While the test engineer has a variety of choices for testing GPS-based position, navigation and timing functions of their integrated GPS receivers, simulation offers the most flexibility, compared to testing with over-the-air signals (“live sky”), or record and replay solutions. Having complete control over GPS signal generation is the only way to have confidence in your hardware and software’s ability to perform – under any condition.
To understand how a GPS simulator works, it is helpful to understand some of the details of GPS signal 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, or soon to be in operation. (See GNSS Terminology Explained for more detail.) 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 synchronized 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.
GPS simulators (also known as GPS signal generators) are radio frequency 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:
Data from the satellite:
- Date/Time via the clock parameters
- Satellite ID (PRN code)
- Ephemeris and almanac
Conditions as seen by the receiver:
- Number of satellite signals
- Power level
- Atmospheric and antenna errors through models
- Multi-path conditions
Position of the receiver:
- Start position (latitude, longitude and elevation)
- Trajectory (motion path)
It is the combination of all these parameters that make up a “scenario” for a GPS simulator. All Orolia GSG models can be used in signal generator mode to generate a single-satellite data stream to verify receiver signal acquisition and validate assembly. Orolia multi-channel simulators can simultaneously generate signals from many satellites. They can be enabled to simulate GPS, or other global navigation satellite systems such as GLONASS, Galileo, BeiDou, QZSS and IRNSS also simultaneously, to test multi-GNSS capability. They also have the ability to generate signals from satellite-based augmentation systems (SBAS); WAAS (North America), EGNOS (Europe), MSAS (Japan), and GAGAN (India). Since the various GNSS constellations currently, or plan to, transmit on other frequencies, Orolia multi-frequency simulators are designed to generate these signals in addition to L1 (and Galileo’s E1 and BeiDou B1) such as GPS L2, L2C, L5; GLONASS L2; Galileo E5, E6; BeiDou B2, B3; IRNSS L5.
GPS signal generators are programmed to adjust signal transit time and relativistic effects to simulate receiver motions to ensure the system is capable of performing under any trajectory. Trajectory is just one parameter included in a scenario. Pre-defined scenarios are loaded into the unit ready for use right out of the box, or edit and save your scenario parameters through the front panel, through uploadable files, or from scenario-builder software, and start. The simulator automatically generates the 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 a SCPI command protocol. Several new capabilities allow for seamless integration of GPS simulation in application-specific test platform where data can be exchanged to synchronize the signals to other simulated data in real-time.
Without a 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. And with “live sky” signals, test parameters would never be repeatable.
Test more parameters, more often, with extreme flexibility for development and manufacturing; save time and money with GPS simulators.
Learn more about our newest GPS Simulator.