GPS receivers continue to be built into more consumer-grade and mission-critical devices, whilst being deployed in ever increasingly difficult applications and environments.
As designs continue to progress with increased capabilities of a multitude of GNSS constellations and augmentation systems, developers and manufacturers need better ways to guarantee the performance of end systems. That’s where a reliable GPS simulator becomes a valuable tool.
While test engineers have a variety of choices for testing GPS-based position, navigation and timing functions of their integrated GPS receivers, a dedicated simulator offers the most flexibility. Compared to testing with over-the-air signals (“live sky”), or record and replay solutions, a simulator offers complete control over the generation of GPS signals. It’s the only way to have complete confidence in your hardware and software’s ability to perform – under any condition.
Understanding GPS transmissions
To understand a GPS simulator, it is helpful to understand some of the details of GPS transmissions. The GPS constellation consists of at least 24 satellites, orbiting every 12 hours, broadcasting navigation data on different frequencies. GPS is just one of several global navigation satellite systems (GNSS) in
operation (alongside GLONASS, Galileo and BeiDou for example). Most navigation applications today use the GPS L1 frequency at the radio frequency 1575.42 MHz. Onto this carrier frequency, satellites transmit identification information and a navigation message that contains synchronised time, the satellite’s orbital data (ephemeris) and data on the expected positions of all the satellites in the constellation (almanac). It is from this data that receivers can accurately calculate its distance from several satellite signals at the same time to achieve its navigation solution through trilateration.
Simulating exact GPS transmissions
GPS simulators are radiofrequency generating instruments that are capable of transmitting the same exact data as GPS satellites. However, the value of a GPS simulator is in the ability to change a wide variety of parameters right from the test bench, including:
- Data from the satellite (date/time, Satellite ID/PRN code and Ephemeris/almanac)
- Conditions as seen by the receiver (number of satellite signals, power level, atmospheric and antenna errors through models and multi-path conditions)
- Position of the receiver (start position, latitude, longitude, elevation and trajectory)
It is the combination of all these parameters that make up a “scenario” for a GPS simulator. GPS simulators from Pendulum Instruments can generate simple single-satellite signals to verify receiver signal acquisition and validate assembly. Some of their simulators can simultaneously generate signals from many satellites, GPS and GLONASS, L1 and L2 frequencies and satellite-based augmentation systems (SBAS); WAAS (North America), EGNOS (Europe), MSAS (Japan), and GAGAN (India). These GPS simulators even adjust signal transit time and relativistic effects to simulate receiver motions to ensure the system is capable of performing under any trajectory.
Pendulum’s simulators off there choice of using pre-defined scenarios, or editing and saving scenario parameters through the front panel. It’s even possible to uploadable text files or use scenario-builder software to customise testing further. The GPS simulator will then automatically generate RF signals for testing GPS receivers using conducted signals through RF connectors and cables or radiated signals using an antenna. Testing can be controlled remotely or automated using a variety of instrument interfaces and an SCPI command protocol.
Without a GPS simulator, attempts to test receivers with over-the-air signals or record-and-replay solutions would be limited to the satellites available at a particular time and place and under current conditions. Testing remote locations or high velocities would be costly, time-consuming or impractical. With “live sky” signals, test parameters would never be repeatable. GPS simulators, however, allows the testing of more parameters, more often, with extreme flexibility for development and manufacturing. Time and cost savings can also be achieved by using a GPS simulator.
GSG Series GPS/GNSS Simulators
Pendulum’s GPS/GNSS satellite simulators are easy to use, scenario-based instruments that combine a powerful, feature-rich platform with industry-leading value and affordability for any requirement for GPS testing.See the range