Hands On: SMI Flex GPS
Professional Surveyor Magazine - February 2003
Al Pepling, LS
John Deere, GPS, NavCom, SMI, Eagle Point—these are some new names, and some familiar names to the practicing land surveyor. John Deere is a name that brings back fond memories of spending summers and some weekends on the farm—an old model B with a hand clutch, and an even older memory of a flywheel-started model with compression release petcocks that you had to close as soon as the engine caught. When I think of "rugged" in the sense of farm equipment, John Deere immediately comes to mind.
In 1999, John Deere acquired NavCom Technology, Inc., and incorporated NavCom GPS on several models of its agricultural equipment. Those of us who have driven tractors can attest to the fact that a GPS unit that works reliably in this application should easily handle the rigors of the land surveying world.
Eagle Point is another company that has partnered with NavCom to deliver GPS technology. In December of 1999, Surveyors Module International (SMI) became a division of Eagle Point. SMI's data collection software was ported to the pocket PC platform and GPS support has been added. Couple the field proven NavCom hardware and firmware with the field proven SMI software, and, better yet, make the GPS interface similar in operation to the familiar SMI-HP 48GX total station data collection interface (to simplify the learning curve), and you have the subject of this month's article, the SMI Flex GPS system.
Quality Design Features
The NavCom RT-3010S units are dual-frequency; combined with the SMI data collection package and software they can produce sub-centimeter-level data. The batteries, receiver, and antenna are housed in a single unit. The black ring atop the gray housing and white antenna is rubber, and the unit is designed to survive a two-meter fall. (Figure 1).
Experience has proven time and again that faulty cables are a source of many GPS data collection troubles. By combining the radios into the housing, at least one cable has been eliminated. There are two "com" ports on the units so that external radios, with more transmitting power, (range) can be used. Good design! There is a cable to connect the Pocket PC device tothe GPS unit for control of the unit and processing. Having spares on hand easily solves the cable issue.
Where I used to work, we tested the cables with an ohmmeter for continuity and resistance. Our GPS crews would go out for two weeks at a time and carry six spares of each type of cable with them. Changing the cables as soon as the signal-to-noise ratio started to deviate usually solved the problem. This is a less expensive solution than reobserving a days' worth of data.
The RT-3010S has L1 & L2 full wavelength carrier phase tracking and C/A, P1 & P2 code tracking with proprietary RTK processing. It features on-the-fly initialization and fast ambiguity resolution. Each unit can be configured as a base or rover, has user programmable output rates, a built in spread spectrum radio, and operates from internal batteries or external power. The removable, rechargeable batteries will power the units for 8 hours on a single charge. (The battery receptacles can be seen on either side of the unit in Figure 1.) The units can be operated from a 12V external power supply, and more powerful external radios can be employed. The units are 10.4" wide x 5.5" high and weigh 5.5 lbs. LEMO connectors handle the I/O communications and power transfer. Operating temperatures are -40ºC, (-104 º) to +55ºC, (+131 º).
Each unit that I reviewed came with a four-place battery charger. As usual, I recommend that you purchase the spares when you buy the equipment. This will enable you to do longer sessions in the summer months. This battery charger is another example of forethought in design for the intended use predicated on experience with batteries by various users. Additional features include interference suppression, patented multi-path rejection, 2 separate WAAS/EGNOS channels, and self survey (position averaging) mode.
Into the Field
We set up in the parking lot where I work and had buildings and traffic to contend with. These units performed very well in this environment. Notice the black "rubber duckie" antenna that hangs down from the unit (Figure 2). The onboard radios are one watt, and external radios can be had with greater power, but with that comes the requirement to obtain FCC licensing for the more powerful ones.
A red or green light on the operating panel indicates whether it is operating as the base or the receiver (Figure 3). A power level indicator for the batteries is another example of design based on experience with the land surveying community. Published RTK accuracy horizontally is <1cm, (0.03') + 1ppm, and vertically <2cm, (0.07') + 1 ppm, which may be conservative in my opinion, and which I will explain later in the article. Real time GPS accuracy is horizontally 12cm, (0.39') + 2ppm and vertically 25cm, (0.82'). The primary focus of this design is RTK, and it does it well.
The environmental case for my Compaq iPAQ 3835 is easy to use and does offer some weather protection, but does not include a heater. The SMI software, in addition to the standard SMI total station interface has the GPS software, too. This makes the conversion from RTK GPS to your total station as simple as disconnecting from one and connecting up to the other. Sweet, simple, and most of all, comfortable!
We have a test traverse around our office that was used for the review of this equipment. Our initial setup was on a line 92.84 feet long, but our two-story building and trees blocks out a major portion of the southern sky. Our planned mission was to orient our system to this control and then collect data typical for an as-built survey. Even though we had seven "birds" on the GPS status screen, the satellite geometry was not conducive to good results from the relatively short occupation times required for RTK. We had tenths, not hundredths.
Improving Space Vehicle Reception
Our remedy was to move the rover to the diagonally opposite corner, which opened up more of the southern sky. The GPS status screen revealed eight SV's and much better SV geometry. A longer initialization time yielded coordinate values within a hundredth of the test traverse values. Results like this are generally associated with static sessions that are post-processed. This explains my opinion that the published accuracies are probably conservative.
RTK data collection for the as-built portion of the review began. One of the enhancements in the SMI software is the ability to see the linework and points on the pocket PC screen as you collect them. After collecting a portion of our parking lot and building we went into the office to download our data. We used the SMI transfer software version 7, and it went without a hitch.
Upon reflection, another useful feature is the ability to see your point positional tolerance on the data collector screenbefore you accept it. This facilitates your decision to increase your occupation time for a tighter tolerance. Remember, this is an early version of the GPS data collection software, and it will get better and better as user feedback filters in for more improvements.
The demand for this SMI Flex RTK GPS equipment precluded a longer period for review. Potential users are asking for demonstrations, and supply has not caught up with the demand. After my time with this system I can readily understand. Users of the current SMI software will have a minimal learning curve. Users of competing data collection software and hardware will have a slightly longer learning curve. Purchasers with total station experience but who have no GPS experience will have a slightly longer learning curve, although not that much longer, thanks to the "total station feel" of the software.
SMI Flex RTK GPS will sort of "ease" you into the ranks of GPS users. Set up your own demonstration and see if you agree! In one word, "IMPRESSIVE!"
Al Pepling practices surveying in Pennsylvania, and is the New Products Reviewer for the magazine.
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