jpower
Cleared for Takeoff
I see. Thanks for clarifying. You think that the following device falls into the same category, then?
OK, that makes sense. What you see from a WAAS output is what Center sees on their screens.
Altitude Alerters
- Thread starter jpower
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I see. Thanks for clarifying. You think that the following device falls into the same category, then?
OK, that makes sense. What you see from a WAAS output is what Center sees on their screens.
alfadog
Final Approach
No, it is more accurate.
jpower
Cleared for Takeoff
But for legality, what Center sees is what matters.
pericynthion
Pre-takeoff checklist
No!
WAAS GPS will give you your true honest-to-god geometric distance above mean sea level to better than 50ft. Unfortunately, separation from other aircraft, ATC clearances, non-busting of airspace etc are all dependent on the pressure altitude, corrected by the altimeter setting. On anything other than a perfect standard-atmosphere day (i.e. every day) this is often completely different, by hundreds or even thousands of feet, from the true geometric altitude!
Until GPS became available, we had no way to accurately measure the true geometric altitude, so all the rules are based on using barometric altimeters instead.
If every airplane has an in-spec altimeter, set correctly, their altimeters will show the same indication for the same true altitude. Thus, everyone is using the same (erroneous) altitude for separation, so you won't hit other planes.
The error reduces to 0 as you approach the ground at an airport for which the altimeter setting is valid, so you're safe on instrument approaches and in the pattern.
There's a couple thousand feet of margin in IFR MEAs over mountains, so you won't hit the ground there either.
GPS/geometric altitude is great for terrain/obstacle warning systems. But because all the airspace and aircraft separation is based on pressure altitude, it's useless for an altitude alerter. Sorry!
alfadog
Final Approach
Meh. Center knows that what you see on your altimeter may not match what they receive from the transponder. Assuming that you do not have an encoding altimeter in the panel, which most do not, I think.
jpower
Cleared for Takeoff
Good thing I posted a clarification.... I thought that's how it worked but did some skimming online that stated otherwise. I should stop skimming things when I'm going to stick my neck out there and say something...
alfadog
Final Approach
Point taken.
So OP's idea is only valid if he duplicates what is already out there and picks up the transponder altitude. Or duplicates the function of an aircraft altimeter in some other fashion. Brings him back to his original transducer idea but I do not think there is much of a market if the pilot is not assured it is close to what ATC has.
pericynthion
Pre-takeoff checklist
Legally the altimeter (when set to 29.92) and the encoded data on the Mode C return must match each other and the true pressure altitude to within, um, 200ish ft (can't find the reference right now). If you're cruising at 9500ft on a summer day your WAAS GPS geometric altitude could easily be above 11000ft. So if you set your WASS GPS altitude alerter to 10k ft and try to skim the top of the Los Angeles Bravo, you'll think you're fine and dandy but Center's alarm bells will be going and you'll have a phone call to make when you get down. You will have made a genuine violation.
pericynthion
Pre-takeoff checklist
Yep. A cheapie pressure sensor running on the cabin pressure is probably good to within a couple hundred ft on most GA aircraft, especially if a little calibration is applied. Good enough for government work. I like the squawk-snooping approach though, it's cuter.
Unfortunately if he tries to build this he'll run into other annoyances - those MEMS sensors are notoriously temperature sensitive, and drift over time... should be a good learning exercise. For a fun, educational project it's totally worthwhile and he might even end up with a useful device for himself. I don't think it's going to make any money.
wabower
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Cellphone apps weren't available when I bought the Airsport, so my thinking wasn't clouded by the potential for such a device. Having now seen numerous apps go south at various times for reasons nobody seems to understand, I'm sure that I wouldn't trade my unit for such technology, but also wouldn't expect for a pilot to spend $600 for an Airsport if an app were available for $20.
Insofar as monitoring the grey-code output for Mode C use, I must maintain current baro settings in the Airsport in order to determine the output that center sees when my uncorrected altitude data is transmitted and their computer makes the correction.
Insofar as monitoring the grey-code output for Mode C use, I must maintain current baro settings in the Airsport in order to determine the output that center sees when my uncorrected altitude data is transmitted and their computer makes the correction.
jpower
Cleared for Takeoff
I agree, mooching off the transponder is pretty nifty. But the pressure sensor I was looking at (BMP085 for those who know electronics) is supposedly extremely accurate (I saw a video of a quad copter hovering within ±3 feet of its altitude using only the BMP085) and has a built in temperature sensor so you can get extremely accurate readings.
I'll probably build one this summer just for kicks and giggles, unless I come up with something else. From what I've heard from everyone, the market potential for such a device is virtually nonexistent. Perhaps I could find a device to replicate at less than half the cost where the market share is significant....
denverpilot
Tied Down
For the smartphone App, if you're not worried about figuring out actual altitude, but just worried about "alerting" if you're off...
Pilot pushes button. App takes an average of GPS altitude over the next [insert appropriate number of seconds here, depending on smartphone published GPS update rate for the API], then just alerts if that changes significantly [insert value for "significantly" here], assuming smartphone GPS altitude is accurate enough for such things.
That code is relatively easy. Not as feature-rich as the device we're discussing, but easy as a simple "I'm off my altitude" alerter.
Having watched the iPhone and iPad GPS info in flight, they're probably good enough for a 50' deviation window either side of the "set point" as an alert.
