How are GNSS and GPS related?
GNSS is the umbrella term for satellite navigation systems, including GPS, BeiDou, GLONASS and Galileo. GPS is one system within that group. Receivers estimate position, velocity and time from satellite observations; multiple constellations and frequencies can provide more usable information.
Pseudorange, carrier phase and ambiguity
Pseudorange estimates distance from signal travel time and includes errors such as receiver clock offset. Carrier phase provides finer observations but includes an unknown integer number of cycles. High-precision positioning must handle ambiguities, observation biases and environmental errors, rather than simply count satellites.
Where do errors come from?
| Error or condition | Effect | Typical engineering response |
|---|---|---|
| Orbit and satellite clock error | Position and time reference biases | Precise products and differential corrections |
| Ionosphere and troposphere | Propagation delays | Multiple frequencies, models and regional corrections |
| Multipath and NLOS | Reflections distort observations | Antenna placement, screening and complementary sensors |
| Obstruction, interference and loss of lock | Missing or abnormal observations | Quality monitoring, fusion and recovery strategies |
| Receiver and installation conditions | Noise, lever-arm and visibility changes | Hardware selection, calibration and field evaluation |
Why are canopy and urban canyons difficult?
Buildings, foliage and walls can block direct signals or create reflections. Receiving a signal does not establish observation reliability. Even a fixed solution should be considered alongside quality indicators. Test open sky, obstruction and recovery separately.
Relationship to HOPO
Pobot Navi combines satellite information with inertial sensing and stereo vision, with mmWave radar on the appropriate configuration. Fusion can help maintain pose estimation, but texture, lighting, motion and obstruction duration remain relevant to field performance.