Thursday, March 31, 2022

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.


Fazal Majid said...

I wonder if you are more exposed because of Denver's altitude.

Chip Overclock said...

Good question. When I believed I was seeing the results of the GPS jamming tests at White Sands, the DoD had announced the testing well in advance via, for example, a Notice To Air Missions (or sometimes "Airmen": it is very old nomenclature ca. 1947) notification. The NOTAM indicated that, at my distance, the jamming should only be visible to aircraft in flight. The issues I observed coincided with the stated test period. I have often wondered if my altitude - 5608' or 1709m above MSL according to my own equipment - made a difference.