Raw Radar for Nav­i­ga­tion

Passage Maker - - Contents - Robert Reeder

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.rose­

Thank you to Starpath School for Nav­i­ga­tion for let­ting us re­pro­duce im­ages from their Radar Trainer 3 soft­ware. More at:

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.

Many mariners lack an un­der­stand­ing of 21st cen­tury GPS vul­ner­a­bil­i­ties and should prac­tice fun­da­men­tal nav­i­ga­tional skills.

Fig. 1 (above); Fig. 2 (be­low); Fig. 3 (right)

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