Globally Corrected GPS (GcGPS) is a technique for improving the accuracy and stability of the Global Positioning System (GPS).
This technique was developed for NASA because they had a requirement for decimeter accuracy in outer space. As a result a totally new concept for differential needed to be developed so they could operate at great distances from any reference sites. The result of this research has proven to be of value to the marine market which does not always have a reference site nearby. It is of particular value to the international operators that prefer to use one system worldwide. This reduces hardware and training costs plus it improves flexibility and safety.
The compact hardware design works well for the dredging industry because they operate both large barges and small survey boats on the same project. The cost of spare hardware is reduced because the same system can be used for various positioning needs on a project. This allows an operator to move from vessel to vessel or to another country and still be comfortable with the positioning system. Since base-stations are not required manpower is reduced as well. This system is called C-Nav”™ and it has proven to be a great asset in many areas of the marine market.
C-Nav does not work like conventional DGPS. Conventional DGPS develops a differential correction message at the reference site and sends that differential out to the user. This is a good technique as long as you stay close to your base station. Until about five years ago it was the only method that we had but now we have another option. C-Nav works differently. You can think of it as fixing the GPS errors at the source (the satellite and the signal path between the satellite and the user) rather than trying to fix the symptoms of the problem at a remote site.
A Global Network
The C-Nav system starts with an extensive network of reference sites strategically positioned around the world. Each reference site is equipped with high quality dual-frequency GPS receivers. In addition many of these same sites are equipped with C-Nav user hardware that is used for real-time system integrity monitoring. Each reference site tracks all of the satellites in view and sends all raw GPS measurement data to two independent network processing hubs (NPH) in real-time. The NPHs receive the raw data and calculate orbit and clock correctors for every healthy satellite in the GPS constellation. These correctors are then broadcast over a network of global communication satellites. The result is a differential correction message that is universally valid anywhere in the world (land sea or space). This technique eliminates the spatial de-correlation problems that limit conventional DGPS systems. As a result this is the first truly global DGPS service available and the accuracies are approaching that of RTK techniques without the hassles of setting up local control or the expense of a person to monitor a base station.
The heart of the C-Nav receiver is a high-quality dual frequency GPS engine with special dual frequency-extended smoothing techniques to allow the use of refraction-corrected measurements. A single tri-band antenna design allows for reception of both GPS frequencies (L1 and L2) and the Inmarsat L-band communication frequencies (1525-1565MHz). The integrated processor receives the refraction-corrected GPS measurements and applies the corrections. The output is an accurate and stable position from a small easy to operate unit. Only one cable is required and the hardware is not only state-of-the-art – it is rugged portable and has proven its ability to withstand the rigors of the offshore environment. .
The C-Nav system is the culmination of ten years of development. It incorporates many technical advantages that set it apart from other DGPS services available today. Additional information can located at www.cctechnol.com/cnav