However, when it is time to test your GNSS equipment designed to operate in space, live sky signal and record and playback are typically not options. You need a GNSS simulator. And not just any simulator: You need one designed to test GNSS receivers used in space-based applications.
Why? Several things make receiving GNSS signals in space different than receiving them on the ground:
- Distance to the satellites
- The vehicle travels above the ionosphere
- Movements are high-speed and non-earth-based
- The space vehicle may be rotating or need to know its orientation
- More satellites are available than on the ground
When evaluating a simulator, it is important to understand its ability to accurately simulate the environment your equipment will be operating in. For space-based applications, any simulator under consideration should provide, at a minimum, the following capabilities:
1. Power Level Adjusted by Distance to Satellite
In an earth-based simulation, the ranges to the satellites do change, but not by enough to make a big difference to the simulation. When the receiver is in space, however, the distance to the satellites can vary greatly and it is possible to receive signals from something other than a satellite antenna’s main lobe. It is important that the power level being simulated can vary based on the location of the receiver and the location of the satellite relative to it.
2. Specialized Ionospheric Modeling
When simulating a scenario where the receiver is on the ground, signal propagation is affected by both the ionosphere and the troposphere. The signal is delayed when is passes through the atmosphere, and that causes errors in the position calculation at the receiver. The receiver uses a model of the ionosphere to try and compensate for this error. This model is received from the GPS signal, or from other sources such as SBAS systems or other correction services. During simulation, it is important to also simulate these delays and any simulator will provide methods of doing so.
Using a receiver in space, though, is a different story. Normally, in space, the receiver will have a direct line of sight through empty space. However, there will be times when the signal is received from a satellite on the other side of the earth. In these cases, the ionosphere still needs to be modeled, so it is important that the simulator supports this type of modeling.
3. Specific Space Trajectories – Orbital and Spacecraft Maneuvering
It can be difficult to specify an orbital trajectory using ground-based waypoints, or the typical six degrees of freedom model. Think about specifying an orbit as x, y, z, pitch, roll, and, yaw! This would be a difficult task indeed. A simulator should support entering trajectories using Keplerian orbit parameters, or through use of a two-line element trajectory format.
4. High Update Rate
Spacecraft in orbit can travel at very high speeds, exceeding 7.5 km/s. To make the proper adjustments to the trajectory, apply the proper models and ensure smooth doppler changes for receiver tracking, the simulation should be running with a high update, or iteration, rate. Choosing a simulator with a 1000Hz update rate will ensure that the simulation is ideal for accurate and realistic receiver testing.
Which Simulator Is Up to the Challenge?
BroadSim SPACE from Orolia makes it easy and affordable to complete and optimize your space vehicles’ GNSS integration, on the ground, from the comfort of your own lab. BroadSim SPACE is revolutionizing the GNSS industry because of its extraordinary flexibility, low cost and ability to achieve rapid development cycles. For more information about simulating space vehicles or advanced simulation capabilities, see the BroadSim SPACE datasheet, visit the BroadSim web page, or get a quote.
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Lisa Perdue is a world-leading expert in testing critical GPS and GNSS systems. She has trained hundreds of engineers and technicians who are responsible for high-reliability positioning, navigation and timing (PNT) applications. She took a lead role in the development of the first GNSS Vulnerability Test System and speaks widely on the topic at many industry conferences. Lisa Perdue is currently the Simulation Product Line Director at Orolia, directing the organization's GNSS simulation activities and contributing to its entire portfolio of resilient PNT solutions. She has more than 15 years of navigation and RF systems experience, which includes 10 years of service with the U.S. Navy, where she was a certified master training specialist.