GNSS Positioning

Seeking Contributors

This section is under development. If you have expertise in marine positioning systems and correction services, please reach out to the admins at omcadmin@ccom.unh.edu to become a contributor!

GNSS visualization

This GPS visualization provides a useful introduction to several concepts discussed below.

Accuracy requirements

Positioning requirements vary widely with survey goals and relative depth ranges. The terms 'shallower' and 'deeper' are relative for this overview, as the specific depth ranges and contributions to total uncertainty vary widely with sensor configurations and acquisition parameters.

In a general and practical sense, GNSS positioning accuracy becomes more significant in 'shallower' water as the survey platform's horizontal uncertainty becomes large relative to the multibeam's projected footprints on the seafloor and related contributions to total horizontal uncertainty (e.g., through horizontal components of bottom detection, refraction correction, etc.).

In this regard, the correct locations of shoals and hazards become critical for safety of navigation in 'shallower' water while the survey platform's positioning uncertainty makes up a more significant portion of the final sounding uncertainty. Conversely, in 'deeper' water, the sonar's footprint for a given beam and contribution to total horizontal uncertainty may significantly exceed the survey platform's positioning uncertainty (thereby relaxing the platform's positioning requirements).

Antennas

[Seeking input on marine GNSS antenna applications, limitations, and best practices.]

Correction services

[Seeking input on correction service applications, limitations, and best practices.]

Several commercial services exist to correct for atmospheric effects and satellite orbits to improve the accuracy of a remote receiver's position. Corrections are calculated from a network of reference stations, adjusted for the remote receiver's location, and then transmitted by satellite or internet to the remote receiver for real-time application (e.g., as shown in this example of precise point positioning (PPP)).

Corrections are broadcast by providers and accepted by remote receiver using common formats such as RTCM and CMR.

The following table outlines several commercial correction services (some of which are compared here) and users across the ocean mapping community.

Note: Service and hardware providers are listed in alphabetical order.

[Additional provider / vessel / user information is welcome!]

Correction service	Applanix POS MV users	iXBlue PHINS users	Kongsberg Seapath users	Notes
Fugro Marinestar	OXR Metal Shark, FK(too)1,2	Seeking input	TGT4, SR5, RR6, EX, FK(too)1,3	Seeking input
Fugro Seastar	Seeking input	Seeking input	Seeking input	Seeking input
NovAtel Oceanix	Seeking input	Seeking input	Seeking input	Seeking input
Trimble RTX Marine	Seeking input	Seeking input	Seeking input	Seeking input
Veripos Spotbeam	Seeking input	Seeking input	OXR	Seeking input

Correction service notes:

1. Different services are used by the same vessel (GPX XP for POS MV and XP2 for Seapath) for redundancy during solar flare events
2. GPX XP corrections are received by the POS MV's Trimble GA-830 antennas (i.e., no additional antenna necessary)
3. XP2 corrections are received via standalone Veripos Spotbeam antenna for application in Seapath
4. Seapath 3710 DGNSS receiver using standalone Fugro AD431-3960 antenna
5. Seapath 3710 DGNSS receiver using one of the Seapath 380's NovAtel GNSS-850 antennas (split signal with Optical Zonu RF-over-fiber system)
6. Seapath 3710 DGNSS receiver using one of the Seapath 380's NovAtel GNSS-850 antennas (split signal with GPSS RF splitter)

Applanix application

Notes for correction services applied to Applanix POS MV systems.

iXBlue application

Notes for correction services applied to iXBlue systems.

Seapath application

Notes for correction services applied to Seapath systems.

DGNSS Receivers

[Seeking input on DGNSS receiver applications, limitations, and best practices.]

Some positioning systems may be able to receive corrections directly through their own GNSS antennas and on-board internet connections. In other cases, either due to hardware limitations or particular vessel configurations, a separate receiver is used to deliver the corrections to the positioning system.

[NOTE: the list below may be turned into a table with references to the correction services table above and/or troubleshooting under the Issues section.]

Commonly installed DGNSS receivers are listed below in alphabetical order:

1. Seapath 3710

High Latitudes

Positioning in high latitudes and polar regions can be compromised by limited satellite coverage, low grazing angles, and more complex atmospheric effects.

[Seeking input on positioning best practices in high latitudes.]