measurement and is not as accurate, over the relatively short duration of a flight, as the relative altitude (relative to the takeoff point, not AGL) derived from barometric measurements.

The GPS-based altitude MSL is not used at all in controlling the aircraft - that is entirely the barometric altitude. There is no AGL altitude measurement at present - perhaps that was your point - I wasn't clear on that.

I misspoke and should have said typical GPS units internally use Doppler effect to not only compute geoposition relative to the earth's surface but also compute altitude normal to its geoposition. Actually, position is computed 3-dimensionally relative to the earth's core and requires signals from at least 4 satellites.

Doppler shift occurs as transmit signals traverse space from each detected satellite location to a receiver. Those shifts are detected as frequency deviation variations used to compute satellite distances. Combining known satellite positions and their distances from a receive allows a GPS to triangulate the receiver's location in 3D space.

So the FC could compare the 3D altitude vector at launch to the 3D altitude vector in-flight to derive a relative altitude differential. Except the earth isn't smooth nor perfectly round (it's actually flat, so the voices tell me) which is why a geospatial vector map is needed to provide a general ground elevation normal to the bird to derive an actual AGL.

I respectfully disagree with you about barometric based altitude derivations being more accurate than GPS derived altitudes. That may have been true in the mid 60's to late 70's, but it is not true today. Barometric pressure is subject to pressure density fluctuations that, given equal elevations or altitudes, change from one geolocation to the next. A lot can happen during a 25 minute flight. Think turbulence and microbursts.

My opinion (which is worth the price you pay for it) is this is why DJI recommends pilots switch from P-mode to A-mode when things go wonky. I think of it as a failover but less accurate solution.

Where we do agree is that while all the necessary telemetry is available to the FC to calculate MSL altitudes, the data is meaningless without either an instrument that can measure distance to the ground (radar/LIDAR/laser ranging), or a geospatial map from which the FC can correlate its position to derive AGL. Ensuring precision through redundancy probably requires both instrumentation and a map.

Just a parting thought, DJI has bottom mounted cameras (on the P4P and other birds) that already optically derive AGL using downward facing cameras by stereoscopically computing the distance between bird and ground. I've read a number of threads regarding flight over bodies of water being problematic due to the lack of sufficient detail to reliably compute distance. Upgrading that system and adding some NV capability certainly couldn't hurt.

But if it were up to me, I would not put a lot of credence into the approach unless the FAA were to amend its guidelines to define "ground" as anything on the earth's surface with sufficient density to appear ground-like (such as tall dense trees, forests, etc.) and DJI solves its various issues with vision distancing. But that's probably a discussion for another thread.

 

[DefiantChild]

Can you explain what the regulation intends us to do? Is it to stay 400 feet above ground? Why? Does planes fly above 400 feet? Sorry I'm not a pilot I'm not really sure what I am supposed to follow even if I want to.

It's designed to cap your flight altitude to under 400ft above the ground or ground based "structures" (buildings, trees, tall angry giants, etc.)

The relevant Part 107 Regulation reads:

"The altitude of the small unmanned aircraft cannot be higher than 400 feet above ground level, unless the small unmanned aircraft:

(1) Is flown within a 400-foot radius of a structure; and

(2) Does not fly higher than 400 feet above the structure's immediate uppermost limit."

The Regulation does not mandate a 400ft flight altitude, rather, you cannot exceed 400ft above the ground or "structure".

I don't recall angry giants specifically mentioned in any definitions so they and large dinosaurs may be subject to interpretation.

 

[sar104]

I misspoke and should have said typical GPS units internally use Doppler effect to not only compute geoposition relative to the earth's surface but also compute altitude normal to its geoposition. Actually, position is computed 3-dimensionally relative to the earth's core and requires signals from at least 4 satellites.

Doppler shift occurs as transmit signals traverse space from each detected satellite location to a receiver. Those shifts are detected as frequency deviation variations used to compute satellite distances. Combining known satellite positions and their distances from a receive allows a GPS to triangulate the receiver's location in 3D space.

