GNSS Simplified for Beginners
This article is intended to give a basic understanding of how GNSS works for those intending to operate GNSS receivers with no prior knowledge. We will start by defining common acronyms and terminology, then explain in laymen’s terms how GNSS works, the expected accuracy of GNSS receivers and how to get the most out of your equipment. Followed by a real world example, using the Emlid Reach RS2+ in conjunction with Emlid Flow, that will solidify your understanding. Using a base and rover pair will ensure high relative accuracy, while using an NGS monument ensures high absolute accuracy.
Common Terms
GNSS, or Global Navigation Satellite System: A general term for satellite constellation that provides positioning, navigation and timing. Global examples are GPS (Navstar) (US), Galileo (Europe), GLONASS (RU), and BeiDou (CN).
Multilateration: Using position and timing data from 4 or more satellites to calculate longitude (x), latitude (y), height (z), and timing.
RMSE, or Root Mean Squared Error: Quantitative measure of accuracy.
RTK: Real Time Kinematics, generally the preferred method.
PPK: Post Processing Kinematics, used in special cases, for example processing LiDAR data
How do GNSS receivers work?
The GNSS receiver will use multilateration to determine its position. Using this method with a single receiver solution will give it a point with a rough accuracy of 2-3m which can be useful for something like your cell phone’s navigation. The accuracy is improved with a GNSS receiver by bringing in a 2nd receiver. Its solution can then be used to correct the position of the first receiver. We can establish a Base by keeping one receiver stationary to maintain a single coordinate that will be transmitted to the other receiver, the Rover. Far greater accuracy can be accomplished with this method. If there are enough satellites for the calculated point, the Rover can then establish a Fixed Solution, which will have an accuracy of 1-3cm in relation to the base.
If conditions do not allow for the calculations to be solved for a Fix, you can use NTRIP for a Float solution. The accuracy of a Float solution can vary up to 1m.
Accuracy: Absolute vs Relative
In GNSS terms, absolute accuracy refers to being accurate to a point on the globe while relative accuracy generally refers to a data set being accurate in relation to its different points. Therefore, we can use a GNSS base and rover pair with poor absolute accuracy yet high relative accuracy. We could achieve both high absolute and relative accuracy if we placed the base over an established monument with a known coordinate and input that coordinate for our base. If that is not an option, we can use observations recorded from our base to post-process our base coordinate using a service like OPUS. Correcting our base position for a data set with high relative accuracy would then shift our data to also give it high absolute accuracy. A third option would be to receive corrections from a permanent base station when establishing your base point.
Example
We start by finding the coordinate of our nearest monument and saving those coordinates as a New Point. We will then stake out with our GNSS receiver, an Emlid Reach RS2+, in conjunction with Emlid Flow and Flow 360. The corrections from our rover will be received from Ohio’s state maintained Continuously Operating Reference Station (CORS) network which provides high absolute accuracy. We located the survey disk that was installed in the monument location 30 years ago within .91cm horizontal and 1.52cm vertical.
The Emlid Reach RS2+ can provide a fixed solutions in seconds and maintain accuracy in challenging environments. Use the Emlid Flow app to handle all your fieldwork including receiver settings, data collection, coding, linework, and stakeout.
References
https://web.pdx.edu/~jduh/courses/Archive/geog481w07/Students/Franczyk_RMSE_Accuracy.pdf