Nearly every pilot flying today uses GPS. This is the confusion it can cause when it fails...
Report: Loss Of GPS Integrity
It's easy to get lulled into a sense of security when you're flying a glass-cockpit airplane. GPS positioning is almost always perfect, making flights today safer than ever. But GPS can fail, and when it does, what's your backup plan?
This SR-22 pilot experienced a loss of GPS integrity flying around Class B airspace. Here's how the pilot handled the situation...
I was attempting to transition southwest to northeast below the Class B airspace when I visually saw the airport and 3 towers. This seemed odd given the MFD map position didn't agree with what I saw with regard to this particular airport and associated towers. I looked at the GPS that was driving the MFD and noticed a message indicating GPS loss of integrity.
After checking for traffic, I immediately started a decent and in a direction to keep me out of a lower Class B shelf ahead. Almost simultaneous to this I opened ForeFlight on my phone to see if I had better reception and coverage for the Class B airspace. I did have proper coverage and got to an altitude I knew was below the Class B shelf. Once stabilized in altitude and direction, I reset the aircraft GPS to see if the loss of integrity signal would go away and it did.
Review: How GPS Works
Over the past 10 years, GPS has overwhelmingly become the preferred method of navigation, especially when it comes to IFR flying. The FAA's entire NextGen system is built around GPS, and almost all of us use it in some form. For an aircraft to get a 3D location, the GPS receiver must get a reliable signal from 4 satellites simultaneously. With GPS available and accurate nearly all of the time, it's become the go-to source for RNAV navigation.
Click here to read how GPS can be NOTAM'd out of service for interference testing.
What Is The Extra "Integrity" Provided By WAAS?
More and more aircraft are WAAS-capable, allowing them to fly more precise GPS instrument approaches. In fact, WAAS-capable receivers can give you a position accuracy of better than 3 meters, 95 percent of the time.
Integrity refers to the usability of a satellite signal, and the ability of a system to provide timely warnings to users when WAAS should not be used for navigation as a result of errors or failures in the system. Beyond providing a correction signal, WAAS provides an additional measurement to your aircraft's receiver. The system effectively increases GPS integrity through real-time monitoring of GPS sources, whereas the accuracy is improved by provided differential corrections from these sources to reduce errors.
Standalone Backup: ForeFlight
ForeFlight has a "profile view" for airspace along your route of flight. It's available to Pro level subscribers (an upgraded version of the standard ForeFlight app). If you're navigating around complex airspace, this tool can significantly increase your sitational awareness.
The profile view combines terrain, obstacles, and airspace along your route into a single profile view, giving you a quick way to double-check that your altitude and route won't interfere with controlled airspace or terrain.
Airspace depicted is from the start to end of your route, left to right. And if you want to get a closer look at complicated airspace, you can zoom in within the profile view to see more detail. Having an EFB in the cockpit with its own GPS receiver is a great backup to fly with.
What Do You Think?
GPS is one of the most valuable tools a pilot has. When GPS fails, what's your backup plan? Have you ever had a GPS failure? Tell us in the comments below.
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