Enhanced-Precision Satellite Navigation: Find Your Location to Within One Centimeter Anywhere in the World

Eric Olson | November 09, 2018

Although global navigation satellite systems (GNSS) like the Global Positioning System (GPS) can produce precise position coordinates for just about any above-ground location on Earth, the technology is subject to a number of errors that reduce its accuracy. [Read: How GPS Works]. These include satellite orbit and clock errors, atmospheric errors as signals pass through the ionosphere and troposphere, and errors resulting from noise in receiver electronics.

Fortunately, engineers have come up with several ways to correct these errors. These enhanced precision techniques greatly improve GNSS integrity and accuracy, in some cases reducing position uncertainties from around 10 m to under 1 cm.

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Differential GNSS

Differential GNSS relies on base stations whose positions have been established to a high degree of accuracy by traditional surveying techniques. The base station receives GNSS satellite signals and calculates its position based on these signals. This position is then compared to the station’s known absolute position and the error between the two is computed. The error is transmitted to the end-user receiver (also known as the “rover”) which it uses to correct its own calculations, resulting in very precise position determination.

As long as the base station and rover are within a few tens of kilometers and both have line-of-sight to four GNSS satellites, the technique is very effective, with accuracies to within 1 m. As the baseline distance between the rover and base station increases, error rises and accuracy falls at a rate of about 1 m per 150 km.

Differential GNSS improves position accuracy using correction data from base stations with known positions that calculate error embedded in GNSS satellite signals. Source: eXtension

One of the earliest large-scale implementations of differential GNSS in the United States was the Maritime Differential GPS (MDGPS) system. The Coast Guard began building MDGPS in the late 1980s to provide minimum maritime navigational accuracy for applications like maneuvering in and approaching harbors. This was necessary to compensate for the degraded accuracy of civilian GPS that resulted from selective availability (SA) — an intentional degradation of GPS signals to reserve the highest accuracy for military users with special receivers and classified cryptographic keys. Although SA was ended by presidential order in 2000, MDGPS continued to be developed, expanding into the Nationwide DGPS (NDGPS) network as a result of differential GPS’s usefulness for correcting other GPS errors like ionospheric delay.

In the end, the U.S. Coast Guard (USCG) decided NDGPS was no longer providing sufficient benefit to justify the cost of maintaining the sprawling network. This was due in part to the fact that minimum maritime navigational accuracy requirements were being met by unaugmented GPS after SA was turned off in addition to the advent and widespread usage of the U.S. Wide Area Augmentation System (a satellite-based augmentation technology that will be discussed later). The U.S. is currently decommissioning NDGPS and plans to phase out the system entirely by September 2020.

Real-Time Kinematic

A surveyor positions a Topcon HiPer V GNSS receiver. Source: Topcon

Another approach to improving GNSS accuracy is the real-time kinematic (RTK) technique. This advanced form of differential GNSS calculates position by measuring the phase of the carrier wave for GNSS signals instead of measuring the pseudo-random noise (PRN) codes that are used to calculate position in standard GNSS implementations.

RTK takes advantage of the fact that carrier phase measurements are much more precise compared to pseudo code measurements, with errors in the range of 1 m for pseudo code measurements and just 5 mm for carrier phase measurements. There are, however, uncertainties in the measurement of the carrier phase known as phase ambiguities. These can be corrected for at the rover receiver through differential measurements incorporating data from a base station whose position is precisely known. Similar to differential GNSS, the rover and base station need to remain within a few tens of kilometers to produce high position accuracy.

RTK can achieve very high accuracies on the order of 1 cm and is often used in surveying applications.

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Satellite-Based Augmentation

Satellite-based augmentation systems (SBAS) expand the effective operational area of differential GNSS to a wider region of coverage. They enhance position accuracy through the use of one or more auxiliary satellites, a set of geographically distributed ground-based reference stations and one or more ground master stations. GNSS signals are received by the reference stations and sent to the master station.

Similar to differential GNSS base stations, the exact positions of SBAS reference stations have been previously established. Thus, the master station can compute to a high degree of accuracy the differential error between the reference station/GNSS satellite pseudoranges and the actual reference station/GNSS satellite ranges. This error is then uplinked by the master station to the SBAS satellites to be rebroadcast. GNSS receivers pick up the SBAS signal along with GNSS signals and incorporate the error corrections into calculations of their own ranges to pinpoint their positions very accurately.

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Australia’s in-development satellite-based augmentation system will enhance position accuracy by calculating error corrections from raw GNSS signals and uploading them to user devices over the internet and via a geostationary satellite. Source: Commonwealth of Australia (Geoscience Australia)

A number of SBAS have been implemented around the world. In the United States, the Wide Area Augmentation System (WAAS) was established to provide highly accurate navigation information for aircraft landing approaches. WAAS specifications require lateral and vertical accuracy to within 7.6 m, although the system often delivers lateral accuracies of 1.0 m and vertical accuracies of 1.5 m.

Similar regional systems include the European Geostationary Navigation Overlay Service (EGNOS), Japan’s MTSAT Satellite Based Augmentation Navigation System (MSAS), India’s GPS-Aided GEO Augmented Navigation System (GAGAN), Russia’s System for Differential Corrections and Monitoring (SDCM) and China’s in-development BeiDou Satellite-based Augmentation System (BDSBAS).

NASA’s Global Differential GPS (GDGPS) provides global timing, navigation and position data with sub-decimeter accuracy. GDGPS also offers the Automatic Precise Positioning Service, which employs an enhanced precision positioning technique called precise point positioning (PPP).

A monitoring station in Rio Grande, Argentina, that is part of the Receiver GNSS Network for IGS and Navigation (REGINA) administered by France’s Space Agency and National Geographic Institute. REGINA is part of a global network of over 400 tracking stations that contribute data for various IGS products such as satellite clock and ephemerides. Among the services IGS offers is its Real-time Service, which provides GNSS corrections that enable precise point positioning. Source: International GNSS Service (IGS)Precise Point Positioning

The structure of PPP systems is similar to SBAS systems, with multiple base stations spread around the world that send satellite clock and orbit corrections to an uplink station that relays the data to a satellite to be rebroadcast to user receivers. The receivers use the corrections in their calculations, yielding position errors of 10 cm or less. Despite a similar structural setup, PPP techniques achieve higher accuracy than SBAS systems because they — like RTK systems — rely on carrier phase measurements instead of PRN code measurements.

PPP systems have an advantage over RTK approaches due to their network of worldwide reference stations, negating the need to setup nearby RTK base stations. PPP systems are effectively a global positioning solution, whereas RTK systems are limited to more localized areas.

The downside of PPP systems is long convergence time, or the length of time it takes to reach accurate position solutions. Elapsed time can exceed half an hour before PPP systems converge to solutions under 10 cm of error.

PPP can be particularly useful for applications that do not require real-time position data, such as land surveying. In these cases, GNSS measurements are captured and saved for later processing. Such post-processing approaches typically produce higher position accuracies than real-time techniques.

It should also be noted that GNSS systems can be combined with inertial navigation system (INS) instruments — including inertial measurement units (IMU) — to provide navigational data at faster sampling rates than GNSS alone or in situations where satellite signals are degraded or nonexistent.

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