Surveying Data Links
Types of TechnologiesBecause data links tend to be grouped with cables, batteries and prisms, buyers often do not recognize the data link technology choices available to them. Today, there are several data link choices. Understanding the relationship between acquisition cost, operating cost and various specifications (such as range, necessity for line of sight, primary use of the electromagnetic spectrum and others) can help the user of these products optimize the system configuration for the needs of his business.
So what are the choices? For RTK GPS, the first and still most common communication technology is UHF (Ultra-High Frequency). To comply with special requirements, a sibling to this technology, VHF (Very-High Frequency), is also used. Other choices today include technology in the 900 MHz band and 2.4 GHz band, and finally, various forms of combining radio modems with cellular phone technology.
Data link choices offered with total stations are UHF, 900 MHz, 2.4 GHz, and in the case of one manufacturer, infrared technology. Generally, for robotic total stations, the manufacturer selects the data link technology; manufacturers may offer more than one technology option based on the model of total station.
The DetailsTo a specialist in radio technology, UHF refers to the band between 300 MHz and 3 GHz. However, in the jargon used by surveyors, "UHF" refers only to "narrow band" technology operating in the area from 450 to 470 MHz. "Narrow band" refers to the fact that the specific channel width used for this technology is limited-as narrow as 12.5 KHz. UHF is applied in RTK, but is also used with total stations with much lower broadcasting power.
According to Rick Gosalvez, product manager for Pacific Crest Corporation, a Trimble company in Santa Clara, Calif., UHF is a "low data rate, long range" technology. "From the onset of RTK, it has proven to be a reliable way of transmitting corrections from a base station to one or more rovers, particularly in areas with difficult foliage and terrain. UHF has proven to be the workhorse for RTK GPS. Wireless coverage between a base and rover is a major concern for surveyors. An inverse relationship exists between the data rate and range. The higher the data rate is, the less energy per bit, and the less robust the signal. UHF data rate capabilities typically meet, or exceed, RTK requirements ensuring reliable performance over long range." However, the operation of UHF links does not come without its problems. In the United States, licenses are required to operate UHF radios. The process of licensing proves complicated and bureaucratic to many surveyors, and is often a stumbling block.
The Federal Communications Commission (FCC), which issues the licenses, applies restrictions to users of the frequencies, including radio etiquette. This means listening first to make sure that a channel is not already in use before pressing the transmit key. This is important since a license does not grant a user exclusive right to the use of a channel or series of channels. Since GPS links broadcast data automatically, there is no human listening before the broadcast is sent. Additionally, RTK GPS is intolerant of delays in the transmission of the corrections-accuracy degrades. One way to make sure the system complies with this FCC rule is by having the ability for the system to "listen" before actually sending a correction. While this works in complying with the rules, it may prevent any correction from being sent out because the channel is busy. Therefore, this requires some planning, patience and monitoring of the channel in high radio traffic areas. Also, voice has precedence over data, for the most part. Thus, a data user has to give way to a voice user wishing to use the channel.*
VHF refers to the band between 30 MHz and 300 MHz. For surveyors it is restricted to the 150 to 174 MHz range, but it is not a widely used technology, in part because of licensing issues and the bulky antennas it requires. Marine and geophysical users make the most use of this technology. Its characteristics are similar to the UHF technology, although range is usually longer. Application is for RTK GPS.
900 MHz technology is often referred to as "spread spectrum, frequency hopping." It is distinct from the narrow band technologies used by surveyors (UHF and VHF). This technology allows a radio link to have a wide band (thus spread spectrum) over which a single connection operates. Within this band, a series of discrete, narrower channels exist. When interference is detected on a particular channel, the "frequency hopping" character of this technology is invoked to rapidly switch to another channel. This hopping can occur with extreme rapidity if required. Thus, it (and the very similar 2.4 GHz technology) is suited to applications where the environment is noisy, but the range expectations are modest. This technology has a distinct benefit of requiring no license to operate in the United States. However, it is highly restricted as to broadcast power, offers less range than UHF or VHF technology, and is often most effective in line of sight, or near line of sight operations. This technology is being used with RTK technology data links as well as total stations. It is common for this technology to be used for communication from a base station to RTK GPS equipment on construction machines as well.
Similarly, 2.4 GHz technology is spread spectrum and frequency hopping technology. It possesses many of the characteristics of 900 MHz technology. Manufacturers often use this option because it may be more widely distributed around the world, as many governments permit license-free operation. Because of its higher frequency, 2.4 GHz technology is a line of sight technology. Application of this technology is similar to 900 MHz.
