GIAA Mailbag: On GPS RTK and the Kodak Grey Card
Q: When using my GPS RTK system in hilly terrain, I sometimes lose the “fixed” RTK solution and cannot continue. Do you have any suggestions, other than relocating my base station, to handle this type of situation? I use my own RTK base station and CORS (Continuously Operating Reference Stations) as applicable.A: Since most radio solutions for this type of application are “line-of-sight,” you may lose reception if you have obstructions between your base station and the surveying location. Relocating your base station is one way to solve this. The use of a “repeater” is another. The repeater option can save you time and energy in these instances. A repeater is a radio link that receives the transmission from the base and then rebroadcasts or “repeats” it to the rover. Low and high power radio modems compatible with the base and rover radios must be used. When you encounter or expect to encounter loss of RTK corrections from your base station, you can power your repeater via battery or cigarette lighter and position it where it can receive corrections from the base station and broadcast the correction signal to your working area. Your ability to use the repeater is optimized if it can be preprogrammed with all of the working frequencies you use in your working area and can change channels (CORS and Personal Base Station) with a built-in selection button or software. This allows speedy setup no matter what base station correction frequency you use. Depending on its design, a repeater radio may also be used to function as an emergency base or rover radio.
Q: I have seen reference to a Kodak Grey Card when some manufacturers state the range of their reflectorless EDMs. What is this card?A: The Kodak Grey Card is used by photo-graphers when an object whose known visible light reflectivity is required. One side of this card, called a “mid-grey,” reflects 18 percent of the incident light visible to humans. The other side is practically white and is 90 percent reflective. The card has come into use by some manufacturers for stating reflectorless EDM range. Keep in mind that retro prisms used with conventional EDMs are relatively standard in light reflectivity. Thus, EDM range can be compared without much regard for the target being observed. With reflectorless systems, the target is variable and thus, so is the reflectivity. The Kodak Grey Card is an attempt to introduce some element of standardization. The question of which side of the card to use is a question yet to be resolved. This is important because the ranges will vary greatly depending on the side used. But first, the card itself must be adopted by all manufacturers. In the absence of a standard some manufacturers have selected the 18 percent reflective side. Keep in mind that with your reflectorless EDM, the infrared reflectivity of objects surveyed can vary and actual EDM ranges may be more or less than specified by the manufacturer.
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