SPRINT is an acoustically aided Subsea Inertial Navigation system for subsea vehicles. SPRINT makes optimal use of acoustic aiding data from acoustic USBL and LBL positioning and other sensors such as Doppler Velocity Log (DVL) and pressure sensors. This improves position accuracy, precision, reliability and integrity while reducing operational time and vessel costs. The system extends the operating limits of Ultra- Short BaseLine (USBL) and can dramatically improve the operational efficiency of Long BaseLine (LBL). The new third generation SPRINT unit offers power pass-through to aiding sensors, reducing cabling and interfacing complexity.
SPRINT INS shares the same hardware platform as Lodestar AHRS and is a combined AHRS (Attitude Heading Reference System) and AAINS (Acoustically Aided Inertial Navigation System). Running both the AHRS and INS algorithms concurrently allows inertial navigation to start or restart instantaneously on receipt of a position update as the AHRS seamlessly provides orientation to the INS on start-up. This avoids the lengthy ‘alignment’ period common to other INS systems. Thereafter, the separate AHRS and INS computed orientation are autonomously monitored as an indication of system health.
All AHRS and AAINS computations are performed within the Lodestar, making the system robust and resilient to communications issues. Lodestar maintains a rolling 15 day backup of all real time navigation and raw data on an onboard SD card which can be retrieved if required. Coupled with concurrent logging of all data by the topside software, data integrity is ensured.
ACOUSTICALLY AIDED INS
Inertial navigation is inherently self-contained, robust with very good short term accuracy but can drift over time. Therefore the INS is aided with complementary acoustic positioning to provide long term accuracy and robustness. The additional integrity offered by inertial integration significantly reduces operational delays during periods of challenging subsea acoustic conditions such as aeration and noise. The precision and update rate of the output allows greater subsea vehicle control and is suitable for ROV station keeping.
USBL aiding is vendor independent using a standard, absolute USBL position message. It can be further optimised with integration to a Sonardyne Marksman LUSBL or Ranger 2 USBL system. Alternatively, tightly coupled LBL aiding from Fusion 6G can be used with either full or sparse (2 to 3 beacon) arrays. LBL observations are optimally time stamped and the rich quality statistics provided with Wideband 2 are used to provide optimal aiding.
SPRINT does not need to be physically co-located with the Doppler Velocity Log. Misalignments are calculated in the field using a freely available calibration utility. This approach allows for more flexible mounting configurations to be considered. A ZUPT (zero velocity) aiding feature is available to provide exceptionally precise positioning during static fixes even if external aiding such as DVL has been lost (e.g. loss of bottom lock).
FLEXIBLE AND EASY TO USE
The SPRINT installed on the subsea vehicle is interfaced using a single serial or Ethernet connection through the vehicle’s umbilical. The SPRINT acts as a subsea multiplexer for it’s own data and all subsea aiding data. Each SPRINT unit is supplied with software to allow easy configuration, monitoring, communications routing and logging. SPRINT can also be provided with a communications hub providing multiple serial/Ethernet ports for input and output.
Accuracy (Secant Latitude)
0.2º (Lodestar AHRS)
<0.1º (SPRINT INS)
<15 minutes in dynamic conditions (Lodestar AHRS)
Instantaneous (SPRINT INS)
Follow Up Speed
900º / second**
Roll and Pitch
0.1º (Lodestar AHRS)
<0.05 º (SPRINT INS)
SPRINT Aided INS
USBL, Depth, DVL, Zero Velocity, Manual Position
2 times improvement in precision over ‘raw’ USBL
USBL and DVL Aided
3-6 times improvement in precision over ‘raw’ USBL