Feature: GNSS Matures

Ashtech’s Olivier Casabianca talks GNSS, how technology is shaping the future of surveying, and what to know before buying.
By Kevin M. Smith

Historically those who have possessed the best technology have been at the forefront of any endeavor. The ancient Romans ruled half the known world because they had the best equipment and most advanced fighting techniques. Johannes Guttenberg revolutionized the publishing industry when he invented a printing press with movable type, and in this last century computers changed everything.

With the advent of satellite communications and ever-more affordable computer hardware and software that expand the capabilities of surveyors, GNSS innovations have been advancing at a rapid pace.

Ashtech’s GNSS product marketing manager Olivier Casabianca was quick to point out that surveyors who have used satellite-based technology have come to the forefront of our profession. “We never saw a surveyor who tasted GNSS go back to traditional techniques,” he said.

Casabianca said that technology advances in GNSS should not create new or unforeseen problems for new users. “There is no particular issue [or difficulty] faced by the new users,” he continued. “Product interfaces [and] software ergonomic[s] have been improved globally for the past 15 years. You don’t need to be an expert to use GNSS…. The key question [a surveyor needs] to answer before buying a GNSS receiver is what kind of survey do you perform, and then choose the right equipment.”

Purchasing Factors

For those who haven’t made the leap into GNSS there are four key questions that need to be answered when considering the purchase or rental of GNSS equipment. Casabianca identified these as:

• Are you working in difficult environments, like cities with tall buildings? Then make sure to have a GLONASS option and a high-quality multi-path mitigation capability.
• Are you working on large survey sites or covering a broad territory? Then invest in a long-range RTK capable receiver (>30km).
• Are you working with access to RTK network corrections? Then buy only a rover with built-in communication modules.
• Do you want your system to operate with your existing equipment, including total stations? Then review the data formats supported by the receiver, including the controller and the software.

As with most advances in technology, GNSS has the potential to improve the productivity of the surveyors using it. With increased production over the use of optical systems comes increased profitability. The benefits of GNSS apply to all kinds of surveying jobs ranging from construction to cadastral and even to seismic.

Antenna Calibrations With more GNSS products available, potential buyers have questioned how equipment will handle the move from relative to absolute calibration, seen by some as a legitimate concern when it comes to processing data, particularly regarding NGS legacy projects. Casabianca says that’s a concern of the past; legacy projects will not be affected as projects move “from relative to absolute calibration. [It] will not introduce any disturbance to Ashtech customers’ processing legacy GPS projects. Since IGS introduced absolute antenna calibrations a long time ago, [our post-processing software] equally well supports both relative and absolute antenna calibrations.” He went on to explain that “there is not much difference between relative and absolute calibration; i.e. the description of relative and absolute calibration is the same, the format is the same, the meaning of the fields is the same. First one takes the Dorne & Margolin element as the reference, while the second try estimates the absolute position of phase center (which in this case would be not a physical point, but would rather look like a ‘cloud’). Moreover, [there are products that] are capable of emulating any kind of virtual antenna to ‘help’ third party rovers to smoothly work with a … base station. Our receivers are capable of broadcasting corrections mapped to an antenna, which will be known by the rovers—even if this is another kind of antenna that is actually connected to the receiver.” Currently, the only issue regarding absolute calibration versus relative relates to real-time precise point positioning (PPP) for precise surveying when “using precise orbits [with] no base station. When this technique matures enough [for us] to expect centimeter-level accuracy in real-time, the use of absolute antenna calibration will be mandatory, but until this happens both relative and absolute calibrations can be used.” Casabianca goes on to explain, “For normal differential RTK processing, relative or absolute calibration makes no difference. Single difference measurements between base and rover will cancel error due to phase center variations, and in our current framework only relative antenna calibration is supported. The receiver has a hard-coded database with available NGS antennae, plus parameters for antennae. The RTK engine processes data from its own antenna and reference receiver’s antenna and applies corresponding information from its database. To provide high flexibility, the receiver also allows the operator to add user-defined antennae to the list and use them in RTK processing.”

