Quick show of hands and be honest, who here flys in ATTI Mode all the time? Most never use it at all, much less as their default method. That means that 99.999% of drone pilots rely on GNSS positioning while flying. Luckily, GNSS systems like GPS & GLONASS work really well most of the time but there are some things can negatively impact its performance and the stability of your aircraft.
What could possibly impact the signal of billion-dollar satellites flying at 12,550mi above the Earth? The Kp Index.
The Kp Index – The Technical Stuff
In simple terms, the Kp Index is a measurement of global geomagnetic activity expressed on a scale of 1-9. In general, we consider that under the level of 4, the geomagnetic activity is “Quiet”, while at 4 it is “disturbed” (unsure/unsettled) and over 4 that we are under a magnetic storm.
This type of interference is caused by things like solar activity and has a huge impact on your ability to safely operate your drone via GPS. According to the popular app, UAV Forecast, Solar activity interferes with GPS signals in two ways, both due to disruptions in the ionosphere.
(1) It decreases the signal-to-noise ratio and affects carrier frequency, causing the receiver to lose lock on some satellites. Instead of 9 satellites, you might lock only 6, or the number might fluctuate from second to second.
(2) It changes the propagation delay through the ionosphere, making GPS positioning inaccurate even if the receiver has all satellites locked.
To fully understand how electromagnetic activity impacts your drone’s GPS, it is important to understand how GNSS systems like GPS work.
How GPS Works?
Your drone’s GPS receiver uses radio signals from known positions of several orbiting satellites (contained in the ephemeris) to determine the range from each satellite, and from these ranges (pseudo-ranges) the actual position of the receiver is determined. The radio signals used in this process must pass through the ionosphere, which introduces a propagation delay that depends on the ionospheric Total Electron Count (TEC) and on the satellite elevation angle above the horizon.
Changes in the electron density, due to space weather activity, can change the speed at which radio waves travel through the ionosphere, causing a delay in the GPS signal and thus making it very difficult for the receiver to calculate distance from the satellite and its location on the earth.
Modern GNSS systems are smart; they try to estimate and compensate for this ionospheric disturbance using an algorithm called the Klobuchar Model (1987), named after famed Atmospheric Research Scientist, John Klobuchar.
However, during times of high electromagnetic activity, the rate of change in TEC is so rapid that the predicted TEC can deviate largely from the true value of the TEC. When large discrepancies appear, the accuracy of the solution can be strongly degraded resulting in loss of accurate positioning capability.
Actual Impact on Drones?
So, how does this impact your drone? That last line says it all;
When large discrepancies appear, the accuracy of the solution can be strongly degraded resulting in loss of accurate positioning capability.
This means that during times with a moderate Kp Index (5-7), a few things might happen:
- Your aircraft may take longer to acquire GPS fix
- Your home/RTH position may not be as precise
- You may lose GPS entirely and be forced into ATTI mode
Again, with a moderate Kp Index, these effects will be pretty minor and you may not even notice.
However, once they get a bit higher (7-9) you could see location errors as large as 100ft. It should be noted though, that Kp Indices in that range are considered a major geomagnetic storm and are very rare. (There were zero occurrences of a Kp Index > 7 during all of 2018.)
The best things you can do to prevent a Kp Index related mishap are to be aware of the Kp Index and to practice ATTI mode regularly. If your GPS signal degrades or fails entirely, you will be forced to complete the mission in ATTI mode. You want to ensure you are comfortable navigating and landing your aircraft in this mode, even with high winds.
The Global Kp Index is a measurement of geomagnetic activity. This activity can disrupt GNSS (GPS) signals as they move through the ionosphere causing GNSS receivers to perform below optimal levels.
A high Kp Index can cause degraded GPS performance and accuracy.
There are three things you can do to ensure the Kp Index does not negatively impact your mission:
- Use an app (like UAV Forecast) or NOAA’s site to check the Kp Index during preflight planning.
- Have a plan on how you will respond to degraded GPS performance and brief it to the flight crew.
- Practice ATTI mode and be ready to use it!
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