GPS World, February 2009

SURVEY CONSTRUCTION Survey Å FIGURE 4 Percentage of the observations with accuracy better than 5 centimeters deep colors as the background and 1 centimeter bright colors at the front was initially planned to last for 24 hours but the GPRS connection stopped after 18 hours of continuous use e lowest availability level was observed during TS6 99 percent which used the voice and data SIM card TABLE 4 shows the detailed availability results Aside from the NRTK epochs some of the observations resulted in DGPS solutions that did not indicate that the NRTK corrections were being received In general it is evident that even when using the normal IP SIM card the availability of the service is still very high greater than 99 percent in contrast with the results of 85 percent obtained in the past tests mentioned at the beginning of the article However the new fi xed IP SIM card demonstrated more stability when performing long periods of continuous observation Impact of Availability on Accuracy The availability of the NRTK solution is a vital index for the good performance of the SmartNet service The accuracy and precision of the observations directly depend on whether the solution obtained is NRTK or not Past tests demonstrated that SmartNet can achieve accuracy generally better than 5 centimeters erefore is the availability of this high accuracy similar to the availability of the NRTK solution shown above To answer this question we evaluated the availability of the accuracy as the percentage of NRTK observations with accuracy better than 5 centimeters We also calculated the percentage of observations with accuracy better than 1 centimeter A summary of the availability of the accuracy results appears in FIGURE 4 in the order of East E North N and Height H Observation accuracy was better than 5 centimeters over 9750 percent of the time in all three coordinate components At the same time the Easting coordinate has a higher accuracy than the Northing and Height components with at least 80 percent of the observations being better than 1 centimeter accuracy It is then quite possible to say that under similar testing conditions SmartNet can achieve accuracy better than 5 centimeters when an NRTK solution is performed Issues for North America As these tests took place in the European wireless environment we now address a few comments on these issues relative to cellular phone service in the United States which is provided on a state by state basis There are many different U S network carriers who provide a mix of CDMA and GSM Typically CDMA has a better overall coverage especially in rural areas The top U S carriers are Verizon Alltel Sprint USCellular and AT T GSM based carriers are AT T formerly Cingular and TMobile They have concentrated their efforts in larger cities especially with respect to their data services but one can find them everywhere Network RTK correction providers willing to offer their services in a particular area would need to research the coverage of the network providers For example in the Southeast Verizon and Alltel are the best networks and they use CDMA Other areas are AT T or T Mobile GSM In Iowa USCellular Verizon T Mobile Alltel and AT T are used to do the network acceptance testing in case one of them didnt work for a particular area but the others did Unfortunately the coverage maps on each of the providers websites are pretty marketing images and dont really reflect the actual coverage One normally doesnt know until actually out in the field trying the service As a best practice customers should choose their cellular provider based on who has the best coverage in the areas where they are primarily going to be working In the United States the most commonly used devices employ CDMA services Although GSM is available for some reason people have migrated to the CDMA versions Contracts are usually bought as unlimited connectivity costing about 60 a month CDMA is the only service used in Canada where GSM has not taken off at all Although available almost 100 percent of NRTK GPS positioning customers prefer CDMA In addition to the provider coverage technology problems which phones may or may not work for each provider also must be considered Technical representatives of each provider should help with the list of phones that would have unlimited data service Bluetooth dial up networking or tethering however just because it can get to the Internet is not a guarantee it will let the instrument bond and connect properly Therefore not only the selection of the proper cellphone carrier but also of the hardware to be used plays an important role in the provision of a reliable NRTK correction service in the United States One of the biggest recent changes is the use of data relay systems or bridges For example the Intuicom bridge is a device that has a cellular modem in it and a regular radio The devices have a Linux brain that can log in to a network and start communication They also have an option of an L1 GPS receiver to start a position This type of device works great when you have several units operating in a small area or if you need to extend to an area where mobile communications are GPS World February 2009 www gpsworld com 18