The place for technical discussions about GA and flying.
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By flyer5
#1895143
GrahamB wrote:
flyer5 wrote: the law is strangely silent on making up your own approaches, so making one up that closely follows a surveyed and properly designed track and profile is probably safer than one to your home strip in the back of beyond.


To keep up currency this time of the year I do night circuits. Although obviously I can see where I am all the time, I do like to use SkyDemon. Just for fun.

It does however raise the question of what is wrong with this model of IFR?: Fly on GPS to the overhead at a safe level. Then descend to join 'the circuit' as if in VFR. Then fly the circuit on GPS, descending down to what you judge to be a safe level. I would have thought, for most airfields/strips in England, you could make a case for anything from 400' down to 250'.

I started making instrument approaches over half a century ago, and you took comfort from some clever people working out all the angles to keep you safe. However billions upon billions have been spent since then on providing a truly miraculous alternative - GPS. Beyond that, your £150 tablet picks up multiple GNSS constellations and knows where it is in the twinkling of an eye. (Don't you remember receivers from 25 years ago that would often take minutes to make their first fix?).

Of course this reticence to accept that high street GPS is much safer than most ridiculously expensive approved GA kit is not the only problem. We also have the huge barrier that you can't fly IMC unless you have an IR or an IMC Rating (or whatever it's now called).

You might then say "What's the point in having a technical solution to flying IMC to your local field if ATC can't ensure separation in the local zone ie there isn't ATC?". Again we have simple, cheap solutions to providing basic surveillance and your local field normally isn't like Heathrow. And - really - do you need highly qualified personnel to keep 2-3 planes apart? I'm always impressed how ordinary folk man the R/T and telephones, and Reception. :)

Ah, well. One day.
By rdfb
#1895182
Typical phone and tablet GPS does no error checking and will report a false position rather than say that it isn't sure. I've been sat at home watching my phone tell me I'm wandering around my neighborhood.

Certified IFR devices don't do this and will report an integrity error in this kind of case.

In an emergency you'll probably be fine using your tablet GPS. But let's say that the probability of it being wrong is 1 in 10000. If tablets were routinely used for approaches in IMC, then that'd be around 1 in 10000 flights in which safety were compromised, with probably some significant amount of CFIT. I don't think that's be good enough.
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By flyer5
#1895325
rdfb wrote:let's say that the probability of it being wrong is 1 in 10000.


This is of course the crux of this argument.

I've never seen my tablets or phones report incorrect positions. ( How many time does your car satnav tell you to turn left now when you aren't at the junction?) Decades ago I can remember a steam GPS 'stopping' but you'd notice that. Making an approach I can't imagine how you wouldn't detect the GPS was having trouble.

The only time I've heard of GPS problems is over the Gulf where various bad actors carry out jamming.

Airliners full of passengers, flying along at 180 knots may well deserve precise monitoring but a lot of us are holding the needle on 70 and can make sudden corrections with the flick of a wrist.

The whole thing doesn't seem to be discussed much. If you have two GPSs displaying CDIs does that virtually eliminate the risk or do GPS errors tend to be systematic ie the software or the satellite signals make errors?

Could we easily get SkyDemon (if it were legal) to adjust the software so it 'notices' potential problems? If you're getting too much difference between constellations it could warn you. Or if your calculated speed or height change in a way that is inconsistent with probable dynamics (eg stops or jumps half a mile), that could give an alert.

Has anybody else experienced situations which would be difficult to avoid?
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By Flyin'Dutch'
#1895334
@flyer5

Have a guess why nobody else has taken up the cudgels and developed this or a similar app.

IFR approved GPS receivers do constellations checks etc, and indicate when the signal is degraded and cannot be used.

Doing an approach with a non approved GPS it is not feasible to use a second one to monitor to reassure you that all is well. a) it presumes the only source for an error is internal and b) if a deviation was to occur, which one do you follow?
By riverrock
FLYER Club Member  FLYER Club Member
#1895339
flyer5 wrote:I've never seen my tablets or phones report incorrect positions. ( How many time does your car satnav tell you to turn left now when you aren't at the junction?) Decades ago I can remember a steam GPS 'stopping' but you'd notice that. Making an approach I can't imagine how you wouldn't detect the GPS was having trouble.

