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Spirent KB Article
Doc ID: FAQ19273
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PosApp: How is curve-fitting used to support GNSS satellite orbital realism?
PosApp v7.00 onward
Historically, PosApp has populated orbital navigation data directly from the satellite motion definition. While this is accurate and computationally simple in the vast majority of cases, some more complex scenarios can expose limitations in this approach. In particular, problems can arise when motion is defined using parameters not present in a particular navigation message, causing the satellite position computed by the user equipment to differ from that simulated by PosApp. For example, GPS Signal Sources file Orbits editor allows A-dot (semi-major axis rate) to be set. This parameter isn't present in the LNAV ephemeris but will be used when simulating satellite motion and in CNAV ephemerides. A receiver using the LNAV ephemeris would see pseudorange residuals increasing away from T_oe.
Curve-fitting is PosApp’s most realistic simulation to date. Although computationally-expensive, curve-fitting is a far more robust and realistic approach to generating orbital data. This mimics the GNSS control segments, adjusting the orbital parameters available in the navigation message in order that users compute a trajectory which best matches the true satellite trajectory over the validity period of the message.
The most advantageous use of this method comes when satellite motion is defined using rolling SP3 files.
As these files will define motion which is not a perfect Keplerian orbit, the curve fitting will exercise all available orbital parameters in order to find the best fit, helping to verify that the receiver under test correctly uses all elements of the ephemeris. Additionally, users will see a realistic, non-zero error in the ephemeris and almanac. These errors are dependent on the type of navigation message: legacy GPS ephemeris errors will be higher than modernised CNAV ephemerides, for example. This allows users to experience directly the benefits of the higher precision used in CNAV messages.
How to apply it?
Navigate to the "General Options" menu, via:
The "Options" tab, for all scenarios
The "Control options" in the scenario tree, for
Enable "Generate nav data by curve-fit
Once applied, further option become available to alter the number of fitting points, choose between different algorithm types, and toggle Jacobian matrix regeneration and field limits
What does it do?
PosApp will interpolate between records and, if required, extrapolate beyond the ends of the file, meaning users will see ephemeris and clock errors which are highly representative of those observed under live-sky conditions. The feature enables users to:
See the increased accuracy of modernised ephemerides
Observe the degradation of accuracy that occurs outside of an ephemeris’ validity period
See the different precision of different almanac messages
Test receiver performance across discontinuities at ephemeris cutovers
Quickly and simply exercise all available orbit and clock parameters in each navigation message
Product : Positioning,SimGEN,PosApp,Positioning