Thursday, March 31, 2022

Supply and Demand

I've been using the Raspberry Pi single board computers (SBC) ever since the Model 1 (see below) came out in 2012. At about US$40 for a board, it is often easier just to buy a new board for a new project, rather than taking apart an existing project and sacrificing its SBC. Today, I'm buying Raspberry Pi 4B boards, still around US$40 each - more if you want more memory, and who doesn't?

Raspberry Pi

For a long time now I've been having an intermittent problem with one of my projects, Obelisk, my WWVB Radio Clock, a desk clock/NTP server I built using a Raspberry Pi 3B and tiny AM receiver board that picks up the NIST WWVB digital time signals from the transmitter near Fort Collins Colorado. Obelisk drops off my home WiFi network, after which it becomes impossible to diagnose since I can't log into it. I power cycle the unit, and it works fine for another month or so until it fails again. Since the clock and display keep running (meaning my software on the 3B is still running), this looked to be a problem with the WiFi chip on the 3B. I have about a dozen Raspberry Pis around the Palatial Overclock Estate that run 24x7, and this is the only one that seems to have this problem.

Obelisk (O-3)

I finally decided to replace the Raspberry Pi in Obelisk. I figured I would just order another 3B board, only to be taught an important lesson about supply and demand: the cheapest 3B I could find online was US$175, and that was used; a new one was over US$250!

That isn't as crazy as it sounds. Aside from the current semiconductor chip shortage, one of the design changes made in the Raspberry Pi 4B was to reverse the placement of the Ethernet jack with the dual USB A jacks at one end of the board. This not only made all of the older Raspberry Pi cases useless with the new 4B, but made use of the 4B problematic to retrofit into a lot of applications that were originally made with older versions of the SBC. (The 4B also added a second micro HDMI port, so those old cases would have been obsolete anyway... which may explain why the Pi architects swapped the Ethernet and USB A jacks.)

Fortunately I have four plastic storage bins of Raspberry Pi paraphernalia in the basement, ranging from my original Raspberry Pi model 1 to, fortunately, several Raspberry Pi 3Bs. They were all in cases, labeled, and ready to be reused as part of whatever project for which they were originally purchased. But in this case, I figured the only reasonable thing to do was to open up one of the cases and extract the 3B. (So long to Tin, a Raspberry Pi 3B that took part in my Roundhouse IPv6 project.)

It only took me a few minutes, and accidentally breaking a nylon spacer, to swap the 3B in Obelisk with this other one. Will that fix the problem? Only time will tell. But for sure it had never occurred to me that the Little SBC That Could would become a collector's item.

Solar Power

I spent part of the late morning - U.S. Mountain time zone plus Daylight Saving Time or MDT (this will become important shortly) - yesterday diagnosing a major failure in my Differential GNSS test bed.

Differential GNSS (or Differential GPS if it just uses the U.S. satellite constellation) is a technique that uses a fixed GNSS base station at a known location to broadcast corrections to mobile, or rover, GNSS receivers. Since the base station is fixed, it knows that any jitter in its position calculation must be due to environmental factors, like weather in the ionosphere, that produce variations in the travel time of the signals from the satellites. Rovers in the same geographic vicinity that are computing their positions using signals from the same satellites as the base station (and so are likely to be subject to the same environmental conditions), can apply the corrections transmitted by the base station to improve their own calculations, even though they are moving and each of their position calculations are constantly changing. Centimeter position precision is possible, versus the precision of many meters typical of uncorrected GNSS. This is why autonomous vehicles cannot just use un-augmented GNSS for navigation - they can't reliably tell what highway lane they are in. Such precision is however perfectly adequate for cruise missiles.

My test rover had completely lost the ability to get a GNSS fix; it could not see any of the satellites in the GPS (U.S.), GLONASS (Russian), Galileo (E.U.), or BeiDou (Chinese) constellations. Meanwhile, the base station was working just fine. Both units run 24x7 at the Palatial Overclock Estate. The rover antenna sits in a south-side (i.e. sun-facing) window of my home office, while the base antenna sits in a skylight above my kitchen, with an excellent view of the sky above but partially shielded by the peak of the roof. 

My initial thought was that either the amplified antenna or the RF section of the GNSS receiver on the rover had failed. I stopped working on it to go out to lunch with the Spousal Unit. I checked on the rover later when we got back, and it was working fine.

I had just about decided that maybe this was an intermittent failure, when I checked my laptop and saw this email from the Space Weather mailing list of the U.S. National Oceanic and Atmospheric Administration (NOAA) that had arrived while I was at lunch:

Space Weather Message Code: SUMX01

Serial Number: 120

Issue Time: 2022 Mar 30 1824 UTC

SUMMARY: X-ray Event exceeded X1

Begin Time: 2022 Mar 30 1721 UTC

Maximum Time: 2022 Mar 30 1737 UTC

End Time: 2022 Mar 30 1746 UTC

X-ray Class: X1.3

Location: N16W41

NOAA Scale: R3 - Strong

NOAA Space Weather Scale descriptions can be found at

Potential Impacts: Area of impact consists of large portions of the sunlit side of Earth, strongest at the sub-solar point.

Radio - Wide area blackout of HF (high frequency) radio communication for about an hour.

17:37 UTC would have been 11:37 MDT. My tax dollars at work, and money well spent, if I may say.

This is not the first time I've detected interesting stuff with my GNSS experiments in Denver. Once, I'm pretty sure I detected pre-announced GPS jamming tests at White Sands test range in New Mexico. Another time, I believe I picked up the testing of a next-generation GPS satellite on a then-unused channel formerly assigned to a decommissioned GPS satellite. 

The U.S. Global Positioning System uses code-division multiplexing. CDM makes extremely efficient use of the radio frequency spectrum, but it is highly susceptible to being trivially jammed. With GPS, this can be done, either deliberately or accidentally, just by RF white noise of the correct gigahertz frequencies. A colleague of mine observed that the effects of this naturally occurring solar weather could easily be misconstrued as GPS jamming. Given the current state of international tensions, such a mistake could lead to unwarranted escalation.

Everyone talks about the space weather, but at least NOAA does something about it.