What is SIS (signal-in-space)

Signal-in-Space (SIS) Explained Technically

In Global Navigation Satellite Systems (GNSS) like GPS, Galileo, and BeiDou, SIS, or Signal-in-Space, refers to the information transmitted by the navigation satellites. This information is crucial for receivers on Earth to determine their location, time, and other navigation parameters.

Here's a deeper dive into the technical aspects of SIS:

Components of SIS:

• Satellite Position: The most critical information transmitted by the satellites is their precise orbital positions. Receivers use this data, along with signal propagation times, to perform trilateration and calculate their position.
• Satellite Clock Bias: Atomic clocks onboard the satellites maintain very accurate timing. However, these clocks might have slight deviations compared to a universal reference. The SIS includes information about these clock biases, allowing receivers to adjust their timing measurements for accurate positioning.
• Satellites Health Status: The SIS might also convey information about the operational health of the satellites. This helps receivers identify and exclude malfunctioning satellites from their calculations.
• Navigation Message: This message contains additional data like ephemeris (orbital parameters) and almanac (information about all visible satellites) parameters, constellation health information, and system announcements.

Transmission of SIS:

• Modulation: The SIS information is typically modulated onto a carrier signal using techniques like Binary Offset Carrier (BOC) modulation. This modulation scheme offers good resistance to noise and multipath interference.
• Multiple Frequencies: Satellites often transmit the SIS on multiple frequencies to enhance signal availability and mitigate the effects of atmospheric disturbances on specific frequencies.

Importance of SIS Accuracy:

The accuracy and integrity of the SIS information directly impact the performance of GNSS receivers. High-precision applications like autonomous vehicles rely on extremely accurate SIS data.

Sources of SIS Errors:

Several factors can introduce errors into the SIS:

• Satellite Clock Drift: Even with atomic clocks, slight drifts can occur, leading to timing errors in the SIS.
• Orbital Perturbations: The satellites' orbits are not perfectly circular due to gravitational influences, introducing minor position errors in the SIS.
• Atmospheric Effects: Signal propagation through the atmosphere can cause delays and distortions, impacting the accuracy of received SIS information.

Mitigating SIS Errors:

Various techniques are employed to minimize the impact of SIS errors:

• Differential GNSS (DGPS): This method utilizes a reference station with a known location to provide corrections for satellite clock and atmospheric effects.
• Receiver Algorithms: Advanced algorithms in GNSS receivers can filter out noise and model atmospheric effects to improve position accuracy.
• Ground-based Augmentation Systems (GBAS): These systems provide real-time corrections and integrity information for GNSS signals, further enhancing accuracy and reliability.

By understanding the technical details of SIS, GNSS engineers can design receivers and infrastructure that leverage the navigation information effectively. This ensures accurate and reliable positioning, navigation, and timing services for various applications.