Interfacing with CAST Navigation GNSS/INS Simulation Solutions


CAST Navigation GNSS/INS simulators shrink the world to the size of your lab. We let you recreate GNSS conditions anywhere on Earth at any point in time and then model reception by any vehicle. However, developing scenarios that replace field testing requires taking control of the vast matrix of variables that influence GNSS reception.

Drawing on decades of experience, CAST has developed user interfaces and visualization tools that streamline scenario development and evaluation.

The interface balances ease of use and complexity, layering at-a-glance summaries of key data over modules that grant total control over every aspect of your simulation.

2D and 3D visualization tools create more intuitive perspectives of simulations that speed scenario development and improve communications.

This guide will explain how the user-focused enhancements in CAST Navigation solutions give you total control over GNSS/INS scenarios and let you observe simulations like never before.

Simulating GNSS Signal Environments

The cumulative effects of many technical and environmental variables determine GNSS signal quality. Many of these elements you cannot control in the field. Even when you can, the realities of geography, budgets, and safety narrowly constrain your field-testing protocols.

GNSS Constellation Variability

Even if everything works perfectly, constellation design and orbital mechanics combine to make GNSS signal strength vary with time and location.

Each constellation broadcasts signals on different frequencies. Within a constellation, each generation of satellites will have unique capabilities and power levels.

Finally, a satellite’s trajectory across the sky and its position relative to the receiver will affect signal strength. For example, receivers at equatorial latitudes will see more satellites than those in high latitudes.

Environmental Effects

A GNSS signal is subject to many environmental influences, from when it leaves the satellite to when it arrives at the receiver.

Ionospheric scintillation and other atmospheric effects are significant sources of GNSS reception errors. At times, these errors can prevent receivers from locking onto signals.

Terrain, ocean waves, or urban structures reflect a satellite’s GNSS signal so it arrives at the antenna at different times and directions. This multi-path interference can degrade a receiver’s positional accuracy.

Interference and Jamming

Not all sources of radio frequency (RF) interference are natural. Industrial activity can inadvertently generate radio interference at GNSS frequencies, reducing a receiver’s accuracy and precision.

In adversarial environments, active jamming can overwhelm GNSS frequencies to degrade navigation system performance.
Vehicle and GNSS/INS system variability

Even with high-quality GNSS signals arriving from orbit, the GNSS/INS system’s design and placement on the vehicle can impact the system’s output.

Conditions during manufacturing and service life can lead two units of the same GNSS/INS hardware to perform differently. Some causes of GNSS/INS performance variability include:

  • Controlled Reception Pattern Antenna (CRPA) element count and layout.
  • Antenna element gain patterns.
  • IMU model design performance.
  • IMU unit actual performance.

In use, a vehicle’s physical motion and the motion defined by its changing GNSS positions are not identical. GNSS receivers, inertial units, and antennas are rarely mounted near the vehicle’s center of gravity (CG). The resulting offsets create a “moment arm” that causes variances between measured and actual positioning.

Designing Accurate High-fidelity GNSS/INS Simulations

Basic GNSS/INS simulation scenarios can assume ideal conditions and technologies. However, most projects need to evaluate some aspect of system performance under realistic conditions.

Sending hardware into the field testing is realistic, but the project’s engineers will have little control over the test environment, much less its reproducibility.

GNSS/INS simulators can provide the fidelity and control projects need to evaluate systems in the lab. Without careful interface design, however, scenario developers can easily get lost in the details.
CAST Navigation simulators combine fine-grained control within simple interfaces that streamline complex scenario development.

Configuring GNSS Signal Quality

Creating high-fidelity GNSS/INS simulations requires controlling two sets of variables. The first consists of the technical and natural sources of error introduced into a signal as it travels from a satellite to a receiver.

Controlling these effects starts with the GNSS constellation. Our simulators can simultaneously generate signals from up to four GNSS constellation types and up to sixteen satellites per constellation.

Although you could choose a default option, the CAST user interface lets you create particular constellation configurations. For example, you can define a combination of GPS Block III and Block II satellites with differing L2C, L5, M-code, and signal power capabilities.

The Constellation Editor screen summarizes how the status of each satellite in the scenario changes along the scenario timeline.

