Mission Accomplished: The Payoff in New GNSS Technologies
The geospatial industry was one of the first commercial beneficiaries of satellite-based positioning. Beginning in the 1980s, surveyors, geodesists, seismic exploration teams and others seized on the nascent GPS technology as a means to operate efficiently over large areas. Today, geospatial professionals worldwide continue to operate at the forefront of GNSS technology.
Satellite positioning has become an essential operational component of countless commercial enterprises around the world. It is indispensable in applications such as emergency response, natural resource management and public safety. This is good news, as diverse, widespread use helps to ensure funding for ongoing operations and enhancement of GNSS ground and space segments. The ongoing expansion and modernization of the system is expected to provide benefits for virtually all users and applications.
Among the first civilian uses of GPS, positioning applications—especially the complexity of high-precision solutions—helped drive GPS innovation. Nevertheless, it’s important to realize that while today the geospatial industry is just one segment of the broader GNSS community, it remains a vital advocate for advances in GNSS technology.
The technology behind satellite-based positioning has grown from early adoption of U.S. GPS satellite signals to include multiple signals from dozens of GNSS satellites as well as augmentations from Russia, Asia and Europe. We can be confident that the growth in GNSS will continue. According to recent launch schedules, we can expect to see about 90 GNSS satellites in orbit by the end of 2015, and more than 120 in the five years after that. In addition, the United States is modernizing its GPS constellation to provide new capabilities and improved performance while increasing satellite security and longevity.
But much of this information is already known. The various GNSS government authorities typically announce plans far in advance of actually launching new satellites. For example, the United States released technical data on the new L2C signals broadcast by its GPS Block IIR-M satellites well ahead of the first launch in 2005. This information enabled GPS manufacturers to implement support for the new signal capabilities into their product development programs. As a result, GNSS users were prepared to utilize the new signals as they became available
Putting New Signals to Work
When it comes to GNSS satellites and signals, more is better, a concept effectively demonstrated by the widespread adoption of the Russian GLONASS system. Receivers incorporating both GPS and GLONASS signal capability can produce positions in urban areas, construction sites and other potentially impeded operating environments where the availability of GPS alone might present challenges. It’s becoming very common to see precise positioning receivers that are capable of tracking Russian and U.S. satellites. For example, the U.S. National Geodetic Survey (NGS) now collects GLONASS observations from approximately 800 continuously operating reference stations (CORS) spread throughout the United States.
The modernized U.S. GPS constellation enhances performance by adding a new civil signal (L2C) to the legacy signals L1C/A, L1- P (Y) and L2- P (Y), which civilian users can utilize via cross-correlation techniques. One benefit from the L2C signal is to enable improved corrections for measurement error due to ionospheric delays. L2C also aids in signal acquisition and tracking in challenging environments. The new third frequency, L5, brings further improvements in ionospheric modeling and multipath mitigation. L5 will also help reduce the time needed to produce precise positions, an important aspect of the critical safety-of-life applications for which it was designed.
Satellites from Europe (Galileo) and China (BeiDou) are now joining the U.S. and Russian GNSS constellations. In 2013, Galileo receivers generated the first position fix using the system’s four operational satellites during the in-orbit validation phase. Galileo plans for 30 satellites to be operating by 2020 while China expects to have 35. Like the United States, both Galileo and BeiDou have publicly released technical signal specification documentation to enable global open access to the GNSS satellite signals for the development of GNSS user equipment. The Galileo Open Service L1 signal is compatible with the U.S. GPS L1C signal (to be available from U.S. GPS Block III satellites), providing a key component of interoperability between the two systems.
The bottom line: more GNSS satellites and signals in the sky can provide wider availability and faster, more robust positioning on the ground—across all levels of precision. Today’s receivers are already showing how this is done.
Bringing Advances Down to Earth
Recent developments in GNSS receiver technology have produced a combination of evolutionary and revolutionary progress. On the evolutionary side, we see smaller, lighter receivers that feature major improvements in tracking and signal processing. New receivers are capable of tracking satellites in multiple constellations, which increases the number of satellites available for the Real-Time Kinematic (RTK) solution.
For example, the Trimble® R10 GNSS receiver is capable of tracking all available civilian signals from the existing and planned GNSS satellite constellations. Additionally, the receiver can track satellite-based augmentation signals (SBAS) from systems including the U.S. Wide Area Augmentation System (WAAS), Europe’s European Geostationary Navigation Overlay Service (EGNOS), Japan’s Multi-functional Satellite Augmentation System (MSAS) and India’s GPS-aided Geo-augmented Navigation System (GAGAN). The unit can also receive satellite-based correction data from Trimble RTX as well as data from ground-based differential systems via cellular phones and terrestrial radio links.
