4th GPS civilian signal goes live

A new GPS civilian signal is now available for use. The new signal is stronger, more accurate, more resilient to interference events, and interoperable with European Galileo system.

Researchers from the Finnish Geospatial Research Institute (FGI) recorded the new civilian signal transmitted by the first GPS III operational satellite.

On Jan. 13 at 21:29 Finnish time, the first GPS III satellite (SVN74) was marked healthy after extensive operational testing in orbit. The satellite broadcasts PRN04 identification codes. It also transmits a new GPS civilian signal, known as L1C, different than the legacy L1 C/A signal used nowadays.

10 times longer

The two signals are transmitted at the same frequency, but L1C codes are 10 times longer than L1 C/A. This makes the signal more robust to interference when multiple satellites are tracked on the same frequency band.

“Marking a satellite health means receivers can use this satellite in their positioning, navigation and timing applications,” said Octavian Andrei, senior research scientist at the Finnish Geospatial Research Institute (FGI). “L1C is the 4th GPS signal for the civilian use.” The Finnish Geospatial Research Institute is a part of the National Land Survey of Finland.

The other three civilian signals of GPS are L2C, L5 and L1 C/A.

For more on the L1C signal, also see First light: Broadcast of L1C by GPS III,

Interoperable with the European GNSS signalL1C signal is transmitted on L1-band at 1575.42 MHz. It is meant to replace the legacy C/A signal in the future. L1C allows for the first time GPS compatibility and full interoperability with signals from other satellite systems, such as E1 signal from the European Galileo.

The interoperability with Galileo is further enhanced by transmission of the inter-system timing biases; that is, the GPS-Galileo Time Offset. All these improvements will bring further benefits and developments to the GNSS market and civilian users in general.

Ionosphere no problem with dual-frequency

Andrei said the new signals means “Exciting times ahead for the civilian users and applications that demand precise satellite positioning and navigation. Most of the effects due to the ionosphere layers of the atmosphere are removed by combining signals from two frequency bands sufficiently apart from each other. This is the case with L1 and L2 or L1 and L5. All these civilian signals are stronger and more robust than ever before,” he explained.

The satellite signals are affected by errors while travelling through the atmosphere. The main errors are due to the ionosphere, which is a dispersive medium and frequency dependent. The latter proves to be actually a significant benefit for the precise applications.

The new signal (L1C, marked with blue) is 3-5 dBHz stronger and more robust than the legacy L1 C/A signal (marked with orange). (Image: Octavian Andrei)

More than 99 % of the ionosphere effect is removed by forming special linear combination of signals observed on two different frequencies. This is the main reason why high-precision is achieved with dual-frequency receivers.

FinnRef network ready for new satellites and signals

“Two GPS III satellites have been launched until now and two more are expected to be launched during 2020. With signals from four satellites, we will also be able to estimate L1C-only positions,” Andrei said.

The first GPS III satellite SVN74, nicknamed Vespucci, was launched on Dec. 23, 2018. The second satellite SVN75, nicknamed Magellan, was launched on Aug.22, 2019. The third and the fourth satellite are planned to be launched in March and July during 2020. The first L1C testing signals were recorded at the FinnRef station in Metsähovi on April 5, 2019.

FinnRef national network includes state-of-the-art multi-constellation tracking stations distributed around Finland. These stations are capable of tracking multiple satellite signals on multiple L-band frequencies from almost 120 GNSS satellites, including the European Union’s Galileo, US GPS, Russian GLONASS, and Chinese BeiDou constellation.

Using new signals often requires updates to station equipment, usually meaning firmware updates on the receiver software. After the new firmware enabling L1C tracking is properly tested, the receivers will be updated and then whole FinnRef will start tracking GPS L1C.