So the FC could compare the 3D altitude vector at launch to the 3D altitude vector in-flight to derive a relative altitude differential. Except the earth isn't smooth nor perfectly round (it's actually flat, so the voices tell me) which is why a geospatial vector map is needed to provide a general ground elevation normal to the bird to derive an actual AGL.

I respectfully disagree with you about barometric based altitude derivations being more accurate than GPS derived altitudes. That may have been true in the mid 60's to late 70's, but it is not true today. Barometric pressure is subject to pressure density fluctuations that, given equal elevations or altitudes, change from one geolocation to the next. A lot can happen during a 25 minute flight. Think turbulence and microbursts.

My opinion (which is worth the price you pay for it) is this is why DJI recommends pilots switch from P-mode to A-mode when things go wonky. I think of it as a failover but less accurate solution.

Where we do agree is that while all the necessary telemetry is available to the FC to calculate MSL altitudes, the data is meaningless without either an instrument that can measure distance to the ground (radar/LIDAR/laser ranging), or a geospatial map from which the FC can correlate its position to derive AGL. Ensuring precision through redundancy probably requires both instrumentation and a map.

Just a parting thought, DJI has bottom mounted cameras (on the P4P and other birds) that already optically derive AGL using downward facing cameras by stereoscopically computing the distance between bird and ground. I've read a number of threads regarding flight over bodies of water being problematic due to the lack of sufficient detail to reliably compute distance. Upgrading that system and adding some NV capability certainly couldn't hurt.

But if it were up to me, I would not put a lot of credence into the approach unless the FAA were to amend its guidelines to define "ground" as anything on the earth's surface with sufficient density to appear ground-like (such as tall dense trees, forests, etc.) and DJI solves its various issues with vision distancing. But that's probably a discussion for another thread.

I'm not quite sure where you are going with all that. A couple of comments:

You have misunderstood the Doppler effect, which is the apparent frequency shift of propagating waves due to the relative motion of the transmitter and receiver. Dopper shift can be used to measure relative velocity, not relative position, which is what GPS does. GPS works by measuring the time of flight of the transmitted signals from the satellite constellation, which gives distance to each satellite. Combined with precise ephermeris data (i.e. the exact locations of the satellites as a function of time) that allows the GPS receiver to compute its own position relative to the Earth's center as you stated.

You may disagree on the relative accuracy of GPS and barometric altitude, and I agree that it is much better than before, especially in the days of SA, but, either way, barometric altitude is what the flight controllers use. In ideal conditions with a clear view of multiple satellites I suspect that GPS altitude would work fine, but in more restricted areas it will not be good enough. GPS altitude accuracy for these kinds of receivers is +/- 10 m or so. Resolution is better than that, of course, but drift is continual, even on the timescale of seconds. Barometric altitude measurement easily resolves a couple of meters and its drift is generally much slower.

The easiest answer for altitude AGL is simply to use a reasonable digital elevation model. That just requires more onboard memory than is currently allocated. Stereoscopic vision is not going to work at altitudes of hundred of feet.

 

[Meta4]

I respectfully disagree with you about barometric based altitude derivations being more accurate than GPS derived altitudes. That may have been true in the mid 60's to late 70's, but it is not true today.

Sorry to bring you the bad news but GPS is really bad for altitude.
You may have a WAAS enabled GPS unit that gives better altitude accuracy but this doesn't work for the rest of the world and I haven't seen any confirmation that DJI employ a WAAS GPS unit. But if they did it still wouldn't help users in most of the world because the WAAS system only works in North America.
Here's what Garmin have to say about GPS altitude accuracy and why GPS altitude data is useless.
How accurate is the GPS elevation reading?
GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, these are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.?

Barometric pressure is subject to pressure density fluctuations that, given equal elevations or altitudes, change from one geolocation to the next. A lot can happen during a 25 minute flight. Think turbulence and microbursts.
Yes ..turbulence and microbursts. How often do you fly in that sort of weather?
The barometer is quite accurate enough for Phantoms (and Cessnas).

