GPS-C: A real-time correction service for Global Positioning System (GPS) data

Technical Information Circular

Introduction

Natural Resources Canada (NRCan) is offering a commercial licence for the distribution of Global Positioning System correction information¹. This wide-area correction service, called GPS-C, has been developed by the Geodetic Survey Division (NRCan-GSD) of NRCan over the last decade.

This technical information circular provides background information on the technology, describes GPS-C in detail, identifies potential target markets and users, and suggests options for data distribution.

It also outlines the licensing process for this commercial opportunity and provides contact information.

GPS-C: Background and History

Who is NRCan-GSD?

The Geodetic Survey Division (NRCan-GSD) is responsible for maintaining the Canadian Spatial Reference System (CSRS) which provides fundamental reference values for latitude, longitude, height and gravity, including earth's orientation parameters and rotation rate in space. All these reference data are essential for accurate positioning and navigation. The Division ensures the availability of spatial referencing information and provides expertise and services to answer client needs.

Rationale and history of the development of GPS-C

Global Positioning System (GPS) technology has been an extremely important tool to maintain and improve the Canadian Spatial Reference System. GPS can provide positioning accuracies in the order of millimetres in a “post mission” mode using high quality GPS receivers and software.

Instantaneous GPS positioning accuracy can be improved more than 10-fold by augmenting GPS signals with simultaneous corrections based on a system of ground reference stations and associated computing and communications systems. In the early 1990’s NRCan-GSD developed such a correction service called GPS·C using a network of 15 permanent base stations distributed throughout the Canadian land mass.  In the intervening years NRCan-GSD has continuously improved the GPS·C algorithms and included more reference tracking stations.

GPS-C was created to provide improved continuous real-time positioning and navigation over Canada with the resulting user positions automatically referenced to the Canadian Spatial Reference System. This automatic reference facilitates data sharing, and integration between agencies, users and across time.

Current use and access to GPS-C

In 2000 NRCan-GSD partnered with its counterparts in all 10 Provinces and 3 Territories to bundle GPS·C into a nation-wide broadcast service using Skyterra’s communication satellite MSAT. This free-to-air, L-band broadcast known as Canada-wide Differential GPS Service (CDGPS) has been operating since October, 2003.  Last fall the collaborating governments signalled their intent to withdraw from financing and operating CDGPS as a free public service.

In 2006 the client base for CDGPS consisted of approximately 1200 end users, with almost 25% from the surveying and mapping community. Since then the numbers have surged to between 6,000 and 9,000 thanks to two manufacturers who integrated CDGPS receivers directly into their GPS boards.

NRCan-GSD will continue to develop and improve GPS-C, but wishes to attract commercial distribution partners interested in the opportunity to exploit the data feed. 

GPS-C: Technical description and unique functionalities

GPS-C is based on the Canadian Active Control System (CACS). The CACS was established to improve the accuracy and efficiency of GPS positioning and to provide direct access to the Canadian Spatial Reference System (CSRS). Precise GPS satellite ephemerides, clock corrections and earth orientation parameters have been produced by NRCan on a daily basis, with precision in the order of 10 centimetres or better. These CACS products allow users to achieve positioning results at the centimetre level with a delay of a few hours or less.

GPS-C is an NRCan-GSD real-time wide-area GPS correction service based on the CACS that includes GPS receiving stations, central processing facility and communications links. It facilitates enhanced real-time positioning providing corrections for the user’s GPS observations. The GPS-C service products include satellite orbit corrections, satellite clock corrections and ionospheric delay corrections.

Features offered by GPS-C include:

• Real-time corrections based on a well distributed network of Canadian GPS tracking stations.
• A highly reliable computing and communication infrastructure.
• Publicly available technical specifications for correction distribution protocol and message content

Moreover, GPS-C already has an established community of users of GPS-C data through an existing L-band broadcast service.

Recent developments at NRCan-GSD now enable the processing of GPS carrier phase observations that provide corrections supporting 10-20 centimetres accuracies for users with appropriate receivers. This is achieved through access to an expanded (beyond North America) global network of real-time tracking stations and sophisticated wide-area network processing software. High-resolution corrections resulting from these new developments can be delivered using the existing GPS-C infrastructure and distribution mechanisms such as CDGPS.

GPS-C: Target Markets and Users

According to a Canadian GNSS Sector Study conducted by Bearing Point in March 2005, Canada’s current domestic GNSS markets² are 5 to 8 million GNSS-enabled devices and the market is expected to grow to 20 to 35 million devices by 2015. The value of the Canadian domestic GNSS market is estimated presently at between $300 and $600 (Cdn)³ million annually.

