Knowing your position is only part of navigation. (Photo: Oliver Montenbruck)
By Oliver Montenbruck and Peter Steigenberger
A year ago, the U.S. Global Positioning System celebrated its silver jubilee upon completing 25 years in operation. Also, it was more than 20 years ago that President Clinton agreed to switch off Selective Availability, thus offering seamless positioning to the civil community. The 10-bit GPS week count experienced its second rollover, and people worldwide got addicted to a ubiquitous positioning capability in those decades. Be it for finding the nearest restaurant or to track a Sunday afternoon bike ride, positioning-related services building on GPS have become an integral part of our daily life. In fact, GPS has almost become a synonym for navigation itself.
One cannot underestimate the contribution that GPS has made to society. It is for sure most deserved that the fathers of GPS were ultimately awarded the highly prestigious Queen Elizabeth Prize for Engineering in the year of the above jubilee. As always, success creates followers, and GPS is no longer the sole player. Next to the Russian GLONASS, two new actors — namely the European Galileo and the Chinese BeiDou-3 GNSS — have mounted the stage. So, users are now offered a choice of four independent GNSS.
However, do we really need so many systems? Isn’t one enough and all others just a waste of taxpayers’ money? The answer to the last question is certainly a clear “no.” Our society already depends on, to a large extent, the availability of positioning, navigation and timing (PNT) services in much the same way we depend on electricity and telecommunication. While mass-market applications such as the ones mentioned above may appear dispensable, there are “hidden” but much more critical applications of GPS, such as synchronizing power lines, stock trading or the base stations of cellular networks.
Clearly, there is a well-justified rationale for nations or groups of nations to build their independent, space-based navigation systems. Well beyond possible military considerations, this is a basic strategic interest for protection of the local economy and of critical infrastructure. Along with these interests, various regulatory conditions may apply that only endorse the use of selected systems for specific applications, such as emergency call systems. Overall, however, all GNSS in place today can be received and utilized by all interested users around the globe.
So, let’s have a closer look at the practical relevance and implications of having four GNSS in parallel for both mass-market and high-end users. The most obvious consequence is certainly an almost four-fold increase in the number of satellites. As of today, the four GNSS comprise more than 100 satellites, out of which 30 to 40 are simultaneously visible and available for positioning at common sites with open-sky conditions. As a rule of thumb, this provides a factor-of-two reduction of statistical errors compared to using only GPS.
Most importantly, however, the prospects for tracking enough satellites for positioning in obstructed sites is greatly improved. The larger number of visible satellites is particularly appealing for GNSS radio scientists who aim to derive temperature and humidity profiles from subtle variations in GNSS signals passing through diverse atmospheric regions. Multiple GNSS allow for better resolution and ultimately benefit weather forecasts.
In terms of positioning, the simple statistical benefits of tracking a large number of satellites are probably outweighed by technological advances in GNSS satellites and ground systems, as well as substantial progress in receiver technology. For GPS, the signal-in-space range error (SISRE) that describes the contribution of broadcast orbit and clock errors to the position accuracy has decreased by more than a factor of three (Figure 1).
FIGURE 1. Evolution of the GPS signal-in-space range error over time. (Image: O. Montenbruck and P. Steigenberger)
For GPS, but also Galileo and BeiDou-3, the use of highly stable atomic frequency standards has contributed to a notable reduction of the error budget of broadcast ephemerides. The same applies for fast upload capabilities, as in Galileo, or the use of intersatellite links in BeiDou-3. With SISRE values of 0.1–0.2 m and 0.3–0.4 m, these constellations enable even more accurate positioning today than GPS and GLONASS (Figure 2).
Figure 2. Signal-in-space ranging errors of the four GNSS. (Image: O. Montenbruck and P. Steigenberger)
However, improvements from new signals and multiple constellations are not only limited to single-point positioning, but likewise apply for precise point positioning (PPP) users. Stable clocks onboard the satellites reduce the update rate and bandwidth for real-time correction users. Digital signal generation units in modernized satellites ensure clean chip shapes in the transmitted ranging signals and reduce the scatter of satellite/receiver biases. Last but not least, the increased number of tracked satellites contributes notably to reducing the convergence time required for successful ambiguity fixing.
Concurrent progress in receiver technology was certainly a prerequisite for being able to track the multitude of new signals that became available with the new and modernized constellations. Compared to early GPS receivers with a few tens of channels, modern geodetic receivers may (or even must) support in the order of 1,000 channels. For mass-market users, the recent introduction of dual-frequency chipsets for mobile phones and car navigation systems marks the most important step forward. These chipsets support joint tracking of signals from GPS, Galileo and BeiDou-3 at the common L1/E1/B1 and L5/E5a/B2a center frequencies. The signals’ chipping rates, modulations and signal power are designed to offer reduced measurement noise, better multipath protection, and improved weak-signal tracking. At the same time, the use of two signal frequencies allows for rigorous elimination of ionospheric path delays, thus removing the biggest contributor to the error budget of low-cost positioning devices.
All in all, the availability of four GNSS means better performance, robustness, diversity and flexibility for navigation users. We should not forget, however, that all GNSS use basically the same core technology and share the same vulnerabilities. We must still give due attention to the challenge of toughening, augmenting and complementing GNSS to meet society’s needs for robust and assured PNT.
Oliver Montenbruck is the head of the GNSS Technology and Navigation Group and Peter Steigenberger is a senior scientist at the German Space Operations Center, German Aerospace Center (DLR).