GNSS architecture
a) space segment
the GNSS satellites, each of which broadcasts a signal that identifies ti and provides its time, orbit and status.
b) control segment
a ground-based network of master control stations, data uploading stations adn monitor stations. in case of GPS, 2 master control stations(one primary and one backup), 4 data uploading stations, and 16 monitor stations
c) user segment
the user equipment that process the received signals.
GNSS propagation
the layer of atmoshpere that most influcences the transmission of GPS signals is the ionosphere(电离层), ionoshperic delays are frequency dependent; and the other layer is troposphere(平流层), whose delay is a function of local temperature, pressure and relative humidity.
some singal energy is reflected on the way to the receiver, called “multipath propagation”.
Antenna
each GNSS constellation has its own signal frequencies and bandwidths, an antenan must cover the signal frequencies and bandwidth.
antenna gain is defined as the relative measure of an antenna’s ability to direct or concentrate radio frequency energy in a particular direction or pattern. A minimum gain is required to achieve a minimum carrier : power-noise-ratio to track GNSS satellites.
GNSS error sources
contributing sources error range
satellite clocks +- 2m
orbit errors +-2.5m
inospheric delays +-5m
tropospheric delays +-0.5m
receiver noies +-0.3m
multi path +-1m
Resolving errors
multi-constellation & multi-frequency
multi-frequency is the most effective way to remove ionospheric error, by comparing the delays of two GNSS signals, L1 & L2, the receiver can correct for the impact of ionospheric errors.
multi-constellation has benefits: reduce signal acquisition time, improve position and time accuracy.
D-GNSS
in differential GNSS(D-GNSS), the position of a fixed GNSS receiver, refered as a base station, which sends the atmospheric delay related errors to receivers, which incorporate the corrections into their positoin calculations.
differential positiong requires a data link betwen the base station and rovers, if corrections need to be applied in real-time. and D-GNSS works very well with base station-to-rover separations of up to 10km.
Real time kinematic(RTK)
it uses measurements of the phase of the signal’s carrier wave, in addition to the information content of the signal and relies on a single fixed reference station to provide real-time corrections, up to centimetre-level accuracy.
the range to a satellite is calculated by multiplying the carrier wavelength times the number of whole cycles between the satellite and the rover and adding the phase difference. the results in an error equal to the error in the estimated number of cycles times the wavelength, so-called integer ambiguity search, which is 19cm for L1 signal.
Precise Point Positioning(PPP)
PPP solution depends on GNSS satellite clock and orbit corrections, generated from a network of global reference stations.
GNSS + IMU
the external reference can quite effectively be provided by GNSS, and GNSS provides an absolute set of coordinates that can be used as the initial start point, as well, GNSS provides continuous positions adn velocities thereafter which are used to update the IMU/INS filter estimates.
for additional combined sensors, such as odometers, cameras vision.
challenges of GNSS in AV
talk from iMorpheus.ai
1) antenna
2) multipath mitigation
3) multi-band, multi-constellation signals
4) integrated navigation (camera )