Getting to the Point with GNSS
Purists will point out that in the 44 years since the GPS program was launched, GPS has nearly achieved the status of a generic trademark when it comes to global navigation satellite systems (GNSS). In fact, GPS is the U.S.-government-owned utility maintained by the U.S. Air Force. GNSS is the more generic term for satellite-based positioning, navigation and timing systems. The number of GNSS systems is expanding even as the technology continues to improve.
Early navigation systems such as LORAN took advantage of developments in radio frequency (RF) technologies like spread spectrum. They were terrestrial systems, and accuracies started at miles and evolved to hundreds of feet — useful for ships at sea, but nowhere near survey level. That would have to wait for satellite-based navigation and positioning systems. Even then, progress took decades and the government policy of Selective Availability restricted the highest accuracies for military and government use. That practice was ended by President Bill Clinton. By then, the government had developed techniques for regional denial of service to deal with security concerns, and high-accuracy global positioning was made available for commercial and civilian use.
Russia’s GLONASS has achieved comparable precision and global coverage, while others are still regional in their coverage. Europe’s Galileo and China’s BeiDou GNSS are expected to be fully operational by 2020.
Reaching for Precision
The basis for geolocation is time and distance measurements based on signals from satellites to terrestrial receivers. Users only need line-of-sight access to enough satellites to give an accurate position. But, it is the level of accuracy that is key.
One of the first challenges is the signal itself. GNSS satellites are typically deployed in medium-earth orbit (12,500 miles). Satellites in the GPS constellation are arranged into six equally spaced orbital planes surrounding the Earth, according to the National Coordination Office for Space-Based Positioning, Navigation and Timing (PNT). Each plane contains four “slots” occupied by baseline satellites, PNT says. This 24-slot arrangement ensures users can view at least four satellites from virtually any point on the planet. Other GNSS systems use similar strategically placed satellite constellations to achieve global or regional coverage.
The satellite constellations are regularly undergoing upgrades and expansion. The U.S. system reportedly has 12 operational legacy satellites and 19 modernized satellites. It is also embarking on a GPS III program, which will not see its first satellite launched until 2018.
The legacy civil GPS signal known as L1 has been joined by L2C, L5 and L1C, meaning there are four signals available for civilian use. Using a dual-frequency receiver allows greater precision — at or above military accuracy levels. The addition of the L2C signal in commercially available receivers enables ionospheric correction. The GPS satellites also transmit on the L2C frequency at a higher power level, improving signal acquisition and reliability in situations such as under trees.
Ionosphere and Augmentation
The atmosphere surrounding the Earth can have varying effects on satellite signals used by GNSS. Specifically, the ionosphere and troposphere can bend or distort the signal. Various augmentation schemes help to correct or mitigate these effects.
Hemisphere GNSS has published a series of white papers on various related topics. Its “Athena GNSS Engine” white paper describes the effect and how it is mitigated through use of an RTK (Realtime Kinematic) engine. The basic argument is that nearby receivers will experience the same error sources. This allows the errors to be cancelled out by differencing observations. RTK employs a reference station with a known location to transmit correction information to the user’s receiver. GNSS receivers can be set up in a similar base and rover arrangement if the user can establish a known location for the base that allows the “differencing observations” used to cancel positioning errors.
Among the government-sponsored or managed augmentation systems are Continuously Operating Reference Stations (CORS), a network of almost 2,000 GPS tracking stations managed by the National Oceanic and Atmospheric Administration (NOAA), tied to the National Spatial Reference System. Where CORS is terrestrial, the Wide Area Augmentation System (WAAS) is a space-based augmentation system (SBAS) operated by the Federal Aviation Administration (FAA). In addition, there are a number of commercial systems.
The most common methods for transmitting the reference station signal are via radio or over the Internet. Modern GNSS receivers have the capability to receive the radio signal and/or a cell phone modem enabling connectivity to the Internet. Dual-frequency GNSS receivers benefit from receiving a second satellite signal on the L2C frequency.
Hemisphere states in its white paper, “As the error sources for both the rover and reference receiver are nearly identical, we are left with very accurate measurements when we difference the signals, which makes the RTK method so robust and efficient.”
Centimeter-level global positioning is only possible with multi-frequency receivers, according to Hemisphere. By observing signals from the same satellite on different frequencies, the effects can be cancelled. The atmosphere is not the only effect which must be mitigated. Satellite orbit error and clock error also add to the complexity of reaching very precise levels of accuracy that make GNSS useful and valuable for land surveying applications.
The troposphere can also contribute to delays, which, along with other factors, affect “convergence time.” This is an issue not only for accuracy, but also workflow efficiency. Signal acquisition time and processing augmentation signals for position resolution can require anywhere from a few minutes to tens of minutes at each set up. Some systems incorporate algorithms to improve convergence time.
In a 2016 article in POB, (Optimizing GNSS Workflow Flexibility, November 2016), Jacek Pietruczanis, director of marketing, GNSS and Imaging portfolio at Trimble, offered a list of requirements to consider relative to GNSS receivers. They should:
- Provide the end user the option to select not only the accuracy and GNSS performance level appropriate for their specific application, and they should also be capable of delivering sub-meter- to centimeter-level positioning accuracy in real time.
- Help achieve a higher level of accuracy in real time using multiple correction sources such as satellite-based augmentation systems (SBAS), virtual reference stations (VRS), and real time kinematic (RTK) and real time extended (RTX) correction services.
- Support multiple satellite constellations and augmentation systems to provide maximum accuracy and positioning performance.
- Be able to be paired with mobile/smart devices on a variety of operating systems and platforms.
- Easily integrate with mapping and GIS analysis software to enable data to be easily collected, communicated and processed, providing the users with high-quality deliverables for the entire organization.
- Be built to withstand harsh environmental conditions that meet military specifications for ruggedness.
Pietruczanis also offered a list of questions to consider, culminating in “what is the desired end result?” And here, the discussion can quickly turn to post processing. Christopher Gibson, vice president of the survey, imaging, GIS, infrastructure, rail, land administration and environmental solutions businesses of Trimble Navigation Ltd., observed in a 2013 article, “Because [knowledge of GNSS post processing] provides a foundation about the 3D world and the sources of error, post processing helps surveyors become better equipped to use both RTK and post processing.” (GNSS Post Processing: A Primary Option, March 2013)
The ubiquity of GNSS has been a challenge and a benefit for land surveyors and geospatial professionals. For a “rough cut” position, nearly any device will do. The popularity of geolocation and its incorporation into the daily lifestyle of more people and into more types of applications pretty much ensures continued growth and refinement. Planned upgrades to satellite systems and greater access to those capabilities as commercial receivers move from dual-frequency to multi-frequency and multiple GNSS systems will keep improvements coming.
Designers are developing technologies that will allow more of today’s GNSS receivers to follow those upgrades. The term being applied is “future proof.” Though the term is a little ambiguous, the goal is to minimize the risk of a current piece of equipment being made obsolete by future developments. In this respect, the rise of bring-your-own-device strategies that incorporate smartphones and tablets as user interfaces for the GNSS has been beneficial. Adding cloud-based services for data management and some immediate post processing steps enhances the capabilities of GNSS receivers in the field. And, the introduction of computer-defined GNSS further improves the ability to update, upgrade and keep current with technology developments in GNSS without a lot of expense to keep replacing hardware.
Correction services are also bound to improve as the government-based programs evolve and satellite systems are upgraded. Subscription services will also improve, especially as providers incorporate access to more services and respond to the rise of open-architecture services.