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Why use a GNSS Simulator?

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Originally Posted On: https://castnav.com/why-use-a-gnss-simulator/

Global Navigation Satellite System (GNSS) simulators pull schedules to the left, reduce costs, and produce more complete analyses for testing and integration projects. Here’s everything you need to know about why military and commercial projects rely on GNSS simulation.

Perfecting GNSS Simulation Solutions

CAST Navigation has earned a trusted position in the industry by pioneering GNSS simulation technology for more than 40 years. We share our expertise here to help those new to the GNSS simulation understand how today’s state-of-the-art simulation technology produces accurate, precise, and repeatable results for even the most challenging simulation scenarios.

What is a GNSS Simulator?

GNSS simulators enable laboratory reproduction of the signals produced by a GNSS receiver’s antenna. Doing so requires advanced modeling of satellite motion, vehicle motion, and other variables that affect signal reception. The resulting radio frequency (RF) output lets projects test receivers under a broader range of simulated conditions than they could outside the lab.

How satellite positioning works

America’s Global Positioning System (GPS), the world’s first GNSS, consists of a constellation of 32 satellites, 24 active and 8 standbys. From medium Earth orbits, the active satellites broadcast their orbital positions and time information. GPS used two carrier frequencies (L1 and L2) in its original form, the first available to civilian users and the second restricted to military use. Over the next few years, modernized GPS satellites will use additional frequencies to improve GPS performance.

Receivers on the ground, at sea, in flight, or in orbit use these satellite signals to calculate their positions. A single-element antenna will detect signals from GPS satellites visible above the horizon. The timing and phase of each signal arriving at the antenna element will vary depending on factors such as:

  • Antenna orientation
  • Antenna placement on the vehicle
  • Vehicle orientation
  • Vehicle motion

The signals arriving at the antenna element form an RF wavefront. The receiver uses this RF wavefront to calculate the receiver’s vertical and horizontal position. A GPS receiver’s positioning data may be accurate within 10 meters when using one signal per satellite.

Positioning accuracy can be improved by:

  • Detecting more satellites
  • Using phased-array antennas with multiple elements
  • Using 2 or more signals from each satellite
  • Augmenting GPS with other space or ground-based systems

GNSS services such as those from Europe (Galileo), Russia (GLONASS), or China (BeiDou) operate similarly. Receivers compatible with multiple GNSS constellations will detect more satellites, allowing them to calculate more accurate and reliable positions.

How a GNSS simulator works

A GNSS simulator is a laboratory tool that recreates the output from an actual GNSS antenna at a specific time and place. To do this reliably, the simulator must simultaneously model:

  • Motion of satellites in multiple GNSS constellations
  • Ionospheric and atmospheric conditions affecting signal propagation
  • Antenna configuration and placement
  • Vehicle motion in 6 degrees of freedom (DOF)

These and other parameters determine the properties of each RF signal arriving at the simulated antenna element. The simulator turns these calculations into an output for the GNSS receiver. Sitting on the bench top, the receiver converts the simulator’s output into the positioning data it would have produced under real-world conditions.

Project teams can evaluate the receiver’s response to the simulator’s output under a range of conditions by changing the simulator’s parameters. Since the simulator records its output as well as the receiver’s output, the resulting analysis can quickly identify error sources.

Other ways to test GNSS receivers

A quick-and-easy way to test a GNSS receiver is to connect an antenna and see what it does. This live-sky testing is simple and does not require additional equipment. Installing the receiver in a vehicle and recording its output lets engineers analyze the receiver’s performance in motion. This approach, however, does not produce robust data and cannot predict receiver performance when deployed.

Limited to conditions during test:

Live-sky testing only observes receiver performance at one time and location. It is limited to the number of visible satellites as well as the environmental and other conditions existing at that time. The test cannot indicate how the receiver would perform under different conditions.

Unrepeatable:

Live-sky testing is not repeatable. All the conditions that exist on one day never quite align the same way on subsequent days. Reproducing anomalous results may not be possible. The test program may never confirm the effectiveness of changes made based on earlier tests.

Analysis complexity and ambiguity:

Live-sky tests control a few variables by choosing where and when to perform the test. Most sources of error affecting the incoming GNSS signals are beyond the project team’s control. Moreover, the test conditions are not reproducible. Any conclusions based on live-sky tests are limited to the test conditions, not the receiver’s overall performance.

Cannot reproduce real-world use cases:

Even if a project conducts multiple live-sky tests, it cannot evaluate all the conditions the receiver will experience. For example, test sites at mid latitudes see GNSS satellites at different inclinations than sites at high latitudes.

GNSS Simulation Advantages

Using GNSS simulators in the lab avoids many of live-sky testing’s limitations. Test programs become more productive when they quickly evaluate many scenarios and produce better analysis than live-sky testing.

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