Work in progress in Altimeter and VSI

Note: Originally published 2016-05-22.

Work has started on the next feature for the Altimeter and VSI version 2.0, and with this, I think the app has the potential to become seriously useful as opposed to something that could just serve as a backup for those instruments.

Many people have asked me to add an alerting feature. This requires quite a bit of redesign and I'm doing this in my spare time, so it will probably be another month or two before it is released.

I didn't want to remove existing functionality, but altitude alerting requires a lot more buttons in its user interface (mainly the 10 digits so that you can type in the desired altitude), so there will be a new screen that will allow entry of this target altitude. You will get a warning when approaching your desired altitude, say 1000 feet away; another warning when inside your tolerance region, say 200 feet away; then you will get warned if you stray outside the tolerance region. The warning will at least be audible and will probably also involve a flashing indicator on the screen.

The good news is that although some people have said they will happily pay for this feature, I have decided that this will be free as well!

I have designed the new screen and am now working on the back-end stuff to support it.

I have also had requests for a "variometer"-type indication that would also be audible and would make a sound based on whether a climb or a descent is in progress, the sound also varying depending on the speed of climb or descent. I want to work on that feature some time in the future.

Finally, I have made the code open source. If the code interests you, you can see it here:

https://github.com/phabvionics/pilot-altimeter

The new stuff isn't on that repository yet, but it will be.

Thanks for reading!

Pilot Altimeter & VSI Version 1.9

Note: Originally published 2015-11-28.

I have added variable-length history for the altimeter and the VSI to this edition. You can select between off, 30 seconds, 60 seconds, 2 minutes or 5 minutes for each instrument. Note that using longer history may result in performance degradation on less powerful devices.

Pilot Altimeter & VSI version 1.8

Note: Originally published 2015-05-16.

Version 1.8 of the Pilot Altimeter and VSI has been released. Here are the changes:

• The graphs no longer lose their contents when the device is rotated.
• Some devices were showing STOPPED all the time, even though it was clear that samples were being received. This has been fixed.
• The port to Android Studio (the new official IDE for Android development) has finally been completed.

I do want to add more to it, but since this update was of value, I wanted to get it out as quickly as possible.

Note that I have selected a staged roll-out in the Google Play Developer Console with 20% of users getting this update initially; I believe these are randomly chosen. The reason I have done this is that I do not have the necessary resources to test on all devices and I want to make sure it's a good quality release before providing everybody with access to it. If you want instant access to it, you can apply to become part of the beta-testing programme by providing your email address.

If no significant problems are reported, I expect to open the doors to everybody to receive this upgrade in about a week's time.

Pilot Altimeter & VSI version 1.7

Not much has been updated for this release. These are the updates:

• The VSI needle was pointing at the wrong angle at least when the graph was enabled and possibly also when the graph was not enabled. That's been fixed.


• In vertical mode, and possibly in horizontal mode depending upon your device (something I could not test), the buttons have been made bigger so that they are easier to press.


• Since I am not using in-app billing, I have removed the permission from the app.

I've tested it, am happy with the changes, and have published it on the Google Play store. Many thanks to Sriram for reporting the bug that I've fixed.

Pilot Altimeter & VSI version 1.6

I have now added the capability to display a rolling graph of the vertical speed as well as the altitude. Both are now enabled by default, but if you installed version 1.5, the altitude graph may not appear unless you enable it in the configuration.

The scaling of the altitude graph is, for the moment, fixed (except for the selection of feet or metres for the altitude readout). The scaling of the vertical speed graph is dynamic; it has a reasonable minimum and will increase as necessary to accommodate the measured history.

This works fine on the Nexus 4, but it might not work well on some slower devices. If this is the case, please let me know and I will see if I can change the app to enhance its performance in these cases. Thank you.

