The emergence of GNSS receivers supporting mult-constellations has kept steady pace with the increasing number of GNSS satellites in the sky in the past decade. With advancements in newer GNSS constellations, two-thirds of all GNSS chipsets and modules currently on the market support multiple constellations and within the next few years 100% of all new devices are expected to so. A new study by the European GNSS Agency (GSA) reveals that the market continues to develop towards fully flexible, multi-constellation GNSS receivers for the mass market and high accuracy professional receivers, with nearly 30% already capable of using the four available global constellations.
More than 60% of receivers also include SBAS, either for higher accuracy or integrity. Regional systems, such as Japan’s QZSS and India’s IRNSS, are also becoming more common. The new GNSS constellations like Galileo and BeiDou have supported open multi-frequency signals from their inception, thus helping drive the introduction of dual and triple-frequency commercial receivers.
Benefits of multi-constellation GNSS receivers
Adoption of multi-constellation GNSS receivers is driven by devices meant for use in closed urban environment and indoors. Although cell/Wi-Fi and related positioning techniques provide some help, GNSS remains the core positioning technology within such environments, the primary reason of multi constellation adoption in mass market receivers.
According to GSA, the benefits of supporting multiple constellations are manifold:
- Increased availability: particularly in areas with shadowing.
- Increased accuracy and integrity: more satellites in view improves accuracy through a better GDOP and integrity through more efficient Receiver Autonomous Integrity Monitoring (RAIM) procedures.
- Improved robustness: as independent systems are harder to spoof.
However, when it comes to adoption of dual frequency, the adoption is only found in the high-precision receivers that demand higher accuracy, finds the study. About a third of all receivers implement more than one of the signals, mostly in high precision.
Although only the L1/E1 signals are currently used in mass market products, the uptake of E5/L5 signals is expected in the coming years as further. The study finds nearly 70% of GNSS receivers are single-frequency (20% supporting two and remaining 10% three frequencies). The most common combination is L1/E1 and L2, followed by L1/ E1, L2 and L5/E5. Not surprisingly, all GNSS receivers use L1/E1 frequency. Furthermore, around 30% have L2 capability, 10% L5/E5, and 1% E6.
The adoption is lower than that of multi-constellation primarily because of cost, complexity and the relative novelty of open signals on multiple frequencies. In the coming years, the ever-increasing demand for a better resilience observed across all applications, and the higher accuracy and integrity needed for automation, will undoubtedly foster a much wider adoption of dual frequency (E1/L1 + E5/L5) solutions, the study forecasts. This is supported by the fact that satellites broadcasting a high quality open signal on L5/E5 are launched at a faster rate and their promising performance in urban canyons is awaking growing interest for double frequency in mass market.
According to the report, the benefits of supporting multiple frequencies are:
- Improved accuracy: Dual-frequency capable devices can estimate and compensate for ionospheric delays.
- Access to RTK and PPP techniques: Although theoretically possible for single-frequency receivers, RTK or real time PPP techniques practically require dual-frequency receivers. Furthermore, the use of triple-frequency receivers will likely enable further improvement of the ambiguity resolution algorithms, e.g. through the TCAR technique.
- Improved robustness: Although rarely advertised, frequency diversity is a basic but very efficient protection against jamming.
The study says new signal designs bring the following significant benefits:
- Improved accuracy: New modulations and higher chip rates enable more precise range measurements.
- Improved multipath mitigation: New modulations and higher chip rates provide additional mitigation for multipath issues.
- Improved sensitivity: Pilot signals (i.e. dataless signals) enable higher sensitivity receivers through longer integration times.
Receivers targeting such safety-critical applications as aviation must wait for new technologies to be proven and new standards or regulations to become available prior to implementing them. With the increasing demand for better resilience across all applications, the need for higher accuracy and integrity that automation demands, adoption of dual frequency solutions (E1/L1 + E5/L5) is expected to grow.
New GNSS constellations and improved old ones
All GNSS constellations are currently working on further developing their systems, either in terms of modernization or initial deployment. While GPS and the Russian GLONASS, both operational, are currently being modernized, Galileo is in the deployment phase and set to be fully operational by 2020. Galileo seeks to develop independent capability in the civilian space as opposed to GPS, BeiDou and GLONASS which are under military control. China is in the process of expanding its regional BeiDou Navigation Satellite System into a global one, which is expected to be in place by 2020. India’s ISRO just finished launching all the seven satellites of the Indian Regional Navigation Satellite System (IRNSS) while the Japanese Quasi-Zenith Satellite System (QZSS) is scheduled to become operational in the coming years.
Based on these GNSS there are several regional augmented navigation systems under the generic name Satellite Based Augmented System, SBAS. As the term augmented suggests, the SBAS is an improvement of the positioning service of the GNSS. All the SBAs including the Wide Area Augmented System (WAAS) in the US, European Geostationary Navigation Overlay Service (EGNOS) in Europe, GPS-Aided GEO Augmented Navigation System (GAGAN) in India, MTSAT Satellite Based Augmentation Navigation System (MSAS) in Japan are all planning upgrades to improve performance. Russia’s System for Differential Corrections and Monitoring (SDCM) and China’s Satellite Navigation Augmentation (SNAS) systems are currently being developed. These systems are primarily meant for aircraft navigation including landing and takeoff stages but are also used for other land and sea based navigation applications. They require additional equipment to augment the basic GNSS receivers.