Understanding GPS/GNSS Drift

Kurt LawtonEducation, GPS, Guidance, Planting, Satellite, Spraying

GPS/GNSS drift, or how accurate your guidance system is over time, is explained in a new piece on www.AlabamaPrecisionAgOnline.com.

The Alabama Precision Agriculture Team discovered that some users of GPS/GNSS-based technologies were not optimizing the GPS/GNSS correction services for their particular field operations (e.g. using the WAAS correction service for planting). It is imperative to understand the different accuracies associated with GPS/GNSS correction services so one can maximize benefits of their precision ag technologies.

Upon returning to the field, a producer may notice discrepancy between what he/she knows to be the crop row where an AB line was previously established, and where the guidance device is suggesting the AB line is located. WAAS and sub-meter correction services may seem accurate during one field operation but be off-track when the operator returns to the field. This result is because there is typically large ambiguity between pass-to-pass accuracy and year-to-year accuracy or GPS drift.

 GPS/GNSS Drift / Year-To-Year Accuracy (Y2Y) / Long-term Accuracy: Drift can be defined as GPS/GNSS receiver (guidance system) accuracy over time. Causes of drift are changes in satellite configuration, operating near trees or other obstacles, and satellite data errors.

 Pass-to-Pass Accuracy (P2P): Represents the short-term (<15 min.) relative accuracy of a GPS/GNSS receiver but does not necessarily reflect long-term accuracy (which includes drift). One can think of this as the accuracy between adjacent, parallel passes made within 15 minutes of one another.

Since manufacturers typically report pass-to-pass accuracy, it is generally used for equipment purchasing decisions. However, this accuracy may not reveal how the guidance or GPS/GNSS-based system will perform relative to the last operation or over the course of time if previously established AB lines are re-used. This result is especially true when AB lines are established for planting and re-used for harvesting.

As mentioned above, GPS/GNSS drift is largely due to the changing GPS/GNSS satellite constellation patterns used by the guidance device to derive positional information. GPS/GNSS satellites are in continuous motion orbiting the earth twice per day in a repeated pattern. It is assumed that the GPS/GNSS satellite constellation and environmental conditions will not drastically change within a given 15 minute time span, thus derived positions using the same satellite constellation and environmental conditions will be closely correlated relative to each other. However, the GPS/GNSS satellite constellation and atmospheric conditions change over just short time periods resulting in different satellites in varying geometric configurations. Therefore, the magnitude of drift expressed by your device is dependent on the correction service used. Using WAAS, potential range of drift is plus or minus 4.7 feet. With sub-meter accuracy, it’s 2.3 ft.; with decimeter it’s 1.7 ft.; and with RTK it’s 1 inch.

Read on to learn solutions…