Skydemon question on vertical profile

[fireflybob]

I have searched the Skydemon website but cannot find an answer to this one.

Am interested to know how Skydemon computes clearance from the base of controlled airspace. Presumable the aircraft vertical distance from the ground is GPS derived given known position and elevation of the ground at that position. But how is the base of controlled airspace derived on the pictorial?

As there appears to be no correction for pressure and/or temperature this would surely mean the base of controlled airspace is in error?

More and more people are relying on these devices these days but assuming position is known the only arbiter with respect to being vertically clear of controlled airspace is the indicated altitude on the altimeter assuming the subscale is correctly set?

By the way I am not a Luddite when it comes to using Skydemon (although I think all pilots should be trained and practised in visual navigation without using GPS).

Thanks for any help.

[nick14]

I thought it used the latest weather updates from the various airports?

[Bobby Hart]

fireflybob, this is Rob, from SkyDemon.


Controlled airspace vertical limits are defined in the AIP by their FL, so there is no need to derive them.


GPS provides the position of the aircraft, and with multiple position reports we can derive speed and direction. SkyDemon then looks at the absolute position of the airspace limits to see if continuing at that same speed and direction would intersect the airspace within the warning time set by the pilot.


Hopefully that answers your question, but if not then please PM me to go into more detail.

[AirborneAgain]

I can't make sense of your answer.

With a GPS, the only altitude you can determine is absolute altitude. Vertical airspace limits are never defined in terms of absolute altitude, only barometric altitude.

So how does SkyDemon know the barometric altitude of the aircraft?

[ChickenHouse]

Before this gets confusing, let us fix the starting point of the discussion, as I believe there is a valid point in it we all have to be aware of.

- Skydemon does present a "GPS height", which is calculated based on the time distance of the GPS satellite signal at the speed of light from the synthetical WGS84 ellipsoid, which serves as a model for the earth shape.
- ICAO compliant aviation charts use WGS84 (currently corrected cycle -EGM96 spec) for AGL/MSL heights, which is why we have to make sure any other chart we compare to also uses WGS84-EGM96.

- aviation routing is based on barometric pressure, usually in GA established by simple linear QNH correction, without fine tuning and temperature calculation.
- deviation from GPS height to barometric height with exact and proper set QNH is usually neglected in GA flights.

If you fly on ISA setting in say an atmosphere with 10hPa off ISA, your altitude is off from geometrical height by about 300ft. In case you scratch too close to a restricted airspace Skydemon may tell you from geometrical height you are fine, but from barometric altitude you are not.

In any case, where you either fly above TL or with a QNH setting off real pressure (which does happen - once in a while you get a "cruise QNH" different from the local pressure) you have to remember that Skydemon is only a supplemental navigational aid and can not be used to judge wether you are legal or not! Skydemon can not replace any relevant cockpit instruments and you have to look at your altitude instrument. Never ever use Skydemon as a glass cockpit replacement, it is not meant and not built to do that.

So how does SkyDemon know the barometric altitude of the aircraft?
@AA: it does not and it does not need to, because it is irrelevant -> there is no simple or wishable way to tell Skydemon in flight which QNH you have been given by FIS. So, please - use your aircrafts instruments, not the electronicky gadgets, for these issues.

But frankly, it is indeed a bit misleading by Skydemon to present the, very helpful in planning, virtual radar also in flight, but if one keeps in mind the limitations - and altitude is NOT a parameter which can be taken serious for flight execution - it is a very good additional navigational aid. The usual GA flight below TL will be fine with this and I guess the vast majority of flights with Skydemon are these low airspace flights.

I assume we will see quite some improvements in gadgets like Skydemon in the very next future, given the fact that smartphones and tablets increasingly receive barometric sensors (which will add the next level of discussion between GA and GA-pressurized). I am looking forward to see a version of Skydemon make use of i.e. the iPhone 6 built in pressure sensor, but I doubt it will ever replace cockpit instruments.

[Whopity]

Nothing confusing, Sky Demon clearly don't know what a Flight Level is!
Controlled airspace vertical limits are defined in the AIP by their FL, so there is no need to derive them.
WRONG WRONG WRONG! A Flight Level can vary by as much as 1200 feet over a typical range of pressure settings.

[dobbin1]

1039 today

[Bobby Hart]

Whopity:

SkyDemon does not "derive" the vertical limits of any given airspace polygon (as the original poster was asking). They are written in the AIP. In fact, they are commonly presented as "GND\FL195" (or similar) which looks a lot like "Ground < x < Flight Level 195" to me. If I am indeed "WRONG WRONG WRONG" then perhaps you could tell me what FL means in this case? Or perhaps you meant to quote another part of my statement?

As an aside, I was under the impression that the whole point of flight levels was that everybody calibrates to the same standard (regardless of what the actual sea level pressure is) so that aircraft are reporting their vertical position to one another consistently. If my understanding of this is wrong, then its a good thing I am in charge of marketing and sales rather than coding development... Regardless, I am more than happy to be corrected.

[AirborneAgain]

Your understanding of flight levels is correct. What I guess Whopity meant is that since a GPS doesn't measure flight levels, Skydemon has to derive the flight level the aircraft is flying on -- and it can't do that without knowing the local pressure and the air temperature between the aircraft and ground.

The difference between GPS altitude and flight levels can easily be more than 1000 feet.