Paddling with GPS-Part 1

April-May 1999

This is an article from WaveLength Magazine, available in print in North America and globally on the web.

by Michael Schmidt

We won't need a compass-we have a GPS!"
I recently overhead this statement in one of our local paddling stores. It is indicative of the sometimes misleading marketing of the benefits for having a GPS receiver with you on the water, in the backcountry or wherever else your travels take you. It is also indicative of the fact that the customers had not taken the time to fully understand the features of their GPS receiver and the technology behind this satellite navigation system. There is no doubt that GPS is a great tool for finding your way on the water or in the mountains but it must be used in conjunction with the existing tools, namely map, compass, altimeter and, not to forget, the know-how to use it correctly. There are already countless examples of how GPS has 'not saved the day'.

GPS Overview

So what is GPS? In 1978 the U.S. military launched the first of the NAVSTAR Global Positioning System (GPS) satellites. The goal was to provide positioning anywhere on earth 24 hours a day, 365 days a year under any weather condition. Twenty years later the world has been given a system that has far surpassed the expectations of the military designers. In fact, non-military users far outnumber military users. Never-the-less it is important to understand that it is still a military system and for this reason does not currently live up to its full potential for civilian use. Fortunately this is starting to change.

The GPS satellite constellation presently (February 1999) consists of 27 satellites in continuous 12-hour orbits around the earth. This is three more satellites than the design configuration of 24 satellites, so coverage is actually enhanced in many areas of the world. Each satellite transmits on two frequencies, L1 = 1575.42MHz and L2 =1227.6 MHz. All the handheld GPS receivers in the outdoor recreation market (and which we will be discussing here) are single frequency (L1) receivers

In order to calculate a position the GPS receiver must obtain simultaneous measurements of distance to a minimum of four different satellites. Most of the newer handheld GPS receivers are capable of making simultaneous measurements to as many as 12 satellites using 12 parallel channels thus improving the reliability of the navigation information. As the receiver starts tracking satellites, status information including the number of satellites being tracked, indicator of signal strength, position reliability and so forth is displayed on the screen. It is important to become familiar with these status screens and understand the meaning of the information being displayed.

Some GPS manufacturers advertise accuracies of 20 or 25 metres. This is slightly misleading because real time GPS accuracy is currently degraded by the U.S. military through a process known as Selective Availability or SA which introduces random noise in the measurements and your position may appear to wander about. The accuracy on each individual position is therefore on the order of 100 metres, not 25. Realistically though, 100 metres is quite adequate for the recreational user especially if you own a receiver that is capable of "averaging" a position, thus improving the reliability somewhat. The good news is that SA is scheduled to be turned off in the not too distant future. The presidential directive of March 1996 states that the U.S. will "discontinue the use of GPS Selective Availability (SA) within a decade". There is presently much pressure to advance that date. Without SA, accuracies better than 25m are easily realizable. (It should be noted that GPS height measurements are slightly less accurate than horizontal coordinates.)

Recreational GPS receivers

The early recreation receivers were capable of providing a position (latitude and longitude, UTM, etc.), speed and heading, store waypoints and possibly a few other features as well. The units presently available provide many more features including larger displays, moving maps, integrated altimeters, radios, e-mail messaging and much more, all at prices that continue to come down. However, the fundamental information is still your position on the water as you explore a fog shrouded coastline in your kayak.

GPS receivers are capable of display ing the position of your secluded beach in any number of different datums or reference systems (e.g. NAD 83). It is therefore essential that your GPS receiver is keyed to use the same datum as your map or chart. Errors of up to 2 kilometres can easily be introduced by trying to use the wrong datum. Take the situation where you are using your GPS unit to navigate in dense fog in open water and are targeting an island to camp on. If your receiver is set to a datum different from the one your maps (and map-derived coordinates) are based on. You run the risk of missing the safety of that sheltered campsite. In a similar fashion you must ensure that the bearing of travel indicated by your GPS is compatible with how you are using your compass-is one bearing to true-north, the other to magnetic north?

We are now going to take a look at a straightforward example of how GPS can enhance your paddling experience.

Trip Planning: During route planning, take the time to scale coordinates of key features along the route from your maps-these may include campsites, sources of fresh water, navigation hazards, landmarks and so forth. Make a list of these 'waypoints' in your logbook and give them each an identifying name. If your receiver comes with software for downloading waypoints you can enter your waypoints into your computer and then download them to the receiver. If not then enter them into the GPS receiver manually. Enroute your receiver will be able to provide distance and bearing to your next waypoint. As part of this process develop a habit of checking all the setup parameters of your receiver including reference datum, declination, etc.-also check your batteries!
Enroute: I would be surprised if any trip followed the exact route planned-serendipity plays a part in any outdoor adventure. Fortunately you can update and add to your list of waypoints as you travel. Depending on the scale of the map you are using and on your plotting skills, it is most likely that a GPS position will be more reliable than the one you entered from the map. Therefore as you paddle you may want to take the time to update the coordinates of your pre-selected waypoints in addition to adding new ones along the way. This will allow you to retrace your route in addition to building a library of waypoints to be used on future trips or passed onto friends.

Putting it All Together: The weather has turned, visibility is down and you are disoriented. It is now time to implement the steps practiced under more benign conditions. Your GPS receiver will provide your position as well as a distance and bearing to your next waypoint. With the use of a compass you can now follow a bearing, stopping occasionally to check your position and, if necessary, assume an updated course. You may also want to save some waypoints enroute in case you want to retrace your exact path.

Unfortunately technology is not infallible, especially under relatively harsh marine environments. GPS receivers can fail due to dead batteries, getting wet, falling overboard and so on. Therefore it is prudent to back up your electronic navigating with the use of your map and logbook enroute. Annotations on a map or in your book could save the day. For example as you enter your waypoint in the receiver, note the coordinates in your log. Also note the bearing on which you have been travelling and the length of time you traveled on that bearing, and so on. These are classical orienteering or navigation skills you must possess-GPS does not replace these skills, but merely gives you another tool to use.

Next Issue of WaveLength: Having now covered some of the background, (the part in the manual most people skip), and a simple example of the use of a GPS receiver, we will go on to examine some of the more advanced features of GPS receivers currently on the market. I will also take time to discuss indicators of position reliability, Y2K and the future of GPS.

Go to Part 2

Michael Schmidt has worked with GPS since 1980. In 1992 he led an expedition to measure the height of Mt. Logan (5959m) using GPS. He will be offering introductory GPS Navigation Courses through Ocean River Sport, Victoria, BC this spring. For more details please contact ORS Adventure Centre at (800) 909-4233 or info@oceanriver.com
Michael Schmidt's email is mschmidt@islandnet.com. ©