Friday, October 02, 2020

Clock Time

POSIX threads (a.k.a. pthreads) - the standard that defines an API and an execution model for supporting threads in a programming language - are old news. In the past twenty years I've written or helped write three different pthread-based frameworks in C++ for different jobs or gigs, and used several other frameworks that others had written. Today finds me finishing up writing yet another pthread framework (YAPTF), this time in C, for my own use. It will be a part of Diminuto, my C-based systems programming library whose open source is available on GitHub. This is something I should have done a long time ago, instead of hacking little separate bits of pthread code into virtually every non-trivial C project that I wrote.

Here's the thing: it wasn't until I was writing a unit test for this new feature that I finally realized why the pthreads API function

int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex, const struct timespec *abstime);

uses the absolute wall clock time (e.g. 2020-10-02T17:38:24.03894879UTC) instead of a relative duration (e.g. 0.50000000s) to specify a timeout (abstime) for the condition that the caller is asking to wait for. Timeouts in other POSIX standard features, like the select(2) system call, are specified as a duration.

It finally dawned on me it was because this allows you to specify a consistent fixed timeout for complex situations that may have to receive multiple pthread signals to satisfy their pthread wait condition.

Here's a code snippet from the unit test. Below, my function diminuto_thread_wait_until calls pthread_cond_timedwait in the pthread library. (Click on the image to see a larger version.)

Screen Shot 2020-10-02 at 11.25.11 AM

The test initializes the thread object (a C struct instance) with a pointer to the function, body7, that implements the thread logic. The test starts the thread with an argument of just a null pointer that is passed to the function. Then the test acquires the current clock time and (effectively) adds one second to it. The test then enters a critical section which locks a pthread mutex associated with the thread. Inside the critical section, if the thread has not yet entered the running state (something the framework changes), the test waits until it receives a pthread signal on the condition associated with the thread from my framework indicating that the thread state has changed. Then it checks again. When the thread is in the running state, the test exits the critical section, unlocking the mutex.

No matter how many times the test waits, the thread state changes, a signal is sent, and the test wakes up to check the state again, the unit test will never wait longer than a total of one second. If more than a total of one second elapses, the wait will return the error code ETIMEDOUT instead of zero, and the test will fail.

This eliminates all the bookkeeping I've had to do in real-time operating systems that have non-POSIX threading models. In those RTOSes, I had to keep track myself of how much time had elapsed, subtract that duration from the total timeout, check that I hadn't used up all the timeout, and if not then wait again. It's not that hard (in fact, Diminuto has features that make this pretty easy), but it does call for more code.

I confess I felt a warm glow inside when I figured this out. There is a joy in understanding a problem that people smarter than you have solved.

Monday, September 21, 2020

Dead Reckoning

 U-blox, the Swiss manufacturer of GPS and GNSS receivers that I've used in a number of projects, has models that incorporate both a multi-band receiver and an Inertial Measurement Unit (IMU). The IMU uses accelerometers and gyroscopes to estimate the movement and rotation of the receiver (and whatever it is embedded in) even in the absence of a usable GNSS signal. This is referred to as dead reckoning. The u-blox NEO-M8U module, for example, uses both the ensemble GNSS fix (if it exists) and the IMU measurements to produce a continuous fusion fix that can be maintained even as the GNSS fix is temporarily lost and then reacquired.

How large is the IMU? It's inside the thumbnail-sized M8U module. Here's is an image of the M8U mounted on a tiny red printed circuit board from SparkFun Electronics, which I in turn mounted in a plastic box on a test fixture, along with a multi-band GNSS active antenna.

Note: Click on any image to see a larger version.


How is this possible? The accelerometers and gyroscopes are examples of a MicroElectroMechanical System (MEMS): microscopic electro-mechanical devices with moving parts, manufactured using semiconductor chip fabrication technology.

The IMU isn't perfect. Because the MEMS devices in the module are fairly noisy, the IMU has to do a fair amount of filtering and debouncing of their output. This leads to a noticeable latency of several seconds for the IMU to respond to changes in roll, pitch (road grade), or yaw (vehicle heading). This latency might explain some of the results I present below.

The IMU requires a short period of calibration at start up for it to determine the vehicle reference frame. The automatic calibration requires that the vehicle initially make turns and travel at sufficient speeds, all with adequate GNSS reception. I had to account for this calibration period during my test drives.

