Global Positioning System (GPS) is the one important equipment on a navigation system. Aviation industry, military, marine, even a delivery service required a GPS (smartphone basis) to locate the destination.
On a marine vessel, GPS is a basic equipment needed to create a waypoint/route, as a position indicator and a data source for the other navigation equipment, such as radar, ECDIS, AIS, etc.
Let’s see on what kind of data that we can get from a marine GPS. Below is a main screen of Furuno GPS GP-170.
- Four information boxes on the top of the screen (FIX, PDOP, RAIM, A. LEVEL) contains the information related with the status and the reliability of GPS receiver.
- On the second row, we get the position information contains latitude, longitude coordinate and the datum reference (WGS84).
- On the right side of the position box, we have time and date information refers to UTC (Universal Time Coordinated).
- Then, on the below screen we have information about Range (RNG), Heading (HDG), Speed Over Ground (SOG), Course Over Ground (COG) and Navigation box (contains the next Waypoint (WPT) and the destination).
Range (RNG) is the distance from the ship to the next waypoint. Only if we have made the voyage route which is containing few waypoints, then range (RNG) and navigation box information will be available. While, heading (HDG) information will be available only if the GPS is connected with gyrocompass/heading sensor (gyrocompass/heading sensor is outputting the heading value to GPS).
If we see back to the screen, the heading (HDG) and the course over ground (COG) has the same exact value. Is this can be happened? Of course yes, this can be happened when the ship is berthing on the port, without any movement for some period of time.
Course Over Ground (COG) is the actual direction of motion (the intended direction of travel). While heading is the direction in which a vehicle/vessel is pointing at any given moment (https://www.applanix.com/news/blog-course-heading-bearing/).
Lets say a vessel has a voyage route through waypoint A, B,C, …, to final destination on the waypoint F. The vessel start to sails, heading to 90 degrees to the east. From point A to B, lets assume that the vessel moving smoothly without failure, so the track can be define as straight line to the east direction. At point A and B, heading and COG value will remains same.
This vessel continue the trip until it reach point C. Unfortunately, the vessel has to deal with strong wind, current, and tide from the south side. To keep the voyage on the track, the vessel then change the heading to 135 degrees against the wind, current and tide. This was happened until point D was reached. The heading was start changed from point C to D (135 degrees). But COG will still remains same (90 degrees) as vector line BC and CD has no change. The measured movement is still 90 degrees to the east.
At point D, the storms has passed. Then, the vessel back to travel with an ideal situation. The next waypoint E is on 135 degrees from point D. So, the heading was kept on 135 degrees as it should be.
At point E the vessel change back the heading as the final destination (point F) was on 90 degrees to the east. While the previous track (vector line DE) was directed to 135 degrees. So even though the heading at this point has changed to 90 degrees, maybe the COG will still showing 135 degrees.
The trip from point E to F is just the same as the trip from point A to B. GPS receiver start to update the vessel position and movement along this trip. As the time passed, the COG will change and showing the same value as the heading (90 degrees) at last point F.
By this illustration, there is one more question. That is, “how quick the COG will change following the latest heading value?”. As we see, COG is always has a ‘delay’ time required, maybe to acquire the position data, then calculate the direction and speed. Is it related with the GNSS chip mounted on each GPS receiver unit?
By the way, Furuno GP-170 is a GPS receiver with 10 Hz position update rate (position updated every 0.1 second) making steady own ship tracking possible. If you are interested with the product, please see following website: https://www.furuno.com/en/merchant/gnss/.
See you!
