WAAS, EGNOS, RTCM, RTK and a sea of acronyms!

Today, I received another question about the difference between differential GPS, WAAS, EGNOS, RTCM, RTK, and other “high end” tools that augment the standard GPS signal. In this blog, I will try to simplify the systems so that they are understandable.

Overview of GPS receiver function
The GPS receiver works because each satellite in the GPS system broadcasts a unique song-code (a bit too simplified; see RTK below). The receiver on Earth has the same codes programmed in. The computer compares the expected pattern to the received pattern and shifts the signal to align. The signal shift can then be converted to a distance, known as the ‘pseudorange.’ By using three or more satellites, the computer can locate where the distances meet on a projected surface of the Earth to get a position. Easy enough. However, there are lots of factors that can influence the reliability of the solution (hence, the ‘psuedo’ range). Some have to do with the position of the receiver. For example, being in a canyon will cause lots of signal bounce, or being under a tree could block the signal from a satellite. Most of the errors, though, are due to the condition of the satellites or atmospheric effects. To erase these extrinsic errors, you rely on a second GPS receiver that is in a fixed, well-located position, known as a “base”. The observed base position becomes offset from its real position because of these factors. Now, if your GPS (called a “rover”) is using the same satellites at the same time, you can apply the base error to correct your position. This involves a bit of calculus, but you can use software to do the calculations for you. Again, you need to know the position of the base, the satellites in use, and the time recorded for the position. The result is the calculation of a differentially-corrected GPS position or “DGPS.”

Differential GPS signal
There are several ways of obtaining a DGPS signal. You can rely on either a fixed base (or set of bases) or you can establish your own base. The next variable is whether the base signal is incorporated directly by the rover (“real time”) or if you use a separate computer to compare the base and rover data after you come in from the field.

In these panels, we see how base stations record the GPS errors, broadcast the information to centralized ground control, who bounces it to the two WAAS satellites. Your receiver can download this data from the WAAS satellites to make real time correction.

WAAS, EGNOS, MSAS, and OmniSTAR
The simplest way is to rely on a system where there is a large network of carefully located ground stations and several satellites. The ground stations calculate their offset and send the signal to a central processing facility which beams the information to one or more satellites who broadcast the signal out. For the acronym fanatics out there, this type of system is known as a satellite-based augmentation system (SBAS). Wide Area Augmentation System (WAAS) is used in North America and a similar system, EGNOS, is used in Europe, and the MSAS system covers Asia. There are also private systems, like OmniSTAR, that are used in parts of the world where the US-based GPS or Russian GLONASS are not available or if you need higher resolution. Most GPS receivers have the capability to read the WAAS satellites and apply the correction without any additional processing, obtaining accuracies in the several meter range. For driving a car or mapping at 1:24,000 scale, this is more than sufficient. Currently, there are 38 ground stations, known as “wide-area reference stations (WRS)” in the US, Canada, and Mexico and two satellites. The western satellite is PRN 135 and the eastern is PRN 138. This status changes more often then you think, so you can check the web to see current status of the system. The benefit is that the signal is free and has wide coverage, the downside is that you need to receive the data from a WAAS satellite and they are not always visible.

In RTCM, established beacons broadcast their position using RTCM language so you can make corrections based on the calculated differences.

RTCM
Another, unrelated system relies on carefully located beacons that broadcast information on their position so you can use the distance to more accurately locate the rover. The standard output was established by the Radio Technical Commission for Maritime Services and is called “RTCM SC-104.” Higher-end GPS receivers are able to receive the RTCM data from beacons and recalculate their position in real-time based on the distance. This usually requires the purchase of a separate beacon antenna to receive the data. A downside to RTCM usage is that the signal degrades the farther you are from the beacon source. However, there are more and more public and private fixed stations (beacons) coming online if you have the software to post-process the data. One of the most widely used systems is maintained by the U.S. Coast Guard, the CORS beacon system. In practice, if WAAS is available, it is more accurate than the beacon system.

Real-time kinematic systems utilize a base and rover who communicate via radios. The measured error at the static base station is used to correct the rover position.

RTK
For very accurate measurements (centimeter to millimeter), it is necessary to establish a close base station and compare offsets from the base station to your rover. The most common way is to use a Real-Time Kinematic (RTK) system. RTK systems rely on the carrier phase signal from the broadcasting satellites. Huh? The “song-code” I mentioned earlier runs at high frequency (known as coarse acquisition or C/A), but the carrier signal is carried on a frequency that is about 1,000 times faster, know as the L1 carrier phase. Why not just use the L1 then? You can’t because it isn’t very unique but more of a simple wave-form. Think of it as a way to fine tune the “song-code.” In an RTK system, you need two receivers. One is the base station and is usually established atop a bench mark. The other receiver acts as a rover. Both can adjust their position using the L1 signal but they need to communicate to get the real-time accuracy. This is accomplished using radios to broadcast and receive the data.

 

Useful Web Resources

http://www.navcen.uscg.gov/ US Coast Guard official site concerning beacons.

http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/ The FAA website about the WAAS system.

http://www.omnistar.com/ The website for the OmniSTAR private GPS satellite system.

 

 

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Tags: DGPS, differential GPS, EGNOS, GPS, MSAS, RTCM, RTK, SBAS, WAAS

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