I also found that I got the best results when the M8U module was leveled and securely mounted with respect to the vehicle. Ultimately, this led me to mount the module on a gimbaled camera mount (like you might use for a GoPro), and to incorporate a spirit level into the test fixture.

But it's kind of remarkable that it works at all. The controller in the device itself does all of the heavy lifting. My software, running on a laptop or small computer like a Raspberry Pi, merely configures the module using proprietary UBX commands, and then reads the continuous stream of data from the device in the form of standard NMEA sentences and UBX messages. The device generates a position fix once a second, and reports useful information about the fix, like whether GNSS, or the IMU, or (more typically) both, were used in the solution.

Test Fixture

Here is the test fixture mounted on the dashboard of my Subaru WRX during a field test.


The module connects via a USB cable to a laptop running some flavor of Linux/GNU and my open source C-based Hazer software package. The USB cable provides both power to, and two-way communication with, the M8U module. As is typical, the active antenna is powered by the M8U via a voltage bias applied to its coaxial cable.

Here is the computer I used in later test runs: a GPD MicroPC industrial laptop running MATE, an Ubuntu-based version of Linux.


You won't want to do any touch typing on this tiny, almost pocket sized, computer, but it was perfect for this application. (Earlier I used an ancient HP 110 netbook running Mint, another Ubuntu-based Linux. It worked fine too.)

Test Strategy

I had to do some pondering as to how to test this device. The most obviously way was to obscure the antenna with something that would block the signals from the various GNSS satellite constellations. I owned several RF-blocking bags - originally purchased when I was testing cellular data modems - that I could have used for this purpose. I did do some early testing using this technique. But I wanted a more dynamic, organic approach that was in line with the M8U's intended use case in road vehicles.

That's when I remembered I lived in a western suburb of Denver Colorado, just east of the Rocky Mountains. Mountains with lots of highway tunnels. Like the short tunnels built in the 1930s along the United States route 6 road just a few minutes west of where I live. Or the ginormous 2.7km (1.7 mile) Eisenhower and Johnson tunnels built in the 1970s, that carry the Interstate route 70 highway through the Continental Divide about an hour west of my home. This project clearly called for a road trip.

My software can be configured to save the current position fix - latitude, longitude, and altitude, along with a bunch of other data - once a second. The data is saved in human-readable form to a file in Comma Separated Value (CSV) format. This makes it easy to post-process using simple scripts or even Excel. I wrote scripts that converted the CSV data into various files in Keyhole Markup Language (KML), an XML-based data format used to annotate place marks and visualize two- and three-dimensional paths in "earth browsers" like Google Earth.

So far I have done four test drives with my test fixture, through either the US-6 tunnels, or the I-70 tunnels, or (as luck would have it) a route that went through both.

Note: All of the screen shots from Google Earth below follow the usual convention of west to the left, east to the right, north up, and south down. In the U.S. we drive on the right-hand side, so in an east-west road, the upper lanes would be west-bound and the lower lanes would be east-bound. The discussion below assumes that Google Earth is pretty accurate in its mapping of latitude/longitude coordinates, and that they use the same datum, or definition of latitude/longitude, as GPS: WGS84.

Here's is a screen shot of one of my test drives as visualized in Google Earth.

Screen Shot 2020-09-15 at 12.13.25 PM

The red line (a LineString in KML) marks my path, and the yellow push pins (Placemarks in KML) mark the important points like tunnels along the way (different post-processing scripts define the push pins differently). The label on each push pin is effectively elapsed seconds. (It's actually the position fix number, but there is a fix recorded every second.)

The total travel duration for this road trip was about two and a half hours. This route took me west through on I-70 through a short tunnel just east of Idaho Springs, then through the Eisenhower Tunnel, turning around at Silverthorne, then back east on I-70 through it's peer, the Johnson Tunnel, exiting onto US-6 through a series of smaller tunnels, and finally back home. I began each test drive with enough meandering around my neighborhood that the IMU should have been able to calibrate. (Whether more meandering would have yielded better results is an open question.)

