USACE Field Research Facility
Field Research Facility
Coastal & Hydraulics Laboratory

LARC Survey | Vehicle | Equipment | Analysis and Software | Photos

The LARC Survey Equipment
The survey hardware consists of the Echosounder, GPS position equipment, a Conductivity, Temperature, Density (CTD) gauge, and computers used for data acquisition and navigation.

The Knudsen 320BP Echosounder

Because accurate depth information is needed as soon as the LARC enters the water and the tires are no longer in contact with the ground, the 320BP Echosounder Transducer has a close proximity option that is capable of measuring depths as shallow as 10 cm. However, the transducer, or fathometer, is mounted 50 cm from the ground on the LARC, which is as close a reading as is required for the surveying done using the LARC system. The echosounder accuracy is determined as 3 percent of the selectable depth range.

The echosounder can be set to survey with ranges from 10 meters to 5000 meters. For much of the coastal surveying which is done using the LARC system, the range is set at 20 meters. The readings taken from the echosounder are sampled at a rate of 10 Hz, or 10 readings per second.

The dimensions of the echosounder are as follows: L 470 x W 371 x D 190 mm and the weight is 7 kg. The data collected can be measured in meters, feet, or fathoms.

The GPS antenna and the echosounder transducer are centered above and below one another to ensure accurate readings.
CTD Profile

Since the echosounder transducer uses sound pulses to map the ocean bottom, the accuracy of the echosounder also depends on the speed of sound in water, which varies with water density. To monitor water density profiles, a conductivity, temperature, and density (CTD) profile is measured frequently during surveying. The Ocean Sensor's CTD gauge used at the FRF is capable of collecting information about salinity and speed of sound from the water surface down to the ocean floor using the CTD profile. At remote locations a self-recording CTD is used. CTD data is collected at many locations throughout the survey area, or a constant speed of sound value is used for analysis. When the CTD profile data are available it is incorporated into the post-data collection software analysis and correction process.

Real-Time Kinematics (RTK)-GPS

The Trimble 4000 GPS System utilizes a base station setup in the operational area at a known reference point on land. The fixed latitude, longitude, and elevation are fed into the base station's memory. The base station then computes its location using signals from 5-9 satellites. By comparing this reading to the known reading, the base station can calculate a correction which can then be sent to the rover station onboard the LARC.

The equipment on the LARC uses a similar setup to that of the base station. A GPS antenna is mounted on the LARC, but is not given a fixed position, since the LARC acts as the rover station. Instead, the LARC GPS system computes its location from available transmitting GPS satellites. After it determines its location, the correction from the base station is received and applied to that computed position for a more accurate positioning. An accuracy of +/- 2cm in all three dimensions is thus achieved for the roving LARC using RTK GPS.

Data Acquisition System

A lap-top PC is used for data acquisition and navigation. The cross-section, or track lines, are pre-prepared and are displayed on an auxiliary screen near the control station for the LARC pilot to follow. Data for the echosounder are acquired at 10 Hz, while data for the RTK-GPS are acquired at the maximum available update rate of 1 Hz. See software and analysis section for collection and analysis information.

The LARC survey system also incorporates Coastal Oceanographics HYPACK Hydrographic survey software into its data collection and analysis. This software acts as an interface between the GPS data and the navigator in order to ensure that the LARC follows the pre-determined survey path. HYPACK software merges GPS and Echosounder data and creates the files which are then analyzed and edited by the FRF's software.