# Accuracy Expectations

For all Cerulean ROV Locator systems, you can expect typical position accuracy of the ROV to be the error-propagation sum of several main uncorrelated errors.

* If you are computing true ROV position using a topside GNSS or GPS receiver without RTK corrections, you will see GNSS position error of typically 3 to 5 meters circular-error-probable (CEP).
* Magnetic compass error, also dependent on the magnetic environment and on the efficacy of the magnetic calibration, typically 3-5 degrees. This error (in meters) is thus approximately **sin(5 degrees) \* slant range (in meters)** probable.
* Angular resolution from the topside unit to the deepside unit, typically 2 degrees. This error (in meters) is thus approximately **sin(2 degrees) \* slant range (in meters)** probable.
* Since the two angular errors are uncorrelated, the combined angular error is about 5.3 degrees probable.
* Another error, slant range measurement \[direct, straight distance between topside unit and deepside unit], is usually insignificant compared to the other errors.
* Not taking into account any other minor error sources, the system CEP is then approximately: **square\_root(5^2 + ( sine( 5.2 degrees ) \* slant range in meters) ^ 2 )**
* It follows that the closer the deepside unit is to the topside unit, the lower the system CEP, with the lower bound set by the GNSS position accuracy.


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