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Russian GLONASS covers GPS gaps

For those farmers and surveyors who suffer when GPS equipment doesn't work like it's supposed to work, GLONASS is here. Da, the Global Orbiting Navigation Satellite System, the Russian equivalent of the U.S. Global Positioning System, is being used to augment GPS, and could enhance the reliability of satellite-based positioning systems.

GLONASS is managed for the Russian Federation Government by the Russian Space Forces and is operated by Russia's Coordination Scientific Information Center.

As with GPS, GLONASS provides specially coded satellite signals that can be processed by a receiver, enabling the receiver to compute position, velocity and time. The signals are free for users.

GLONASS has apparently worked through some political and funding problems, and today has a constellation of 12 operational satellites orbiting Earth. The first GLONASS satellite was launched in 1982. Three more were launched in December 2004.

Up to now, GPS and GLONASS have operated as unintegrated systems. But occasionally unreliable GPS signals renewed efforts to combine them to increase the number of satellites available to U.S. users.

The opportunity was first seized by Javad Ashjaee, a former employee of Trimble born in Iran. The U.S.-educated engineer found a way to integrate the signals from GPS and GLONASS into one receiver and designed a unique antenna for the receiver.

In 2000, Topcon Corporation purchased Ashjaee's Javad Positioning Systems, with offices in San Jose, Calif., and Moscow, Russia. Ashjaee's GPS/GLONASS systems are being manufactured by the newly formed Topcon Positioning Systems.

Receivers that integrate GPS and GLONASS are available for real-time kinetic positioning systems. An RTK system uses a base station located in the field for accurate positioning to within a centimeter — both horizontally and vertically.

This is different from a sub-meter accurate system which relies on a differential signal such as Wide Area Augmentation Signal to enhance satellite signals for accuracy to within a foot.

While using GPS and GLONASS together can help both sub-meter and RTK users, the most important — and cost effective — application is for RTK uses such as levee surveying for rice and soybeans, land-leveling and auto-steer.

Essentially, with RTK systems, you can drive a vehicle across a field and know exactly where you are on Earth as well how far above Earth you are — to within an area the size of a pencil eraser.

John Dice, with Topcon, California, which markets the GPS/GLONASS receivers under the Topcon brand, said there's nothing special about GLONASS other than it increases the number of satellites available and improves satellite geometry.

“To get the most accurate position, you want to have a certain number of satellites in the correct position — at least five — and you want scattered rather than clumped positioning of satellites.”

Here's one way to visualize how scattered is better, according to Dice. “If you were sitting at a table and you dropped a string straight down from the ceiling, you have one distance from the ceiling. Say you pulled another four strings directly above you within a 2-foot to 3-foot diameter. If you held the other ends of those strings together above your head, you could move that string back and forth a little bit, meaning the precision is not as good.

“But if you went to the corners of the room and pulled strings to that point, you have a tight position. That's kind of how it works with satellites. That's where simply having the availability of more satellites with GLONASS is helpful. It's not because they're Russian satellites. It's simply the fact that there are more satellites. If we had 35 satellites in the U.S. constellation, that would be great, too.”

In levee surveying, a GPS-RTK system can pinpoint exact elevation changes, according to Terry Browning, owner of TOPO Solutions, Bono, Ark., and a Topcon dealer. Adding GLONASS makes it consistently accurate.

“With a satellite system, you're constantly losing and gaining satellites. As your constellation changes, the loss of accuracy can be significant, especially if you're concerned about the vertical reading. The vertical reading is twice as inaccurate at the horizontal.

“If you get off two-tenths of a foot in the vertical, which could happen if the conditions aren't right with a GPS-only system, you're all the way to the next levee if you're surveying levees. During the worst parts of the day for GPS, it will jump off a tenth or two. When it does, it's worthless for surveying levees.”

Tree lines can also cause problems for GPS users, according to surveyor David Bader with Welty and Bader Land Leveling Inc., and AgScan Designs in Bastrop, La. For the past four months, Bader has collected elevation and horizontal positioning using a Topcon RTK system with GPS/GLONASS capability. He still uses laser technology for field cuts.

“There is no difference in overall vertical accuracy between GPS alone versus GPS with GLONASS. But you can work longer in the field with GLONASS/GPS simply because there are more satellites to choose from to maintain your RTK fix. When you get in a tight spot when there aren't enough GPS satellites for the base and rover (receivers) to get a RTK solution, there's a GLONASS satellite to help you pull through.

“GPS/GLONASS isn't perfect by any means. I have lost my fix around a tree line with GPS/GLONASS.”

The same variations in satellite alignment can also cause variances in light bar systems on tractors used for field activities which use sub-meter accuracy, according to Justin McCuiston, precision ag consultant, Farm Service, Inc., Hoxie, Ark. “Several of our farmers are using sub-meter light bar systems to plant fields using grain drills. We have accuracy to stay within 6 to 12 inches. But during bad times of the day for GPS, we have farmers telling us they can't get within 6 feet sometimes.”

“Many times, these errors are diagnosed as problems with the equipment, when it's really a satellite geometry problem,” Browning said. “The only way to get around it is adding more satellites. The way it's been done is with GLONASS. When the entire GLONASS constellation is up, trees and buildings that can block signals won't get in the way any more.”

Browning added, “Even though there are no GPS/GLONASS sub-meter receivers currently available, GLONASS would benefit sub-meter receiver users.”

Browning noted that satellite positioning companies offer free software programs that predict GPS satellite arrangement to help users determine optimum times to schedule a survey or other operation. “But farming is not the type of job that can wait a couple of hours.”

Bader said he doesn't rely on prediction software nearly as much as he used to, now that he's using GPS and GLONASS. “There might be two hours out of the day when GPS accuracy is not good enough. With GPS and GLONASS, it might be 10 minutes. I don't see as many bad times during the day with GLONASS and GPS.

Topcon's RTK system consists of two GPS/GLONASS receivers, a rover and a base station, a small tripod with 3-foot legs and a range pole. The rover receiver can be removed from the survey vehicle and placed on the range pole to survey areas that are unreachable by the surveying vehicle.

There are no cables or wires between any of the components offered by Browning thanks to a wireless communication technology called Bluetooth.

A ruggedized tablet PC that Browning supplies with the system also has a voice recognition software option which allows users to perform commands without keystrokes.

“There are no cables and you have internal batteries, so the system is very mobile,” said Briley Tomlinson, Craighead Farmers Co-op, Jonesboro, Ark.

“For people who are scared to death of computers and technology, you can operate this equipment exactly the same way you did your laser equipment,” Browning said. “Just set up, set your number, and watch those scrolling numbers; they act just the same as an analog laser receiving mast.”

Other uses offered by Browning include a levee surveying tool for contractors. “If you pull into a field you're not familiar with, find the high point in the field, drive around the inside perimeter of the field, and the program will predict where the levees will end up on the outside of the field. You still have to drive the inside contours.”

The system will also allow users to record where levees are run, according to Browning. “You can come back to the field any year after that and it will show you exactly where the levees are. When you rerun the levees, you don't even need the base station. We can use WAAS.

“You can plant your rice, pull the levees and sow the levees that day. Then your levee rice matures relatively about the same time as your other rice matures.”

The Topcon receiver can be configured to use only the GPS or GLONASS signals or both, according to Browning. The Topcon GPS/GLONASS system lists for $36,000.


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