Sunday, September 01, 2019

Geotagging While Airborne

While uploading my vacation photos to Flickr, I was surprised that one photograph that I had taken from the window of the 757 was geotagged.


When I use my iPhone 7 in airplane mode, apps usually complain that GPS location isn't available. That's because typically the GPS receiver is integrated into the cellular radio - GPS being a requirement in the U.S. so that emergency services can locate you when you dial 911 - and when GPS isn't available (for example, when you're indoors), your phone may use cell tower triangulation and WiFi router identification to determine your location. In airplane mode, I'd expect all of these radios to be turned off.

My iPhone was in airplane mode, but I had just the WiFi radio enabled to use the in-flight WiFi - mostly, I confess, as an experiment, having worked on business aviation products that provide that service. Apparently this was enough to enable the GPS receiver, but, I assume, keep the RF section of the cellular radio turned off. This tells me something interesting about how iOS manages its RF resources.
(Querying the Salmon of Knowledge, as I've come to call internet searches following my recent travels in Ireland, one article says that the iPhone has a GPS receiver separate from the cellular radio. Another says that in more recent versions of iOS, GPS is never disabled in airplane mode.)
Flickr typically displays the geotag, if available in the metadata in the photograph, as a tiny map with a place name underneath it that is a URL that does a search for photographs with a nearby geotag. But in this case, it was just a square of blue labelled with "A mysterious place with no name".


When I looked at the URL, it had the latitude and longitude encoded as parameters.

I pasted 56.798622, -16.282175  into Google Maps, and got a result properly way out in the Atlantic.


Dropping into satellite view, I got an orbital image of where the 757 was when I took the photograph.


Readers of my blog (all two of you) may recall that I've done some work integrating my own GPS software with Google Earth and remotely tracked my travel with a moving map display as I drove my automobile around, with a Raspberry Pi running my software and a GPS receiver and an LTE modem sitting on the dashboard [Better Never Than Late]. It hadn't really occurred to me until now that you could do something similar with an aircraft and create your own moving map display if you had internet access to Google while airborne.

Of course this is exactly what the aircraft does with its own moving map display, except that the map is onboard in some box in the avionics bay. Although I didn't work on that particular feature, at least two of the business aircraft products for which I was one of the developers did exactly that, using the GPS coordinates provided by the aircraft navigation system over an ARINC 429 serial bus.

Saturday, July 27, 2019

Time Flies Again

Friday, the European Union Aviation Safety Agency, the EU's equivalent of the U.S. Federal Aviation Administration (FAA), issued a revised mandatory Airworthiness Directive for the Airbus A350-941 jet airliner. Quoting from AD 2017-0129R1, the EASA says:
Prompted by in-service events where a loss of communication occurred between some avionics systems and avionics network, analysis has shown that this may occur after 149 hours of continuous aeroplane power-up. Depending on the affected aeroplane systems or equipment, different consequences have been observed and reported by operators, from redundancy loss to complete loss on a specific function hosted on common remote data concentrator and core processing input/output modules. 
This condition, if not corrected, could lead to partial or total loss of some avionics systems or functions, possibly resulting in an unsafe condition. 
To address this potential unsafe condition, Airbus issued the AOT to provide instructions to reset the internal timer. Consequently, EASA issued AD 2017-0129 to require repetitive on ground power cycles (resets).
This means exactly what you think it does: you need to power cycle your A350-941 aircraft no less often than every 149 hours unless and until a software fix is applied.

So: what's special about 149 hours? The AD doesn't say. Let's see if we can figure that out using a little firmware forensics.

Multiply 149 hours by 60 to get 8940 minutes.

Multiply that by 60 to get 536400 seconds.

Multiply that by 1000 to get 536400000 milliseconds.

Finally, multiply that by 4 to get 2145600000.

We could equivalently have multiplied 536400 seconds by one million to get microseconds, then divided it by 250 to get the same value.

So what?

2145600000 is perilously close to 2147483647 or 0x7FFFFFFF in hexadecimal, which is is (231 - 1), the largest positive number represented in two's compliment binary form that you can store in a 32-bit variable.

So I feel pretty confident in making this prediction: somewhere in the A350-941 firmware or software there is a 32-bit signed variable that is incremented every 250 microseconds. After doing so for exactly 149 hours, 7 minutes, and 50.91175 seconds, the very next 250 microsecond clock tick will make that variable overflow and its value will transition from positive to negative as it increments from 0x7FFFFFFF to 0x80000000.

Wackiness ensues.

(An alternative hypothesis is a 32-bit unsigned variable which is incremented twice as often, or every 125 microseconds. It eventually wraps from 0xFFFFFFFF to 0x00000000, similarly confusing an algorithm.)

Long time readers of my blog will recognize that this counter rollover bug is similar to the one I previously described in the Boeing 787 Dreamliner; that bug could result in the loss of all electrical power on the aircraft after 248 days.

Commercial aircraft in service can stay powered up for a long time. They transition from aircraft power to ground power when at the gate, maintaining power so that the interior can be cleaned, the galleys restocked, the air conditioning kept running while boarding, and so forth. Then they go back to on-board power until the flight lands and the cycle repeats.

248 days is a long time. But keeping an aircraft powered up for 149 hours seems not only plausible, but likely. So how was this not uncovered during testing?

Counter rollover is a class of bug that I spent a significant portion of my career ferreting out of firmware and software for telecommunications systems - the kinds of systems expected to run 24x7 with extraordinarily high reliability - during my time at Bell Labs. It kept cropping up in the products I was helping develop (sometimes, admittedly, in my own code), and also in other vendors' products with which we or our customers were integrating our equipment.

