In Real Time: Real-time GNSS Positioning Field Guide
Real-time GNSS (RT) technology is a fast, efficient positioning and navigation tool that can yield survey-grade coordinates used in a variety of applications.
But how confident do you feel about the data you are producing with your real-time gear? It is “black box” technology, after all. The data collector is telling you that your position is good to 0.01 meter horizontal and 0.02 meter vertical with a root mean square (RMS) of 0.015 meter. How reliably can you count on that? Are the numbers showing precision, or are they showing accuracy--and at what confidence level? What would you expect if you take another shot at a different time or with different weather? What are the factors that might be affecting your data, anyway? Would the new real-time network (RTN) in your area give you better results? Is there any way to have real confidence with a RT-established position?
There may be methods that you use to consistently achieve the same results--and I’d like to hear about them. While some very good RTN articles have been published, it would be beneficial to address the user’s perspective. This new column will provide recommendations for how a field RT survey should proceed in order to capture data at various tiers of precision with a high degree of confidence.
In the mid-90s, we started to use RTK by sporting a 45-pound backpack (Z-XII receiver, two lead acid batteries, spread spectrum radio--all on a steel mounting board in a backpack, plus a whip radio antenna that trimmed the lower tree leaves as you walked, cables running to the rover pole with a 15-inch diameter “whopper” ground plane antenna--a really smooth RT rover machine, eh?). Now we can RT survey all day without needing a chiropractor. I hope my experience with several different GNSS manufacturers’ gear will bring you some benefit in the field.
Why Single-Base RTK?With the burgeoning number of RTNs sweeping across the United States, one would think single-base RT is passé. There are well over 80 RTNs in the United States, with about 37 state Departments of Transportation involved in either their planning or operation. The truth, however, is that single-base RT will likely be around for years to come. Many users still employ legacy RTK equipment using radios for data transfer, and there are also many areas without cell tower coverage, leaving no data transfer vehicle readily available to the RTN user.
Additionally, many RTNs start as closest-base networks--until subscriptions increase enough to warrant the expense of purchasing network software--and thus operate in a single-base fashion for an unspecified time. Furthermore, there are applications, such as precision agriculture or machine guidance, that will utilize local base stations for several more years.
The Seven C'sThe best methods for RT users have been broken down into seven categories for this series. Since NGS is an office of the NOAA, these categories have been dubbed “The Seven C’s” (or “seas”) of RT positioning. Specifically, the categories are: checking, communication, conditions, constraints, coordinates, collection and confidence. Each column in this series will address one or two of the seven C’s. The first one is below.
C No. 1: Check Your RT Gear.
- Prior to a RT campaign, adjust the bull’s eye bubbles on the base and rover poles.2
- Make sure the base radio and receiver batteries are fully charged. A weak base radio battery will limit communication range. Make sure you have spare rover batteries with you.
- Raise the radio antenna as high as possible. When you double the antenna height, you can increase range by 40 percent. Typically, an antenna height of 5 feet would equal a 5-mile range; a height of 20 feet would equal an 11-mile range. Use a low-loss cable for heights above 25 feet.
- Use a dipole (directional) antenna for special applications, such as corridor surveys.
- Reach into poor communication areas, such as valleys, with a repeater.
- Make sure cable connections are seated properly and tightly.
- Check the “real” height of a fixed-height tripod and rover pole at the beginning of the RT session.
- Make sure the base is occupying the correct mark, and enter its coordinates and height into the data collector.
- Secure the base station tripod feet if possible (using sandbags, a foot tether, etc.)
- Decide on the base station’s security. Is a “babysitter” needed?
The Four Cardinal Rules for RT Positioning
- Always have redundant locations on important points.
- Always get a check shot before collecting new data and with a new initialization.
- Always have a strong, continuous communication link to the base during a point location.
- Avoid multipath conditions whenever possible.
- The current version of the single-base user guidelines can be found at www.ngs.noaa.gov/PUBS_LIB/NGSRealTimeUserGuidelines.v1.1.pdf.
- See SECO Tech Tip No. 13, “Setup a SECO Precise GPS Antenna Tripod," www.surveying.com/tech_tips/details.asp?techTipNo=13.