Tuesday, May 16, 2023

ECS Without GPS

We might assume an electronic charting system (ECS) is only useful if we have a GPS connected to it so we can track our boat across the chart. But the main components of the system are actually the echart navigation software and a set of echarts. The GPS is just a luxury. We can do a tremendous amount of sophisticated navigation, both easily and accurately, without the GPS.

In fact, the crucial part of a well-navigated voyage is done before the boat pulls away from the dock. We can do a rough layout of the route for quick time and distance estimates, and then zoom in along the route and fine tune each of the waypoints to optimize their use underway. This usually calls for choosing them such that they correlate with good visual or radar targets along the route. Then give each waypoint a name, not just a number. 

These programs also have built-in tide and current predictions, so we can estimate anticipated speeds made good (SMG) along the route, and then print out a route plan with individual route-leg courses and distances and ETAs all tabulated. This is prudent preparation, with or without GPS to track us across the chart. All US echarts are free, so now there is no reason not to have all possible charts of interest at hand for any route. See chart download options at starpath.com/getcharts.

In this note we look at a couple ways you can navigate with echarts without relying on GPS–or put another way, if you happen to lose your GPS, don’t shut off the ECS. It remains your most convenient and accurate means of chart navigation.

Also keep in mind that essentially all ECS include some form of DR mode with which you can set the boat icon at your best estimated position on the chart and then turn on DR and set the speed to match your knot meter and heading to match what you are steering. Your ECS will then plot out your DR track. Your job is to then update heading and speed as they change. 

We look here at a couple samples of navigation that might occur on the chart in Figure 1 that we must  solve using our ECS without GPS.

Figure 1. It is night, in light fog. The boat is near buoy VD and wishes to enter Baynes Channel, about 3 miles to the West. NOAA current predictions call for a current of 1.5 kts flowing toward 135T. Broadcast Notice to Mariners has just informed us that the light on buoy VK is not showing and the buoy may be off station. We have no valid GPS signals to rely on. We have a calibrated knotmeter, compass, and depth sounder. We are left to use traditional navigation to get into this channel, taking care not to get set down onto the rocks that buoy VK was supposed to mark. We are under power at 6 kts. The magenta shade on the chart means that area is in a separation zone between two vessel traffic lanes. 

First we look at what course to steer to make good a due West track correcting for the tidal current. We know we must point into the current, but how much? We do not have the benefit of a nice COG and past track from a working GPS, so we will rely on theoretical values... and then monitor the rocks by other means. The goal is to have more than one check on our navigation at all times if possible.

Course to Steer (CTS) to Correct for Current

The solution is illustrated in Figure 2. We could do this plotting on a chart, or indeed on a blank paper using standard plotting tools, but this can be solved faster with electronic plotting tools available in any ECS. One procedure is outlined below.

Figure 2. Solution to the CTS problem, which can be worked on paper charts or on an ECS.

CTS Procedure

Step 1. From your DR position, draw a line in the direction you want to make good (CMG) with a length longer than the knotmeter speed (boat speed) — 270 T in this example.

Step 2. From the DR position, imagine the boat drifting with the current for 1 hour. Mark the end point of that drift on the chart — 1.5 nmi in direction 135T in this example.

Step 3. From that drift position, set your dividers to the anticipated boat speed, and swing an arc to find where your speed for one hour will put you onto your desired track. Mark that point on your desired track — radius = 6.0 nmi in this example.

Step 4. The direction of that line, drift point to intersection point, is the CTS that will maintain a CMG of 270 at a boat speed of 6 kts in a current of 1.5 kts in direction 135 — CTS = 280T in this example.

Step 5. Measure the length of the line from the DR position to the intersection point and that will be your speed made good (SMG) as you crab along the the desired track — 4.9 kts in this example.

Solving this vector triangle following these steps can be done numerous ways with an ECS, depending on plotting options offered. It could all be done with one 3-leg route. 

Starting at the DR position, use the ruler tool to find a point that is farther than your boat speed in the desired direction, and drop a mark there. Then start the route at that mark with leg 1 back to the DR position;  leg 2 from there to the drift location; and finally from there find the point on the first line that is your boat speed from the drift point; and end the route there. You know the final point has to have length equal boat speed, so just monitor that read-out as you move the cursor along the line. Then use the ruler tool or measurement tool to find the lengths and directions you need.  Here is a video example of this method.

