GNSS Channels: What are and why are they so relevant?

Posted on 24 August 2023

Definition and Usefulness

In general, GNSS geopositioning satellites emit signals consisting of three parts. A carrier signal, one or more pseudorandom noise codes (PRNs), and a navigation message. The last two are modulated by the carrier using a technique known as phase modulation.

Table of Contents

LGNSS channels allow the antenna of a GNSS receiver to detect the signal coming from a given satellite, and from a specific band. The GNSS antenna is able to receive, classify and follow them continuously. GNSS receiver systems perform the entire process of Acquisition, Tracking and Decoding.

The Acquisition process determines which satellites are visible by the receiver and to which constellation it belongs, that is, it detects the presence or absence of the satellites in sight. In addition, it performs a first estimate of the code and carrier phase delays

Carrier phase: The difference between the carrier signal generated by the internal oscillator of a receiver and the carrier signal coming from the satellite.

This estimation is subsequently improved in the Tracking process, giving rise to the second stage of the signal processing unit. Here the local search for more precise estimates of code and carrier phase delays, and their variations, is performed.

Generally, the links of GNSS signals are modulated by a navigation message, and this contains the moment in which the transmitted ephemerides were transmitted, information about the general state of the system, approximate satellite orbits, error correction, among others.

Multiband and Multi-frequency GNSS Receivers

The bands have to do with the frequency ranges that allow us to synchronize with the signals of the constellations. Each satellite sends several radio signals at different frequencies and for the range codes and navigation message to travel from the satellite to the receiver they must be modulated with a carrier that allows the signals to arrive with sufficient power.

The real distance between the satellite and the GNSS receiver is not the same at the measured distance, generating an error in that magnitude, as there are waves of different frequencies, there will be different paths between the satellite and receiver, where through a linear combination these errors are corrected quickly obtaining more reliable Satellite-Receiver distances.

Therefore, using several wave signals from different carriers would help us speed up and improve the calculation of the position of a receiver, even in environments of poor satellite visibility.

Precision

The greater the number of channels, the more satellites can be tracked simultaneously, increasing data redundancy. In this process, the accuracy does not increase linearly, but this improves to a certain number of satellites, then becoming almost constant.

Taking into account, for example, 10 satellites, in each of the 4 available constellations (GPS, GLONASS, GALILEO AND BEIDOU) and 4 frequencies for each constellation, we have approximately 160 channels, which are enough to be able to acquire the frequencies of all the satellites of the different constellations available.

It should be mentioned that the number of frequency bands does not directly affect centimeter accuracy, therefore, a Single-band GNSS Receiver could obtain similar accuracy as a Multiband one, as long as they are in ideal environmental conditions and do not exceed distances greater than 15 km between both receivers during the comparison.

Multiband receivers capture several frequency bands from satellites, achieving a fixed or Fix solution in seconds and maintain robust performance even if visibility of the sky is partially blocked. In addition, they work with longer baselines since it allows to correct ionospheric errors (Example: up to 60 km in RTK and 100 km in PPK).

The Mettatec X5 GNSS Receiver has the best of both worlds, it is multi-frequency and multiband with enough channels, allowing great accuracy at low cost. Here are some of its features:

Number of channels: 184 channels
Multiband GNSS receiver with centimeter accuracy
Works in PPK, RTK and NTRIP mode
GPS, GLONASS L1/L2, Galileo E1/E5b, BeiDou B1I/B2I, QZSS L1/L2C
Compatible with software or applications such as: SWMaps, Field Genius, Survey Master, SurvPC and any GIS software that supports NMEA via Bluetooth
Works in RTK mode with LoRa Radio up to 10 km