GPS Navigation System and Space Weather
GPS Navigation System and Part 3 of a 3 Part series on the impacts of space weather on GPS navigation system accuracy and performance and also on your HF/SSB radio communications.
SPACE WEATHER AND GPS
The ionosphere is well known for the effects it has an HF/SSB and Ham radio, but a lot less known in boating and yachting circles are the effects on marine GPS navigation systems.
It is an important source of range and range rate errors for users of GPS 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 to 3 meters.
It can also 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. Understand the affects and how to deal with it,
GPS Navigation System and the Ionosphere
The GPS signals pass through the ionosphere but can suffer propagation delays. This can result in ranging errors of tens of meters in extreme ionosphere conditions, but typically it is 5 to 10 meters. If you are in the region be aware of this, don’t assume the GPS receiver has failed. They used to use Differential GPS (DGPS) for signal error correction but that system is now discontinued. 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 boat GPS systems 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 to 400km.
When Does GPS Navigation System 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 and 2025 is the next peak of the so and predicted 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 marine GPS receiver. Signal enhancements also occur, but the 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 GPS receiver lock threshold and must be continually re-acquired. All marine GPS are affected by space weather. Understand the affects and how to deal with it. You can check out the following website for up to date forecasts and if you use Twitter, it can be updated every hour or so when there are coronal mass events, solar wind events and also radio blackout events. Check out the NOAA Space Weather Prediction Center.
How Widespread is Interference
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 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. 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 marine GPS receiver 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 GPS marine. 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 loose GPS signal lock, or simple unexplainable inaccuracies. May you have solar fair winds and good GPS fixes.
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