GNSS/INS Simulations of High-Latitude Operations


Warming oceans are making the Arctic more navigable. This creates opportunities for shipping, fishing, and other industries. A warmer Arctic also becomes more strategically competitive. Over the past two years, every American military service has updated its Arctic strategies to reflect this new reality. The Arctic environment will present many challenges for expanded operations not least of which is the performance of positioning and navigation systems.

We will explain how the GPS and inertial systems that work so well at mid latitudes run into trouble as you approach the poles. Evaluating equipment performance in the Arctic is difficult and expensive. GNSS/INS simulations can help evaluate systems and operational scenarios quickly and affordably.

Why does GNSS/INS degrade in high latitudes?

Polar regions present unique challenges that extend to GPS positioning and inertial navigation. Some of these challenges are natural and others are due to choices made when the poles, and the Arctic Ocean in particular, were less accessible.

GPS orbits limit performance

Every GPS satellite orbits in a plane that is tilted 55° relative to the equator. A constellation of 24 satellites in six orbital planes ensures that four or more satellites are visible almost anywhere on Earth. In ideal conditions, some satellites will be overhead with others distributed around the horizon.

As you travel above 55° latitude, however, GPS performance begins to degrade. No satellites pass overhead and by the time you reach the pole, GPS satellites only rise 45° above the horizon. As a result, GPS performance — vertical accuracy in particular — declines as you approach the poles.

Multi-constellation GNSS receivers can mitigate this effect by supplementing GPS signals with signals from GLONASS (64°8’ inclination) and Galileo (56° inclination) satellites. However, the geometry of orbits still applies. More satellites may be visible, but they will all be grouped near the horizon.

Gravity makes polar navigation difficult

Earth’s gravitational field is not constant. Since Earth isn’t a perfect sphere, our planet’s rotation makes gravity about 0.53% stronger at the poles than at the equator. Mountains exert a stronger pull than oceans. And Earth’s gravitational pull over oceans changes with the tides.

These variations affect GPS satellites. While they orbit at a nominal altitude of 10,900 nautical miles, they could be higher or lower depending on their positions above the Earth. Gravity variations also create relativistic effects that impact GPS signals. Orbital perturbations and relativistic errors are understood and accounted for by the GPS system.

In the polar regions, however, Earth’s gravitational field is under-measured. There is no gravity data for some areas of Alaska which impacts the accuracy of sea-level measurements and the reported elevation of mountains or airports. The combination of less accurate GPS altitude data and less accurate mapped elevation data increases the risk of navigation errors.

Ionospheric interference degrades performance

Day-to-day interactions between solar particles, Earth’s ionosphere, and its magnetic field are built into the GPS models. But more extreme events can degrade performance or disrupt reception entirely.

At high latitudes, ionospheric instabilities are more likely to scintillate a GPS satellite’s radio signals. If the signal’s phase and amplitude change too much, positioning becomes less accurate and signal lock becomes more difficult.

Solar flares and other space weather events also produce ionospheric interference. In polar regions, where the Earth’s magnetic field lines converge, the ionosphere extends lower into the atmosphere. The impact of space weather events on high-latitude GPS reception can be much stronger and last longer.

Inertial navigation is complicated at the poles

The navigation infrastructure in polar regions is underdeveloped. Maps and charts are not as complete or accurate. As we’ve seen, poor gravity measurements produce poor elevation data. Location data are also inaccurate. According to NOAA, the last time Alaska’s northern and western shorelines were systematically mapped was more than sixty years ago. Undersea sounding charts are even less complete.

The infrastructure used to augment GPS at mid-latitudes is also underdeveloped. For example, EGNOS and similar GPS augmentation services rely on geostationary satellites that are not visible above 70° latitude.

Complicating matters further, navigation algorithms that reference the poles become less accurate at high latitudes. A degree of longitude is more than 60 nautical miles wide at the equator, 28 nautical miles at the Arctic circle, and converges to zero at the pole. Inertial systems must use a different frame of reference for polar navigation.

Simulate Arctic and Antarctic operations with CAST Navigation

GNSS/INS simulations allow for the exploration of a scenario space that would be too difficult and expensive to perform near the poles. Running more simulations and more varied simulations can enhance projects including:

  • Identifying IMU/EGI behaviors in sub-optimal conditions.
  • Simulating Arctic air, land, and sea operations.
  • Modeling alternative navigation methods.

CAST Navigation offers a full range of simulation capabilities including GNSS, Inertial, CRPA antenna, as well as Jamming and Interference. Our proprietary technology delivers precise, repeatable results for real-world simulations. Users can customize scenarios to reproduce high-latitude conditions and model the effect of land or sea environments.

Contact CAST Navigation to learn more about simulating high-latitude operations.

December 30, 2021