Raw Radar for Navigation
Bye, Bye, Birdie: When GPS Fails
Since 1991 when the Global Position System (GPS) first became operational, we have become ever more dependent on it for navigation. A quarter of a century later, boats and ships routinely set sail across oceans (or even across harbors) with no consideration given for how they would navigate home without it. We have always known—or should know— that GPS could either be degraded or taken offline entirely, but with a few rare exceptions, it has proven as dependable and steadfast as the sunrise. For decades, we have come to depend on it, but recently we have acquired a better understanding of and appreciation for the vulnerabilities of the 24 satellites that make up GPS. Military, commercial, and civilian aviators and seafarers are now scrambling to relearn the fundamental navigation skills that were once the staples of our trade. Here are some techniques and tools available to us as powerboaters in the event of a sudden loss of GPS while underway. For this first installment of Bye Bye Birdie, I will present the use of radar and chart plotters for navigation, absent GPS data for position, heading, or time.
In this scenario, we will assume that our boat is equipped with a small-craft radar (x-band or broadband, it does not matter) and an electronic chart plotter, both interfaced with GPS and a satellite compass for position and heading, respectively. With the loss of GPS and the satellite compass, all heading and position inputs to these, and interfacing between these, is lost. Radar will revert to a headsup display, and the charting will lose all reference to own ship. In this scenario the vessel happens to be in Puget Sound, west of Blake Island in the fog, when the satellites fail. The charting software used for this demonstration is Rose Point Coastal Explorer, and I will be simulating the radar with Radar Trainer 3 from Starpath. However, any combination of radar and chart plotter should work for this example.
At time 00:05, with the radar’s Variable Range Marker, we measure the distance to the northwest point of Blake Island to be 0.57 nautical miles. Note that we will be using radar ranges only, and ignoring the relative radar bearings that are available. Besides the added risk of error converting from a heads-up to true, simply by the way radio waves propagate radar ranges are far more accurate than radar bearings. If we only have one radar target, we would utilize a range and bearing to it, but this is rarely the case in pilotage waters.
In our chart plotting program, we place a New Mark at the northwest point of Blake Island, and create a range circle of 0.57 nautical miles around it.
Next we find that Orchard Point is 1.27 nautical miles and then plot that. (Fig. 1)
Then we find that Harper Head is 1.31 nautical miles and plot that as well. Where the three range circles intersect is our time 00:05 radar fix. Label this as well with a New Mark. (Fig. 2)
From the 00:05 radar fix (Fig. 3), use the chart plotter’s Range and Bearing Line function to set up your dead reckoning. In this case, the vessel is making 12 knots and we are using six-minute fix intervals to keep the math easy (six minutes = 1/10 of an hour, so in that time you will cover onetenth of your speed in knots, or 1.2 nautical miles). Our magnetic compass heading (whether standard or Fluxgate—without GPS—this is our best heading reference) is 050°. Note that one of the advantages in this scenario of using electronic rather than paper charting is that the NOAA electronic charts will usually have more accurate magnetic variation than their paper counterparts. Place a mark at your time 00:11, Dead Reckoning position. At this point it is fine to clear your old range circles from the screen.
At 00:11 (Fig. 4 , page 24) we repeat the process. Orchard Point is now 1.69 nautical miles, Restoration Point is 1.74 nautical miles, and the end of the breakwater at the Blake Island marina is 0.86 nautical miles. Where these ranges cross is our time 00:11
radar fix. We see that there is a little more than a knot of northerly set and drift.
Now, simply drag the range and bearing line Dead Reckoning (DR) to the 00:11 mark, for the next six minutes of DR, to time 00:17. Rinse. Repeat. Of course you can use this same technique with paper charts, remembering to use the magnetic compass rose for the dead reckoning. With practice you will likely find that even with functional GPS, this method of radar chart navigation is quicker, easier, and more accurate than plotting either visual bearings from a hand compass or plotting raw latitude and longitude from GPS. This works equally well in clear weather or foul, day or night. Good watch, and following seas! Q
Robert Reeder is a 1600-ton master in the Seattle area. He drives passenger ferries for a living and drives tiny little sailboats for the pure, unbridled fun of it. Robert never goes offshore without a sextant, chronometer, and two GPSs.
Special thanks to Rose Point Navigation for allowing us to reproduce their charts. More online at: www.rosepoint.com
Thank you to Starpath School for Navigation for letting us reproduce images from their Radar Trainer 3 software. More at: www.starpath.com
We have always known—or should know, anyway—that GPS could be degraded or taken offline entirely, but with a few rare exceptions, it has proven as dependable and steadfast as the sunrise.