Sat Navs know where you are planning to go, so guess you are staying on route when the signal is degraded .
How long does it take your car sat nav to realise you are on a different route from what it told you to follow? Some built in ones will use other car features (speed, distance, steering turn amount) to enhance (especially so they work in tunnels) but phone based ones obviously can't do this.

Last time it got this wrong for me was yesterday - there are roadworks near me it wanted me to head towards, I took a different route to avoid them. It took a while before it realised I was on a different route.
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By flyer5
#1895551
Flyin'Dutch' wrote:@flyer5
IFR approved GPS receivers do constellations checks etc, and indicate when the signal is degraded and cannot be used.

As you know, you only need 4 satellites to get a fix. Even decades ago I can remember getting 6-8. Modern chips with multiple constellations - how many are they picking up: 12? I assume the GPS averages all the possible combinations. You see where I'm going here: in extremis, in 1990, you might only have 4 satellites and they might be on awkward positions to give a good calculation. And one might be compromised. So you needed to take care. But in 2022, I can't imagine how you would ever get a degraded signal. I'll leave the reader to work out how many fixes you can get from 12 satellites but if they weren't enough then what catastrophe had befallen the Earth would probably be more worrying than the loss of your GPS fix.

It's about the biggest challenges to flying. A big one for sure is the eye-watering cost.

However closely following that is how often you are frustrated because you can't guarantee VMC there and back. The number of times you wouldn't have visibility of the runway at lets say 350 feet is a tiny fraction of the year. The IMC Rating is not a rating for IMC but rather a skill test of instrument flying in the late 1960s. Why do they bother with VOR, ADF and ILS but totally ignore a technology which is million times better, a million times easier for pilots, available in a million times as many locations, and therefore a million times safer?
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By Flyin'Dutch'
#1895557
The technology is there and is being used.

It is just unfortunately that the UK is very slow in the uptake of GPS IAPs and the appropriate adjustment to make these possible outside of full ATC control.

If there would have been the progress, which has been there across the globe in various permutations, we would not have this discussion and on crappy days we would all be flying GPS IAPs to 'small-town' airfields.
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By flyer5
#1895998
I was watching a YouTube review of a drone. Fantastic thing. Did 60 mph, would stop if it came to an obstacle and would avoid hitting the ground. When its battery got low it went into fly-home mode and automatically went back to the point of take-off. Of course it was relying on GPS so didn't quite hit the same spot. It was four feet off!!. Personally I'd take that anyday. The film was in Hawaii which I don't supposed is especially well served by satellites but it was pulling in 24!*

* well that's what the tester reported. Seems unbelievable to me. Perhaps you can pick up three constellations?
By cotterpot
FLYER Club Member  FLYER Club Member
#1896031
When its battery got low it went into fly-home mode and automatically went back to the point of take-off.


So flying in a straight - ish line it could never use more than 50% of the battery it is carrying?
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By Dusty_B
FLYER Club Member  FLYER Club Member
#1896142
flyer5 wrote:I was watching a YouTube review of a drone. Fantastic thing. Did 60 mph, would stop if it came to an obstacle and would avoid hitting the ground. When its battery got low it went into fly-home mode and automatically went back to the point of take-off. Of course it was relying on GPS so didn't quite hit the same spot. It was four feet off!!. Personally I'd take that anyday. The film was in Hawaii which I don't supposed is especially well served by satellites but it was pulling in 24!*

* well that's what the tester reported. Seems unbelievable to me. Perhaps you can pick up three constellations?


With four global GNSS constalations (GPS, GLONASS, Beidou, and Galileo) 24 satellites shouldn't be a problem for most modern receivers.

4 feet off is great.
But what happens when the signal is lost? Reflected? Jammed? Or spoofed?
DJI will switch to hover mode if it detects a loss of GNSS position. Can't do that in a aeroplane! Can they detect a loss of integrity? Can they detect an totally erronious position?
User avatar
By flyer5
#1896144
cotterpot wrote:
When its battery got low it went into fly-home mode and automatically went back to the point of take-off.


