Specialized Applications Tap Power of GNSS
Although some might argue that the world got along just fine before the development of GNSS (Global Navigation Satellite Systems), probably a vast majority would point out the numerous benefits of having easily accessible location and time information. These systems are a reliable and accurate source of data that creates enormous opportunities for efficiency in business and everyday life, and the full extent of their influence is yet to be seen.
Impact on Geospatial Industry
To say that GNSS has had a major impact on the growth enjoyed by the geospatial industry would be an understatement. There are literally billions of electronic devices used today that rely on the signals, and with better location information, many new applications are feasible such as self-driving cars and drone deliveries. Traditional surveying methods are also being augmented with GNSS, which results in time savings, and higher levels of accuracy. Geospatial companies such as Quantum Spatial go far beyond basic data acquisition by applying analytics in creative ways to support decision making, so improved data inputs lead to more useful results.
“The availability of GNSS caused a complete paradigm shift in how we do all of our work in a cost-effective way,” says Evon Silvia, solutions architect at Quantum Spatial. “The real advantage of the new satellite systems – GPS III and Galileo – is not just increased accuracy, but more importantly that you can achieve that accuracy faster and more reliably at a lower cost. More satellites in range help us get the signal faster – in minutes instead of many minutes – and on a large surveying project, that really adds up. Ninety nine percent of our groundwork uses GNSS.
“As GNSS becomes more accurate, specialized sectors become more interested,” continues Silvia. “Geospatial data used to be collected at ZIP code or county level, then by the mid-20th century the technology allowed for more detail that identified location by block – and now we can drill down to a corner or a building on a block, and even by person.”
Although several-meter accuracy is fine for some applications, such as a person navigating to a restaurant, there are other applications that require finer detail. Self-driving cars need to know not only what road they are on, but which lane they are in. Engineers must work at centimeter-level accuracy to achieve rigorous quality and safety goals. The next generation GPS III will offer direct signals that result in higher accuracy for everyone.
How Did We Get Here?
The U.S. Global Positioning System (GPS) was originally developed by the military, with the first Navigation System with Timing and Ranging (NAVSTAR) satellite launched in 1978. Today the 30 operational satellites circling the globe transmit radio signals for two levels of service — Standard Positioning Service, available at no charge to everyone, and Precise Positioning Service, which is reserved for the U.S. military, U.S. Federal agencies, and U.S. allies.
Prior to 2000, the U.S. Air Force sporadically applied Selective Availability (SA) to GPS signals to intentionally degrade the accuracy of civil signals. However, to better support safety-critical navigation applications, particularly for aviation, SA was discontinued and is not a feature in the GPS III satellites.
Each GPS satellite is equipped with an atomic clock that keeps exact time; the time and location of the satellite is transmitted in radio signals and captured by GPS receivers, which may be in a cellphone or watch, on an airplane, car, or boat, or on another satellite. GPS measures the transmission time of signals from a satellite to a receiver. When a receiver is in contact with four or more satellites, 3D geographical information (x,y,z) can be determined. Signal errors are mitigated by adding even more satellite observations, and further reduced by using Continuously Operating Reference Stations (CORS) or third-party service providers. Also, the U.S. Coast Guard runs a maritime differential GPS network that improves accuracy to 1‒3 meters. Surveyors requiring sub-meter accuracy utilize a combination of methods to achieve 3‒5 cm accuracy.
Most new equipment used today operates with signals from international constellations of satellites, in addition to GPS. The other systems most often accessed are GLONASS from Russia, BeiDou from China, and Galileo from Europe, expected to be fully operational in 2022. Other countries also use U.S. GPS, although national security concerns have motivated them to develop independent systems. The more signals there are, the more reliable navigation is, so complementary systems benefit everyone and offer a “Plan B” if problems arise with a specific constellation.
GNSS for Disaster and Climate Change Monitoring
An emerging field of study indicates that GNSS could be a useful tool for monitoring natural disasters because the GNSS signals are altered by changes in the atmosphere. At Oregon State University, research is being conducted by Dr. Jihye Park on hurricanes and typhoons. Park applies advanced algorithms to GNSS data to help forecast and track extreme weather.
“My research concentrates on hazard monitoring using GNSS,” explains Dr. Park, civil and construction engineering assistant professor at Oregon State University. “GNSS is capable of precisely measuring the water vapor pressure in the atmosphere. Recently, my research team analyzed the spatial distribution of water vapor pressure before, during, and after hurricanes and a typhoon, and found that it could be useful to predict the path of the weather event.
“Since GNSS stations already exist in many countries ‒ the U.S. alone has more than 3,000 base stations ‒ and the data is freely available, this existing infrastructure could be used to develop valuable warning systems in places vulnerable to destructive storms,” says Park.
Park is also researching GNSS-Reflectometry, which is an excellent tool for monitoring sea levels. By using GNSS data near the coast, researchers measure the long-term water level change that is related to climate change. This approach is also useful for monitoring flooding and inundation.
“Recently a lot of research articles show that the GNSS-R can provide the water level output with very good agreement to tide gauges,” said Park. “Because GNSS returns the positioning result based on the terrestrial reference system, this technique is considered an even more useful tool for inundation monitoring as compared to tide gauges that provide output in their own datum.”
At the University of Michigan, studies are underway to assess the effectiveness of using GPS signals to forecast the maximum wind speed of hurricanes. This type of weather data is being collected by NASA’s Cyclone Global Navigation Satellite System (CYGNSS), a fleet of eight small satellites equipped with GPS receivers launched in 2016. The longer wavelength of GPS signals allows them to penetrate to the center of a storm and reach the ocean’s surface. The CYGNSS satellites receive the reflected signals, which are used to calculate and forecast wind speed. Advanced knowledge of storm strength is beneficial for public safety.
As an early warning system for earthquakes, tsunamis and volcanic activity, the Geospatial Information Authority of Japan operates the GNSS Earth Observation Network (GEONET) that includes over 1,300 base stations. The first stations were installed in 1993, and today the system provides near-real-time monitoring of potential and ongoing natural disasters based on received GNSS signals.
Adapting to Centimeter Accuracy
With the gap between consumer-grade and survey-grade accuracy continuing to shrink, the two will eventually converge. However, it will be important to understand the science and not rely entirely on the equipment. To create repeatable consistent surveys, users will still need to be knowledgeable about geodesy, tectonic motion, effects of the atmosphere, etc. These are not common knowledge to consumers but should be part of the knowledge base of licensed surveyors.
“Continuing education, training, and a higher level of understanding about geodesy is needed to be consistent in the work being done, to understand the fine and nuanced issues that come with the highest accuracy,” says Silvia. “Although we can measure a position very accurately for a particular time, the point itself is moving year over year due to tectonic motion. Consistency between years requires awareness of how everything works in relation to each other, not just better equipment. When the standard for GPS was ten meters, a centimeter shift wasn’t noticeable, but now it is relevant.”