Pilot Altimeter & VSI version 1.5 released

I have just uploaded to Google Play the latest version, 1.5. It adds a rolling graph of altitude over the last 30 seconds. This is the first change to the program I have made that I consider really exciting. I can't wait to take this one flying and find out exactly how bad my level flight really is! :-)

You have to select it in the configuration before it displays, otherwise it displays just the same as in previous versions. I did that on purpose because it takes longer to update the display if the graph is also present, and I didn't want that to cause a problem for anybody.

Note that you can now find details of my apps on Google+ here:

https://plus.google.com/communities/102881120171139152924

Pilot Altimeter & VSI version 1.4 released

Version 1.4 adds the capability to display the altitude and vertical speed indication in metres and metres per minute respectively. If you want the display shown that way, you can select it in Settings. The standard display is in feet and feet per minute.

Android Pilot Altimeter & VSI v1.3

I just released version 1.3. Release notes are as follows:

Added QNH and QFE buttons that remember the pressure setting used. (You use QNH for altitude and QFE for height above aerodrome.) These two buttons just act as memories; if you don't need that feature, just use either.

If you select Pressure Alt, the -10, -1, +1 and +10 controls become disabled. They are re-enabled if you select QNH or QFE.

Unit selection is now in the configuration screen rather than accessed through a button on the main screen.

Android Pilot Altimeter & VSI v1.2c

I released version 1.2c a couple of days ago.

With the help of Andrew Dobie Tanpaw for which I'm very grateful indeed, I eventually found that the timestamp returned by SensorEvent differs between various Android versions.

It seems to be the case that on earlier Android versions, the timestamp refers to nanoseconds since the time the device was booted; in later Android versions, the timestamp refers to nanoseconds since the epoch (start of 1970).

I determined empirically that the timestamp on my Nexus 4 refers to nanoseconds since 1970 and produced a compatible timestamp when the altimeter was being drawn, which was used for the purposes of filtering. Unfortunately this did not work on some devices because the timestamp did not have the same point of reference, and it caused the program on those models to crash.

Since it is never officially documented what the timestamp refers to, I have taken the decision to leave out filtering when the altimeter is drawn; filtering now occurs only when samples come in.

I have also added the capability since the last release of moving the program to SD card where appropriate.

Android Pilot Altimeter

This page describes my first published Android app!



What it does!

The app functions as a barometric altimeter and vertical speed indicator, similar to the instruments you find on aircraft. Note that it requires your Android device to have a pressure sensor. If no pressure sensor is present, it will load, but it will display No Data. It will NOT use GPS at this time.

Just as you can do with a conventional altimeter, you have a "Kollsman window" into which you can "dial" a datum, and the altimeter will then show height above the pressure datum specified in feet. You can choose whether to use hectopascals (equivalent to millibars) or inches of mercury. There are +10, +1, -1 and -10 buttons that allow you to control the setting, and there is a Pressure Alt button that allows you to rapidly select 1013.25hPa (the international standard atmosphere [ISA] mean sea level pressure) and in that case the altimeter will report the nearest flight level in addition to the pressure altitude in feet.

The altimeter display shows the height above the datum numerically in feet, rounded to the nearest foot, and there is a single hand that shows hundreds of feet. I chose this display because it is least likely to be misinterpreted; the type of display with two or three hands is easier to misread and has lead to accidents. The VSI display shows the vertical speed in hundreds of feet in the usual way, plus a numeric representation that is rounded to the nearest foot per minute.

How much does it cost?

Nothing. This one's on me. If it proves popular, I plan to add features to make it more useful, and I might make a small charge for the enhanced version.

What's the point?

Why would you use this? I can't guarantee that your hardware works OK, and as such, it's not approved for aircraft navigation purposes; I have included a warning as such on the display. If you do use it for aircraft navigation, you accept all risks associated with that use.

Aside from the warning, it might work fine; it might even work better than "approved" types. During my pilot training, I have flown in at least one aircraft that had an altimeter that was out of calibration, and the needle used to stick. A backup would be useful for that scenario. I also sometimes fly aircraft with two altimeters, and their readings might be as far as 50' or so different when set to the same setting!

