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Frequency management is obsolete. Here’s why.
12 May 2022
In the Technical Services Team here at Sonardyne, we are often asked to assist project managers with a frequency management plan for their field-wide acoustic positioning solution. Often these are made up of a mix of LBL, Sparse LBL and USBL systems from a mix of vendors. They will be used for tracking multiple targets, such as manned vessels for dynamic positioning or towfish work, uncrewed surface vessels (USVs) requiring position aiding, or ROVs and AUV/UUVs, for general positioning, and structures or pipelines requiring tracking during installation.
These days, with digital systems, the term frequency management is now obsolete. Digital systems do not operate on a single frequency and the detection of a signal is now based on the recognition of a code. Different manufacturers of acoustic systems use different techniques to encrypt the codes on to a base carrier frequency, preventing cross talk between systems from different vendors.
While there is no chance of detecting signal from a different vendor’s system, any signal that is not the one you are looking for, is effectively noise. More noise means less SNR, the fundamental for signal detection. This can be mitigated by using appropriate transmission powers and update rating settings for the operation at hand.
There are still some rules to set
While frequency management is now no longer required in the traditional sense, there are still some rules that need to be set for vessels and transponders in a field. Each transponder, whether it be a mini-transponder or a Compatt, must have a unique acoustic address. This will ensure that any system using the transponders will be able to tell which one it is talking to by the Individual Interrogation Signal (IIS) it uses to speak to the transponder, or the Individual Reply Signal (IRS) it receives back when using a Common Interrogation Signal (CIS).
Wideband 2 and 3 use the same base code structure, so should be considered the same addresses for this purpose.
When using a CIS to interrogate transponders, it is necessary to make sure any vessel in the area uses a unique CIS. Problems occur when multiple vessels use the same CIS, as the transponders will be triggered to reply whenever they hear their CIS, which could have been sent from either system. This means that when a system receives an IRS, it will not be able to distinguish when that reply was sent and therefore not be able to reliably compute a range.
If a vessel has multiple independent systems on board, such as two ROVs using LBL, it is likely that they will be using the same reference Compatts to compute their positions. As a Compatt could traditionally only be programmed to respond to one CIS, systems would need to be time synchronised to prevent both trying to interrogate the references at the same time, causing something like the CIS confusion mentioned above.
With the increase in the processing power inside Compatts, it is now possible to program a Compatt to respond to four CISs, on top of its standard CIS, effectively making it five Compatts in one. This is known as “multiuser”. It allows up to five independent systems to use the same references at the same time and keep track of what was sent when, maintaining proper operation.
So, to summarise, make sure all Sonardyne transponders and Compatts have an address that is unique in that field. Any vessels using a CIS must use a CIS that is unique in the area. If you want to share reference Compatts between different independent systems at the same time, use multiuser.
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