Sonardyne Wideband^® Technology

Introduction

Over the last 10 years satellite navigation systems like GPS have transformed positioning and surveying. Within a few short years Differential GPS has consigned the radio based systems formerly used by the offshore oil industry for precise surface positioning at sea to history. The offshore survey and construction market has matured and grown over the last 20 years and the future focus of operations in deeper waters is prompting renewed interest in the further development of underwater positioning systems.

A key design requirement for GPS was that transmitted signals were ‘covert’ in so much that they could only be detected by users equipped with dedicated receivers. This is achieved in the GPS system by using signals that are modulated with binary codes. GPS receivers contain replicas of the codes transmitted by individual satellites which are compared with the received satellite signals. These are converted from analogue to digital format and compared with the replica using a correlation function. This modulation technique is also used to transfer a considerable volume of additional information to the user such as satellite status and orbit predictions.

The processing power required to perform signal processing and code correlation in these early receivers meant that the units were bulky and power-hungry. Recent advances in high-speed digital computer technology and the increasing application of digital signal processing in mass applications like mobile telephony have driven the development of small, low power consumption Digital Signal Processors (DSP).

Tone Burst vs. Wideband

The majority of acoustic positioning systems used for subsea positioning over the last 30 years were developed using analogue electrical circuits and simple pulsed narrowband continuous wave signals. These ‘tone burst’ systems are fundamentally limited in terms of the number of unique operating channels. They are also subject to ‘fade’ and lack resolution in reverberant environments where multiple signals may arrive at a receiver within a short time interval. In a tone burst system optimal ranging precision requires the use of very high frequencies and short pulses which are subject to greater attenuation, lack power and hence have a limited operating range. The simplicity of the signal architecture limits range resolution as this is determined by the transmission bandwidth. Range resolution is thus effectively noise limited as the signals lack bandwidth and power. 

Timing resolution using Tone Burst and Wideband signals

(Above) Tone Burst Signal: Coarse resolution of filter response to uncoded signal is more susceptible to degradation by noise
(Above Right) Wideband Signal: Code correlation function results in far greater precision and resistance to noise

In a wideband system the signal spectrum is spread such that the frequency bandwidth is greater than the information rate. One way that this spreading can be achieved is through the use of binary code sequences, as in GPS. In this manner the transmitted signal occupies a greater bandwidth than the message it conveys and the transmission appears as a wide band of frequencies as opposed to a signal on a discrete carrier. The wideband system is able to make many more unique signals or channels available to the user than the narrowband system. In this respect it is similar to terrestrial digital television which can provide many more channels than the “analogue” systems using the same available bandwidth. It is possible to operate 10 or more “coded” wideband channels and a tone channel using the same “tone” or carrier frequency. Using the correct signal processing it is perfectly possible for them to co-exist. As well as having superior ranging precision than a tone burst equivalent the wideband system can resolve signal paths in time according to the inverse of the bandwidth and is also less affected by noise. A wideband signal can have a long duration and hence contain a large amount of energy to mitigate noise at the receiver but it still retains bandwidth.

Sonardyne has developed the Fusion range of flexible modern hardware platforms that use Digital Signal Processing technology to support both traditional “tone burst” and wideband signals. This equipment has been extensively operated over the last two years in an extreme range of environments from the icy waters of the Grand Banks to the straits of Singapore and the immense pressures of deepwater West Africa and the Gulf of Mexico . Fusion transponders and transceivers are fully backwardly compatible and support new functionality through software upgrades.

Benefits of Sonardyne Wideband Technology

High accuracy positioning

Deep water oil field developments require highly accurate seabed positioning that can often only be achieved using Long BaseLine (LBL) acoustic positioning techniques. The limitations of tone burst signals meant that obtaining the highest positional accuracy in the last generation of LBL equipment required the use of the Extra High Frequency band. The use of EHF systems in water depths in excess of about 500 metres is complicated by the fact that it is not possible to range to seabed transponders from a surface vessel due to the attenuation of the high frequency signals. Even if it were possible to receive the signals a separate, dedicated transceiver would be required since the transponders operate outside the Medium Frequency band employed by the majority of USBL systems. The increased ranging precision offered by Wideband signals means that it is possible to obtain positional accuracies at MF that were previously obtainable only at EHF. This has the combined benefits of extending the range of high accuracy positioning and rationalising equipment inventories.

Results from deep water field trials December 2004

(Above) Baseline measurement between MF Compatts using wideband signals indicates 1cm precision over 780m range.
(Above Right) MF Compatt 5 units were deployed in seabed stands in 1300m water depth

High speed robust telemetry

A key benefit of wideband technology lies in its application to telemetry where tone burst systems offer limited scope for optimisation. Sonardyne has developed a proprietory robust high speed wideband telemetry scheme that is specifically designed for the real-time transfer of the relatively short data packets that are commonly associated with subsea navigation. In addition to being 10 times faster than the fastest tone burst telemetry scheme the robust wideband link incorporates forward error correction and all of the recognised benefits associated with correlation signal processing in terms of immunity to noise and multipath. In contrast to the schemes employed in many acoustic modem products it does not require the overhead of a training sequence which reduces the latency associated with the data. This makes it more appropriate to real-time monitoring applications such as the acoustic telemetry of gyrocompass and attitude data for navigation. Wideband acoustic telemetry offers significant improvements in the efficiency of deep water operations by greatly increasing the update rate for the positioning of structures as they are deployed to the seabed.

Multi operation use

The non-interfering properties of wideband signal architectures effectively resolve the interference problems that were a feature of conventional acoustic positioning systems. The use of wideband signals greatly simplifies the support of multiple simultaneous positioning operations within the same frequency band and within interference range. This has significant implications in deep water oil field developments where acoustic systems have an increasingly important role to play.

System performance in the presence of multipath

Tone Burst Signal: Filter response showing the effects of destructive interference resulting from two overlapping uncoded signals arriving at the transducer
Wideband Signal: Code correlation function allows the separation of the same two paths using wideband signals. Both direct and multipath signals can clearly be seen

The majority of deep water drilling rigs, construction and survey vessels are permanently equipped with Ultra-short Baseline (USBL) acoustic positioning systems. These systems are used both for dynamic positioning and for tracking the positions of Remotely Operated Vehicles and other subsea targets. As economics generally preclude the use of a drilling vessel for a subsea construction program additional vessels are employed to complete the installation of seabed infrastructure in deepwater oil field developments. In many cases the development schedule often results in concurrent drilling and installation activity which dictates the use of additional vessels for installation operations. Simultaneous operation of USBL systems that are limited to tone burst technology within interference range of another similar system can result in acoustic pollution that may significantly affect positioning performance on both vessels. Deep water drilling rigs are reliant on the integrity of their acoustic positioning systems and the potential implications of interference with the operation of these systems are extremely serious. Using wideband signals in drilling rig USBL systems effectively mitigates this risk and enables better utilisation of the available bandwidth.

Conclusion

Any attempt to implement a system that emulated GPS underwater would be technically and commercially compromised by the differences between atmosphere and ocean and the expense and complexity of the measurement solution. However, the wideband signals that are used so effectively in GPS do offer significant advantages in the accepted techniques of USBL and LBL. Systems like Sonardyne’ Fusion offer a low risk route to a step change in performance through the support of wideband signals and an integrated navigation and communication system that addresses the contemporary requirements of the offshore survey, construction and drilling industries.