Comparison of L-Band Correction Services for Marine Applications

If you’re looking for a marine Global Navigation Satellite System (GNSS), it can be difficult to determine the right solution for you. The purpose of this article is to make choosing the right L-Band correction service for your marine operation as easy as possible.

Positioning Solutions

There are four primary solutions available: Private Real Time Kinematic (RTK), Subscription-based Real Time Kinematic (RTK), Differential GNSS (DGNSS) and Precise Point Positioning (PPP).

Generally speaking, RTK, both provided by a privately owned reference station or obtained from a third party service provider, is the most accurate, because it resolves in real time the complex geometry of a tridimensional vector between the known and accurate position of a base station and the desired accurate position of a rover static or mobile station. Often, in the best conditions, RTK provides centimeter-level accuracies.

The privately owned reference station RTK mode requires a user to setup two GNSS receivers, the first one on a survey mark for which the position is accurately known and the second that measures new positions. The secret resides into a permanent communication between the two, whether it comes from a digital radio apparatus or by the means of Internet and a cellular data link. The information transmitted from the reference station provide the rover station what it needs to obtain the orientation and distance separating both instruments, resulting into an accurate new position for the rover in relationship with the already known position of the reference station.

The third party provided RTK mode uses the same geometry logic and algorithms to find accurate positions of a rover GNSS receiver but instead of using a privately owned GNSS receiver as a base and communication apparatus, it uses Internet and a netwrork of reference stations operated by a service provider. Many of such services offer some sort of improvements by interpolating corrections from the nearest stations to reduce the incertainties related to the distances between them and the client’s rover. This type of service makes it such that a user no longer needs to work with two GNSS receivers, only one being required, most often including a cellular data link to access the service provider’s correction services. Recently implemented high-end such services are now using satellites on what is called the “L-Band”, requiring a special capability at the GNSS receiver end to receive correction data on this frequency from other geostationary satellites. The accuracy varies but is generally still at centimeter level if an open sky is available and a sufficient number of satellites can be received.

Differential GNSS (DGNSS) can be provided by a few sources. Initially, such services, requiring a separate digital radio receiver, were provided by many country’s Coast Guards to increase the accuracy of maritime navigation but these services have been mostly replaced by a few geostationary satellites that act at the same time as GPS satellites, but also providing corrections. This service is called SBAS (Satellite Based Augmentation System). In all cases, DGPS provides only pseudo-distances corrections on each satellite and positioning accuracies that can only reach one meter in the best conditions.

Precise Point Positioning (PPP) is the latest and most complete corrections method. It uses a better knowledge of satellites ephemeris (their accurate orbits), of ionospheric and tropospheric delays in the user’s region and finally, wherever possible the resolution of frequency carrier’s phase ambiguities in comparison with fixed ground stations. PPP services are provided by “L-Band” satellite-based corrections with convergence times required to obtain higher accuracies that can vary with many factors, notably the distance separating the user from permanent pursuit stations. The PPP mode advantage is the possibility to obtain centimetric levels of accuracy in remote territories where referencing to geodetic networks is impossible.

Offshore vs. Near-Shore

If your project operates within a small range and near the coast, such as inshore surveys, inshore hydrography, dredging, environmental surveys, mapping or construction including placement of materials, breakwaters and pylons, a near-shore DGNSS or RTK solution is likely your preferred choice.

If your situation doesn’t permit or prefer the use of a GNSS base or communication link to the base station, you’ll likely want a PPP correction service.

The accuracy and convergence time you need will depend on a large number of factors, such as whether you need precision navigation through congested marine traffic, are travelling narrow passageways or the precision required for your marine construction or drilling project. PPP is ideal for hydrographical surveying vessels, fishing vessels, seismic exploration, dynamic positioning for vessels from DP 1 to DP3, dynamic positioning for drilling and construction, including rig installations and moves, route surveys and repair of subsea structures.

Depending on the service provider, DGNSS and PPP may be offered as near-shore and offshore options. Near-shore is substantially more affordable but is limited to 10-20 km of the coast depending on the provider. If you’re looking for L-Band corrective services, you have three primary choices: Atlas, NovAtel and Fugro.

Correction Services

Fugro

Fugro is specialized in all three and divides their services into three categories:

• Starfix is (primarily) a PPP based system aimed at offshore construction vessels, pipe and cable laying, dive support and more. Starfix has a variety of different service levels, with up to 3 cm accuracy (using a combination of PPP and other systems), with some geographical restrictions at the highest accuracies.
• Seastar is a dynamic positioning DGNSS service aimed at platform supply vessels, diving support vessels drilling rigs and more. Seastar services are available by accuracy (ranging from sub-meter to 10 cm) and satellite system.
• Marinestar^® ooffers phase, DGPS and DGLONASS services designed for vessels travelling busy waterways, such as hydrographic vessels and dredging vessels as well as MMS operated vessels to save fuel and improve efficiency. Marinestar® is accurate to 10 cm horizontal (95%) and 15 cm vertical (95%).
• Oceanstar^TM is an onboard decision support system for large vessels (e.g. cruise ships, ferries, bulk carriers etc.) that enhances berthing operation and navigation in confined waters. It will allow you to achieve an accuracy of 5-10 cm.

NovAtel

NovAtel has also specialized in all three services, but their DGNSS service is limited to coastal areas.

• NovAtel’s RTK solution provides accuracy of 1 cm + 1 ppm horizontal and 1 cm vertical (up to 40 km baseline range) but requires a privately owned base station and radio or communication equipment between it and the mobile receiver.
• Novatel’s DGNSS service provides 40 cm accuracy up to 100 km of range.
• Two Novatel PPP solutions:
  • NovAtel TerraStar-C is accurate to 5 cm RMS horizontal and 6.5 cm RMS vertical with 20-40 minutes convergence time.
  • NovAtel TerraStar-L is accurate to 40 cm RMS horizontal and 50 cm RMS vertical with less than 5 minutes convergence time

Atlas

Atlas has specialized in PPP with three solutions:

• H10: with 4 cm RMS horizontal accuracy and 12-20 minute convergence times
• H30: with 15 cm RMS horizontal accuracy and 4-5 minute convergence times
• H100: with 50 cm RMS horizontal accuracy and near instant convergence times

In conclusion, if the project you’re working on is restricted to a short geographical range near the coast, where you are able set up an RTK station or use GNSS base stations, RTK or DGNSS is likely the best solution. It will provide you instantaneous convergence times and the highest accuracy (RTK). If you’re operating near-shore, but aren’t able to or prefer not to use a GNSS base station, a near-shore PPP L-Band corrective service is likely the best solution. Your specific PPP corrective service will depend on the convergence time and accuracy you require. If you’re not near-shore, offshore PPP or DGNSS corrective services are often the best choice, with the specific package depending on the convergence time and accuracy you require.