Road Trips

Here is a satellite view of the eastern end of the Eisenhower/Johnson tunnels on I-70. The yellow push pins here mark where there were changes in the position fix - a change in the number of satellites in view, or a total loss of GNSS navigation with a fallback onto the IMU. I'm entering the west-bound Eisenhower Tunnel at the top, and later exiting the east-bound Johnson Tunnel at the bottom.

Screen Shot 2020-09-21 at 8.41.08 AM

When I entered the Eisenhower Tunnel at the top going west,  I quickly lost GNSS reception until it was completely gone and I was relying solely on the IMU at the last of the west-bound push pins. Later, on the return leg, when I exited the Johnson Tunnel at the bottom going east, having turned around, it took a few moments for the module to reacquire GNSS signals at the first east-bound push pin, but then it quickly acquired more satellites by the last east-bound push pin.

A similar story is found at the western end of the Eisenhower/Johnson tunnels, exiting the west-bound Eisenhower Tunnel at the top, entering the east-bound Johnson Tunnel at the bottom.

Screen Shot 2020-09-21 at 8.40.40 AM

It took a few moments to reacquire the fix upon exiting the tunnel when heading west. Going back east on my return leg, I lost the GNSS fix quickly upon entering the tunnel. 

I traversed the Eisenhower and Johnson tunnels on three different test drives, each with slightly different results.

Screen Shot 2020-06-22 at 1.19.49 PM

In screen shot above, at the western end of the tunnels, the purely IMU position fix of my west-bound track drifted so far left (down), it crossed the east-bound track and made it look like I was in the wrong tunnel (which is impossible). When the module reacquired GNSS, it quickly corrected the fix to put me in the correct lane.

Screen Shot 2020-09-03 at 12.54.01 PM

In the screen shot above, its hard to say whether the IMU position fix drifted since I don't know exactly the path of the two tunnels through the mountain range, but it seems likely. (There is a pronounced bend in the tunnels at the eastern end, which you can see from the orientation of the eastern entrance relative to the western entrance.)

Screen Shot 2020-09-15 at 12.15.02 PM

Once again, there is an obvious drift in the screen shot above that makes it look like the tunnels merge at the western end (they do not). It's corrected as soon as I get near enough to the tunnel mouth that GNSS is reacquired.

There were even more radical corrections on the far shorter tunnels along US-6. (Updated 2020-10-03) Note, however, that both of these tests shown below were performed before I came up with the more stable test fixture on which I mounted the GNSS module; results after that change were noticeably better. This illustrates the importance of having a secure mounting. (End Update)

Screen Shot 2020-06-15 at 1.44.39 PM

On the western end of the tunnel above, you can see the sudden and abrupt correction made as I exited the tunnel going west-bound. It looks like I was smoking the tires on the WRX as I made a pair of bootlegger turns.

Screen Shot 2020-09-15 at 12.19.16 PM

Above, on another tunnel along different section of US-6 going east bound, you can see a similar sudden correction.

But some of the test runs in some of the tunnels looked nearly perfect.

Screen Shot 2020-09-03 at 12.52.54 PM

This tunnel is far shorter than, and not as deeply buried as, the Eisenhower/Johnson tunnels. Going back and examining the original CSV file, I see in this particular case that a GNSS fix was maintained for about half way through the west-bound tunnel, and for about a third of the way through the east-bound tunnel. In both directions, the position fix was then made solely using the IMU until I exited the tunnel.

Parking Garage

Another idea I had to test the IMU was by driving around inside a four-level parking garage at a shopping mall just a few minutes away from my home. This is what that path looked like after I converted the CSV into KML and imported it into Google Earth.

Screen Shot 2020-09-17 at 10.14.11 AM

Even driving around in the lowest two levels, then parking at the lowest, darkest, level of the garage, where no direct sunlight was visible, I was able to get a GNSS fix with at least six satellites (the minimum necessary is four if you want to also solve for altitude). The M8U used both the GNSS and the IMU for a position fix, but in this case the IMU was continuously able to be checked and if necessary corrected using GNSS.

Screen Shot 2020-09-17 at 10.34.31 AM

Looking at a close up, tilted, view of the parking garage in Google Earth, the path deviates enough that it looks like my WRX flew out of the parking deck and (somehow) back in again in a couple of places. This could be a lack of precision in the position fix of the M8U (GNSS isn't perfect either), an error on the part of Google Earth, or maybe I should audition for the next Fast and Furious movie. If I were to fly out of the parking deck and back in again, the Subaru WRX would be the car I'd use to do it.