Part of systems engineering - the art and craft of discovering, defining, and specifying requirements for a product, and insuring that they are met - must be deciding how long a product or component is expected to run before it must be power cycled. No human artifact is perfect, and nothing runs forever. I dimly remember during my mainframe days that IBM recommended periodically rebooting OS/360 on our 360/65 because of control block degradation; a euphemism, I suspect, for memory corruption or counter overflow.

But 149 hours? That does not seem like a very long time to me.

Monday, June 17, 2019

This Is What You Have To Deal With

I'm passing this anecdote along not because I'm asking for help (although any insight would be welcome), but as an example of what a low-level real-time development looks like.

I spent several days troubleshooting what I still assume to be a bug in my software that deals with output from Global Navigation Satellite System (GNSS) - of which the U.S. Global Positioning System (GPS) is one example - receivers, regarding the synchronization of the NMEA, UBX, and RTCM state machines with the input stream from the U-blox UBX-ZED-F9P receiver (a U-Blox 9 device) on my development system named Nickel (Intel NUC5i7RYH, Ubuntu 18.04.1).

NMEA is the typical output of GNSS receivers, consisting of "sentences" of printable ASCII characters. UBX is a proprietary binary format used for messaging to and from GNSS receivers manufactured by U-blox. RTCM is a standard binary format used for messaging to and from GNSS receivers that support differential GNSS (DGNSS). When a receiver like the F9P generates all three types of output, your software has to be able to grok all three formats and switch between them seamlessly.

I tested the same code on Cadmium (NUC7i7BNH, Ubuntu 16.04.5) and saw the same symptom: every tens of seconds or so my software would loose sync with the input stream than regain it shortly afterwards.

I could not reproduce the problem on Gold, an ARM-based Raspberry Pi (Broadcom BCM2837B0 SoC, Raspbian 9.4) using exactly the same application software and the same receiver hardware, nor on Bodega or Mochila, also ARM-based Pis (also Broadcom BCM2837B0, Raspbian 9.8).

So I ran the standard Linux utility socat on Nickel to save about a minute's worth of NMEA data (UBX and RTCM weren't enabled on the F9P for this test). I ran my software against the file (so: no real-time constraints) and recorded the Sync Lost messages that included the offset into the data where the errors occurred. At this point I was pretty sure I'd found a bug in the state machines in my software.

Then I did a hex dump of the original file generated by socat, checked at those offsets, and lo and behold the NMEA stream was corrupted: periodically a spurious handful of bytes, looking suspiciously like a fragment of an NMEA sentence, was inserted at the end of a valid NMEA sentence.

So this corruption occurs without my software being involved at all.

I ran my software on Nickel using the GlobalSat BU353W10 receiver (a U-Blox 8 device), and I saw similar occasional loss of sync due to corruption of the NMEA stream. As before, the same software and GPS hardware on a Pi worked without problems.

I updated Cadmium to Ubuntu 19.04 and observed the same misbehavior with the BU353W10. I updated the BIOS firmware on Cadmium to the latest version and it still lost sync every tens of seconds.

I built the same software and used the BU353W10 receiver on Mercury (Dell OptiPlex 7040, Ubuntu 14.04.4) and saw sync lost every tens of seconds.

The evidence points an issue either with the USB hardware or firmware on all of my Intel boxes, or with the USB software stacks in many versions of Ubuntu (even though both Ubuntu and Raspbian are based on Debian, making it likely that their USB stacks share a common provenance).

I'm not buying it. Either seem unlikely. What is the likelihood that I'm the only developer in existence to have run into this? Gotta be pretty much zero. And my search-fu has turned up nothing.

I'd like this to be a bug in my software - and I am still assuming it is, perhaps a flaw shared with socat - because then I could fix it. But its root cause remains a mystery.

I use the Intel-based NUCs as my development systems (the one Intel-based Dell box is a legacy development system), and the Pis as test systems, typically deploying to ARM-based systems in the field.

I have no clue at this point what this is. But when I figure it out - and I hope that I do - I will have learned something useful.

Update (2019-06-19)

Just by way of providing actual evidence, here is a snippet extracted from about one minute of data collected via the command
cat /dev/ttyACM0 > FILE
running on Nickel, the Intel NUC5i7RYH running Ubuntu 18.04.1, from a BU353W10 (U-blox 8) USB-dongle GPS receiver:


The 7A\r\n ('7', 'A', carriage return, line feed) inserted between two sentences is obvious. It has the appearance of a checksum and standard end matter of another NMEA sentence (and in fact there are several sentences following this corruption that happen have 0x7A as their checksum).

Keep in mind that these corruptions don't appear when I run the same test on an ARM-based Raspberry Pi. This makes me wonder if there is another process that runs on the NUC but not the Pi that is consuming data from the same port. This isn't as crazy as it sounds; I have inadvertently run redundant instances of my gpstool software or the gpsd GPS daemon and run into similar issues (yes, I already checked for that). However, running the commands
fuser /dev/ttyACM0
lsof | grep ttyACM0
(which displays what process has a particular file open, and all open files and the processes using them, respectively) only show the cat command when I run this test.

An interesting difference between the Ubuntu NUC and the Raspbian Pi is that on the former it isn't necessary to configure /dev/ttyACM0 as a serial port (baud rate, raw mode, etc.) in my application; on the latter, it is (although it doesn't seem to matter to what I set the baud rate).