In the first article of this series on real-time GNSS positioning (POB November 2010), we looked at checking your RT gear before proceeding with field work--the first of NOAA’s seven C’s covering best methods for RT positioning. In this article, we’ll cover the second C: Communication between the base reference station--from a real-time network server or user setup--and rover.
All high-precision RT work is differentially based at this time. That means your rover will obtain its position as a differential Cartesian X, Y, Z vector from the coordinates at a base position referenced on the Earth-centered, Earth-fixed (ECEF) WGS 84 datum of the GPS system (and transformed PZ 90.02 Russian Federation datum used for the Glonass [GLN] constellation). From this ECEF X, Y, Z coordinate, we may derive local coordinates such as NAD 83 latitude, longitude and ellipsoid height using established transformation parameters and ultimately provide local projected plane coordinates, such as in our State Plane Coordinate System or in a user-adopted low distortion projection (LDP). Such transformations may be done in your GNSS unit or data collector by selecting the desired output or back at your office using various software packages. A robust communication link is needed to acquire the corrections to the observables at the base (as in single-base methodology) or interpolated to a position near the rover (as in RTN solutions).
There are several methods to choose from when considering rover communication. UHF radios (typically in the 450-470 MHz frequency band) or, less commonly, VHF radios (30-174 MHz frequency band) are generally used for a user-setup base station or to extend RTN corrections across a project site, which may be done for machine guidance. The Federal Communications Commission (FCC) requires frequencies to be allocated by fee-based (private) or free (public safety) registration. There are commercial services that will complete the requisite FCC paperwork for you−for a fee. Some rovers will come configured with a few frequencies, but you must still register these with the FCC. Base radios usually operate at 25-35 watts, and rover radios are usually low wattage (around 2 watts). A communication range of 10 km is good (and could be up to 25 km in theory), but UHF range can be limited in urban areas where interference and frequency usage are high. Some rover internal radios are broadcast capable (perhaps 0.5 watt power) for a few kilometers.
The user should remember that with radio communication, voice always takes precedence over data; therefore, your broadcast radio will cease its data transmission until the voice traffic is no longer heard. Obtaining a handful of frequencies rather than one or two will help mitigate this problem. For example, any voice transmission by a trucker on the interstate very close to your rover’s frequency or on a harmonic can walk over your data and knock out your communication. For this reason, it’s worthwhile to have a scanner at the base to find a clear frequency before you venture several miles away to collect data. But as Murphy’s Law has it, it’s possible that as soon as you depart the base location your frequency will be walked over, and you’ll experience an intermittent radio link. Remember one of the four cardinal rules (discussed in the previous column): Always have a strong, continuous communication link to the base during a point location.
Low wattage spread spectrum radios are robust but have a limited range of communication. They do not require FCC licensure since they typically transmit around 0.5-2 watts of power (and, by definition, may frequency hop or cross a wider range of the frequency spectrum than the actual data transmitted), but should be thought to provide only line-of-sight range. Many are now installed internally in the RT unit.
Wireless data modems (such as cell phones) are typically CDMA, GSM and GPRS (see the list of acronyms below) communication formats using TCP/IP over cellular provider networks. SIM cards can be used inside a data collector or cell phone, or an external data modem can be set up for connections. A monthly fee is charged and varies by data usage and cellular carrier. This format is used for extended range single-base RT and especially for RTN communication. It is recommended the user obtain a static IP address rather than a dynamic IP address solution from the carrier. If communication formats or “rules” are in place at a base or RTN server, the consistent static address will remain “in the loop” and allow the data packets to find the rover rather than changing with every reboot or break in communication. Additionally, no one else will be using your static rover address, and it will remain robust and exclusively available, assuming internet connectivity. In good cell coverage areas, this is the most bang for the buck, because it allows ranges of more than 25 miles from a base.
Two points to remember with this range for single-base solutions are that you must maintain a “fixed” solution rather than a float solution for survey grade accuracy (which with distance becomes increasingly difficult to statistically resolve for the rover), and there is a 1 part per million (ppm) component in the position error budget at the 68 percent confidence level (1 ï¤) correlated to distance from the base. There are many areas of the United States where cell coverage is limited or unavailable. This would then eliminate the use of RTN and data modems in general for extended single-base ranges, pushing the user back to setting up UHF radios to complete RT work from a known and trusted base point.
To ensure robust communication, keep the following considerations in mind.
For UHF radio communication, always keep the base radio battery fully charged. Reduced battery power means reduced communication range. Many RT users set up their base stations with a lawn tractor battery because it provides extended power time for the radio while being more portable than a car or truck lead acid battery and at a reduced cost. The best power on the block would be a deep cell marine battery−great power for a long time and fully rechargeable (but very heavy). As stated in the first column, remember to raise the base radio antenna as high as possible (but below 25 feet if not using a low-loss cable).
For the best communication without hitches and bugs−and cursing−keep the firmware updated. This is mainly essential for rovers in an RTN where duplex communication with an RTN server is necessary.
For single-base RT, don’t leave the base site before you have a robust communication link established. However, don’t get too close to the base when initializing and starting the radio communication. Keep the rover about 30 meters away, if possible. Furthermore, having the rover too close to the base when initializing can be a problem when using Bluetooth between the data collector and the base receiver or rover receiver. Make sure the Bluetooth is connected to the correct receiver.
Remember to keep robust communication while locating a point. Don’t use intermittent communication conditions to establish “important” points. Important points can be thought of as any point that will be used to establish further data. These could include project densification monumentation, photo control points, passive monumentation located to constrain a project and critical design features.
As many of you know, the configuration and startup of communication for RT can be challenging and was not addressed here due to the unique requirements of each manufacturer. Regardless of the general nature of this information, I hope to hear from some of you about your experiences as we move through the series of articles. In the next “C,” for “conditions,” we will look at multipath, space weather, tropospheric conditions, satellite availability, using Glonass and other topics that make us better RT data providers.
Communication AcronymsCDMA: Code Division Multiple Access
GSM: Global System for Mobile Communication
GPRS: General Packet Radio Service
IP: Internet Protocol
SIM: Subscriber Identity Module
TCP: Transmission Control Protocol
UHF: Ultra High Frequency (FCC 300 MHz to 3 GHz)
VHF: Very High Frequency (FCC 30 MHZ to 300 MHz)