GPS Interference 

GPS interference has atmospheric origins as well as space weather.  The ionosphere is well known for the effects it has an HF and Ham radio, but a lot less known in boating and yachting circles are the effects on boat GPS.  It is an important source of range and range rate errors for users of marine GPS satellites where high accuracy is required, in particular the offshore oil industry. Ionosphere range error can vary from a few meters to tens of meters; with tropospheric range error at a peak can be up to 2-3 meters.

It can alter rapidly in value, changing over one day by one order of magnitude. In practice the troposphere range error does not alter more the + or - 10% over long time periods. These effects are called atmospheric scintillations. 

GPS Problems and the Ionosphere

The GPS signals pass through the ionosphere but suffer propagation delays. This can result GPS interference with ranging errors of tens of meters in extreme ionosphere conditions, but typically it is 5-10 meters. If you are in the region be aware of this, don’t assume the boat GPS has failed. DGPS is also subject to the effect. These equatorial spread (ESF) irregularity events are often associated with plasma bubbles that characterize the unstable state of the night-time equatorial ionosphere and are subject to much ongoing research.

Ionosphere plasma bubbles are a natural phenomena consisting of large scale regions of space within the atmosphere, and were first detected in Brazil in 1976. Bubbles are known to interfere with satellite communications in the frequency range VHF to 6GHz, and also are known to interfere with marine GPS causing position errors.

The bubbles are highly aligned with the earths geomagnetic field lines along which they may extend 1000’s of kms and across geomagnetic field lines they measure 100-400km. 

When Does GPS Interference Occur

They occur after sunset and exist at nights only, and activity generally is more active in frequency of occurrence during periods of maximum solar activity so 2000/2001 are scheduled to be very active. Irregularities in the ionosphere produce both diffraction and refraction effects, causing short term signal fading, which can severely stress the tracking capabilities of the GPS receiver. Signal enhancements also occur, but the marine GPS user cannot get any benefit of brief periods of strong signal.  Fading can be so severe that the signal level will drop completely below the marine GPS receiver lock threshold and must be continually re-acquired.

How Common is this Activity?

The equatorial scintillation region can cover up to 50% of the earth. Strong scintillation effects in near equatorial regions are observed generally 1 hour after local sunset to local midnight. If precise measurement using GPS-marine can possibly be avoided during 1900-2400 hours local during periods of high solar activity and during months of normal high scintillation activity the chances of encountering the effects are small.  There are also seasonal and solar cycle effects that also reduce chances of encountering scintillation in near equatorial regions. In the period April to August the chances are small of significant scintillation in the American African and Indian regions.

Where Does GPS Interference Occur?

In the Pacific region scintillation effects maximize during these months. September to March the situation reverses.  The regions where the strongest scintillation effects are observed are Kwajalein Island in the Pacific and Ascension Island in the South Atlantic. The occurrence of strong amplitude scintillation is also closely correlated with the sunspot number; in years with near minimum solar activity there is little if any strong scintillation effects on boat GPS in equatorial or low latitude regions.   So next time you set out on a protracted offshore voyage, make sure you check up on the Space Weather Forecast, it just may have some affect on your boat GPS. The HF predictions are a useful guide to what's ahead, and checking may save you some needless worry about the equipment failing, or beeping as you lose GPS signal lock, or simple inexplicable inaccuracies.  Look for more valuable boat GPS information at fishing and boats.