My opinion (which is worth the price you pay for it) is this is why DJI recommends pilots switch from P-mode to A-mode when things go wonky. I think of it as a failover but less accurate solution.

Why? What advantage would atti mode give you?
I've never seen this DJI recommendation. What's the source?
When things get wonky, going to atti mode would reduce the chance of bringing the Phantom back for most people.

Where we do agree is that while all the necessary telemetry is available to the FC to calculate MSL altitudes,

You are on your own there. The Phantom has a very simple altitude measuring system that has no way of calculating MSL anything.

 

[DefiantChild]

I'm not quite sure where you are going with all that. A couple of comments:

You have misunderstood the Doppler effect, which is the apparent frequency shift of propagating waves due to the relative motion of the transmitter and receiver. Dopper shift can be used to measure relative velocity, not relative position, which is what GPS does. GPS works by measuring the time of flight of the transmitted signals from the satellite constellation, which gives distance to each satellite. Combined with precise ephermeris data (i.e. the exact locations of the satellites as a function of time) that allows the GPS receiver to compute its own position relative to the Earth's center as you stated.

You may disagree on the relative accuracy of GPS and barometric altitude, and I agree that it is much better than before, especially in the days of SA, but, either way, barometric altitude is what the flight controllers use. In ideal conditions with a clear view of multiple satellites I suspect that GPS altitude would work fine, but in more restricted areas it will not be good enough. GPS altitude accuracy for these kinds of receivers is +/- 10 m or so. Resolution is better than that, of course, but drift is continual, even on the timescale of seconds. Barometric altitude measurement easily resolves a couple of meters and its drift is generally much slower.

The easiest answer for altitude AGL is simply to use a reasonable digital elevation model. That just requires more onboard memory than is currently allocated. Stereoscopic vision is not going to work at altitudes of hundred of feet.

You are correct that Doppler effect is used to measure velocity, but it is also used in the determination and refinement of satellite to receiver ranging. Calculating GPS satellite distance is far more complex than R=CΔt. If GPS sats were geosynchronous, we might not be having this discussion. The fact they have an orbital velocity introduces a Doppler effect into the satellite transmission that impacts ranging. For a more technical discussion:

Doppler Shift | GEOG 862: GPS and GNSS for Geospatial Professionals

(In fact, the entire GEOG862 lesson set is a fascinating read, but I digress.)

The subject of Doppler in this particular forum has no real bearing on the OP or my assertion that DJI has done its consumer base a disservice by failing to provide in-flight AGL though it has the technical means to do so.

 

[Meta4]

DJI has done its consumer base a disservice by failing to provide in-flight AGL though it has the technical means to do so.

Perhaps you didn't notice me saying this is total nonsense.
Your Phantom has pretty well the same altitude measuring system that most of the single engine Cessnas and Pipers have down at your local general aviation airport.
If what you are saying was true, they would all have something to tell them their AGL height too.
As it is, this is one of the least expensive solutions for them: GRA™ 55
It costs $6300 + the display unit.

DJI can't provide you with an AGL height. They absolutely do not have the technical means to do it.
If you have such an unrealistic idea of things, you'll never understand why things are the way they are or how difficult your dreams are to achieve.

 

[DefiantChild]

Sorry to bring you the bad news but GPS is really bad for altitude.

. . .

Here's what Garmin have to say about GPS altitude accuracy . . .

Sorry, but I disagree. What Garmin has to say about their own products does not reflect what the rest of the world is doing, especially in the world of GIS. If what they said had any real merit, companies like Trimble would have been laughed out of the survey business long ago.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations.

Wrong. There are a host of reasons for error, most of which have been eliminated through math, highly improved engineering, and deregulation. One of the biggest issues was clock synchronization between satellites and receivers. Much of the problem was addressed when satellites bearing atomic clocks were placed in orbit. Clock sync was such a significant problem that a millionth of a second deviation in clock sync introduced a 100 meter error. Improvements have dramatically improved accuracy.

