Woods Hole Science Center operates both single and dual frequency GPS receivers. These receivers can operate as stand-alone or
DGPS (differential corrected systems). DGPS utilize corrections broadcast from U.S. Coast Guard Reference Stations. Accuracies are
on the order of 5 7 meters stand-alone and 2 3 meters differential.
Additionally, dual frequency Real-Time Kinematic (RTK) systems can provide positions to within 2 3 centimeters in real
time or in post-acquisition processing, and are currently utilized in our nearshore geophysical surveys. These systems rely on
setting up a base station over a location that has been surveyed, such as a geodetic benchmark. Corrections taken from the satellite
constellations are then transmitted from the base station to a remote receiver on the survey vessel via a radio modem. The
information sent from the base receiver to the remote location can be used to precisely locate the remote receiver. For our purposes,
precise elevations can be made above an established benchmark, which in turn can give us real-time water level heights within a
survey area. This information is critical for our bathymetric and swath bathymetric data sets (see the section at right on swath
interpretation and software).
|RTK base station established with a survey-grade antenna. Satellite data are received for a few days and logged on a computer. These data are then processed to
establish the corrections for that location. These corrections are then transmitted to the remote receiver, usually aboard one of our research vessels.
This base station is located on St. George's Island to provide corrections for USGS research in Apalachicola Bay, FL, 2005.
||RTK transmitter. The antenna is set-up with a receiver, radio modem, and battery pack contained in the metal box at its base. This transmitter broadcasts the kinematic
correction from the base station to the receiver on the boat. A direct line-of-sight within 10 km, is necessary between the transmitter to the receiver onboard the boat. This
transmitter is located on a bridge to complement a USGS survey in Apalachicola Bay, FL, 2005.
|Another example of a navigation station for USGS field work offshore Massachusetts, 2003-2004. This land station was
set up in Gloucester, MA. The location of the RTK base station (tripod to the right) was positioned in a surveyed site. In this instance, the transmitter, which must be in line of
sight to the offshore survey area, is located nearby (antenna on tripod to the left) .
|Navigation setup on board the R/V Connecticut for USGS field work, 2005. The blue box is the RTK navigation receiver which
incorporates the kinematic corrections sent by the base station via the transmitter antenna. An L1/L2 antenna on board the ship is connected to the receiver unit. Also shown are the
power supplies for the receiver and ancillary gear.