Overview
Surveying is a tough business. There are no excuses. The highest precision and accuracy are required as a matter of course, and because time is money where surveying is involved, clients demand fast results. It is no wonder that GPS Real Time Kinematics (RTK) survey systems, which can measure location on a scale of centimeters in moments, are taking over today's survey marketplace.

Choosing the right RTK surveying system also is a tough business. RTK surveying provides military-like GPS accuracy with ordinary GPS equipment by constantly computing the error between the GPS-determined location of a fixed site with the site's known location and transmitting these real-time correction factors to nearby mobile GPS receivers, thereby increasing their accuracy. The key element in any RTK system is a data communications link–most commonly some type of short-range radio–modem system–that reliably transmits the correction factor data throughout the site of a survey. Surveyors need a data link that is highly reliable, one that works the first time and every time, and one with sufficient range to cover a wide variety of applications. And because surveying is a business like any other, cost is an object.

This buyers guide provides you with what you need to know to select the best data link for your RTK survey business. A typical data link system includes radio-modem transmitters and receivers, antennas, cables, and batteries to power the system in the field. A variety of fully integrated radio solutions are available that provide cost-effective service at various levels of system performance.

See Comparison Chart by Product:

		PDLHPB Comparison
		PDLLPB Comparison
		PDLRVR & PDLRXO Comparison

Key Factors
Paying attention to a few simple factors and selecting a data link system that suits your surveying needs and business strategy enables you to improve the performance, enhance the cost-effectiveness, and increase the user-satisfaction of your RTK survey system.

Consider the following technical parameters when selecting a radio modem for your RTK applications:

• RF output power
• Receiver sensitivity
• Antenna gain and siting
• Feed-line losses
  

RF Output Power
The first radio system specification to understand is the specification for RF output power. A radio-modem data link functions just like a miniature version of a large commercial radio or TV station, and the RF output power indicates the transmitter's basic ability to propagate the signal to further distances.

Depending on the region or country, the RF output power is limited by regulations. In the U.S., the limit for RF power in survey itinerant operations is 35 Watts. In Europe, the regulations vary from country to country.

In general, the higher the RF output power, the better. The cost of higher RF output power is higher overall system power consumption. Larger batteries are required for operation, batteries may require more frequent recharging and/or replacement, and the integration of high and medium power radio transmitters may be more difficult.

Rule of thumb:  Go with the highest power RF output available.

Receiver Sensitivity
The next fundamental system specification to understand is for the radio receiver sensitivity. The more sensitive the receiver, the better its ability to receive signals from distant transmitters.   Receiver sensitivity is derived from the core design of the radio receiver. The true art in radio receiver design is to select and implement a design that enhances the ability of the receiver to resolve very weak signals.

In addition to the sensitivity provided by its core design, the ability of a receiver to receive weak signals can be enhanced by other means. The most cost-effective approach is forward error correction (FEC). This is an advanced technology that improves receiver sensitivity by adding redundant information to the data signal in a manner that allows a receiver to detect and correct errors that occur from weak signals. Radio receivers designed for high sensitivity always include FEC.

In general, the higher the receiver sensitivity, the better. The cost of increased sensitivity comes from the higher quality components that are required, as well as the greater care required in tuning the receiver and verifying sensitivity performance.

Rule of thumb:  Go with the highest receiver sensitivity available that uses FEC technology.

Antenna Gain and Siting
Antenna gain refers to the focusing of the RF energy (either the signal emitting from a transmitter or being picked up by receiver) and is generally represented in terms of dB with respect to either a theoretical isotropic antenna (dBi), or a dipole antenna (dBd). Antennas measured with dBi gain rate are commonly utilized for general-purpose portable systems or mobile whip antennas, and those showing a dBd gain rate are commonly used for fixed base stations and higher-quality installations.

In addition, antennas are divided in two categories: directional and omni-directional. Each type of antenna offers a different radiation pattern. Directional antennas (yagi) have an increased vertical radiation pattern with a decreased horizontal radiation pattern. High-gain omni-directional antennas have an increased horizontal pattern with a decreased vertical pattern.   Directional yagi antennas allow the use of relatively low power radio transmitter to send data over long distances; they are ideal for point-to-point fixed location applications. Omni-directional antennas are recommended when the signal transmission between the transmitter and the receiver is constantly changing. These types of antennas are best for mobile point-to-point or point-to-multipoint communication systems.

Complementary to antenna gain is antenna siting. It is important to position any antenna to optimize line-of-sight within a communications system. Always place the antenna on the highest point available. At a minimum, set the antenna to at least ten feet above the terrain using an antenna mast.

Rule of Thumb: Select the type of antenna that best fits your application and the one that offers the highest dB gain. In addition, set up your system in the highest possible location to minimize obstacles between the transmitting and receiving systems.

Feed-Line Losses
Also important to consider for system specifications is feed-line loss. Decrease of the signal starts from the moment the signal leaves the transmitter. Most fixed radio frequency data communication systems have the radio equipment connected to the antenna through a coaxial cable. This cable can be a source of signal loss (about 1 dB per connection in addition to cable attenuation) and should be optimized for best system performance.

The feed-line cable is the source of at least two types of feed-line losses. One is when the connection of a coaxial cable to the antenna port of the radio acts as an impedance boundary reflecting the RF energy back to the transmitter. It is important to use coaxial cable and connectors that can minimize or eliminate the impedance mismatch. The second type of feed-line loss is the attenuation of the signal as it propagates along the length of the cable. Attenuation of the signal can be caused by the leakage of RF though imperfect shielding of the cable as well as resistance in the cable conductors.

Rule of Thumb: If direct connection of the antenna to the radio modem is impossible, use coaxial cable and connectors that are impedance-matched with the radio equipment, and make sure to use the shortest length of cable.

Note: All these considerations apply to all radio system setups, including the receiver.

For additional details on antenna height specifications and performance in different types of terrain, please read The Guide to Wireless GPS Data Links.

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