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Australia | The word "drift" is a word that sums up the ever changing 'errors' which affect the accuracy of GPS positioning, in particular SBAS 'float' positioning (WAAS, Omnistar XP & HP, SF etc). Each GPS receiver manufacturer deals with drift using a variety of software techniques in an attempt to mask the practical impacts such as line-jumps and curves. These techniques vary in terms of their effectiveness in different situations, but they are all battling the same problem in different ways. I have to say that Hemisphere/Outback have been focusing their R&D on L1 "float" positioning in agriculture for years and do a good job. Novatel have recently released their L1 GLIDE solution which is also excellent at cutting the effects of drift.
The only way to get rid of drift is to go to what's called an 'integer' fix solution. I won't go into the science as I will embarrass myself, but essentially instead of a 'floating' position, the GPS receiver 'fixes' its position and keeps it locked in. This is a well known strength of RTK receivers and what makes them 'driftless' at least within their accuracy specification. The drawback of RTK is that while the accuracy is excellent, they can quite easily 'lose lock' on the satellites and provide no position at all.
That is why many farmers prefer to have a drifting L1 float position over a driftless RTK fixed solution. They prefer to keep working with a bad position than put up with the historical downtime associated with RTK.
This is changing rapidly. All the GPS manufacturers are working on smart ways to make RTK more available and have mostly succeeded. Glonass is an excellent way to increase the number of satellites used to calculate the position and so reduces dropouts. DOT RTK networks will also make things more convenient and reliable. Even simple techniques like Deere's RTK extend keep customers operating but on lower accuracies.
One suggestion would be to upgrade the standard GPS antenna's on the Trimble units which may make a significant difference. L1 positioning benefits greatly from high quality antennas.
Cheers
John | |
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Outback vs. Trimble drift
