Re: Roughtly converting DOP to metric error

speleoluc@xxxxxxxxx wrote:
Hi,

My cave survey freeware connects to a GPS for surface tracking. When I
connect to a Bluetooth GPS, the GGA and GSA NMEA sentences provide
various flavors of DOP values (PDOP, HDOP, VDOP). How can I provide
users with a rough error value in meters (as most GPS units do) from
these unitless numbers?


Luc Le Blanc
http://www.speleo.qc.ca/Auriga

See: http://edu-observatory.org/gps/gps_accuracy.html

Estimated Position Error (EPE) and Error Sources
EPE (1-sigma) = HDOP * UERE (1-sigma) (1)

Multiplying the HDOP * UERE * 2 gives EPE (2drms) and is commonly taken as the 95% limit for the magnitude of the horizontal error. The probability of horizontal error is within an ellipse of radius 2drms ranges between 0.95 and 0.98 depending on the ratio of the ellipse semi-axes. User Equivalent Range Error (UERE) is computed in the tables lower on this page.

EPE (2drms) = 2 * HDOP * SQRT [URE^2 + UEE^2] (2)

HDOP (Horizontal Geometric Dilution of Precision), GDOP, PDOP and VDOP are determined by the geometry of the current satellites visible above the receiver's mask angle with respect to user receiver's antenna. DOPs can be degraded (made larger) by signal obstruction due to terrain, foliage, building, vehicle structure, etc.

URE (User Range Error) is an estimate of "Signals in Space" errors, i.e., ephemeris data, satellite clocks, ionospheric delay and tropospheric delay. These errors can be greatly reduced by differential and multiple frequency techniques. Differential correction sources include user provided reference stations, community base stations, governmental beacon transmissions, FM sub-carrier transmissions and geosynchronous satellite transmissions.

UEE (User Equipment Errors) includes receiver noise, multipath, antenna orientation, EMI/RFI. Receiver and antenna design can greatly reduce UEE error sources--usually at substantial cost.

Position error can range from tens of meters (recreational) to a few millimeters (survey) depending on equipment, signals and usage. Professional mapping and survey equipment often includes user-settable minimum thresholds for SNR, mask angle, DOP, number of SVs used, etc.

.

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