Researchers at the Netherlands' Delft University of Technology (TU Delft) have produced the first double-difference measurements from Galileo satellite signals, tracking both GIOVE-A and GIOVE-B satellite with Septentrio GNSS receivers.

GIOVE-B, the second Galileo In Orbit Validation Element (GIOVE) satellite, joined GIOVE A in orbit April 27; in May GIOVE-B began broadcasting the Galileo L1 signal using a specific optimized waveform, a multiplexed binary offset carrier – the signal that will be interoperable with the L1C signal to be used in future Block III GPS satellites.

TU Delft researchers made the measurements during a 1.5-hour window of simultaneous visibility of both GIOVE-A and GIOVE-B on July 6. This enabled them to collect simultaneous ranging measurements of the L1 open source signals broadcast by the two Galileo satellites, as well as signals from 12 GPS satellites and 2 EGNOS satellites.

"With Galileo pseudorange measurements taken by AsteRx1 receivers, using the binary offset carrier (BOC) modulation on the L1-frequency, the Galileo double difference carrier phase cycle ambiguity could be precisely estimated, and consequently fixed to an integer value, with large confidence," according to Christiaan Tiberius and Hans van der Marel from TU Delft

The 24-channel AsteRx1 receivers provided by Septentrio were off-the-shelf units with the necessary firmware provided by the company. Researchers set up the two receivers with a short baseline, which enabled them to perform pure Galileo double-difference calculations. Leuven, Belgium-based GNSS equipment supplier Septentrio has been involved with the design of Galileo test user receivers since the beginning of the program.

Double-difference measurements are a key to real-time kinematic (RTK) applications, and Septentrio suggested that the achievement at TU Delft will pave the way for multi constellation RTK receivers. The AsteRx1, described by the company as a low power, high-end single-frequency GNSS receiver, can track both GIOVE and Galileo in-orbit validation signals, and has demonstrated the capability to calculate position and timing using both types of Galileo signals, according to Septentrio.

Delft researchers first began tracking GIOVE B on June 10th. The satellite was still undergoing payload configuration at that time, but in a limited time span, ranges could be measured already using the transmitted signal in the L1-frequency band (1.57542 GHz) with experimental firmware from Septentrio running on a AsteRx1 GNSS receiver.

First analyses show that the ranging accuracy is better than with GPS, according to TU Delft. While Galileo is using the same civilian frequency as GPS, it employs a more sophisticated signal modulation, and this allows for an improvement in accuracy, according to the university researchers.