These capabilities for receiving GNSS signals and augmentation data enable the Trimble R10 operator to utilize multiple approaches that enable a single receiver to produce real-time positions with up to centimeter precision. The new receivers also implement significant improvements in handling multipath. By reducing errors or biases due to multipath, the systems can help decrease unpredictable behavior in RTK and post-processed solutions.
The second advance in receiver hardware comes from the enormous increase in onboard computing power. Core receiver functions are handled by specialized ASIC (application-specific integrated circuits). These dedicated, enormously complex chips manage the GNSS satellite tracking and signal processing to gather and deliver information to the data processing engines. The engines, which are large computer programs, run on high-performance microprocessors to perform the millions of computations needed to produce precise RTK positions.
The evolutionary improvements in receiver hardware have facilitated a revolutionary change in the software that utilizes the GNSS satellite signals. Because of the fast, powerful central processing units (CPU) in a GNSS receiver, it’s possible to utilize sophisticated new software to compute precise positions in real time. The new approach to real-time positioning is providing faster operations and increased precision reliability in the field and office. To achieve this, we need to introduce a new way of thinking in the field.
A New View on Precision
For nearly two decades, surveyors using RTK have looked to achieve a “Fixed” solution on their RTK rovers. This solution indicated that the RTK processing engine had selected a set of integers to represent the number of full wavelengths to the satellites it was tracking. With that information, the engine could produce centimeter precision in measuring the baseline between the reference and rover receivers. The transition from float to fixed solution when the so-called integer ambiguities were resolved was also accompanied by a sudden improvement in the displayed values for the precision of a measured point. When the fixed solution appeared, the surveyor could begin working at centimeter precision.
The processing engine would then hold the solution, using the fixed integer values to compute positions of new points with as few as two epochs of data on a point. But if an obstruction or other issue interrupted tracking, the fixed integers would be lost. The positioning reverted to the float solution until a new solution could be computed (or re-initialized).
This process has remained fundamentally unchanged since the early 1990s. Now, by utilizing the power of the new processors, Trimble GNSS software engineers are taking a new and better approach. The new technique, HD-GNSS, provides more information and control to the field operator. Rather than waiting for the abrupt transition from a float to fixed solution, the operator receives continuous updates on the precision of the computed real-time position during a rapid convergence phase. Whenever the precision meets the requirements for the project at hand, the operator can record points and information. Built into the Trimble R10, the new HD-GNSS engine incorporates triple-frequency pseudorange and carrier phase data into the RTK computations. The system resolves the integer ambiguity continuously, adding new satellites as they become available while accommodating temporary signal loss from satellites previously used in the solution.
With HD-GNSS, solving the integer ambiguity takes place together with the rapid convergence of the position to a precise solution. During these computations, the system continuously updates the values for positional precisions. By removing the float-fixed dichotomy, HD-GNSS lets the surveyor see the positional precision throughout the convergence process and decide when the precision is acceptable for the task at hand. Rather than waiting for the “Fixed” indicator to appear, which can take considerable time with legacy systems working in difficult environments, the operator can collect points as soon as the precision reaches the desired level. In addition, HD-GNSS reduces the risk of selecting an incorrect set of integers, which could display unrealistically good precision on inaccurate positions. The processing engine is also less susceptible to multipath, which—as one would expect—appears as a degradation of position.
The HD-GNSS technology is available not only for RTK. Trimble Business Center software (TBC) uses HD-GNSS for post-processed static and kinematic solutions. While the engine uses the same integer search techniques as the field version, the office processing facilitates more data filtering and selection as well as use of precise orbital ephemerides. These approaches are especially useful when working with longer baselines and observation times.
The Wait is Over
As GNSS modernization continues, it’s likely that triple-frequency receivers capable of tracking multiple constellations will become the norm for RTK and high-precision users. Following the successful February 2014 launch of the fifth GPS Block IIF satellite, the U.S. GPS constellation contains 12 L2C-capable satellites; this addition keeps the United States on track to fully implement the L5 signals via GPS Block IIF and Block III satellites.
According to U.S. National Geodetic Survey (NGS) Deputy Director Neil Weston, all GNSS users should embrace the new capabilities. “The benefits of having three or even four mature constellations will be quite significant,” Weston said. “More observations, redundancy and compatibility are the most obvious benefits. We will also see different services being offered such as guaranteed integrity of signals, safety of life systems and the supporting infrastructure, faster solutions, improvements to receiver autonomous integrity monitoring (RAIM), improved modeling, and of course benefits to the consumer: more receiver capabilities and options.”
The performance of today’s GNSS receivers and software illustrate that the long-touted benefits are now at hand. As GNSS users manage their equipment, it’s essential to put the new capabilities into purchase planning. By using new satellites and signals (HD-GNSS uses L2C and data from all tracked satellites), users can achieve results faster and with more confidence than ever before.