Written by Raiana Kelly
Satellite navigation relies on a global network of satellites that transmit radio signals from medium earth orbit, an altitude of approximately 2,000 to 36,000 kilometers (1,243 to 22,300 miles). Satellites used for navigation are equipped with atomic clocks that provide extremely accurate time and these atomic clocks are synchronized to a universal time standard. The signals they emit are used by receivers on earth to calculate their exact distance by measuring the time delay between satellite signals.
A monitoring station, or ground control station, provides a map of satellite positions at any given time - this is referred to as an almanac. It’s the almanac that provides the receiver with the satellite location information used to trilaterate its position.
Most users of satellite navigation are familiar with GPS, a collection of 31 satellites developed and operated by the United States. Each of the 31 GPS satellites in orbit is equipped with an atomic clock that provides extremely accurate time. Receivers on earth determine the location of satellites and the time of their broadcast through a combination of signals from at least four satellites.
By computing the difference between the time of signal reception and the time of signal broadcast, a receiver computes its own three-dimensional position by determining the distance from itself to the satellites. Once the receiver has information about the ranges from three signals, the data from a fourth satellite enables the receiver to determine the latitude, longitude, altitude, and time without the need for an atomic clock within the receiver itself. Depending on the quality of the receiver, basic GPS service can achieve as good as 7.0-meter accuracy, 95% of the time, anywhere on earth.
While GPS and GNSS are related, GPS is only one component of GNSS and GNSS is not limited to the satellites within the GPS constellation operated by the United States. GNSS refers to a collection of multiple constellations: GLONASS, operated by the Russian Federation; Galileo, operated by the European Union; BeiDou, operated by China; QZSS, a regional system operated by Japan; and NavIC, a regional system operated by India. All of these providers have offered free use of their respective systems to the international community.
While GNSS is used in collaboration with GPS systems and all GNSS receivers are compatible with GPS, not all GPS receivers are compatible with GNSS. Because GNSS receivers are capable of communicating with satellites beyond the GPS constellation, GNSS receivers offer increased accuracy and reliability.
What GPS and GNSS have in common is that they each consist of three segments: the Space Segment (satellites), the Control Segment (ground control stations), and the User Segment (GNSS or GPS receivers). As seen in the figure below, satellites are broadcasting radio signals toward earth at all times. These signals are picked up by receivers and monitored by ground control stations that continuously track satellites and enable information from earth to be transmitted back to the satellites.
The addition of GNSS constellations to GPS improved reliability and accuracy, but it was still not accurate enough for high-precision mapping and navigation applications. To solve this shortcoming, the FAA (Federal Aviation Administration) introduced the WAAS (Wide Area Augmentation System) in 2003.
WAAS works by providing base station monitoring systems across the United States and then uploading code-based differential correction information back to geosynchronous satellites, which then re-broadcast corrections along the same L-band frequency that GPS utilizes. This idea allowed GPS accuracies to approach 1-meter accuracy across most of the United States and parts of Canada and Mexico.
SBAS, like GNSS to GPS, simply expanded WAAS to include systems outside of the United States, such as Europe’s EGNOS, Japan’s MSAS, and India’s GAGAN to name a few. SBAS can now achieve an accuracy of 1-meter across the United States and the majority of the world with differential corrections to multiple constellations.
While SBAS corrected GNSS communicates with more satellites than a single satellite constellation and allows for greater accuracy, it’s still prone to error from ephemeris inaccuracies, satellite clock inconsistencies, ionospheric delay, and tropospheric disturbance.
RTK is a GNSS technique that enhances the accuracy of GPS positioning by using a fixed base station on the ground as a known reference point to calculate and transmit real-time carrier signal-based corrections to a moving receiver, or rover. The receiver then uses the data from both the satellites and the base station to compare and cancel any errors in measurement, providing a much more accurate position.
RTK technology is used in various fields that require high-precision positioning, including surveying and mapping, construction, precision agriculture, autonomous vehicles and robotics, and drone surveying.
Much like the relationship between GPS and GNSS, dual-frequency RTK systems are more reliable and accurate than single-frequency RTK systems. This is especially true for challenging environments where there is greater signal interference, such as urban canyons and mountainous regions, because single-frequency RTK systems are more susceptible to calculation errors as a result of ionospheric delays and multipath effects.
The difference between the accuracy achieved by single and dual-frequency RTK is a result of how the two communicate with satellites. Single-frequency RTK utilizes one frequency band when communicating with satellites. When combined with a base station, this single band provides centimeter level accuracy.
Dual-frequency, as its name suggests, communicates using two frequency bands. This allows more than one measurement per satellite, and thus more data is transmitted to the receiver to correct computational errors. The end result is increased availability and applications in a diverse range of conditions.
Multi-frequency RTK operates much like dual-frequency RTK except that it utilizes three or more frequency bands to offer exceptional accuracy, faster initialization, improved signal availability, anti-spoofing, and enhanced reliability. For the best performance and greatest possible accuracy, choose a GPS or GNSS receiver capable of multi-frequency RTK and a reliable RTK service provider.
Consider your accuracy needs, budget, and working conditions when choosing between single, dual, and multi-frequency RTK options. If you’re working in challenging environments or require sub-centimeter accuracy, then dual-frequency or multi-frequency RTK is recommended for their superior accuracy and reliability.
Like RTK, PPP corrects satellite positioning data to offer enhanced accuracy. Unlike RTK, PPP uses a limited number of base stations for its corrections, so it’s not instantaneous. With PPP, the error corrections are happening on the receiver or rover device, resulting in longer convergence times. The accuracy it can achieve is impressive, but many professionals using GNSS receivers do not have time to wait an hour for convergence to occur.
For more information on PPP vs. RTK and real-life data comparing the two, check out our blog post Precise Point Positioning Service: A Blessing and a Curse.
Across industries, geospatial technology like GPS, GNSS, RTK, and PPP is changing the way we live, from precision agriculture, utility and natural resource management, delivery logistics, construction, autonomous vehicles, and more.
If you’re interested in learning more about how GNSS and RTK can meet your needs, Tri-Global Technologies has experts with decades of experience ready to help! From GNSS receivers, accessories, RTK correction services, and professional GIS consultation, Tri-Global can put precision in your hands and help you collect and manage your data with confidence.
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