Could also be built right into something like Foreflight or WingX. Flash the screen (invert color map) once an altitude is "set" by the pilot via a tap on the altitude box in the HUD, if they're off by more than 50'... 100'... whatever you want to set it to. Add a sound if desired, but most of us don't have the iPhone/iPad plumbed into the audio system. (I say that, but I sometimes do... usually playing tunes from one of them into my Aux port on the intercom.)
It'd be a "low hanging fruit" feature to add to any of the current crop of iPad/iPhone Apps... as long as they could find an unobtrusive yet intuitive way to "set" the altitude set point.
Also might be so easy it'd be a market killer for your proposed device, with as many iPads are running around in the recreational aviation market. It'd be nearly useless (as others have pointed out) for a pressurized cabin.
Pilot pushes button. App takes an average of GPS altitude over the next [insert appropriate number of seconds here, depending on smartphone published GPS update rate for the API], then just alerts if that changes significantly [insert value for "significantly" here], assuming smartphone GPS altitude is accurate enough for such things.
That code is relatively easy. Not as feature-rich as the device we're discussing, but easy as a simple "I'm off my altitude" alerter.
Having watched the iPhone and iPad GPS info in flight, they're probably good enough for a 50' deviation window either side of the "set point" as an alert.
Could also be built right into something like Foreflight or WingX. Flash the screen (invert color map) once an altitude is "set" by the pilot via a tap on the altitude box in the HUD, if they're off by more than 50'... 100'... whatever you want to set it to. Add a sound if desired, but most of us don't have the iPhone/iPad plumbed into the audio system. (I say that, but I sometimes do... usually playing tunes from one of them into my Aux port on the intercom.)
It'd be a "low hanging fruit" feature to add to any of the current crop of iPad/iPhone Apps... as long as they could find an unobtrusive yet intuitive way to "set" the altitude set point.
Also might be so easy it'd be a market killer for your proposed device, with as many iPads are running around in the recreational aviation market. It'd be nearly useless (as others have pointed out) for a pressurized cabin.
jpower
Cleared for Takeoff
Yeah, that's very true. There's another device that was on the market a while back (10 ish years, maybe) that allowed you to do just that--set it for your current altitude and it would flash at you if you deviated more than some given altitude. It wouldn't have much application for IFR pilots, though, because some absurd percentage of altitude excursions happen by forgetting to stop climbing or descending. But I could get that built and selling for $50 per unit, when the other was selling for $200. Maybe that would have a market niche? Perhaps interesting to pilots routinely do VFR cross countries?
alfadog
Final Approach
You do not want to factor in nonstandard temperature as neither the altimeter nor the transponder does nor do I guess ATC does.
pericynthion
Pre-takeoff checklist
The temp sensor built into the BMP085 isn't for compensating for nonstandard atmospheric temperature. It's to help correct the greatest component of the bias in the sensor's measurement of barometric pressure, which varies with temperature.
It's turtles all the way down, I swear!
It's turtles all the way down, I swear!
jpower
Cleared for Takeoff
What was that I said earlier? Oh right.
I should stop skimming things when I'm going to stick my neck out there and say something...
alfadog
Final Approach
Turtles on top of turtles! Haven't heard that reference in a while. :wink2:
Everskyward
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I never knew there was such a thing as an altitude alerter until I started flying better-equipped airplanes. The thing that I like about it is not so much that it's tells you when you are off altitude but it gives you a visual reminder of the altitude you are supposed to be climbing or descending to without writing it down. It also warns you with 1,000 feet to go. Maybe in a slower climbing or descending airplane 500 feet or 200 feet to go might be more useful.
wabower
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Ironically, my plane has one of those too. I've never used it since the Airsport does so much more and I already owned it as well. But if anybody wants a fly-off to see which they like better, I'm your man.
Piloto
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James
For altitude alert function you do not need super accuracy since the pilot would be using the A/C altimeter for altitude holding. Setting a 200ft warning window would be enough to warn the pilot of the incoming target altitude.
José
For altitude alert function you do not need super accuracy since the pilot would be using the A/C altimeter for altitude holding. Setting a 200ft warning window would be enough to warn the pilot of the incoming target altitude.
José
denverpilot
Tied Down
Forgot to post that the feature request is turned in to Foreflight.
(I didn't mention it in the request, but they could easily add it for heading too.)
(I didn't mention it in the request, but they could easily add it for heading too.)
alfadog
Final Approach
jpower
Cleared for Takeoff
For which feature?
alfadog
Final Approach
jpower
Cleared for Takeoff
Teach you to broadcast your idea for an invention
Haha yeah, it's no problem though, considering there's not much of a market for a standalone device. All this pondering has made me think of another device
, though, that I think does have a market....
alfadog
Final Approach
Haha yeah, it's no problem though, considering there's not much of a market for a standalone device. All this pondering has made me think of another device
Cool. Good luck.
alfadog
Final Approach
The feature would be an altitude alert as he describes - it would alert you when you stray from your present altitude (or heading).
jpower
Cleared for Takeoff
OK, makes sense. An altitude alerter with no preset warning for climbs or descents.
I've seen one that flashes when you get within 500', don't know what model it was.
Personally I wouldn't spend money on any altitude device except an altimeter unless it was driving the autopilot.
wabower
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Airsport has standard pre-select warning tones, same sequence and protocols as you would expect from autopilot. The altitude deviation warning is a three-tone system with the highest pitch note at the beginning of the sequence if descent is required and at the end of the three-tone sequence if a climb is necessary to return to the selected altitude.