What good is the Pressure Sensor on the Stratus 2s?

Discussion in 'Avionics and Upgrades' started by wazooda, Oct 18, 2016.

  1. wazooda

    wazooda Filing Flight Plan

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    I have a Stratus 2s with Foreflight and utilize CloudAhoy for debriefing while I'm learning to fly.

    I shoot video from a mounted GoPro that enables me to see both out the window and the instrument panel.

    In reviewing my footage, I noticed that the altitude shown in the cockpit is always different than what CloudAhoy (via the Stratus 2s flight data logging) shows for altitude.

    I posted this question on Foreflight's Facebook page and received this answer:

    "Although the Stratus 2S can detect pressure altitude, the altitude values that are saved in a Track Log are from GPS altitude, which is why the altitudes in the playback differ from your cockpit recording."

    So.... what good is having a pressure sensor then? Is GPS altitude more accurate?

    To be honest, I'm a bit perplexed by this subject....
     
  2. arnoha

    arnoha Cleared for Takeoff

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    Neither is accurate. I'm not sure how useful the pressure sensor is, either. I think it's mostly used for warning about cabin altitude.

    GPS altitude is an order of magnitude less accurate vs. lateral position. With WAAS, it's still pretty good and I'd trust that. The Stratus, of course, has WAAS.

    The pressure sensor will tell you cabin pressure. Cabin pressure is not calibrated, so it could be pretty off. On top of that, is the Stratus altitude corrected for local altimeter setting? I'll bet it's not. That can also vary the reported altitude significantly.

    It was cheap to add, so they added it. They had to add something to make it an S, right? (The S is actually mostly redesigned internally to be more rugged, last longer, and has a better charging connection. But with no new features, there wasn't much to sell.)
     
  3. DGlaeser

    DGlaeser Pre-takeoff checklist

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    GPS altitude is not MSL, so 'more accurate' depends on what you are looking for. I don't know if they correct PA for the local altimeter setting, so the altitude display may be PA, not MSL.
    I only have a Stratus 1, so I don't know if the EFIS-like display on FF (which requires the 2/2S) uses PA in the altitude display or not. The AHRS package probably needs altitude info even if GPS is not available.
     
  4. BigBadLou

    BigBadLou Final Approach

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    Wait, what does the GPS altitude mean then? I always saw a big difference vs MSL in the airplane and did not know what to attribute it to.
    I always expected to see GPS altitude pretty much match the altimeter with proper setting in Kohlman's window but that was never the case. Even on a standard pressure day.
    Got a source of explanation for the lazy folks like me? :)
     
  5. Clark1961

    Clark1961 Touchdown! Greaser!

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  6. BigBadLou

    BigBadLou Final Approach

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  7. wazooda

    wazooda Filing Flight Plan

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    Thanks everyone for the replies.

    So... followup question regarding accuracy.

    Since the Stratus 2s has AHARS, wouldn't the altitude, no matter how it is derived, need to be quite accurate?

    Or put another way, if I'm in IFC and using the AHARS with synthetic vision, doesn't the altitude need to be very accurate to avoid terrain?
     
  8. Unit74

    Unit74 Final Approach

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    How damn close are you tryIng to fly to terra firma? Look on the IFR low charts and take a look see what the MOCA is. Fly above that and you won't hit a thing except another airborne thing. The waas gps should put you within 9ft of actual altitude. Is that not good enough?
     
  9. cshavit

    cshavit Filing Flight Plan

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    When importing from Stratus, CloudAhoy converts the GPS altitude to MSL. The altitude displayed in CloudAhoy is MSL.
     
  10. Unit74

    Unit74 Final Approach

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    That has nothing to do with his question, but thanks for regurgitating that from his original post.
     
  11. cshavit

    cshavit Filing Flight Plan

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    To debrief, the best altitude would be the one that was displayed by the altimeter in the cockpit, which is barometric of course.

    About the usefulness of Stratus pressure sensor: not very useful for debriefing. For one thing, CloudAhoy would need to know the altimeter setting, which is modified frequently by the pilot per ATC or ATIS/ASOS. But even if (theoretically) we could get over this hump, still the pressure inside the cockpit has significant inertia; while changing altitudes, the pressure inside the cockpit will take a while to equal the baro pressure outside.
     
  12. wazooda

    wazooda Filing Flight Plan

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    Hmmm.... I don't *think* I made a qualitative statement about the data not being "good enough".... but I'll go back and check.

    Being new to flying, I thought I'd try to learn something from people who know more about this subject than I do.

    I apologize if my question somehow impugned the integrity of the Stratus 2s.
     
