Guidance Accuracy and Signal Choices ? Part 2

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Wallace: The GPS Guru
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In Part 1, we discussed some things you need to know about GPS equipment manufacturer accuracy claims. We also looked at the accuracy and performance of a free WAAS correction signal for guidance packages. In this article we’ll cover the subscription based and RTK (real time kinematic) systems, both of which offer improved accuracy over WAAS.

OmniSTAR and StarFire-2 are subscription-based signals that are corrected using a land-based reference station. The accuracy of the signal depends entirely on the signal you purchase. OmniSTAR accuracy is listed under a 2-Sigma accuracy level. You choose from 3 levels of accuracy: OmniSTAR VBS (6” - 8” Pass-to-Pass),OmniSTAR XP (3”–5” Pass-to-Pass),and OmniSTAR HP (2” – 4” Pass-to-Pass) accuracy. StarFire-2 is listed as a 1-Sigma (2” Pass-to-Pass). Currently I’m not able to find a full 2-Sigma accuracy listing for StarFire-2, but from experience it is relatively close to the OmniSTAR XP accuracy.

These subscription signals all have pro’s and con’s, but the biggest benefit is they offer is an enhanced accuracy level over WAAS which makes it more suitable for row crops without the need for an in-field base station. The biggest disadvantage to all these signals is it takes time for the signal to gain accuracy (we call this converge time). It can take anywhere from 5 – 25 minutes to secure an accurate signal depending on your geographical area. If you pass behind trees, buildings, or drop into a valley even for a little bit and you lose your correctional signal, it takes time for the signal to re-converge to the accuracy you are paying for.

Depending on the signal this could be anywhere from 1 – 25 minutes. This re-initialization of signal can be very frustrating in some geographical areas where trees and hills are an issue. In these situations there is really no perfect solution with these types of signal. The best advice I can offer is if you are considering this type of signal, have your local dealer bring you a system so you can try the signals on your fields and in your geographical area. Determine if waiting for signal is going to be an issue.

RTK (Real Time Kinematic) signal uses a single reference base station located within a close proximity to your land. The base-station calculates real-time corrections and sends this information by radio from the base station to your cab. The RTK System is based on the abilities of the radios and your environmental conditions to transmit the signal from the base to your cab to give you the accuracy of the systems.

RTK accuracy is listed as sub-centimeter accuracy. Instead of measuring accuracy based on pass-to-pass or 15 minute intervals, RTK can be accurate to + / – 1” and repeatable year-to-year. There are 2 types of RTK bases. Mobile bases are stations that are moved from farm to farm, field to field, and are usually set up on a tripod or monument in the field corner or by the gate. Fixed base stations are mounted on a permanent position on your farm so signal can be transmitted over your farming operation. The higher you mount the station the better!

Fixed bases can be tied to a network providing coverage over a wider area using multiple stations. RTK networks are becoming more and more popular as the expense of the base stations are being shared by multiple growers. RTK accuracy is very reliable as long as the base does not move. Should the base move 1” to the south the field reference data would also shift 1” to the south. This makes it extremely difficult to obtain repeatable RTK signal with a mobile base.