GPS for Mariners, 2nd Edition: A Guide for the Recreational Boater

By Robert J. Sweet

The operator's manual that should come with every boater's GPS receiver or chart plotter.

• Language: English
• Publisher: McGraw-Hill Education
• Release date: Apr 8, 2011
• ISBN: 9780071744621

Book preview

navigation.

CHAPTER 1

GPS Navigation Basics

GPS (global positioning system) is a marvel of engineering that uses a constellation of satellites, a system of supporting ground stations, and coded radio transmissions all to do one simple thing—tell you exactly where you are. From there, the tiny computer in your GPS receiver uses that information to determine your direction, your speed, and the distance, course, and time to your next destination.

It is up to you to choose that destination wisely. A car GPS navigation unit guides you to your destination over a sequence of paved roads, preselected because they are suitable for your vehicle. On the water, where potential perils lurk below the surface, you must create your own courses (roads) using information provided by charts. Navigating to your destination on the water requires you to select and track your direction and progress. This is the fun and challenge of marine navigation, and GPS is one of your most valuable tools, provided you know how to use it. This book walks you through the steps. You don’t need to know a whole lot about how GPS works to use it effectively, but a little background might be helpful.

HISTORICAL PERSPECTIVE

ENTERING A HARBOR. Harbor entry is a navigation challenge when visibility is limited. Having a GPS unit onboard can be a great help and comfort because it assists in making your passage safe. Marine GPS navigation, unlike land-based GPS use, requires constant coordination of the position provided by your GPS with your nautical chart as well as a visual check of your position.

I became involved with GPS in the latter 1970s as a program manager and business-unit director for a major contractor developing GPS for the U.S. military. At that time, the air force had program responsibility for what was—and still is—called NAVSTAR GPS. At first, Congress was reluctant to fund the program, but the civil channel on GPS helped sell them on it. (GPS was designed to have two distinct frequencies—one with a precision code for military use and one with less accuracy that could be further degraded for civil use.) Little did any of us anticipate today’s wide use of GPS by virtually everyone. GPS has truly revolutionized the concept of navigation.

ORIGINAL U.S. AIR FORCE NAVSTAR GPS IMPLEMENTATION. Designed for the military, GPS is a robust and reliable system providing highly accurate position information with the support of ground and monitoring stations to keep the satellites updated. (U.S. Air Force)

GPS SATELLITE CONSTELLATION COVERS THE EARTH. GPS satellites are deployed in six orbital planes inclined 55 degrees from the equator. Of the thirty-two existing satellites, at least twenty-one are operational at any given time. The U.S. Air Force controls the satellites and moves the spares to fill holes in coverage. Each satellite orbits the Earth every twenty-four hours. (U.S. Air Force)

HOW GPS PROVIDES POSITION

The sole function of GPS is to provide position via information received from its constellation of satellites and supporting ground stations. Everything else is calculated within your GPS receiver.

GPS satellites move constantly around the Earth, with their orbits inclined so they appear to move diagonally. Several satellites follow the same orbital path along an orbital plane, spaced evenly as they circle about 11,000 miles above the Earth’s surface. Six separate orbital planes are used. In all, some twenty-one or more GPS satellites are active at any given time, providing a net of coverage around the entire globe. Depicted from space, the constellation of satellites can be imagined as a swarm of bees. Typically, as many as a dozen satellites may be in view from your location, each moving across the sky. Some appear to rise while others sink below the horizon.

Orbiting GPS satellites date back to the 1970s. They have proven to be extremely reliable and usually outlive their ten-year planned lifetime by a considerable margin. The technology of the satellites has evolved, and each new block (group of satellites) has expanded features and better performance.

GPS was the first satellite navigation system, but it is not the only one. The Russians operate a system called GLONASS, which uses different frequencies and orbits and requires a special receiver. The Europeans are in the process of launching their own system, called Galileo, which is intended to inter-operate (work with) with GPS, but that’s still in the development stage. Galileo is scheduled for deployment around 2014 and is intended to expand ultimately to thirty satellites. China is also developing its own system, called Compass. The United States GPS, developed and managed by the U.S. Air Force, is evolving as well. As GPS has become more crucial to such uses as aircraft navigation, it has had to become more accurate and reliable. The air force is working to add new civil frequencies to improve signal reliability and accuracy for a number of applications with aircraft navigation and services within cities, where signal blockage is a challenge. The new frequencies will also support operations with Galileo. Some of these new frequencies are available on a limited number of satellites, largely for testing. The Block III satellites, which are scheduled to begin launch in 2013, will be equipped to transmit these new frequencies while still transmitting the Legacy L1 signal, which has been used for years. To take advantage of these new features, you will need updated receivers, but you will still be able to use older GPS receivers as well.

