Navigation Systems Lecture 7: GNSS Measurements and Error Sources

This set covers vocabulary and technical concepts related to GNSS error sources, signal propagation, and satellite orbit calculations as discussed in Lecture 7 of Navigation Systems.

Discriminator

A component in a GPS receiver that converts misalignment signals ($E-L$) from the Early, Prompt, and Late correlation process into a feedback signal for the tracking loop.

Pseudorange

The estimated distance between the receiver and a satellite, denoted as $R$ or $\rho$, which includes the true range plus errors from clock offsets, atmospheric delays, and multipath.

Tropospheric Delay

The signal delay caused by the non-charged part of the atmosphere ($T$), divided into dry and wet components; it is typically expressed as a Zenith value scaled by elevation angle.

Slant Total Electron Content (STEC)

The total amount of charge encountered by a GNSS signal as it passes through the ionosphere along a specific slanted path from the satellite to the user.

Integer Ambiguity ($N_L$)

The unknown whole number of carrier cycles between the user and the satellite that must be resolved in phase measurements.

Klobuchar Model

A half sinewave model used by GPS during the day to approximate and remove about 50% of the ionospheric contribution to signal error.

Cycle Slipping

An occurrence where a receiver's tracking loop temporarily loses its lock on the signal phase and re-locks one or more cycles away, biasing the phase measurement.

Ionospheric Scintillation

Rapid shifts in the phase and range of a signal caused by complex 3-dimensional distributions in the ionosphere, which can lead to deep fades ($40\,dB$) and loss of lock.

Code-Carrier Divergence

The effect where the ionosphere increases the apparent range for code measurements while decreasing the apparent range for carrier phase measurements (carrier phase advance).

Signal Group Delay

The absolute or relative delay a signal experiences while passing through electronic equipment, such as satellite components or the user receiver and cabling.

Phase Center Variation (PCV)

The slight movement of the point in space that effectively receives the signal, which varies depending on the direction from which the signal arrives at the antenna.

Carrier Phase Windup

An effect in circularly polarized signals where the rotation of the satellite or the receiver antenna causes an apparent change in the measured carrier phase.

Ephemeris

Immediate orbital data transmitted in a $50\,bps$ bitstream from each satellite, used to calculate the satellite's position in space at a specific time.

Receiver INdependent EXchange (RINEX)

A neutral format developed to standardize the exchange of core GNSS information including time, pseudorange, phase, and orbital parameters.

Friis Formula

A mathematical representation used to determine the cascaded noise floor of a system, highlighting the importance of placing a high-gain, low-noise amplifier at the first stage.

Second Harmonic Perturbations ($J_2$)

Gravitational harmonics mapped onto an orbit used to account for the Earth's non-uniform mass distribution when calculating satellite positions.

Saastamoinen Model

A specific tropospheric model used by the Galileo system to calculate atmospheric delays.