Better Signals, Better Receivers
Improved satellites in the sky and advanced electronics on the ground translate into the best year ever for GPS signal users.
September 11, 2008
Accuracy and repeatability continue to improve, agrees Rhett Schildroth, ag product marketing manager with Topcon Positioning Systems. "Enhancements to the U.S. WAAS signal have improved submeter applications in most parts of the U.S. and even parts of Canada," he says. "Dual constellation receivers that are able to track both the U.S. GPS satellite constellation and the Russian GLONASS constellation improve positional accuracy and increase up-time in the field." He also points out these additional satellites allow users to maintain their signal when traveling close to tree lines and other obstructions.
"Block IIR-M satellites with more stable rubidium clocks are being launched at a high rate, additional signals such as the L2C are being added to the new payloads, and efforts to improve the broadcast ephemeris have yielded better overall performance," says John Deere Engineer Curtis Hay.
John Deere is the only ag equipment manufacturer that operates its own global, space-based differential GPS network, says Hay. The StarFire Network is a collection of ground stations across the world that uploads differential corrections to six orbiting satellites at geostationary altitudes.
"Over the last several months, Deere has upgraded this network by leasing additional satellites and improving redundancy at the network control centers," Hay explains. "This translates to better network coverage around the globe and better reliability within the network infrastructure."
One thing to watch out for, though, is that a couple of WAAS satellites will be retired sometime this summer, reports Marlin Melander, marketing manager, Raven Flow Controls. The move may have ramifications for receivers that are locked onto one of the retirees. Users will need to reconfigure their receiver to lock into a different signal or satellite, and Raven will be sending out bulletins to distributors on the details.
Raven is flying high with its new Phoenix line of receivers. The best seller this spring has been the Phoenix 200 ($1,600), a submeter receiver capable of using WAAS or e-Dif, says Melander. In the event a WAAS signal is unreliable or there isn't one available, e-Dif actually calculates its own differential correction for the satellites that are in view, he explains. The technology was developed by the Hemisphere GPS/Outback group and is currently available in that company's Crescent receivers.
The Phoenix 300 ($3,995) is capable of receiving WAAS, CDGPS (Canadian DGPS), and OmniSTAR VBS, XP, and HP. It's selling extremely well in Canada and can be used in the northern tier of states in the U.S. within range of picking up CDGPS, says Melander.
An eye to upgrading is key to Trimble's technologies this year. Both the AgGPS 252 ($2,995) and AgGPS 332 ($2,995) can work with WAAS and subsequently be upgraded to work with OmniSTAR XP and HP signals — as well as go all the way to RTK, says Siefken. "The 252 is generally the standard receiver we would sell with our AutoPilot systems."
Midwest Technologies' receiver line is shifting a bit as older products are replaced, says Rich Gould, vice president of product strategy. One new niche item geared for the ag market is the RX 410 ($3,463), which, at presstime, was scheduled to ship in just a few weeks. Equipped with new electronics, the 410 replaces the RX 400. It handles multiple correction signals — WAAS, VBS, and Coast Guard Beacon.
The newly released RX 500 ($5,500) is a step up from the 410 and offers OmniSTAR XP and HP as well as WAAS correction capabilities. With the OmniSTAR HP signal, it gives reliable 5- to 10-centimeter accuracy and is very attractive for autosteer applications, says Gould. "It is extremely stable and its accuracy is enough for just about any field operation," he explains.
Indeed, some technology developments are happening so fast, ag guidance manufacturers are racing to keep up with them. Hemisphere GPS released its BaseLine guidance system last year, but discovered components that "really made it quite a bit better in accuracy" just a few months later, says Jeff Farrar, North America sales manager for Outback Guidance. Enter the BaseLine HD (high definition) at $8,490, designed to replace mechanical disk markers. It features a data transmitter, internal battery, and integrated base station that can be operated from temporary tripod sites. Several BaseLine Rovers or vehicle-mounted GPS receivers can be serviced using single unit.
Midwest Technologies' guidance system, the CenterLine 220, has the receiver built in. Gould says this approach has benefits and drawbacks: There's one less component in the cab, but future performance upgrades are more of a challenge.
Advice From Experts
â– Consider future upgrades. Look for a system that can grow in repeatability and accuracy as the demands of your business grow, says Schildroth. Several receiver systems on the market today allow operators to start at modest price and upgrade later. "They usually have submeter accuracy out of the box and then can be upgraded to decimeter and even centimeter level accuracy by purchasing correction signals or adding a base station," he explains. One example is the Topcon MapHP.
While most guidance system packages come with a receiver, the customer does indeed have the option of using a brand of their choosing. For instance, the X20 ProSteer system from Topcon is an open architecture system that can work with any brand of positioning receiver.
â– Check into accuracy. Trevor Mecham, North American business manager for ag with Leica, encourages receiver users to ask manufacturers about the difference between on-the-ground and receiver engine accuracy. This helps operators gain realistic expectations for performance in the field. "And users need to understand what level of accuracy they're going to need for their operations. Don't overbuy or underbuy," says Melander.
Gould believes there's still a place in the market for WAAS-only receivers. "At $1,500, they're less costly and so reliable these days," he points out.
â– Price precautions. Mid-Tech's Gould is realistic on pricing when it comes to receivers. "You get what you pay for," he says.
â– Firmware updates. Siefken advises users to go to the Web site of the receiver manufacturer to make sure they're running the latest version of the receiver's firmware. "Like getting any other piece of quality equipment field-ready, it's part of the checklist for spring preparedness."
â– Watch for standards. Some good news is ahead for receiver buyers. The American Society for Agricultural and Biological Engineers is working to standardize testing procedures for receivers. "Each company uses its own terms and statistics when describing their receivers," says Gould. "That can make them hard to compare."
There's no question about the accuracy and repeatability required for strip-till work. The residue content on the fields can be high; hence, visibility for planting and input placement is low. Siefken describes a field where a grower might plant and fertilize into strip-till rows, then needs to come back to apply postemergence products, running applicators between rows. Operators can try a number of the newer dual frequency technologies, including RTK (available from a number of suppliers), OmniSTAR, and Starfire. Outback's Farrar says BaseLine HD also offers the technology needed for strip-till operations in nutrient placement. Growers can decide which option offers the accuracy they want.
New acres planted to corn may be a boon to the guidance market, especially in strip-till situations. "A lot of cooperatives have created strip-till application programs as a service for sale," Siefken notes.