Article, November 2004
Global Positioning System --
Where Exactly Are You?
by Joe Schmitt,
Tampa Bay Computer Society, Florida
). All three of these companies make good products. Garmin has the largest market share with Magellan in second place. Often the brand loyalty of the owners is akin to the IBM and Macintosh debate. But regardless of the differences, all the units operate in much the same manner and it comes down to personal preference more than anything else. The manufacturers have similar products to each other and are reliable and rugged. Most of these models that are designed for outdoor use are, as a result of their intended use, built very well. Most handheld models are waterproof and shock proof.
This is the first of a three article series on the Global Positioning System. The first discusses what the system is and how it works. The two subsequent articles will delve into receivers and uses to civilians.
For centuries, man has relied on the most rudimentary of tools to navigate. Things like a compass or sextant, and later a timepiece, were the best he had. In the past, the most difficult task of a ship’s captain was to maintain the location and heading of a ship pounded by winds and currents. As time went on, those tools were perfected and improved. Though they worked well, they were not perfect. It was a time consuming process to mark position; early airplane pilots often navigated using existing landmarks and roads because the speed of their craft made traditional methods cumbersome.
A system called Loran (Long Range Navigation) was developed in the 1940’s that utilized radio pulses which were projected on board a ship using a cathode ray tube, similar to your computer monitor. Though accurate to a degree, this system was expensive and offered only a limited area of use.
The global positioning system is a revolution in the way we navigate today. The global positioning system is a set of satellites that just like Loran that use radio signals to determine position. Popularly known as GPS, the system uses a “constellation” of 24 satellites in high orbit to determine position in latitude and longitude. In addition to position, the system also can determine altitude and speed.
GPS was first implemented in the early 1970s as a way for the military to improve its ability to navigate and position vehicles. This system eventually evolved to include civilian as well as military uses. In the early days of GPS the signal was intentionally diffused to deteriorate the accuracy of civilian receivers. This use of selective availability as the Department of Defense (DOD) calls it, rendered civilian receivers accurate only within a hundred yards or so. Imagine having the unit telling you make a turn on the next road after you’ve passed it. In 2000, President Clinton took a serious look at the system and its potential to the civilian population. Selective availability was disabled on May 2, 2000 making the system accurate to within thirty feet or so.
So how does all this work? Well, as mentioned previously, there are 24 satellites orbiting the earth that broadcast radio signals. To better understand the system, a simple idea of two-dimensional navigation must be explained. Suppose you are in a field with one of those sign posts that point to a bunch of different places with distances. The top arrow of the sign says Montgomery, Alabama – 139 miles. The next arrow down says Savannah, Georgia – 217 miles. The third arrow points to Nashville, Tennessee – 231 miles. Now sit down at your favorite map and draw a circle around Montgomery with a radius of 139 miles. Next draw a circle around Savannah and Nashville with the radius the distance on our imaginary sign post. Where those three circles all intersect is roughly Atlanta, Georgia. Easy enough to understand, huh?
GPS works with the same principal. The receiver determines its location by its distance from the satellites. Hold it one minute! But those aren’t satellites stationary? Well the satellites move on a predictable path and within a consistent time frame. Inside each GPS receiver, is a programmed almanac of the position of each satellite based on time. Each satellite broadcasts a signal which moves at the speed of light. By measuring how long the signal takes to reach the receiver, distance from the satellite is determined. Using three or more satellites the receiver can then triangulate its position very much the same way we did with our imaginary signpost.
These kinds of accurate calculations with radio signals require precise timing, so on each satellite is an atomic clock which is monitored and corrected by ground stations. Using the accuracy of the atomic clock, a code pattern is broadcast from the satellite at a starting point. The receiver knows what time the signal left the satellite based on the portion of the code it is currently receiving. Measuring the time it took for the signal to reach the receiver by comparing the segment of code received with the time in the receiver, the distance to the satellite is determined. The distances from three or more satellites are compared and the position is formulated. That position is then displayed on your receiver as a set of coordinates, or graphically on an electronic map.
Using this pattern the receiver checks and adjusts its internal quartz clock to match that of the atomic clock. So, in addition to knowing where you are, you also know the precise time. Imagine having an atomic clock in your pocket! The receiver also can determine speed by measuring its change in position over time. This is a three dimensional arrangement and so altitude is also calculated.
This is all great and wonderful, but there are some issues. Gravitational pulls from the sun and moon affect the orbits of the satellites. Changing atmospheric conditions also can affect how well the signal travels from the satellite to the receiver. To combat this, the DOD has several ground stations that constantly monitor the signals and make adjustments in the satellites as necessary. In addition, there are two geo-stationary satellites which are in a fixed position in the sky on both sides of the earth. These systems are called WAAS, or Wide Angle Augmentation System. This system’s sole purpose is to correct for the issues that affect the accuracy of the broadcast code.
