- SPECIAL REPORTS
- THE MAGAZINE
In our surveying discipline, we have five field crews that are each equipped with standard equipment including a total station, level and four-wheel drive truck. Each crew is also equipped with a laptop computer for transferring data via their cell phones to and from the office. We have a network that allows each field crew to access our office server for sending and receiving both pictures and data files. This saves a great deal of time by allowing the field crews to minimize their travel time. They can stay out on the jobsite when they need more information, or they can quickly download their information for those “need it right away” jobs.
From Radios to Cell PhonesFour years ago, we began using two Leica GPS RTK SR 530 systems. These systems are great tools but we found that the radio links between the base stations and their rovers seriously limit these systems. When these systems work, they are very good, but when they do not, they are very frustrating. We have had radio interruptions from being out of range, being over a hill and being overridden by other transmitters.
Since our firm is based in the highly populated area of Minneapolis and St. Paul, and RTK GPS use in large cities often involves much radio interference, we knew that a cell phone link would be much more reliable than a radio link. For the last two years, Leica has been talking to us about using cell phones to relay information between the base stations and the rovers. This would have dedicated two cell phones to each GPS system, one for the base station and one for the rover. It sounded great and in line with our progressive nature, but our interest was limited due to the cost of airtime. We estimated that at five cents per minute, we would pay around $400 per month—per system! We would also only be able to run one rover from a base station. Now, however, Leica has developed a new cell phone system that uses dedicated modems to transmit data and costs about $100 per month. We are able to run as many rovers as we want off of the same base station.
With the help of Ari Silkey and Gary Belschner from Leica Geosystems, we installed a system that incorporates the cell phone with the Internet. This new system transfers the RTK data from our roof-mounted base station to a dedicated Internet site, which is accessible from virtually anywhere in the world. Our rover receives data via a specialized cellular modem made by AirLink Communications Inc., a national wireless solutions provider. Loucks Associates is the first private firm in the upper Midwest area to establish a base station using the cell phone/Internet technology.
In our area, the radio link limits our area of coverage for quality horizontal and vertical control to about a four-mile radius. With this new cell phone system, our coverage increased to an 18-mile radius. We have established a very accurate latitude, longitude and elevation on this base station by tying into the Minnesota Department of Transportation First Order control network.
The time savings on this system is incredible. Our field crews now leave the office with their control already established. They will spend far less time breaking down sections, establishing bench marks and setting control.
The New System DefinedBase station
We have replaced the radio in our Leica model SR530 GPS system with an Air-Link Raven CDPD (Cellular Digital Packet Data) phone modem. On the base station side of the system, we have permanently mounted a Leica model AT502 L1/L2 dual-frequency, high-accuracy GPS antenna on the roof of our building. The antenna is connected to the base station in our main server room with a 100-meter cable. The base station is cabled to a dedicated computer that is connected to the Internet with a high-speed modem.
The GPS base station receives data from the GPS satellites, then the data is sent through the SR530 and funneled through a Com port to the computer. The computer, using a program called TCP Com, transfers the real-time data to a dedicated IP (Internet Protocol) address on the Internet. This is where it becomes available for use by the rover.
On the rover side of the system, we have simply replaced the radio receiver with the cell phone modem. The rover receives the real-time data from the base station through the cellular phone network instead of the radio link. So, wherever the rover can receive CDPD cellular coverage, it can receive the data from the base station. The CDPD part of the cell phone network is a dedicated area of the cell phone network designed to transfer data. Once a link is established on the rover the connection is fixed. As long as you stay in CDPD coverage area, you will be receiving the data from the base station.
In Minnesota, we typically work on the county coordinate system and in NAVD88 elevations. With a GEOID model and the county coordinate systems installed on the rover, we now usually receive accuracy of ±0.03 feet +1ppm for horizontal and ±0.06 + 2ppm feet for vertical. And with a range now extended to 18 miles, we are constantly looking for new ways to utilize it.
The additional costs of this new system were minimal, about $4,000. The operating costs are $60 per month for unlimited cell phone service and $40 per month for a dedicated Internet site. Now that the base station is up and running, it requires no maintenance and operates 24/7.
Endless AdvantagesAll of the seven counties that comprise the Twin City metro area have established county coordinate values for most of their section corners. For many of our surveys, we can compute the section breakdown and the boundary corners before the field crews leave the office. The crew’s typical procedure is to check into two or three section corners for horizontal control, check into a local bench mark for vertical control, set control around the site for future work and begin searching for monuments using the stakeout feature of the system.
The advantages of this system are endless. Knowing that our control is established and ready for use, we will be able to save time on every type of job we do. For construction staking jobs, we won’t be concerned about losing our control points due to construction, we will no longer have setup time for those rush times when the backhoe is waiting for stakes and we will be able to send one person onto the sites for quick checks.
For boundary surveys, we will have our computations done prior to arriving at the site and our initial field time and energy will be spent verifying control, not establishing control.
For topographic surveys, we will have a bench mark system established prior to arriving on the site, we may begin locating topography prior to boundary work for those “need to be done right away” jobs and we may send one person to obtain simple preliminary work and save the costs of sending a full two-person crew.
For work with Geographic Information Systems, we may cover many areas in different locations without the setup time on each site and we may send one person to obtain simple work and save the costs of sending a full two-person crew. And for photo control, our horizontal and vertical control is already established and the crews can begin immediately.
A Plan for the FutureWe intend to purchase enough additional equipment to have two permanent base stations and one rover for each field crew. We currently have one base station operating at our Minneapolis office with plans for establishing another one at our St. Paul office.
We believe that the key to success in this business, and any other business, is client satisfaction. With this new technology, we are confident that we will maintain our high standard of quality, shorten production time and continue to serve our clients with competitive costs.