Even with constant advances in technology leading to lower costs, the cost of GNSS equipment can still be prohibitive for the individual surveyor, admitted Casabianca, but no more so than buying the equivalent optical equipment. “Depending on the level of economic development of the market,” he said, “the cost of the GNSS equipment can still be a restraint. However it’s standard now to find premium brands offering GNSS systems at the same cost as a total station. The price democratization is a reality with premium brands as well as with new players that are offering entry-level GPS/GNSS solutions.”

The concerns regarding the cost of adding new equipment and of even changing one’s way of working creates a natural resistance in the market. “But it’s a … barrier that productivity and increased profitability potentials usually clear rapidly when people start to look into a GNSS solution,” said Casabianca.

GLONASS Consequences

For those users currently operating GNSS equipment, several recent developments may have an impact. These include the possible consequences of the splashdown last December of three Russian GLONASS satellites.

At the time of the splashdown, Russian officials announced that there would not be a significant impact to the effectiveness of the current GLONASS and that the system would still provide users with more than enough coverage to get accurate information. Casabianca said that it is Ashtech’s understanding that GLONASS, in spite of being three satellites shy of what was planned, will continue to strengthen the GPS constellation for professionals in places that are problematic, such as in urban canyons and under foliage.

Casabianca noted that Ashtech was early to the game, introducing GPS with GLONASS technology back in 1996 when the Russian constellation was almost complete and operational under its original design parameters. “Then GLONASS started declining, and in early 2000 people were thinking GLONASS to be dead,” he explained. “Then five years ago GLONASS started rising again, and today we are again close to a fully operational constellation, and without this splashdown, GLONASS would now be declared as fully operational. Such incidents are regrettable, but the reality is that the next GLONASS satellite launch is already scheduled in March 2011, and there is no doubt that GLONASS will be declared complete and fully operational very soon.”

Bandwidth Changes

Closer to home, the GPS community in the United States is dealing with a telecommunications firm that has petitioned the FCC for permission to broadcast co-primary terrestrial wireless services in the L-band frequency that has been typically reserved for space systems, radio navigation, and satellite services. In short, they want to use the same frequency used by GPS surveying equipment. Casabianca explained, “this company wants to broadcast in those frequencies from powerful terrestrial transmitters, reallocating from space to a terrestrial broadband service that could effectively overlay the GPS signal for millions of users in large metropolitan areas where this company seeks to operate its service across the United States. It is extremely hard to predict what could be the impact. This is indeed a topic [that surveyors and the businesses that make GPS surveying equipment] need to pay attention to.”

At press time, this petition had received a waver by the FCC to proceed, which has the potential to create problems caused by less bandwidth dedicated to surveyors. This might be a short-lived problem. Within the next five years it is expected that the L-5 frequency will become part of the portfolio of frequencies available for the commercial use of surveyors, opening the option of dual- and triple-frequency systems. Unfortunately, the first surveying tools to take advantage of the additional frequency could be expensive. But the reason for that expense is the same reason why the L5 frequency could remain the province of surveyors.

Casabianca said that the research and development of a new dual-frequency L1/L5 receiver is going to represent a hefty investment, which consumer goods-manufacturers are likely to avoid due to a probable lack of return on investment. He also pointed out that size, power consumption, and the cost of L1/L5 devices will likely be prohibitive to companies in other industries.

“My point of view is that an L5-only product will be of interest only to specific professional markets, like avionics for instance,” said Casabianca. “But the main change for the GNSS market will be the GNSS products offering extending to triple-frequency systems. For professionals, the triple-frequency-user benefits will be tangible, such as faster acquisition, enhanced quality of the solutions, and better coverage in difficult environment. This will expand the product portfolio for every market not only in terms of features but also price-to-performance choices as single- and dual-frequency systems will remain available. The combination of frequencies will also convey a global increase of performance that will open new markets for RTK applications.” For more information visit www.ashtech.com.

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Kevin M. Smith is editor of this magazine.

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