Ashtech's Locus System

Being very curious about the ability of single frequency GPS, I eagerly accepted an opportunity from Mike Margolis to review Ashtech's Locus system. Mike put me in touch with Bill Martin, who hails from "down east" Caribou, Maine. Bill and I arranged to meet one weekend this past October in order to let me try out the Locus system.

The equipment arrived at my office before Bill. Eager to see the new equipment, we took the units out of the box, hastily read the pictorial, and inserted the "D" cell batteries that came with it. Compared to our dual frequency Z sensors and controllers, the lack of cables and the small size made us skeptical, as did the absence of the controller. It did, however, come with an instructional video on its use which I watched the night before Bill's arrival.

Bill arrived on Saturday. Unfortunately, the weather precluded dry field testing so we opted for the office and the new software that accompanies the Locus system. Our office has a DOS-based copy of PRISM95, so the newer Windows-based product was most impressive. Bill ran through the rudiments, like mission preplanning and setting processing parameters.


Keeping Sidewalk Superintendents at Bay
Sunday morning, the first item on our agenda was a short static session. Bill set up the two-meter rod/tripod at a small park just north of town. Two meter poles keep operators' and sidewalk superintendents' heads from blocking the antenna reception of satellites. Our second unit was set on a tripod. The next step was to record the antenna heights, unit serial numbers (the last four numbers of the serial number allow you to identify the receivers in the Processor software), and other point occupation data. The Locus processor software has a sample Locus Field Observation Log sheet that you can print out and copy.

Ashtech's field experience is evidenced in the antenna height measuring devices that come with each unit. The first is a collar that gently twist-locks on the antenna. The measuring tape is hooked onto an attached lug. The tape is cut off from a standard unit and has a plumb bob-like tip on it. You hook it over the antenna "plate," and with one hand you can put it on the point, and then lock the extended tape and read the height. About the only blunder you can make now is to misread the tape.

Next, we turned on the receivers. A laminated Quick Reference card is part of the accessories. It explains the four LEDs and the operation of the Locus. The four "D" cells are good for about 100 field hours at 68F. For static surveying, that's about it. Just let the units "cook" for the required time for the type of survey you are conducting. The right LED glows green to let you know there is ample battery power. The next light from the right is the satellite LED which flashes red if it is tracking but not logging a satellite. It will flash green when it does log an SUV. It flashes the number of satellites, either red or green, pauses momentarily, and then flashes the number of satellites again. You can watch them go from red to green to know how many you are logging.

LEDs Indicate When Enough Data Has Been Collected
Moving to the left, the next LED blinks green when you are logging data and at the interval you are logging the data. Finally, the leftmost LED blinks to let you know when you have collected sufficient data for the vector between the two units dependent upon length. One blink for <5km, two blinks for <10km, three blinks for <15 km and four blinks <20km. To make sure, count the blinks on the last unit you turned on. Except for the infrared port that's about all there is to using the Locus in the static mode.

By now, experienced users of data loggers may question how to get site data into the Locus or into the software. One method is to manually enter the data from the log sheets at the time it is being processed. This is not difficult or time consuming. Another method is to synchronize the HP48GX with the Locus software card aboard to the unit you are logging and then entering the site data. You can then download the handheld to your PC via inferred link. Of course, you can do both to detect field blunders.

I was impressed with the ease of set up and the lack of cables of the Locus system. I have learned from experience that lack of cable care can cause blunders, even a loss of session, if not addressed properly. Just careless coiling, over time, can degrade the radio signal between the antenna, receiver and data logger. Proper precaution and attention will keep cable-related issues to a minimum. When we had one blink on the occupation timer, we stopped our static session so Bill could demonstrate the Kinematic survey procedure.

Kinematic Sessions Use Unique Bar as Baseline
Locus Kinematic sessions require initialization on a bar. It also requires the use of an HP48GX with PC card software provided with the kit. You set the two receivers up on the bar and then synchronize the 48, first with the base unit, and second, with the rover unit. While the units are acquiring lock, you can set up the 48's cloth case on the carbon fiber meter pole. It snaps on the plate, which can be rotated to a position that is comfortable for you to work with.
For our demonstration, we set the recording time for points at ten seconds. The user can walk up to a feature which is to be collected, input the feature code, press the start key and hold steady while the timer counts down to zero and then beeps. Then, move onto the next and if it is the same feature code, just accept the default and hit the start key. The software automatically increments the point numbers. The system also permits the use of more than one rover.

Next came a loss of lock exercise. Just before we walked into the trees, a point was set (a PK or any well-defined point will do) to reinitialize on. We walked to the tree with the most leaves and forced a loss of lock. Properly warned, we went back to the set point, input the number and regained our lock. It is not necessary to go back to the initial setup with the bar. A few more points were collected and then the rain came.

Back at the office, we downloaded and processed the data. We brought up the download screen, plugged the inferred module into the serial port, powered up the receiver, and watched the session files come up on the left-hand window. Windows users will be familiar with the drag-and-drop procedure that is used to transfer them to a disk for processing. When all the session files are loaded, the user can turn the receiver off and repeat the procedure with the next one. When you get to the 48GX logger, it has to be held very close to the infrared module to enable communications. Seems HP designed it that way to prevent enterprising students from passing answers back and forth during testing.

Using Log Sheets or a Controller
Next, the user can replace the default IDs with the proper site IDs from the field logs if that method is preferred. It is important to log session start and stop times on the field log sheets to match them with the time signatures of the receiver files. This is the time to enter antenna heights and other site information. Then you click on the processing icon in the tool bar and sit back.

The software can handle RINEX files and vectors from other Ashtech software. It has adjustment capability and analysis. There are tabs for files, occupations, sites, control sites, vectors, repeat vectors, loop closure, control tie, adjustment analysis and network relative accuracy. It also has zooming capabilities for graphics. Users may work in geodetic, State Plane, UTM, or local grids.

There is data-filtering and the ability to set parameters prior to processing. Data can be output to any system of your choosing. The output is also formatted to blend well with popular word processors for client reports.

Manuals for the receivers and for the software are provided. They are well-written, and liberally illustrated, with a flow chart on how to do an adjustment. An appendix goes into more detail on how and why to perform various activities. The results of the demonstration were impressive. Vector residuals, at the 95% level, were all subcentimeter. The mission of the Locus product is to make it less complicated than earlier generation GPS and at the same time relatively inexpensive. From my perspective they have succeeded.


Al Pepling practices surveying in Pittsburgh, Pennsylvania, and is the New Products Editor for the magazine.

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