So flying in a straight - ish line it could never use more than 50% of the battery it is carrying?



They are normally geofenced, and would go out of range, so normally come back to the start point.

You don't (normally) fly this GPS manually. You command it to do things. I guess GA flying will be like this in years to come.

https://www.bing.com/videos/search?q=fpv+drone&view=detail&mid=C9BBE60B3E68EE96E947C9BBE60B3E68EE96E947&FORM=VIRE
By Lefty
FLYER Club Member  FLYER Club Member
#1896202
There is an app called ILS, made by “Sparcliff Ltd” and is available on the Apple AppStore. Or www.ILS app.com

The developer can be contacted at :
Jason dot brown at sparcliff dot co dot uk


I have it on several Apple devices and it works really well.

It offers an ILS style guidance right down to the threshold - with distance to threshold countdown and Outer and Middle Marker displays (and audio). On practice approaches it is rarely wrong by more than 10 ft horizontally or 20-50 ft vertically.

It is set to a default glide slope of 3 degrees, which, while convenient for the mental maths of 330ft per nautical mile, is a lot shallower than most GA aircraft use on a normal VFR approach. The big gotcha here is that this would put you a great deal lower on the approach than you are used to. However the real danger is that the final approach track to (non instrument) runways have never been surveyed for obstructions and therefore someone trying to follow a 3 degree glide slope is at a massive risk of hitting trees, buildings or other obstructions. For example at my home airfield, a 3 degree glide slope would put you really dangerously low on 5 of the 6 runways.

The developer can be contacted at : Jason dot brown at sparcliff dot co dot uk

However, as others have said, it has no capability to indicate a loss of accuracy - or even a loss of position, so I would never consider using it for an actual approach in hard IMC. it does however have a “Distance to Threshold” display that counts down. If GPS were lost, the distance countdown would freeze, thus giving you some indication that it had failed.

Even those with an IR or an IRR rarely need to fly an approach to below around 500ft aal. So the cut off at 500’ on SkyDemon and the Garmin Handhelds shouldn’t be a problem. If you are not visual at 500’ and 1.6nm - then you should perhaps question whether this is such a great position to be in.

Nice as a fun thing
Nice to have for horizontal positioning (as backup to your installed nav kit and / SkyDemon)

If you suffered an electrical failure, it would be useful to have a self powered Device to help you get down safely.

NOT SOMETHING TO BE USED AS A SUBSTITUTE FOR A PROPER ILS OR APPROACH ENABLED GPS.

NB, as far as I know,
(1) this is not the app that Timothy Nathan used and (2) his accident was a simple case of poor decision making, and nothing to do any app he might have been using.
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By PeteSpencer
FLYER Club Member  FLYER Club Member
#1896209
Nobody's mentioned RAIM, which while we can still steal EGNOS we can have it done automatically,.

However when that particular plug is pulled we won't even be able to check RAIM manually.....

The mention of Timothy has got me thinking as I remember him offering to do an 'ILS ' into Knettishall for me but I think nothing ever came of it-I certainly didn't hear back.

But blow me down, Knettishall is coded into this app as a fictitious EGKI airport with correct approaches on a Google map and approach directions .
This clearly derives from the name jestingly given to it -London Knettishall International - by the late Keef :Older forumites will fondly remember. ’LKI’ fly-ins though the epithet is rarely used now. :(
One degree out but heck who's complaining. :wink:
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By flyer5
#1896260
PeteSpencer wrote:Nobody's mentioned RAIM


The impression I get is RAIM takes calculations from several combinations and draws conclusions about which single satellite might be suspect. If it can see 6 satellites it can determine a correct fix, and potentially exclude the rogue satellite from further calculations.

However if the GPS is taking an average of all the fixes then I don't see why you need RAIM. With high numbers of satellites in view, the plain average would surely be pretty good even with the duff one in the calculation.

I wonder if anyone knows how this works in practice. Does SkyDemon just take a simple position output from the device or does it have access to individual fixes? Do typical devices take averages of all available satellites? Do typical devices do RAIM of any sort?