This fall, Sokkia Corporation introduced the SRX, a fully robotic total station with several market-leading features. On Sept. 19, 2006, President Hitoshi Ito of Sokkia Co. Ltd. unveiled the instrument at the North American headquarters in Olathe, Kan., in a room full of Sokkia distributors and other guests. Complete with a new color scheme and styling treatment, this flagship of the Sokkia total station line possesses many new and interesting features--some of them unique to surveying products.

The Windows CE-powered total station boasts a bright, crisp display with clever color designs for easier reading and bold characters for showing principal data lines, such as horizontal and vertical angles and slope distance. The instrument's overall profile appears more compact than previous Sokkia models; it is about 5 mm shorter than the 230RM model. Also notable in its profile are the Bluetooth 1 antenna and a window on the detachable handle that opens to reveal the receiving optics when the instrument is in search mode for the prism. The instrument is available with angular accuracies (DIN 18723) of 1", 2", 3" and 5"; product designations will be SRX1, SRX2, etc.

For this profile, I was able to interview Sokkia marketing personnel as well design engineers from the factory who were directly involved in the development of the SRX. Some of the details that follow are unlikely to be found in product brochures.


The SRX boasts improved Sokkia EDM reflectorless technology--called Red-Tech EX--featuring a coherent red laser EDM that uses phase modulation to generate accurate distances. Resolution is potentially increased by using a fine measuring wavelength of only 0.8 m, corresponding to a modulation frequency of 185 MHz. The instrument uses a total of five measuring frequencies. The optics of the instrument for projecting the laser (which also serves as the pointing laser when desired) keeps the laser spot at a smaller dimension at longer distances (~500 m) without making it too small for short-range measurements. When switched to prism mode, a diverging lens is added that broadens the EDM beam to +/-3.5 arc minutes to enable easier finding of the prism at all distances. New for Sokkia is a feature that monitors the random errors in the EDM measurement, and increases the number of repetitions until the desired standard deviation is reached. Under difficult measuring conditions, this means that measurements can take longer, but the user is assured of their precision.

The instrument is specified as having a range of 500 m, with a minimum distance of 0.3 m when measured reflectorless to the white side of a standard Kodak Gray Card. Single prism range is 6,000 m. EDM accuracy is specified at +/-(3 mm + 2 ppm) without prism, and +/-(2 mm + 2 ppm) with prism, except for the SRX1 model, which has a distance accuracy of +/-(1.5 mm + 2 ppm).


The angle measurement system relies on Sokkia's tried and true absolute encoder system, but with a new twist to the design. Sokkia's engineers developed a patent-pending Independent Angle Calibration System (IACS) for the SRX. By virtue of having known angles that are fixed in the encoder system, the instrument can self-calibrate its encoders and apply corrections to the observed value after a complete observation of all its graduations over 360oon the horizontal and vertical circles. Sokkia claims to be able to reduce angular errors using this methodology to below 1 arc second; in fact, the smallest least count available with the SRX1 and SRX2 models is 0.5". Because this self-calibration system does not rely on external references, Sokkia claims that the instrument can be used as a standard to calibrate other instruments.

Robotic Functions

In the auto tracking department, Sokkia has added some nice touches. The tracking system covers a broader area than what is covered by looking through the telescope. So the system will acquire the target even when the prism is near the telescope field of view but not within it. The auto pointing (or target centering) function uses a high speed area CCD (charge coupled device) running at 200 frames a second to recognize and calculate the relationship of the target's position to the cross hairs. Then, depending on the user's preference, the system will either (1) calculate the angular offset to the target and record the current angle values after adjusting for those offsets as the measurement data, or (2) pass the information to the servo motors to move the cross hairs exactly to the point before recording the angular values. Sokkia's engineers claim that both methods produce equivalent results. The former method is obviously faster, but many surveyors prefer to see the cross hairs on the target before they are satisfied that the measurement is correct.

Sokkia's technology for the tracking function is patent-pending as well. Taking advantage of the CCD's fast imaging, the system compares the terrain with the target present in the foreground to quickly center on it. This is combined with sophisticated predictive software to find "lost" prisms. Incidentally, the prism is the ATP1 360oprism (also patented), which appears to look like competitors' prisms, but is significantly different. First, it has a -7 mm offset. Some may see this as a negative, but it is necessary due to the physics involved in reducing angular errors (both horizontally and vertically) when sighting to it that are inherent in other 360oprism systems. Hopefully, more technical information on this will become available as Sokkia gears up for worldwide distribution of the product.

Communication between the total station and data collector to "drive" the total station uses Bluetooth 1, an implementation of Bluetooth technology that has not previously been offered in total stations. It allows communication per the Bluetooth standard up to 100 m. According to Sokkia's engineers, this implementation allows ranges to several hundred meters.

To use the SRX in addition to the ATP1 prism on the pole, a data collector is offered with Bluetooth 2 (10 m range) or a cable to connect to the RC-PR3, which is also on the pole. The RC-PR3 has a Bluetooth 1 transceiver, a Bluetooth 2 transceiver, antenna, batteries and an infrared fan beam transmitter. When operational situations require directing the total station to find the prism, the "on-demand" system turns on the fan beam while simultaneously informing the total station to search for the fan beam. The receiver mounted in the handle of the total station then detects and provides feedback to the servo motors to turn the instrument in the appropriate direction until the regular auto tracking technology comes into play. Time to target is minimized by the incorporation of an electronic compass and clinometer inside the RC-PR3, providing angular information so the instrument always rotates in the shortest direction towards the target.

Additional Features

Although this product profile is not intended to be a comprehensive description of the SRX, the following is a list of additional features that are found on the instrument.

The SRX is environmentally protected to an IP64 standard. Its variable speed tangent screws are electronic with mechanical resistance to feel more like mechanical tangent screws. Additionally, the total station features 64 Mb of onboard memory with more than 20 Mb available for data; serial and USB (slave and host) ports; a one-quarter VGA backlit touchscreen display; 32 alphanumeric backlit keys that can be dimmed; Compact Flash card support for up to 1 Gb; and a green/red guidelight for stakeout work with variable flash speeds so that it can be used even if the rod person is colorblind.

Two batteries are furnished standard with the instrument, enabling two to four hours of operating time per battery. When the SRX is in "native mode," workflow for functions such as basic measurement, stakeout, and station and backsight setup can be viewed as a flow chart, and the step that the user wants to access can be selected simply by touching the appropriate box in the chart.