Keywords : posidonia

POSIDONIA

POSIDONIA: ULTRA SHORT BASELINE POSITIONING SYSTEM

POSIDONIA is a system dedicated to the positioning of manned, teleoperated or autonomous underwater vehicles down to 6,000 m depths.

Fields and conditions for use

  • Fields of operation
    • long profile follow-up
    • rough seabed
    • noisy environment (support ship)
  • Conditions
    • survey mode (vehicle beacon on responder or transponder mode)
    • sea state £ 5
    • sound velocity profile measured and known (using XBT 700 Sippican probes and Levitus data)
    • vehicle operating in a maximum conical angle of ±45° under the support ship
    • maximum water depth= 6 000 m

Performance

DGPS is highly recommended for vessel surface positioning. When it is performed without DGPS, the greatest error on positioning comes from the GPS.

At ± 1s, 0,5% of the slant range is the navigation dispersion value given by the manufacturer (68 % of calculated points are inside a 1s-radius circle).

At ± 2s, the operational navigation dispersion is 1% of the slant range (95% of calculated points are inside a 2s-radius circle).

Operational limits

  • Conical working area under the support ship
    • nominal: ± 30° (above, dispersion increases)
    • maximal: ± 45°
  • Signal to noise ratio (acoustic signal power received/noise power from the ship received by the array):
    • 30 < S/B < 40 dB for a normal dispersion
    • 20 < S/B < 30 dB for a degraded dispersion (coefficient from 2 to 4)
    • loss of points when S/B £ 20 dB

Features

POSIDONIA is made up with 2 sub-systems:

  • Beacon sub-system
    • 4x1 RTT vehicle/mobile beacons
    • 4x1 RTT fixed point beacons
    • 4x1 ET/RT mini USBL beacons
  • Ultra short baseline sub-system
    • flush or deployable acoustic array
    • electronic cabinet
    • operator workstation (including IHM)
    • acoustic array and electronic cabinet's connecting cable

Based on measures performed by USBL combined with chirp signals (linear frequency modulation), POSIDONIA makes possible the positioning of underwater vehicles to a maximum depth of 6,000 m.

Working frequency

  • From 8kHz to 14kHz for interrogations with monochromatic signals
  • From 14,5kHz to 17,5kHz for reception with modulated signals

Constraints

  • Define calibration trajectories:
    • after the array deposit
    • if the navigation dispersion seems to degrade
    • if one shipborne equipment is replaced (attitude control system, DGPS).
  • Use a HDMS attitude control system

Partners