GPS BLOCK II SATELLITE. Block II satellites, notable for their extraordinary performance and agility, were the first widely deployed GPS satellites; they were built by Rockwell Corporation. GPS encompassed the first large constellation of satellites deployed for any mission. (U.S. Air Force)

HOW GPS WORKS

The concept is simple. It is based on the time it takes satellite signals to reach the GPS receiver on your boat. Each satellite sends a coded radio transmission, which is picked up by one of your unit’s receivers a short time later. The satellite, which knows its own precise location at the time it sent the transmission, provides that data as part of a message embedded within the transmission.

Your GPS unit’s internal clock recognizes the time, and since it knows how fast radio waves move, it can convert this signal transit time into distance from the satellite. All this happens in a few hundredths of a second. It does the same thing with a second satellite, and then a third and a fourth satellite, each with an assigned receiver. Imagine that of each of these satellites has a string attached to it, each with its own unique length. The place where the strings meet is your location. This information is shown graphically in the accompanying illustrations.

GPS BLOCK III SATELLITE. The Block III is a new model with enhanced capabilities. (Lockheed Martin)

THE ACCURACY OF GPS

How accurate is your GPS position? The short answer is: very accurate. With the enhanced systems described below, accuracy is 10 to 15 feet.

GPS offers better accuracy to the military than to civilian boaters. You’ve probably heard about smart GPS-guided weapons that can fly into a building through a window. The military has its own special frequency, which is reserved for their use. There is also a civil frequency, which we boaters use. The civil frequency was degraded by something called selective availability until May 2000 so that an enemy could not make use of our GPS system. The result was an error of up to 300 feet. Once selective availability was ended by a presidential order (it is not likely to return), civilian GPS position became accurate to within about 35 feet.

SIGNALS ARE TRANSMITTED FROM SATELLITES TO USERS. GPS satellites know their positions. Each satellite transmits a coded signal to the Earth telling the GPS receiver its identity, its precise location, and the exact time the signal was sent. The receiver compares the time the signal was sent with the time it was received to compute the distance from the satellite to the receiver.

UNCERTAINTY OF DISTANCE DUE TO GPS RECEIVER INACCURACY. Inaccuracies in the internal clock of a GPS unit and propagation errors distorting the path of the signal result in a band of uncertainty regarding your precise location. The circle is in fact a wide band, shown here in a light color. You know you are located somewhere within this band.

POSITION INFORMATION FROM A SINGLE GPS SATELLITE. The transit time of the signal from a single satellite to the receiver can be converted to distance. All possible points on the Earth at that same distance from the satellite result in a circle whose center is directly below the satellite.

That’s a big improvement, but it’s not good enough for navigating a harbor, and it’s certainly not good enough for aligning an airplane for landing, so solutions were developed by the U.S. Coast Guard and the Federal Aviation Administration that have greatly improved accuracy. More on this below.

To understand the solution, it helps to understand the problem. The radio waves that satellites transmit cannot travel in precisely straight lines through the Earth’s atmosphere. They are bent by layers of electrons in the upper atmosphere that are created by radiation from the sun. Therefore, the signal paths are not perfectly straight, and may not reflect the exact distance between the satellite and your location. The result is an inaccurate location reported on the GPS.

The solution to this problem lies in ground stations that identify the satellite errors and transmit them to your GPS unit, which then applies corrections to produce a more accurate position. Differential GPS (DGPS), which was created by the Coast Guard, uses individual stations that transmit to you by radio. You need a separate antenna and a DGPS receiver that is connected to your GPS to apply the corrections. The Coast Guard DGPS has stations located along the coasts and major waterways.

Another solution, the FAA’s wide area augmentation system (WAAS), is even more effective since it networks an entire national grid of ground monitoring stations from all across the country and is not limited to the coasts. WAAS was initiated to support aircraft need for greater accuracy. It evaluates the whole grid of information and transmits a table of corrections up to two geo-synchronous satellites (see graphic, page 11). These satellites remain stationary above the equator, one over the Atlantic and one over the Pacific. Each retransmits the data downward directly into boaters’ GPS units. Most new GPS units are able to receive and process this information directly without needing additional equipment—in essence, for free. Once loaded into your GPS unit, these corrections significantly improve the accuracy of your position to about 10 to 15 feet.