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  13. mfisher

    mfisher Pre-Flight

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    By posting here?!? :rofl:
     
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  14. BigBadLou

    BigBadLou Final Approach

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    Yes, he's a new user and naively thought that all of us here are nice. I know, rookie mistake, right? ;)

    OP, don't worry about negativity, it comes from our pilot arrogance and know-it-all-ism.
    You gotta sift through posts and find the useful ones. Also, you can block threads and even members. How nifty, huh? :)
    Enjoy your stay here, you just need to grow thicker skin.
     
  15. wazooda

    wazooda Filing Flight Plan

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    Thanks. I appreciate your thoughts on the matter.

    And just for the record, as a former prosecutor, trust me when I say I have a thick skin.

    I just don't suffer fools and little men behind keyboards. ;)
     
  16. Ravioli

    Ravioli Final Approach

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    Hopefully you have a good CFI that will do two things:

    1) put a limitation on your solo endorsement that you may not take all that electro-bable-**** on board while flying alone. You've got better things to do when you're learning to fly, and debriefing yourself is more of an Anthony Weiner exercise than a flight training exercise.
    2) Sit you in a chair at the flight school for as long as it takes for you to correctly describe the concepts of Pressure Altitude, Density Altitude, Indicated Altitude, MSL, AGL.

    And when you use that search thingy you'll find that many people advise that you learn to fly with as little gadgetry as possible. And that I am among them.
     
  17. Theboys

    Theboys Line Up and Wait

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    I use the pressure altitude to reference my cabin altitude. I fly a pressurized plane mostly and it shows the altitude inside plane which I usually find to be within 25 ft of my other gauges which show altitude of cabin. Also shows how fast altitude changes when it's dumping or I'm climbing. Kind of interesting. Figure it's a good addition to the other safety's if have to warn me of pressure dump. I hand my iPad to passengers while flying a lot to let them follow along. They seem to enjoy it and I have plenty of other gauges to keep me informed.
     
  18. wazooda

    wazooda Filing Flight Plan

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    For clarity, I should differentiate what I do in the cockpit versus my intellectual curiosity.

    When flying, the only gadgets I use is the Stratus 2s to collect flight data and a GoPro. I don't fly with an iPad at this point and the c172 I train in is all steam gauges.

    However, I disagree about the value of debrief. I find CloudAhoy, with synced video, EXTREMELY useful for going back after a flight and watching my, for example, pattern work. Being able to hear my CFI's corrections as well as the ATC audio in addition to all the telemetry and an out the cockpit view has been very helpful, at least for me, to know what I need to focus on in my next lesson and to not make the same mistakes twice.

    But to each his own I guess....
     
  19. DGlaeser

    DGlaeser Pre-takeoff checklist

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    If you are in IMC using a Stratus as your flight instruments, it had better be an emergency!
    AHRS shows ATTITUDE and HEADING (hence the name), Altitude and Airspeed are shown on the 'EFIS-like' display, and they are used in the algorithms to verify attitude. Altitude accuracy is important, but you should be well above terrain - especially in IMC, unless you are on an instrument approach - which you should never do using a Stratus (well, except in that emergency, and then you'd better get below the clouds before altitude accuracy for terrain avoidance becomes an issue) :eek:
     
  20. Theboys

    Theboys Line Up and Wait

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    I had emergency one time and lost everything so used the iPad. Was the beginning of my best landings. Worked pretty well.
     
  21. Unit74

    Unit74 Final Approach

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    And you probably did learn something. Fly the MOCA and you are in a safe obstruction clearance altitude.

    Quit trying to read into some non-existent inflection based off written word. You are stepping onto a hobby dominated by type A personalities. Go with the flow and you'll be fine. Get panty twisted at the drop of being challenged and you'll get tore up from the floor up. Sharks can smell blood in the water from a long way away.

    Post something others vehemently disagree with, make a stand and then and watch Awel hell break loose....happens a lot.
     
  22. Unit74

    Unit74 Final Approach

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    And ....ahegm.... For the record (you should understand this part), Kenny Rogers said it best......


    Know when to hold em, know when to fold em. Know when to walk away. Known when to run.
     
  23. luvflyin

    luvflyin Final Approach

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    I'm wondering how accurate it is. I'd like to hear from you that have them. Set your altimeter to 29.92 every now and then when you're up there and compare. Better yet, get a friend with one to go along and compare the two of them
     
  24. BigBadLou

    BigBadLou Final Approach

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    Do they provide any alerting function? It would be nice to have one in case the cabin altitude climbs too high.
    It would be also nice to have a general altitude alert. My autopilot does not have altitude preselect so having an alert 1000-to-go, 500-to-go and maybe 100-to-go while I am busy in the cockpit would be great.