I think the program could be useful for getting your head round how the pressure works in the atmosphere. Reading books about mean sea level pressure (QNH), field elevation (QFE) and flight levels will click far better if you have an altimeter to hand and you can play with it to see how it responds.

I personally consider that besides the training purpose, the app MIGHT be useful as an emergency backup altimeter and VSI, with a few caveats that I detail below. My suggestion is that you try it out at your airfield with the QNH and QFE settings and check the results are close to what you expect (taking into account of course any deviations in height compared with that of the runway), and then try it out on a flight and see how it compares to that of a purpose-designed altimeter.

Things to be aware of.

The altimeter of your aircraft is a device that takes one input: air from the static port. If that were to become blocked and there are no leaks in the pipes, the reading of the altimeter would stop changing. Some aircraft allow you to set an alternate source of static air for this eventuality, which is usually cabin air. If you do that, you have to be aware that cabin air has a slightly lower pressure compared with the static air pressure outside the aircraft while it is flying, and therefore the altimeter will over-read. The degree to which this will occur is detailed in the pilot's operating handbook. This will not affect the VSI.

The effect might not be much (a PA28 handbook stated that the error will be less than 50 feet unless otherwise placarded), but it is something to be aware of. In addition, you may find that further errors occur because of the use of cabin ventilation and such use may be restricted if you want the best accuracy from the altimeter; the pilot's operating handbook will probably advise.

Using this altimeter app would be like using an altimeter with alternate static air (cabin air), so you need to take this variable into account.

If you are lucky enough to have a pressurised aircraft, while you are pressurising the cabin, the altimeter will under-read and not be any use at all for estimating your height.

The altimeter display is filtered in such a way that if the pressure was measured perfectly, a change from one steady state to another would be reflected by 99.7% of the change over a period of five seconds (technically it is five time constants). You can control that filtering by adjusting the settings; I have provided five options including a "no filtering" option. The more filtering you have, the more accurate will be the indication, but the slower the altimeter will respond to changes. For aviation purposes, I think weak or medium filtering would be good.

The VSI display is also filtered in such a way that its indication lags a climb or descent by several seconds. It is no different from a real VSI in this regard. However, its response to changes in vertical speed is likely to differ from that of a real VSI. It is not possible to control the filtering of the VSI at present, and the instrument is to be considered experimental.

How it works!

The app gathers pressure samples from the barometric sensor. It optionally filters the samples as explained above. It then calculates what indication would be shown on an altimeter using the formulae given here:

http://williams.best.vwh.net/avform.htm#Altimetry

Note that the height reported will be subject to exactly the same errors that a purpose-designed altimeter is subjected to. If the air is colder than the ISA (15 deg C at mean sea level with a lapse rate of 1.98 deg C per 1000 feet), it will be denser, and the pressure will fall off at a more rapid rate with increasing height, causing the altimeter to over-read. If the air is warmer than the ISA, it will be less dense, and the pressure will fall off at a lower rate with increasing height, causing the altimeter to under-read.

Testing

Updated 31st May 2013. I am currently on holiday in Saundersfoot, UK. Today, I went to the beach. Obviously, the altitude at the beach will be very close to mean sea level. I obtained the Cardiff QNH through the most recent METAR available and the display was showing -4 feet while I was on the beach.

Unfortunately I am only able to test on the hardware I own. The only Android device I own that has an altimeter is a Nexus 4. That is what the app is tested on and that is what I consider to be the reference platform for this app. I am aware that at least one person is having difficulty on one other platform, that being the Samsung Galaxy S3. The app may or may not work on other devices.

Comments please!

I designed this app for pilots as you can clearly infer from the name. If you are a pilot and you try it, please report your experiences.

Whilst this app is designed for pilots, if you are not a pilot but you find the app useful for any reason, I would also like to hear from you, especially if you have any feature requests. They might make it into a future version of this app, or I might publish a separate app for them.