Course to Steer by Quick ECS Solution

Danger Bearings

In the above scenario, we cannot be certain that the current predictions are correct, so we need a way to monitor our progress to be sure we stay off of Fulford Reef, whose buoy (VK) is now missing. One way is to set up a danger bearing using Cadboro Point Light, which is clearly in view, near dead ahead. 

Using an ECS, we can, in seconds, measure an accurate danger bearing to the QG (Quick flashing Green) light that will keep us off the reef, as shown in Figure 3. We see that this bearing should remain less than 254 M (we are assuming a 20ยบ E variation). We watch it with a bearing compass as we proceed, after first checking that it is now due west (250 M) as it should be. If the bearing gets smaller we are slipping north of our track; if it gets larger we are getting pushed south. If it gets larger than 254 M, we are headed for the reef, as shown in Figure 3.

Note that this type of measurement requires a good bearing compass such as the hockey puck or equivalent.

Figure 3.
Selecting a danger bearing to the QG light dead ahead at the start. The 0.5 nmi range rings mark distance from the starting point, which helps for quick DR chart plotting from log readings. Note on this older chart, there was a US buoy there with name FR for Fulford Reef, but that was later replaced by a Canadian buoy VK shown in Figure 1. The reef name is also different on newer charts.

Line of Soundings Navigation

In this hypothetical situation, the only thing we have left to use is our sounder. This is not an ideal route for depth sounding navigation, but even in this case there are still a few guidelines that can assist our navigation. The main point for now is to show how powerful the ECS is for setting up sounder navigation. To do this accurately we need tide height, which we also get from the ECS program with the click of a button.

Figure 4. Corrections to be applied to the sounder readings so they can be compared to the charted depth contours. In the following, we assume these corrections were made so we are working with corrected depths.

Figure 5. A line of soundings made from an ECS program. The data can be accumulated in an automated plot or recorded by hand.  Range rings at 0.5 nmi set on the starting point helps monitor the correlation between depth and DR position. Though not the best bottom for sounding navigation, we still have a few things to note. First the depth should drop off rapidly, then carry on slowly deepening. If it should go up at about 0.5 - 0.7 nmi off (marked red in the plot), we know we are getting pushed south. Then if we are right on track, we should see the 19-fathom bump at about 1.7 nmi off (marked green in the plot)   

 ECS Line of Soundings Procedure

Whenever the water is not too deep (meaning our sounder works—often referred to as "within soundings") and not too flat, we can usually gain useful nav info from a line of soundings. Here is a fast and accurate way to set this up using ECS.

(1) Look ahead to see if depths or contours are favorable and if so choose a couple to monitor, and make rough estimates of what you expect if you are where you think you are.

(2) Check tide height to figure net correction to sounder reading, i.e., if sounder is 2 ft below water level with 5 ft of tide, then sounder reading in feet  + 2 - 5 = charted depth expected.

(3) Note time, speed, and heading, and log preferably, and start the plot of depth vs log. The goal is to have a log reading or time for each notable sounding change.

(4) After crossing the contours of interest, figure distances between key depths or contours and use ECS plotting tools to locate most likely track around the contours, as shown in the example below. Range rings on movable marks is one way to check for closest path across the contours.


Example of Line of Soundings Fix using ECS

With paper charts you can draw a line on transparent paper in the right orientation, and mark off the measured depths using the same miles scale as the chart.  Then slide the transparency around till you match your measurements. Line of Soundings Fix 2, is a video example of the paper solutrion.

These are just a couple chart navigation exercises that are nicely solved by ECS without GPS. There are many. Besides the range and bearing tool for standard plotting, a versatile option that is often overlooked is the ability to set multiple range rings on any mark or waypoint, or your vessel icon. This has numerous applications, we have shown only one.

The navigation procedures described here are adapted from our textbook Inland and Coastal Navigation, 2nd Edition.

Line of Soundings Fix 2


Unknown said...

The post says: "Tidal current predictions are for 1.5 kts setting toward 135 T at the time along this route." However, it looks like the set of the current is 315 T in Figure 1. Is 135 a transposition typo for 315?

Unknown said...

Never mind, I see that 315 T and 135 T are actually reciprocals.