Now with a simple receiver, John Q Public can use a multibillion dollar DOD system. At this time it is estimated that there are approximately 200 civilian users for each military user of the system!
Next month, I’ll discuss the different types of receivers available to you as a consumer and some neat things that can be done with them. In the mean time, check out these interesting sites:
Official page for GPS management Gps.losangeles.af.mil
NASA page on GPS Gpshome.scc.nasa.gov
U.S. Navy GPS Timing Operations Tycho.usno.navy.mil/gps.html
Block II specifications: http://www.spaceandtech.com/spacedata/constellations/navstar-gps-block2_conspecs.shtml
This is the second of a three article series on the Global Positioning System. The first discussed what the system is and how it works. The second and third articles will delve into receivers and uses to civilians.
In my last article I discussed how the Global Positioning System (GPS) works and how it has changed navigation. GPS utilizes a set of 24 or more orbiting satellites broadcasting a coded signal that is used by a ground receiver to pinpoint location, speed, and distance while moving. I would like to delve into the numerous receivers available on the market to the civilian user. Essentially there are four types of receivers available.
The first is the military model. This operates on entirely different frequency than their civilian counterparts. The very accurate military units are on what is known as the P-code and Y-code and should be accurate within a foot or so. They include mapping features similar to consumer models.
The second type is surveying/Geographic Information System (GIS) models. Quite a bit larger than consumer models, they can easily be the size of a small briefcase. They are accurate to within one centimeter. This accuracy is obtained after being in the field through post-processing. The data gathered is stored and analyzed in a lab to compensate for atmospheric errors from the satellites.
The third type, for commercial transportation, is used to navigate ships at sea and aircraft. Often these units are tied into an autopilot system used to maneuver the vehicle. The units are quite large and permanently installed in the control panels of the vehicle.
The fourth type, consumer models, will be the focus of this discussion. The consumer model GPS unit can be permanently installed in a car or be handheld. There are units designed for joggers and bikers, those specifically for boating, and hiking models as well.
There are literally hundreds of different models available for you to choose from. The very basic models start at approximately $100 and go up from there. Some characteristics are common to almost all GPS receivers. They are able to store a waypoint, which is a location defined by its latitude and longitude marked in the unit’s memory. All have a display screen that may contain a digital map and a back light. They also are able to tell you the time, location in coordinates, speed, and distance from waypoints. Some are able to connect to a computer to transfer data, and some display altitude.
Beyond these basic functions is where choosing a receiver can be a mind-boggling mess. The activity you want a GPS for will determine which features to look for. There are three major manufacturers of GPS receivers: Garmin (http://www.garmin.com), Magellan (http://www.magellangps.com), and Lowrance (A href="http://www.lowrance.com">http://www.lowrance.com
My receiver is made by Garmin. The model is GPSMAP 60c (http://www.garmin.com/products/gpsmap60c/). It is a bit larger than a cell phone and has a color display.
This model runs on 2 AA batteries – you’ll find that most handheld models run on 2 or 4 AA batteries. With normal usage, I can get about 30 hours of use with my rechargeable batteries. The unit has an external power cord that can be plugged into a cigarette lighter socket to save on those batteries. This model is a mapping unit, so I can upload maps to it and display my location on that map. Additionally, this unit supports auto-routing, which is a feature that allows the unit to give you turn-by-turn directions on the road.
With most units there are some basic features that you should be aware of. First the display screen has several different modes for display. The most common is a satellite page. This displays the satellites in graphical format. As you can see, the satellites are numbered and the ones brightly colored are those the unit is currently receiving a signal from. The bars below show signal strength and status. The higher the bar is the better the signal. The bars that are solidly colored indicate that the unit has locked onto the satellite. The outlined bars indicate the unit is receiving the signal but has not locked onto it yet. This screen in particular displays the latitude and longitude coordinates. Just to left of those coordinates, the accuracy of the unit is displayed. The gray sphere with a red center shows the direction the unit is currently moving.
The next screen shown is also in most models. It is the waypoint management screen. Waypoints are a location in the real world stored in the unit as latitude and longitude, thus allowing you to locate a place to go or show distance from. This image shows the names of some waypoints I have stored in my unit. They can be listed either alphabetically or by proximity. As you can see by the highlighted point, the unit also displays its direction and distance to the selected point. After selecting the name and hitting enter, the details of the waypoint are displayed.