When Clinton lifted the restrictions on consumer grade GPS receivers, the industry was free to manufacture precision units instead of crippleware. This too had a significant impact on accuracy.

The earth blocks out satellites needed to get a good quality vertical measurement.

So do other obstacles like trees, buildings, and mountainous horizons. That is (1) why there are many more satellites in orbit today than 10 years ago, and (2) why drones incorporate barometric, visual, accelerometric, gyroscopic, and other telematics into their flight controllers that impact altitude accuracy.

Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions.

There is no "vertical datum". You have satellites and receivers occupying points in Euclidian space. The technology ranges distances between those points three dimensionally. The math does not differentiate between x, y, or z components.

Why? What advantage would atti mode give you?
I've never seen this DJI recommendation. What's the source?
When things get wonky, going to atti mode would reduce the chance of bringing the Phantom back for most people.

Really? If you have to ask the question then you've never experienced a runaway bird operating in P-Mode whose compass decides to run amuck or whose FC experiences some other unexpected malfunction. The only solution in some cases is to flip the controller out of P and actually pilot the bird with some skill.

What does any of this have to do with the original thread topic? I'm talking about finding solutions to complying with the FAA's AGL altitude Guidelines which requires DJI to rethink their defective restrictions and up their engineering game so we can all operate without worrying about whether we're operating legally.

 

[Meta4]

Sorry, but I disagree.

Disagree all you like but the facts remain.
Consumer grade GPS gives a highly inaccurate altitude and DJI only use GPS for horizontal positioning.
But what would I know? I've only been using GPS professionally and for recreation for 20 something years.
A trimble survey grade differential GPS isn't something you want to pay for or have your Phantom lifitng.

Really? If you have to ask the question then you've never experienced a runaway bird operating in P-Mode whose compass decides to run amuck or whose FC experiences some other unexpected malfunction. The only solution in some cases is to flip the controller out of P and actually pilot the bird with some skill.

If you have a compass error in flight (only one of a large number of potential problems) your Phantom has already dropped into atti mode before you think of flicking the switch.
But that still makes no difference to the compass issue that is causing the problem.
In atti mode whether you switch to it or the Phantom does, you still depend on a good compass to be properly flyable.
I very much doubt DJI have ever recommended this action as it isn't going to be much help.
Perhaps you picked it up from one of their people on their forum ... if so, that counts for nothing.

I'm talking about finding solutions to complying with the FAA's AGL altitude Guidelines which requires DJI to rethink their defective restrictions and up their engineering game so we can all operate without worrying about whether we're operating legally.

No ... to comply with FAA guidelines you need to do what most single engine pilots do .. a bit of mental arithmetic and be aware of your surroundings.
No other drone maker is doing what you tell us is simple ... perhaps there's a reason for that?

 

[Meta4]

When Clinton lifted the restrictions on consumer grade GPS receivers, the industry was free to manufacture precision units instead of crippleware. This too had a significant impact on accuracy.

Actually, it wasn't about restrictions on receivers at all.
Turning off SA was all that was needed to make consumer GPS much more accurate - not making better GPS units.
I was on a hill working with GPS the day it was turned off and noticed it immediately.

So do other obstacles like trees, buildings, and mountainous horizons. That is (1) why there are many more satellites in orbit today than 10 years ago

By my counting there were 35 GPS sats (including reserves) up there in Aug 2007 and there are 31 now.
Perhaps you aren't a reliable authority on GPS???
Here's an example of how (in)accurate consumer GPS is when it comes to altitude.
This is a plot taken taxiing around a major airport for 6 minutes.
The track would be level at about 35 ft + 13 ft above sea level but it's all over the place despite there being plenty of sats in a good spread.

The lesson is that GPS is NOT accurate for altitude, which is why DJI don't use GPS altitude.

 

[DefiantChild]

Perhaps you didn't notice me saying this is total nonsense.

