Lodestar Hardware Platform
The core of the system is the Lodestar platform. Lodestar was released to the market in 2007 as a premium quality, survey grade Attitude and Heading Reference System (AHRS) for surface and subsea applications. Using the same hardware platform, Lodestar is now configured for tightly integrated, acoustically aided INS operations for DP applications.
Lodestar makes use of three ring laser gyroscopes that measure the angular rate and three accelerometers that measure the specific force of a moving platform. The highest quality, field proven sensors have been selected for use due to their performance, high mean time between failure (MTBF) and ease of export (non ITAR). These sensors have highly stable error characteristics and are compensated for temperature variation, making them ideally suited to DP applications.
The INS sensor outputs are combined mathematically to compute the position, velocity and attitude of the vessel. The output is extremely low noise and very accurate in the short term but slowly degrades over time. Therefore it is necessary to seamlessly aid the INS with complimentary acoustic positioning observations.
Acoustic Aiding Input
Marksman DP-INS uses a tightly coupled integration of range and bearing measurements from at least three transponders to aid the INS and control integration drift. Tight coupling makes optimum use of Sonardyne Wideband acoustics to exploit and enhance the positive characteristics of the inertial sensors. This adds robustness, particularly in marginal acoustic conditions, as a full acoustic observation set is not continuously needed in order to aid the inertial navigation unit.
Sonardyne’s latest Sixth Generation (6G®) vessel-based transceivers and subsea transponders maximise the benefits of the system by providing the most precise and reliable acoustic aiding input. Whilst only one transponder is needed to aid navigation, two or three are typically deployed to optimise the robustness for drilling applications. Before the system is used operationally, the vessel’s GNSS receiver is used to calibrate the real-world position of the transponders. Once this is complete, the system no longer requires GNSS as an input. Previous generation Sonardyne transceivers and transponders are also compatible with DP-INS.
The Lodestar INS and acoustic transceiver are interfaced to, and powered from, the Navigation Sensor Hub (NSH), which in turn is connected to a dedicated Navigation Computer running the Marksman software. DP-INS is fully integrated into the Marksman system so an additional workstation console is not needed.
The Navigation Computer is a powerful, purpose-built unit that has been specifically designed to run Sonardyne’s family of acoustic and inertial navigation software applications. The operator interface can be fully integrated within DP consoles from all the leading manufacturers. Alternatively, a separate rack-mountable monitor and keyboard can be supplied. Dual-screen and touch-screen options are also available.
The vessel’s Marksman software displays both the vessel’s LUSBL position and the INS position in an intuitive format alongside essential status information. The display gives the system operator access to all the information required to monitor DP-INS performance without significantly increasing their workload.
The Lodestar and USBL transceiver are connected through to the bridge where they interface to a Navigation Sensor Hub (NSH). The NSH provides power and serial communications via a junction box at the deployment pole or machine. For the Lodestar, 1x CAT6 or better is needed for INS communications and 1x copper pair used for power. For the LUSBL transceiver, a second CAT6 is used for acoustic transceiver communications and a second copper pair used for power.
Connections to external sensors such as ships gyros, VRUs and GNSS are all on the bridge making installation easy with no routing of sensor data over long cable runs through the vessel.
Transceiver and Lodestar Deployment
In order to achieve optimum system performance, it is important that the inertial and acoustic systems experience the same dynamic motion. Therefore the Lodestar INS and vessel’s acoustic transceiver should, wherever possible, be co-located on the same deployment pole using either a through-hull or through-tube arrangement.
Sonardyne’s design of through-hull deployment machine has a rigid, one piece Inconel clad steel pole that hydraulically lowers and raises the transceiver through a gate valve. A sea chest with inspection hatch allows for ease of installation and cleaning of the transceiver.
To allow the Marksman transceiver and INS sensor to be fitted to older vessels, Sonardyne also offers a through-tube method of deployment. As through-tube poles are less rigid than through-hull deployment poles, it is recommended that GyroUSBL is used, a unit which integrates a HPT acoustic transceiver and Lodestar INS in a single mechanical housing.
GyroUSBL offers further operational savings as the unit can be pre-calibrated and delivered to the vessel ready to use. Unlike conventional acoustic transceivers, GyroUSBL doesn’t need re-calibrating if it is removed and refitted for maintenance or even transferred to another vessel.