It shows its coordinates and elevation. The highlighted “CAR” point is located in Grand Canyon National park just as I was beginning a hike. Click on this link to see a map. This screen displays the symbol I have selected as well as the direction and distance from the current location. I have options to delete the point, display it on the map, and navigate to it.
If I were to navigate to it, I would get this screen.
The giant arrow points to the destination and of course the distance is displayed. The new item we see here is the speed. If I was moving when this picture was taken, you would also see the time it would take to reach the waypoint.
Across the top are several icons. These items are from left to right, the battery strength, 3D navigation status, USB connection status, and backlight status. The 3D navigation is particular to this model. It indicates that the receiver is getting a strong enough signal to find altitude as well as latitude and longitude. The arrow and compass move as the unit moves. As it is now, you would have to change your direction to the left a bit to head directly to the CAR waypoint. If you were to be moving away from the waypoint, the arrow would point down and the 12 would be at the top showing your heading as 120°.
The next screen displayed is the map screen.
This is showing the current location of the unit with a white arrow. The pink line going in the NW direction is the direction to the waypoint, and the line to the waypoint is a little to the left of where the arrow is pointing. The red line is a track or trail. The track displays where the unit has moved. In this case it came down the street to my home. As you can see, it looks like I was driving in my neighbors’ yards. The black lines are the streets and in this case the black blotches are ponds. The accuracy of unit displayed me off the street when in actuality I never left the road until I pulled in my driveway. In the lower left is the scale of the map. Above the map is data that was shown on the previous screen. The data above the map can be hidden as to provide a bigger map area on the display.
The GPS is also able to keep a travel log. As you look at the next screen, you can really get an idea of what kind of data the unit gathers.
It has a trip meter and odometer. Also it keeps track of moving data. The max speed the unit has read is displayed. The moving time is blank as this data was gathered on a long trip and eventually it surpassed the field’s ability to display. The moving average and overall average speed are great features. Before I had the GPS I would often occupy my mind while driving trying to figure out these very same averages. I doubt I ever got this precise. Elevation is also displayed.
Another useful feature of most GPS receivers is routing. Routing is the ability to string together waypoints to form a path. For example look at the next screen shot.
In this route are several points within a preplanned trip. From here in Tampa Bay we are going to St. Louis, Missouri. Each point defines a leg of the trip. When you navigate this route, the first point that the GPS directs you to is Atlanta. When you get to Atlanta, it changes the destination to Chattanooga. This continues until you have arrived at your destination. This is a great feature for driving as it allows you to keep updating the GPS with the correct directions but frees you from having to manipulate the GPS while driving. Most receivers show you destinations in a straight line, “as the crow flies.” A few of the more expensive units, such as this one, can auto-route. Auto-routing is a feature that navigates along roads loaded into the map as opposed to an as-the-crow-flies path. It displays turn-by-turn directions to your destination so that all you have to do is input the final destination. The unit then tells you which highways to get on and where to get off.
So how do you know what unit to buy? There are literally hundreds of different models to choose from. The unit that is right for you depends entirely on what you’re going to use it for. If you primarily want it to navigate a boat, airplane, or car there are large screen models that run off the vehicle’s electrical system and permanently mount to the console. If you want it to go hiking there are handheld models that can load topographic maps. The models range from very simple receivers that just stream data to large display models that can talk and direct you to the next exit verbally. Additionally there are other features like removable memory cards that map information can be loaded on. For a good place to find models available take a look to Joe Mehaffey, Jack Yeazel, and Dale Depriest’s http://www.gpsinformation.net site.
If you would like to try a GPS without purchasing a unit, go to Lowrance’s http://www.lowrance.com/software/pcsoftware/demos.asp site for a GPS simulator. They are listed by model and will install software on your machine. Although this will not actually lock onto satellites, it offers a great way to get a feel for their features and how they work.
So now you’re interested, right? But what would need one these things for? Just to give you an idea of the many uses: · Find your exact location on a map · Find where you left the car · Use to find your way back after getting lost in the woods · Tie it to the dog and see where he goes at night · Mark that great fishing spot · Find out where the cruise ship or airplane you’re on is · Calibrate your speedometer · Tell exactly how long to you get there · Use it a trip meter · Use it as a compass (when moving) · It’s fun to use
As you see there are tons of ways to amuse yourself. One of my favorite pastimes with the GPS is Geocaching (http://www.geocaching.com). This is a scavenger hunt with a GPS. A box is hidden in an interesting location like a city park. Inside a weather-proof box is a logbook and some trinkets. The coordinates of the “cache” are posted on the website providing details on the type of box, terrain, and notes from prior hunters. Because the GPS is only accurate within 10 feet or so, the cache can be quite difficult to locate.