To provide you with an accurate position, your GPS solves a set of simultaneous equations for four unknowns: latitude, longitude, altitude, and time. In order to do that, it needs information from four independent known quantities—that is, from four separate satellites.

REFINING OF GPS POSITION WITH TWO SATELLITES. Decoding the signal from a second satellite results in a new distance and a second circle on the surface of the Earth with its center under the second satellite. The two independent and intersecting circles narrow down the user’s possible position to two locations. You know you are located at one of the two intersections of the two circles, but a problem with clock calibration will create uncertainty and produce circular bands of some width rather than sharp lines. Your GPS screen will indicate that the GPS is still acquiring satellites.

A THIRD SATELLITE PRODUCES A 2-D FIX. Adding the signal and the corresponding computed distance from a third satellite narrows the region of your position to one unambiguous area, as shown by the yellow circle on the diagram. By assuming your altitude to be at the Earth’s surface, your GPS unit will adjust its internal clock until the circles formed by the three computed distances converge to a point. This 2-D fix will provide a position to within 100 feet or so; it can be used for navigation, but you should take care in the absence of signals from more satellites.

REFINING THE ACCURACY WITH A FOURTH SATELLITE. Signals from a fourth satellite enable the GPS receiver to refine its internal clock and to establish a 3-D fix by computing a value for altitude as well as latitude and longitude. This is needed because of the uncertainty resulting from each satellite (depicted by the intersecting wide bands). Using the fourth satellite permits refined adjustment of the GPS unit’s clock and results in a very accurate 3-D position, as shown by three narrow black lines within the shaded intersection of the three bands, with your fix position located at the black dot.

Terms Used with GPS

Position: Your present location, measured by a latitude and a longitude

Course over ground (COG) or track: Your present direction of travel over ground*

Speed (speed over ground, or SOG): Your present speed over ground*

Course: Your intended course over ground from the point where you activated a waypoint to that now active waypoint

Waypoint: A location identified by coordinates and a name

Active waypoint: The waypoint that you are navigating to

Leg: The straight-line path between two waypoints

Active leg: The leg you are currently navigating, also considered to be your course

Off course, crosstrack error: The distance of your present position laterally from the course line either left or right (port or starboard); updated continually

Bearing: The direction from the present position to the active waypoint; updated continually

Distance: The distance from the present position to the active waypoint; updated continually

Course to steer: The direction to steer to the active waypoint; direct path, does not consider obstructions

Turn: The number of degrees left or right (port or starboard) to steer directly toward the active waypoint

Velocity made good: The effective rate of closure toward the active waypoint; the vector component toward the waypoint even if the boat is moving in a direction other than toward the waypoint. Particularly useful in sailing

Estimated time of arrival: The estimated clock time upon reaching the active way-point, based on present speed

Estimated time to go: The estimated time required to reach the active waypoint, based on present speed

Route: The sequence of waypoints and legs from starting location to destination

Estimated time at destination: The estimated clock time upon reaching the destination waypoint, based on present speed

Estimated time to destination: The estimated time required to reach the destination waypoint, based on present speed

* These values may not match the heading of the boat (the direction of the bow) or the boat’s speed through the water because winds and currents alter these values. COG and SOG are the true motion of the boat as reflected on a chart.

As stated earlier, there are a dozen or so satellites in view at any given time. Most GPS units have a dozen or more individual receivers, each tracking a separate satellite as it passes by. The satellites stay in view for quite a while because a full orbit of each satellite takes about twelve hours, but you won’t be there to see it for long because you, on Earth, have moved on. Your unit continually selects the best combination of four satellites to provide an accurate position.

What is best? That depends on two things—signal strength and satellite position. Satellite position is important; four satellites directly overhead may provide strong signals, but they don’t provide much triangulation of position. (If you know how to plot a position on a chart, you already know why triangulation is important. We go over that later.)

The best result is attained when satellites are widely spaced. Your GPS unit checks the quality of a fix by computing a factor called geometric dilution of precision (GDOP). The closer the GDOP number is to 1, the better the fix; a GDOP of 3 is poor. Your GPS unit’s computer selects the satellites that produce the best combination of signal strength and GDOP (see graphic, page 11). Your unit makes all these decisions by continually updating and adjusting.