At any point during the simulation, you can create a Constellation Event that alters the satellite’s performance by changing, for example, its transmission power or navigation data availability.

Scenarios can also account for natural, urban, and adversarial sources of RF interference. In addition to atmospheric effects, you can introduce multi-path reflections due to terrain or sea state.

Adding a CAST Jammer system to your simulator lets you incorporate discrete RF interference sources into a scenario. These could be static industrial sources or an adversary’s vehicle-mounted jamming systems.

The CAST Jammer interface lets you configure all aspects of the interference source, from activation timing to power levels to a jamming vehicle’s motion throughout the scenario timeline.

Finally, 3D terrain models let CAST simulators model the effect of geography on signal reception. For example, mountains will block signals from low-flying jamming sources as well as low-elevation GNSS satellites.

Configuring GNSS/INS Technology

Accurately modeling GNSS/INS hardware performance is the second half of simulation fidelity. CAST simulators give you control over the effect vehicle geometry, vehicle motion, and the vehicle’s GNSS/INS technology has on signal reception.

You can choose from a pre-configured selection of air, sea, land, and space vehicles. These configurations include offsets defining the GNSS/INS systems’ positions relative to the vehicle’s CG, thus accounting for differences between vehicle and GNSS/INS system motion.

For example, a ship’s GNSS antenna mounts to a mast tens of meters above the ship’s CG. CAST simulators will generate signals matching the complex path the antenna traces as the ship maneuvers in rough seas.

You can further refine the vehicle configuration to create simulations more representative of the system under test. Rather than modeling GNSS reception by ideal antennas, for instance, you can specify each antenna element’s gain patterns to model its phase center more realistically.

Finally, CAST GNSS/INS solutions let you define the vehicle’s motion through the simulated terrain.

From the scenario’s User Motion Generator module, you can insert a maneuver event at any point along the scenario’s timeline. Each event allows you to alter the vehicle’s altitude, attitude, and motion properties.

Benefits of Granular GNSS/INS Simulations

With total control over the variables affecting GNSS/INS performance, laboratory simulations offer significant benefits over field testing.

Laboratories don’t suffer from weather delays or permitting issues. Simulated vehicles are always where and when they need to be and never run out of fuel. As a result, simulations let projects achieve their goals faster.

Laboratory testing can be more comprehensive than field testing. Simulators can reproduce GNSS conditions at any time, anywhere on the planet, allowing projects to evaluate scenarios that would be unaffordable or impossible in the field.

Simulator solutions produce accurate, repeatable results resulting in more robust analyses. Controlling every variable affecting GNSS reception lets engineers evaluate equipment performance and quickly identify the root cause of anomalous results.

Real-Time Simulation Visualizations

While data is the core product of any simulation, sometimes numbers can’t tell the whole story. CAST Navigation simulators have 2D and 3D graphical tools that provide a visual perspective of each scenario.

For example, overlays on aeronautical charts provide a traditional view of a scenario’s flight path. Pilots can review these charts and provide feedback during scenario development.

CAST simulators include a 3D graphics engine that visualizes scenario runs in real-time. These visualizations display key elements, including vehicle trajectory through terrain, satellite positioning, jammer location, and jammer sphere of influence.

3D visualizations allow observers to spot misconfigurations and opportunities for improvement. For instance, a pilot observing a scenario run could suggest more realistic maneuvers in low-level flight through mountainous terrain.

These graphical tools have additional applications. When incorporating GNSS simulations in mission planning, the visualizations offer a more direct way to evaluate a vehicle’s path through terrain and jamming sources.

Visualizations are also valuable tools for communicating a project’s results. Charts, images, and videos reinforce essential points to make reports and briefing materials more effective.

Designing GNSS/INS Simulators for Results

Over the past four decades, CAST Navigation has led the way in GNSS/INS simulation. Our commitment to excellence goes beyond simulator technology. CAST solutions deliver excellent customer experiences.

By developing an intuitive user interface, we simplify the configuration of complex scenarios and streamline the iterative testing process.

Our visualization tools let you produce more refined simulations while helping you communicate results more convincingly.

Explore the rest of our site to see how CAST solutions combine:

Contact us to learn how CAST Navigation’s GNSS/INS simulation solutions can accelerate your lab’s performance.

May 27, 2023