Since the iterative least-squares algorithms typically used by GNSS receivers to compute the overdetermined solution is more accurate the more satellites it uses, and in open sky the M8U is able to use many satellites from several constellations, it is possible that in some circumstances in the garage the IMU could actually be more accurate than the GNSS when only a very few satellites are visible and they are close enough in their orbits that Dilution of Precision is an issue.


The integrated Inertial Measurement Unit in the NEO-M8U added to the accuracy of the position fix when GNSS signals were completely unavailable, but only for a short amount of time. The longer the device went without GNSS, the more the IMU position fix drifted.

The IMU might be quite useful for its intended application with typical road vehicles. I wouldn't use it to navigate in any coal mines. But it works fine over short periods of no satellite reception. And it's remarkable that it works at all.

Tuesday, June 16, 2020


Here's the problem I had to solve: my Differential Global Navigation Satellite System (GNSS) base station runs in "survey-in" mode for days in order to achieve a positioning resolution down to a few centimeters. This procedure takes so long that the Raspberry Pi 3B+, on which my gpstool software runs to process the output from the u-blox ZED-F9P GNSS module, is powered by an Uninterruptible Power Supply (UPS).
 (Click on any image to see a larger version.)

gpstool has a useful, simple, text-based real-time output display. I'd like to keep an eye on the display as it runs. But the Raspberry Pi base station runs headless - sans display, keyboard, or mouse - tucked inside a narrow drawer near where its antenna is mounted in a skylight. To avoid disrupting this lengthy operation, gpstool runs as a daemon, carefully disassociating itself from any human interface device, and insulating itself from Linux/GNU software signals that might interfere with it. How do I track its progress?

The typical approach in development projects on which I've worked in the past is to log copious text messages to the system log, a service provided by Linux/GNU that saves such messages to a file or files in a protected system directory, which is managed by a privileged syslog process that itself is a daemon. gpstool makes use of this facility. But the rate at which the state of things change in the GNSS module and in gpstool is frequent enough to be a kind of firehose of data to the syslog. It would be a lot more user friendly to carefully ssh into the Raspberry Pi - an action which itself is not without some risk - and use some kind of command line tool to bring up the real-time display, then later discard it and log out, all without interfering with gpstool itself.

This seemed to me to be a common enough problem that instead of merely implementing some specialized solution in Hazer, the git repository home of gpstool and my other GNSS-related software, I should implement it in Diminuto, my git repository containing a general purpose C-based Linux/GNU systems programming library and toolkit. Diminuto underlies Hazer and many of my other projects. 

This article describes what I did and how it works.

Step 1: The Application Programming Interface (API)

The Diminuto observation API provides the following function calls for applications like gpstool. (I'll explain what these library functions do under the hood in a bit.)

FILE * diminuto_observation_create(const char * path, char ** tempp)

The application calls diminuto_observation_create, passing it the path name of an observation file to which it wants to write its real-time display. The library function returns a standard input/output file pointer that the application can use with standard C library calls like fprintf to write its display. The function also provides a pointer to a character string that the application is responsible for providing to subsequent API calls.

FILE * diminuto_observation_commit(FILE * fp, char ** tempp)

When the application is finished with the observation file (all the output for its current display iteration has been written to it), the application calls diminuto_observation_commit with the original file pointer and the original variable containing the pointer to the character string. Once the observation file is committed - and not before - it is visible in the file system to other software, and to humans via the ls command. The library function closes the file pointer and releases the storage associated with the character string, so the contents of the two arguments are no longer useful. The library function returns a null file pointer to indicate success.

FILE * diminuto_observation_discard(FILE * fp, char ** tempp)

Should the application want to discard the current observation file and its contents, it calls diminuto_observation_discard. The observation file is never visible in the file system, and any data contained in it is lost. The file pointer is closed, and the storage associated with the character string is released. The library function returns a null file pointer to indicate success.