Cellular phone technology is probably the most complicated to describe, as the term is a "catch-all" for various forms of cell phone technology in use in the United States today. Various acronyms exist in this realm including CDMA, CDPD, GSM and GPRS (see our online glossary for more on these). These are all cellular network provider-specific technologies used to divide the spectrum into channels, transmit digital information, sometimes in packets, and to establish IP (Internet Protocol) addresses. When using digital cellular phone technology, the manufacturers of data link devices for surveying instruments can provide users with two options. One is commonly referred to as a digital cellular modem, a device embedded in the GPS receiver consisting of cell phone technology to access the cellular network and the modem. The other common way of using cell phone technology is to have a modem, either internally or externally, on the GPS receiver and then connect via cable or Bluetooth to a cell phone that could also be used for voice communication. Application of cellular phone technology today is primarily for RTK GPS corrections; however, it can also be used for transmitting data files to or from the office, or between two data collection systems in the field. While it can theoretically be used for total station robotic control, it is not very practical to do so today since it involves a lot of "dead air" time.
Another way to use cell phone technology for RTK is to use the phone to dial in to an IP address. At the other end, users have the equivalent of a website that delivers the corrections to the IP address. The rover's cell phone link then extracts the correction data and sends it to the processing system at the rover to use. The source of the corrections for IP-accessed data can come from a single base station, similar to the "conventional" UHF setup. In this case, the cell phone technology at the base station delivers the data directly to the IP address accessed by the rover cell phone. As networked RTK technology becomes more common, it will be possible for users to only possess rovers with cell phone technology, which access a portal that delivers corrections. Usually for a fee that may be on a subscription basis, the RTK network provider computes corrections for the user and provides them as needed.
Also in the jargon of surveying are the concepts of circuit-switched and packet-switched data delivery. In circuit-switched, a single, unique link or channel is set up, even if it is cell phone (which may be internally packet-switched) to deliver data from one point to another. In packet-switched, the physical link is less important and the information, sent in packets, may be sent through a variety of routes, depending on congestion, so long as the delivery of data from the source to the user is comprehensible. This is done by establishing the more abstract IP address as the portal.
Infrared (IR) technology can also carry data between a robotic total station and the controller. In this case, an IR channel is established and it is then modulated in the same way a radio wave is modulated to transfer the data. Infrared requires an unobstructed line of sight to work. It requires relatively low power to operate, and is license-free. Topcon is the only manufacturer that offers this option.
Compatibility between systems is not feasible, though some users have devised ways of "converting" data from one technology to another. Even the mixing of brands within a technology must be carefully approached to ensure that there is a "language" spoken and understood by all the transmitters and receivers (and modems). It is becoming more common today for a business to have more than one technology available for data links, especially with RTK GPS, so that users can switch to another if trouble with one link is encountered.
The FutureWireless data links in surveying systems are expected to grow in the future. "We can expect to see developments in surveying systems similar to those in the WiFi world," says Joe Brabec, chief technical officer of Topcon Positioning Systems (Livermore, Calif). "Wireless communication is going to be an even bigger part of total station and GPS surveying in the future years." Andrew Hurley, area sales manager for the southern region at Leica Geosystems (Norcross, Ga.), expects that: "As wireless technologies become ubiquitous, it will become common to see office warehousing of field data occurring in real-time, making the necessity of an in-field data repository only a temporary one. It is likely that radio technology will make the ability to have updates and upgrades to software being used in the field to occur as needed, instead of having to wait until the equipment is back in the office"¦ perhaps even to occur automatically as we are used to seeing with some of our office software products."
* These are only some of the issues, rules and protocols that license holders are obliged to follow. The rules for using any licensed radio technology (and license-free radio technology, for that matter) is not insignificant. Users will do well to understand and follow them.
Sidebar: Glossary of Cell Phone AcronymsCDMA: short for Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
GPRS: short for General Packet Radio Service, a standard for wireless communications which runs at speeds up to 115 kilobits per second, compared with current GSM (Global System for Mobile Communications) systems' 9.6 kilobits. GPRS, which supports a wide range of bandwidths, is an efficient use of limited bandwidth and is particularly suited for sending and receiving small bursts of data, such as e-mail and Web browsing, as well as large volumes of data.
GSM: short for Global System for Mobile Communications, one of the leading digital cellular systems. GSM uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency. GSM was first introduced in 1991. As of the end of 1997, GSM service was available in more than 100 countries and has become the de facto standard in Europe and Asia.
IP: (pronounced as separate letters) short for Internet Protocol. IP specifies the format of packets, also called datagrams, and the addressing scheme. Most networks combine IP with a higher-level protocol called Transmission Control Protocol (TCP), which establishes a virtual connection between a destination and a source. IP by itself is something like the postal system. It allows you to address a package and drop it in the system, but there's no direct link between you and the recipient. TCP/IP, on the other hand, establishes a connection between two hosts so that they can send messages back and forth for a period of time.
TDMA: short for Time Division Multiple Access, a technology for delivering digital wireless service using time-division multiplexing (TDM). TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. TDMA is used by the GSM digital cellular system.