Not at all. I've completely disregarded it. You're entitled to your personal opinions.

Your Phantom has pretty well the same altitude measuring system that most of the single engine Cessnas and Pipers have down at your local general aviation airport.

I've trained in and flown Cessnas, Pipers, and King Airs. You're comparing apples to oranges. The missions for fixed wing piloted craft have nothing to do with multi-rotor remotely piloted UAVs. The rules for piloting a manned aircraft are functionally and operationally different than piloting unmanned UAVs. Sure there are some similarities in instrumentation but from what I've personally observed, the instrumentation in modern UAVs are light years ahead of what you find in most piloted (small - single/twin engine) fixed wings.

As it is, this is one of the least expensive solutions for them: GRA[emoji769] 55
It costs $6300 + the display unit.

How did we ever fly without $6,300 radar altimeters and $15,000 display consoles? Your point of reference is extraordinarily narrow. Your talking about helicopter and fixed wing applications at altitudes best suited for radar technologies using equipment whose pricing is heavily burdened by avionics standards, compliance requirements, and liability considerations.

At 400ft AGL, there are plenty of inexpensive (sub $100) LIDAR systems that would adequately serve the needs of most UAV applications.

DJI can't provide you with an AGL height. They absolutely do not have the technical means to do it.
If you have such an unrealistic idea of things, you'll never understand why things are the way they are or how difficult your dreams are to achieve.

I highly doubt that you're qualified to state with any factual certainty whether DJI can provide an AGL altitude or what their technical means and capabilities are. Nor do you know me or my capabilities well enough to opine about what you may think are my "dreams" or my ability to achieve them. I suggest you speak for yourself. You may have been beaten into submitting to someone else's ideas of what is and what is not possible. I have no obligation to do so.

 

[DefiantChild]

Actually, it wasn't about restrictions on receivers at all.
Turning off SA was all that was needed to make consumer GPS much more accurate - not making better GPS units.
I was on a hill working with GPS the day it was turned off and noticed it immediately.

By my counting there were 35 GPS sats (including reserves) up there in Aug 2007 and there are 31 now.
Perhaps you aren't a reliable authority on GPS???

Probably not. But I have a fairly adequate grasp of the tech and became an enthusiast back when dinosaurs roamed the planet.

I'm not sure what the count was in 2007 or how many were actually operational. If I'm not mistaken, 8 IIR-M sats were launched between 2005-2009 and roughly 11 or 12 IIF sats were launched between 2010-2016.

I was also under the impression that GNSS integration at the consumer level occurred within recent years upping the aggregate number of available sats. If you know differently, I'll defer to your greater wisdom.

My recollection from ancient times is that consumer grade units were single frequency devices, had limits on the number and quality of receivers, and generally took a very long time to lock in enough sats to compute position. In contrast, military units were dual frequency, and contained more receivers making them far more accurate. I also seem to recall that manufacturers were required to cripple devices as to altitude and velocity so they couldn't be used to pilot ballistic missiles.

Then on the satellite side SA caused the intentional streaming of errors to further reduce accuracy. So I think there were a multitude of factors in play though I agree that turning off SA was a big step in the right direction.

None of that applies today. I look at the accuracy of my iPhone's GPS, or the GPS built into my Kenwood TH-D74 and am amazed by their accuracies (considering their chipsets have grown smaller while accuracies improve.) I believe GPS.gov reported that while 9.7 meter accuracies were achieved 95% of the time, actual performance was much better and closer to 1 meter.

Don't want to argue the horizontal vs vertical accuracy issue or whether DJI only uses the horizontal data. What I'm saying is that with the right chipset they could and should harness the altitude component and use it with other telemetry and ground elevation maps to determine "true" AGL altitude (within acceptable margins of error.)

 

[AuctioneerChris]

If I take off on top of a 10,000 foot mountain and fly out over a valley I'm at 10,400 feet above the ground but only 400 foot above my take off point. .

You were 10,400 feet above mean sea level, but never 10,400 above the ground.