FILE * diminuto_observation_checkpoint(FILE * fp, char ** tempp)

Should the application want to keep the current observation file before it commits or discards it - an action that might be stimulated by a human operator doing something like sending the application a SIGHUP or "hangup" software signal (a common idiom in the Linux/GNU world, and one used by gpstool) - the application calls diminuto_observation_checkpoint. A new file appears in the file system that has the name of the original observation file appended with a microsecond-resolution timestamp. This checkpoint file persists in the file system with whatever data was written to the file pointer between the time of the create and the commit or the discard, regardless of when the checkpoint function was called.

Step 2: The Application Task Loop

The Hazer gpstool command line utility implements a task loop in which it reads and processes data from the GNSS module; about once a second, it pauses to update its real-time display.

At the top of the task loop, gpstool calls diminuto_observation_create and gets a pointer to a standard I/O file object. As it processes information from the GNSS module, it writes to this file pointer.

Screen Shot 2020-06-16 at 11.32.54 AM

The contents of the file looks something like this once a complete display has been generated. When this file is complete, gpstool calls diminuto_observation_commit and the observation file containing this display is now visible in the file system. Then gpstool loops back to the top of the task loop, calling diminuto_observation_create again.

Should gpstool receive a SIGHUP software signal, it makes a note of this fact, and eventually calls diminuto_observation_checkpoint.

When gpstool exits, it calls diminuto_observation_discard to clean up any uncommitted temporary file that may have existed from its final, partial, iteration of the task loop.

Step 3: The Library Implementation

This is how the Diminuto observation module is implemented.

  • Dynamically allocate a character string containing the observation file path name appended with the string "-XXXXXX". The use of the observation file path name is important, as it insures that this character string will name a file that is in the same directory as that of the observation file.
  • Use the standard mkstemp function to create a file using the character string as its name, but automatically replacing the "XXXXXX" with a randomly generated character sequence like "qx03ru" (actual example) that guarantees that the file is unique in the target directory. The standard function returns an open file descriptor (fd) for this new file.
  • Use the standard fdopen function to create an open standard I/O file pointer for this descriptor.
  • Store the pointer to the allocated character string in the provided variable.
  • Return the open file pointer to the temporary file for success
  • Recover the original observation file path name by truncating the added temporary file suffix from the character string in a second dynamically acquired character string.
  • Close the file pointer using the standard fclose function. This has the desirable side effect of flushing the standard I/O memory buffer to the temporary file.
  • Use the standard rename system call to rename the temporary file to be the observation file. Because the two files are in the same directory, this system call performs this action atomically: the temporary file disappears as if it were deleted, and the observation file appears with the full intact contents of the data written to the temporary file by the application. (rename performs its action atomically provided the source and destination are both in the same file system; being in the same directory is a simple way to insure this.) The rename system call replaces any existing file with the same name, so any prior observation file is deleted from the file system as a side effect. There is never a time when a partially written observation file is visible.
  • Free the character string that contains the temporary file name.
  • Free the observation file path name that was recovered in the first step.
  • Return a null file pointer for success
  • Close the file pointer using the standard fclose function.
  • Delete the temporary file using the standard unlink system call.
  • Free the character string containing the temporary file name.
  • Return a null file pointer for success.
  • Read the system clock using the Diminuto function diminuto_time_zulu.
  • Create a new file name by truncating the mkstemp suffix from the character string that is the name of the temporary file, and append a new suffix that is a UTC timestamp like "-20200616T161740Z958048" (actual example), in another dynamically acquired character string. Note that this contains the year, month, day, hour, minute, second, and microseconds, in an order that will collate alphabetically in time order.
  • Use the standard function fflush to flush the standard I/O buffer for the temporary file out to the file system.
  • Use the standard link system call to create a hard link - a kind of alias in the file system - between the temporary file and the checkpoint file name. Because the two files are in the same directory, this action is also done atomically: the checkpoint file appears in the file system containing all the data that is in the temporary file. Because it is a hard link, as the application continues to write to the temporary file, the data will also appear in the checkpoint file (which are, in fact, the same file, now known under two names). When the temporary file is either committed or discarded, the checkpoint file and its contents will remain.
  • Free the checkpoint file path name.
  • Return the file pointer to the temporary file for success.
Step 4: The Observation Script

We now have a mechanism that gpstool, or any other application, can use to create a sequential series of output files. How to these files get displayed?

