Home   News

Wallace: The GPS Guru

Mar 12, 2009

Guidance Accuracy and Signal Choices – Part 2

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.

RTK accuracy can be used in so many ways to improve the performance and efficiencies of your farming operation, whether it is in the reduction of stress do to guidance and auto-steer, repeatability for controlled traffic and controlled application methods. The cost of RTK equipment may look over-whelming at first but take a good look into the accuracy and the repeatability and you might find ways to justify the system in your area and with your cropping practice. There may even be an RTK Network in your area or other producers or companies that would be interested in pursuing this.

Choosing which level of GPS accuracy to utilize is completely up to you, but it doesn’t take long to find out that accuracy is addictive, and the more accurate you can be, the fussier you become. With accuracy comes great crop improvement possibilities, and opportunities to help decrease your input costs and increase your bottom line profitability.

This could be as simple as reducing overlap when applying fertilizer while maintaining yields. Another potential use is strip-till applications where we could row-inject fertilizer and reduce the rates but still supply optimum nutrient levels to the plants.  There are many opportunities in agriculture today to improve the way we manage and operate our farms. The right level of accuracy sourced using the right correction signal for your operation is an important part of the precision ag strategy for your farm. If we do it right, we can farm smarter and be more profitable in the end.
Jordan Wallace is a partner in GPS Ontario, a North Gower, Ontario based company. He handles sales, service, and 24 / 7 technical support. Jordan is also working with Trimble Engineering to help test new products and firmware slated for future release. Jordan spends a lot of time working on new and improved ways to advance the precision ag industry. He sees improved compatibility between components and technical support as key topics. GPS Ontario is an industry leader in precision agriculture, providing specialized and compatible equipment and information for progressive producers.

Jordan Wallace
GPS Ontario
6558 3rd Line Road South
P.O Box 456 North Gower
K0A2T0, Ontario, Canada
Office (613) 489-2932
Cell (613) 229-6377
Support (613) 327-6377
Mike PPT 905*5*52389

This commentary is for informational purposes only.  The opinions and comments expressed herein represent the opinions of the author--they do not necessarily reflect the opinion of  This commentary is not intended to provide individual advice to anyone. will not be liable for any errors or omissions in the information, or for any damages or losses in any way related to this commentary.