Ensuring Aviation Safety by Adjusting for Risk in GPS DataMay 2019
Topics: Air Navigation, Aviation and Aeronautics, Performance of Systems, Avionics, Sensing and Signal Processing, Risk Management
Most of us now rely on the Global Position System (GPS) for navigation and location assistance in our daily lives. But did you know GPS signals are affected by the atmosphere and space weather, such as sunspots and solar flares?
Extreme atmospheric phenomena can cause vast inaccuracies in the measurements GPS-dependent devices produce—errors as great as 50 meters (164 feet) or more. A mistake in satellite position parameters uploaded by GPS ground support personnel can cause an arbitrarily large error depending on the mistake.
If the device receiving the GPS signals is the smartphone you're using for driving directions, you might miss your turn-off, but your life and property aren't at risk. But when pilots use GPS information to land a plane, inaccuracies in the data create a serious safety hazard.
In Singapore, MITRE has worked on a system to ensure that pilots are receiving accurate position data from the GPS aboard their aircraft—or to let them know when not to trust that information.
Alternative Methods for Ensuring GPS Accuracy
Because of the safety factor so crucial to the world of international aviation, GPS and other Global Navigation Satellite Systems (GNSS) are not enough on their own. They must be backed up with technology that provides the pilot with additional information.
These come in three varieties:
- On-board Augmentation Systems (ABAS)
- Satellite-Based Augmentation Systems (SBAS)
- Ground-Based Augmentation Systems (GBAS)
Prior to GPS, aviation relied solely on ground-based navigational aids such as the Instrument Landing System (ILS). That system emits radio beams from the end of the runway. Pilots can then use the beams to help them guide the plane to the runway.
But ILSs have limitations: Installation can be costly because of siting criteria and the complexity of the ILS antenna system. Airports must install ILS "hold short" lines on taxiways to prevent aircraft on the ground from interfering with the ILS signal.
At airports with limited space, this restriction can create additional congestion. If an aircraft on the ground inadvertently passes the ILS hold short line, it could cause an aircraft using the system to land to abort that landing and perform a go-around.
The vertical component of the system, called the glide slope, works by bouncing the signal off the ground. Thus, the area in front of the runway must be cleared of snow or the system must be set out of service. For these reasons, airports around the world are considering SBAS and GBAS as an alternative for ILS.
"GBASs enhance the efficiency of airport operations," explains GPS navigation systems engineer Tim Cashin. "Unlike the more common ILSs, one GBAS can support multiple runways at an airport. GBASs also provide greater flexibility in the glidepath angle—the angle of descent of an approaching aircraft. That can enable approaches to runways with difficult terrain or other obstacles."
Ground-Based Augmentation Systems Provide a Double-Check of GPS Information
GBAS is a precision-landing system that uses GPS receivers, or ground stations, at surveyed locations to check the real-time accuracy of the GPS signals the aircraft is receiving. These ground stations are typically placed at the airport, so any errors they sense should be very close to the errors that would affect aircraft approaching the airport for landing.
However, sometimes phenomena in the Earth's ionosphere will produce errors that are different at the GBAS ground stations than aboard the landing aircraft. This means the pilot may be relying on incorrect information during the runway approach.
"The ionosphere is the largest source of error for GPS measurements," Cashin says. These anomalies are worse in equatorial regions, where adverse atmospheric phenomena are stronger and more prevalent. That can cause significant errors in the position the plane would compute.
Because of this potential for erroneous measurements—and the safety hazards they introduce—scientists and engineers understand they must take these errors into account when designing GBASs.
Risk of Ionospheric Anomalies Higher Nearer the Equator
When the Civil Aviation Authority of Singapore (CAAS) decided to pursue a GBAS system for the country's busy Changi Airport, they knew they first needed to capture the full range of ionospheric behavior that could affect the GBAS at Changi. They then wanted to build enough safety margin into the integrity monitors to keep the system safe. They asked for MITRE's help with that work.
"Because ionospheric anomalies are worse near the equator, this is a much more significant risk in places like Singapore than it is in the United States," Cashin explains. "For that reason, GPS data needed to be collected, and we needed to characterize the anomalies, before the GBAS system could be certified."
Our engineers accomplished that task in several ways. First, they installed four data-collection ground stations at the airport. Using those stations, they recorded GPS measurements for a year, completing the work in 2017.
They also obtained nearly two years' worth of data from the Singapore Land Authority (SLA), which operates a network of ground stations across the island nation.
"Solar activity goes through an 11-year cycle, and these ionospheric anomalies are stronger and more common during the peak of that cycle," Cashin says. "The last solar maximum was from roughly 2011 to 2014, so we wanted that data and were able to obtain it from the SLA." That data was key in identifying some large anomalies caused by atmospheric changes.
"The combined data from the two networks provided us with a statistically significant picture of ionospheric variability over Singapore Changi Airport."
Building Integrity Limits into the Model
In all, our engineers analyzed approximately 180 anomalous ionospheric events. They used their findings to create an ionospheric threat model for Singapore. This model defines limits on the size and speed of the disturbances that could affect GPS reception at Changi.
By determining the extent of the risk that potential anomalies represent, our researchers could build those risks into the model to help ensure safety.
"We can't control the atmosphere but using our analysis we were able to develop a model to enhance the safety of GBAS," Cashin says. "We know that the anomalies we observed are possible in Singapore, so we created limits that bound the worst-case scenarios, plus provide an additional safety margin."
When employing the model, the GBAS effectively tells the aircraft to not use certain combinations of satellites that could produce unsafe position errors. As long as the aircraft only uses approved satellites, the pilot can safely fly the approach.
"This task was a key prerequisite that now allows CAAS to move forward on the installation and certification of a GBAS at Changi Airport," Cashin says. "That aligns with their goal to continue being at the forefront of aviation in Southeast Asia."
And to keep skies safer, a core MITRE mission.
—by Marlis McCollum
Explore more at MITRE Focal Point: Transportation.