Whether it is for precision farming, aerial spraying of forests, aerial bombing with fire retardant on fires or capturing airborne remote sensing/geophysical data, the common requirement is for real-time high precision positioning. Real-time access to GPS-C is expected to benefit any industry that requires instantaneous and continuous accurate spatial positioning including:

Agriculture	Security and Tracking	Mapping
Oil and Gas	Submarine Telecommunications	Forestry
Civil Engineering	Inshore and Coastal Applications	Environment
Mineral Exploration	Mining

The potential also exists for using GPS-C corrected data for real-time transmission of synchronous-time transactions.

By making GPS-C corrections available for distribution, NRCan is creating a means for a wide user base to generate, use and operate within the national coordinate reference system, thereby facilitating use, sharing and integration of spatially generated information.

Example: Impact of High Precision GPS data on Precision Farming

As an example, a tractor operator using a 35m spray boom will want to reduce pass-to-pass overlaps at the edge of the boom to 15cm to reduce waste and pollution.  Using augmented GNSS, tractor operators are able to limit overlap and spray at night or pre-dawn when wind speed is low and spray drift will be reduced to a minimum.  Augmented GNSS use in farming activities also had an unintended consequence by reducing farm accident insurance claims.  Augmented GNSS can even be used for more sophisticated precision farming applications where fertilizer, lime and herbicidal inputs vary according to local field conditions and for which high precision real time positioning is essential.  The business case for precision farming has been so well-established that GPS is an integrated component on most new tractor sales.

Data Distribution Options

Options for distributing GPS-C data could include:

• Acquire, under a separate licence, the right to operate an existing Canadian-based distribution service called Canada-Wide Differential GPS Service, or CDGPS (currently operated by the Canadian Council on Geomatics).
• Develop and operate a new space-based broadcast service to distribute GPS-C data.
• Develop and operate an Internet Protocol-based service that uses wireless cell phone infrastructure.
• A combination of the above or other solution in keeping with the intent of this offering.

1. Canada-Wide Differential GPS Service (CDGPS)

Two key design features of CDGPS are the extent of its broadcast coverage and its ability to operate under intermittent, line-of-sight conditions, such as moderate tree canopy, a common condition throughout Canada’s boreal forest land.

As of May, 2007 the CDGPS coverage area extended across North America beyond the 200 nautical mile exclusive economic zones off the Atlantic, Pacific and Arctic coasts (Figure 1).  The Canadian Coast Guard and Canadian Hydrographic Service have used CDGPS as far north as 78° N latitude.  The most significant outliers in CDGPS’ Canadian coverage are Ellesmere Island and parts of the Beaufort Sea.

CDGPS users in the red area can expect to receive GPS·C corrections for all GPS satellites tracking at least 10° above the horizon; in the orange area, for 95% of those (Figure 1).

For optimal transmission of GPS·C in blocked line-of-sight conditions CDGPS:

• Transmits at 5X the speed of the U.S. Federal Aviation Administration’s Wide Area Augmentation Service (WAAS) giving it more opportunity to capitalize on momentary openings in the blockage, and
• Employs Reed-Solomon code for forward error correcting messages that may have been corrupted by intermittent blockage.
• Fast corrections are re-transmitted to provide a second chance to receive blocked/missed messages.

Licensing rights can be acquired and negotiated through the Canada-wide Differential GPS Service (CDGPS), under the Canadian Council on Geomatics.

2. New space-based multi-cast service

This option could be a different method of a space-based broadcast to users potentially utilizing a different satellite for broadcast of GPS-C information.  Re-use of the existing CDGPS infrastructure may be an option to consider.

3. Internet Protocol-based service

This option would make the GPS-C information available on the Internet. Users would need an internet connection to receive the information.

To support wireless internet access to GPS·C through existing wireless infrastructures as an alternative to either CDGPS or to multi-casting from a communications satellite, GSD operates a User Datagram Protocol (UDP) server.  The IGS (International GNSS (Global Navigation Satellite Systems) Service) Real Time Working Group is recommending the User Datagram Protocol over the more familiar TCP/IP, valuing transmission speed and efficiency in real time applications over the reliability of TCP.

Not designed to act as a national GPS·C internet service, this UDP server can feed 3rd party re-broadcasters who would be expected to localize the GPS·C corrections and serve them up to subscribers of commercial wireless internet providers using another existing international radio standard, NTRIP – Networked Transport of RTCM/Internet Protocol.

4. Other Solutions

A combination of the above options or other solutions will be considered in keeping with the intent of this offering.

Interested?  Your Next Steps

• A detailed technical information package is available upon request using the contact information below
• Interested companies are encouraged to request a Applicant Information Package for their use in preparing and submitting a commercial proposal for GPS-C.  The Applicant Information Package may be requested using the contact information provided below.

Contact

Information Management and Client Services
Geodetic Survey Division
Natural Resources Canada
615 Booth St Ottawa ON
K1A 0E9

Telephone: 613-995-4410
Fax: 613-995-3215
Email: information@geod.NRCan-RNCan.gc.ca

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