Omnitech Electronics' AHLS (pronounced. ‘alice’) is a directional acoustic array, processing module and acoustic modem (optional) combined in a single navigation sensor. The system is mounted inside an AUV nose cone and provides a long-range acoustic bearing (LRAB) to a ‘homing’ beacon. LRAB is available in two models: DAHLS, for larger Deep-water AUVs and ‘CAHLS’ a Compact model for smaller, shallower depth Ivers2 family of AUVs (or similar).
In addition to providing a bearing to a homing beacon, both models of AHLS feature a user-defined set of communication messages or command states that can be embedded in the homing beacon to the AUV. These decoded command states are sent, from the AHLS to the vehicle control computer with the LRAB bearing and can be used to signal changes to the mission, return and hold messages, emergency surface commands, etc. These messages function over the full range of the AHLS and allow operating vehicles and mission control abilities from distances much greater than traditional acoustic communication devices.
Some typical usage scenarios include:
Under Ice: Vehicle can home to a recovery hole on a moving ice sheet or vessel. Mid-mission changes: If weather, presence of another vessel, or other conditions force the launch vessel to leave the area before the AUV can be recovered, AHLS messages can be used to signal a ‘mission change’ to the AUV. In shallow water, the AUV can rest on the sea bed or in a low power state until the vessel returns and a ‘resume’ or ‘come home’ message is received. The operators can send these controls from tens of kilometers range vs the few kilometers typical with most acoustic communications. Control in Poor Conditions: AHLS messaging and homing signals are designed to operate in very diverse acoustic conditions. AHLS has been used in conditions ranging from shallow harbours where traditional acoustic communications failed to ice-covered arctic waters alongside the ice-breaker CGGS Louis St-Laurent.
When paired with a compatible acoustic modem, AHLS can provide updates and corrections to absolute, geo-referenced vehicle position. As the AUV approaches the beacon, or after reaching deployment depth, the DAHLS can, optionally, use a deployed field of acoustic modems as an ‘underwater positioning system’ to acquire accurate absolute vehicle position while at depth, without the need for an accurate synchronized time reference. This Short Range Localization system (SRL) can be used to correct vehicle position when the vehicle does not have a bottom lock, and to correct navigation system error in long deployments. SRL has been tested to accuracies better than 5 meters of absolute vehicle position or within the error of the surface buoy GPS measurements.
AHLS was designed to assist Canada’s Arctic Explorer AUVs for National Resources Canada’s Project CORNERSTONE. The AUVs are used to gather hydrographic quality bathymetric data of the seabed under the ice in Canada’s Arctic Ocean. For these missions, the AUVs were deployed through holes cut on floating ice sheets. The AUV conducted 500 - 1000km missions, under the ice, and returned to the mission endpoint. However, the ice sheet can drift by tens of kilometers during the mission. The LRAB homing beacon is placed in the water at the new endpoint and LRAB, onboard the vehicle, provides the AUV with a bearing to the source. AHLS has provided bearings at distances greater than 75 km from the beacon in real deployments.
Building on the success of Project Cornerstone’s AHLS, Omnitech Electronics designed a smaller, more compact, version of AHLS for use with smaller, more common, AUVs. The CAHLS is designed to fit inside vehicles with a hull diameter of 147mm (5.8 in) or less, such as the Ivers2 family of AUVs, and has a depth rating of 250 m.
More information on LRAB and Cornerstone can found on the project page and its associated references.