The Linux kernel has a facility called inotify, which can be used to monitor file system activity and report it to an application. Lots of existing tools use this facility, like the udev mechanism that supports the hot-plugging of peripherals and the automatic attachment of removable media like USB thumb drives. Most Linux distros have a package of user-space utilities, inotify-tools, that provide command line to this facility.

Diminuto has an observe shell script that calls the utility inotifywait in a loop with the appropriate parameters so that the script is told when a file with the name of the observation file appears in the observation file directory as a result of a move operation. The implementation of the diminuto_observation_commit function emulates what the mv command does, and so it triggers inotifywait to emit the name of the observation file whenever a commit operation is performed. The observe script captures this name and emits it itself to whomever is running it, then loops to call inotifywait again.

Note that observe doesn't actually display the observation file. It has no idea what the observation file contains, or how an application like gpstool wants it displayed, or to where. It just watches for the file to show up in the file system.

(By the way, the observe script has its own SIGHUP implementation. So while the Hazer gpstool uses diminuto_observation_checkpoint to checkpoint the observation file, the Diminuto observe script provides a similar function.)

Step 5: The Rendering Script

To actually display the observation file, Hazer has a peruse script that includes a lot of Hazer-specific context about which Diminuto knows nothing. The Hazer peruse script merely calls the Diminuto observe script with the path name of where gpstool will create the observation file, based on the gpstool -H (for headless) command line option. It pipes the output of observe into a pipeline that reads the file name when it appears, clears the terminal screen, does some minor pretty-printing post-processing of the contents off the observation file, and copies it to standard output. It does this every time a new observation file by that name is moved to the target directory (even though it replaces an existing observation file).

This separates the processing of the input from the GNSS module from the output of the real-time display. I can ssh into the Raspberry Pi running a long-term survey-in as the Differential GNSS base station, fire up the peruse script, check on its progress, and then control-C out of the peruse script, with no impact to gpstool.


base.csv  base.out                         base.out-ee02oB
base.err  base.out-20200608T190630Z025023

This is an actual directory listing from a long term base station survey that's running right now.
  • base.out is the latest committed observation file;
  • base.out-ee02oB the current temporary file being written that will replace it once committed;
  • base.out-20200608T190630Z025023 is a checkpointed observation file.
In addition, there are some other files generated by gpstool.
  • base.csv is a dataset of GNSS solutions in Comma Separated Values (CSV) format;
  • base.err is the file to which gpstool is redirecting its standard error output;
  • contains the process identifier of gpstool used to sent it a SIGHUP signal.

The ability to checkpoint observation files is so useful that I use this mechanism even when I'm not doing a long-term survey. Just yesterday, Mrs. Overclock kindly served as my co-driver as we tested the u-blox NEO-M8U, another GNSS module which includes an Inertial Measurement Unit (IMU). The tiny board-mounted module's IMU contains a gyroscope and accelerometers implemented as a Micro-ElectroMechanical System (MEMS). This can be used to approximate the module's location even when the satellite signals cannot be received - like when we drove through a series of highway tunnels on route US6 west of where we live near Denver Colorado.

I wrote a script that combined gpstool using the Diminuto observation capability, with the peruse script, and another script, hups, that sends gpstool a SIGHUP signal any time any key was pressed on the laptop running my software. This made it easy for Mrs. Overclock to capture the real-time gpstool display in a series of timestamped files, for example as we entered a short tunnel about 215 meters in length, and moments later when we exited it.

Screen Shot 2020-06-15 at 1.44.39 PM

Here's a visualization from Google Earth, produced using the Positioning, Navigation, and Timing (PNT) data captured by gpstool about once per second, converted into a Keyhole Markup Language (KML) file by another Hazer script, then imported into Google Earth. (The red continuous visualized path is not a product of the observation and checkpointing mechanism; but that mechanism was used to identify the locations marked by Google Earth with the yellow push-pins.)

The IMU tracked our path from east (right) to west (left) as we went through the tunnel. (You can see remains of the old pre-tunnel road in the satellite imagery too.) As we left the tunnel and the GNSS signals were re-acquired, the NEO-M8U determined that the IMU had our location a little off and corrected it.

I assure you that we didn't do two sudden tire-smoking turns as we exited the tunnel. Although had we done so, my Subaru WRX would have been the vehicle in which to do it.