Guest Post By Joseph Kunches

Most people know that disturbed space weather conditions can adversely impact GPS to various degrees. The Sun can roil the ionosphere and make signal propagation from satellites to the ground tricky or impossible.

The good news is that space weather, in general, is abating now. Solar Cycle 24 is in its decline. This cycle began in 2008, reached maximum in 2014, and now is falling fast.

Sunspot numbers are the traditional way of measuring cycles. More critical is the activity the Sun generates, flares and Coronal Mass Ejections (CMEs), that affect the workings of navigation systems. Those disturbances also quiet as sunspots fade.

The graph at the head of this post (also here) also shows Cycle 23, which maxed in 2000. The current cycle was quite weak and benign when eruptive events were counted. That was good for GPS users as refractive and diffractive errors from a disturbed ionosphere were few.

The not-so-good news is that history reminds us a solar minimum does not necessarily mean solar nil. In late 2006 Cycle 23 was far into its decay, even farther than where we sit now. Yet on December 6, 2006, the Sun produced a most remarkable eruption, when a strong active region yielded flares and CMEs that caused problems for GPS users over a period of a week. A never-before-seen characteristic of that sequence was a radio burst of one million flux units at 1415 MHz (near L1), with the key attribute of being right-hand circularly polarized (RHCP), that hampered codeless and semi-codeless GPS operation on the dayside of the earth for a short time. This event was completely unpredictable. In fact, forecasters can’t predict radio bursts, much less how bursts will be polarized which can greatly reduce or worsen impacts.

Those who rely on GPS signals should remain aware of solar activity and the conditions that impact their operation through the next few years. The most prudent will, of course, be ready with plan B should solar disruptions appear without warning.


Joseph Kunches ([email protected]) is an internationally recognized expert in space weather and provides services to PNT users on space weather. He writes for the Washington Post on space weather issues as a contributor to the Capital Weather Gang.  He has served in senior leadership positions at NOAA’s Space Weather Prediction Center, was Director of Space Weather Services at ASTRA, and a past Secretary of the International Space Environment Service (ISES).