What does all this mean to you? It means that you need to make sure your GPS unit, and particularly its antenna, has a reasonably clear view of the entire sky and that it is pointed upward. The antenna is designed with broad hemispherical coverage, but a tilted antenna can lose some satellite signals because its coverage is above the horizon on one side and into the ground on the other. You are likely to miss those satellites near the horizon away from the antenna. The coverage issue is also important in antenna placement; any blockage, especially metallic, can interfere with satellite signals. These signals are extremely weak as they reach you on the ground, and blocking them will reduce their strength or even eliminate them altogether. Most GPS units have a satellite screen that acts like a signal meter to help you with positioning the antenna. (See Chapter 2 for a discussion of the screen.)

GPS PROVIDES A 3-D POINT IN SPACE. The purpose of GPS is to provide a precise position in the form of a 3-D position in space. Each satellite transmits information about its identity and location, plus a time signal. The receiver makes note of the time the signal is received to determine how far the signal traveled. With signals from four or more satellites, the receiver can deduce position and precise time.

PROPAGATION ERRORS IN GPS SIGNALS. These errors are introduced by the bending and shifting of a satellite signal’s path through the atmosphere; the errors are small, but they prevent GPS from being accurate enough to navigate in harbors and narrow channels. This kind of error is uncontrollable, but it can be corrected.

GPS MONITORING RECEIVERS. Monitoring receivers are fixed ground stations that are located in precisely known positions and receive the same GPS satellite signals that you do. By knowing the correct location, the station can unravel errors attributable to each satellite. Corrections for these errors are then transmitted to your boat, where your GPS applies them to its final position solution. Although the corrections apply to the location of the ground station rather than to where you may be, they help considerably since you are likely to be close enough to benefit. The objective is to provide greater accuracy for harbor and channel operations than basic GPS affords.

NAVIGATION PRIMER

24-Hour Time/Universal Time/Daylight Saving Time

Most navigators use the 24-hour clock. In this system, time begins at midnight at 0000 hours and extends to 1200 hours at noon. What would be considered 1 p.m. becomes 1200 hours plus 0100 hour, or 1300 hours.

Universal time (UT), sometimes called Greenwich mean time (GMT), is based on the meridian running through Greenwich, England. It corresponds with 0° longitude. Since the Earth moves eastward, local time to the west is earlier than at Greenwich. For example, on the East Coast of the United States, the local time is five hours earlier than UT. Most GPS units allow you to enter an offset because GPS time is based on Greenwich. The offset for the East Coast is thereby minus five hours. Other GPS units allow you to select the region, and the offset is applied for you.

24-HOUR CLOCK. This clock shows an alternate value for each hour. Since the clock has two 12-hour cycles per day, the second cycle continues the count using 1300 for 1 p.m., 1400 for 2 p.m., and so on. This system is easier to use when comparing times for different locations and is the common reference for marine navigation.

For most regions, the offset is one hour less during daylight saving time. This means that during daylight saving time on the East Coast of the United States, the offset is minus four hours.

Time zones each span 15° of longitude. The UT time zone centers on the meridian running through Greenwich, England, and extends to 7.5°E and 7.5° W. For example, the next time zone to the west is centered at 15° W, extending from 7.5° W to 22.5° W, and so on. Each time zone adds an additional hour, later to the east and earlier to the west.

TIME REFERENCED TO GREENWICH, ENGLAND. Universal time (UT) is needed to set your GPS to the present local time. This graphic illustrates the number of hours that must be subtracted from universal time (UT), also called Greenwich mean time (GMT), to get local time for the United States. During daylight saving time, you need to add back an hour.

You may have wondered how your GPS unit knows where the satellites are. When you first power up, your GPS unit looks for satellites until it locks on to one or more of them. It then decodes the messages they send. From this, it knows what other satellites should be in view and can acquire them. If you shut down your GPS and restart it in the same location, it assumes you are where you were when it was last operating, and it can acquire satellites more quickly because it knows what satellites should be in view at this time. If you start your GPS at a different location, you will notice that it takes longer to get its position fix. This is because of the extra time needed to acquire the first satellite since it may be looking for the satellites that were in view the last time it was used.

POOR SATELLITE GEOMETRY. Satellites that are bunched together may produce less-accurate positions because they are too close together for effective triangulation. If all the available satellites are closely spaced, the fix is adversely affected.

Using three satellites, if only three were available at the time, your GPS unit would assume that its altitude is zero and compute latitude and longitude and precise time.