Once found, you sign the logbook and exchange an item inside for one that you brought. The item is often of little monetary value. I often bring matchbox cars or maybe spare batteries for the GPS. Click on the link for the site and enter you’re zip code and check out how many are near you!
In addition to the using the receivers as stand-alone units almost all of them can tie into your computer to expand its capabilities. Next month I will show you all the programs that are available to you and some great resources on the web for maps and waypoints.
GPS and Mapping Software
This is the last in a three article series on the Global Positioning System. The first explained what the system is and how it work, the second dealt with GPS units, and the third discusses the various software available to interface with the units.
While out on the road or trail, the GPS can be indispensable for navigating. It tells you where you are, where you’re heading, and can track distance and speed. When you arrive at an interesting locale, you can mark it with a waypoint. Now all that is great, but one of the more interesting aspects of GPS ownership is the ability to link it with your computer for additional functionality.
In a unit that displays maps, only the most major roads and surface features are pre-loaded. This is known as the base map. The base map cannot be erased and is particular to the region that the unit is purchased in. For example, my Garmin receiver is loaded with a North American road map that contains US highways and interstates. This is great for a long trip, but doesn’t really help you around town. Luckily, additional specialized maps are available.
Each of the manufacturers have street level mapping available for purchase; unfortunately, the maps available are almost strictly proprietary. For my Garmin unit, there are instructions on the web to make your own maps, but is a long complex process involving multiple pieces of software. I could dedicate an entire article to that process alone. Another limitation with the mapping software comes with the opening of new roads. None of the software can be updated with the latest roads, so you must purchase the newer version at full price. Be careful though, sometimes it takes a few versions for the newer roads to be displayed at all. If you can live with data that is a couple of years old, places like EBay can be great for getting a deal on some older version.
Even if you do not have a GPS with mapping capability, quite often that receiver can be tied to a computer to process the data it receives. Most units utilize a serial cable to connect to a computer. Although a serial connection is much slower than USB it has been a standard with receivers for a long time and manufacturers are slowly coming out with USB capable units.
In addition to the proprietary software you can purchase from the manufacturer, there are pieces of software that can transfer waypoints and tracks from a GPS. A great waypoint manager, EasyGPS (http://www.easygps.com), is a free utility that allows you create, edit, and transfer waypoints from your computer. It works with Garmin, Lowrance, and Magellan receivers. Creating a waypoint with the receiver itself can be a long process since the unit does not have a keypad, and you will have to scroll through menus and numbers to set the coordinates. EasyGPS allows you to do this on your desktop computer with the keyboard.
Its bigger cousin, ExpertGPS (http://www.expertgps.com), displays your waypoints and routes on topographic maps and aerial photos. This software is not free, but you can download a demo. ExpertGPS uses an internet connection to retrieve maps as you scroll around.
There are several places you can get maps for free on the internet. You may already be familiar with some internet mapping websites for street cartography. Arguably one of the most popular is MapQuest (http://www.mapquest.com), but also popular is Microsoft MapPoint (mappoint.msn.com) and Maptech Mapserver (mapserver.maptech.com). In addition to these street map websites, many others offer topographic and aerial photos. Maptech offers topographic, nautical, aeronautical, and aerial photos, though they’ll want you to register.
For topographic maps, TopoZone (A href="http://www.topozone.com">http://www.topozone.com), is a great site that displays maps in various resolutions and datum. TopoZone allows you also to search by geographic features such as mountain peak names as well as city names, states, zip codes, and coordinates. Another excellent site that combines topographic and aerial photos is TerraServer USA (terraserver.microsoft.com). At this site you can flip between maps and photos.
Although these sites are helpful for viewing and printing maps, they do not offer any sort of data transfer with the GPS receiver. For that kind of functionality you must install some software. A great free application available for download is USAPhotoMaps (jdmcox.com). This program installs on your machine and then uses an Internet connection to download topographic and aerial photos. It can be linked with a GPS to transfer waypoints, tracks, and routes. The maps are compiled from free sites on the web. Be aware, though, that maps are quite large in file size and a slow connection will make getting the maps a long process. Once downloaded, the maps are stored locally on your computer so be sure that you have enough hard drive space.
The next realm in mapping software is independent mapping programs. Two of the most popular stand-alone street mapping programs is Microsoft’s Streets & Trips (http://www.microsoft.com/streets/default.asp) and Delorme’s Street Atlas (http://www.delorme.com/streetatlasusa/default.asp). Both programs offer detailed street mapping in the US and have the ability to link in various ways to the GPS receiver.
Streets and Trips allows you to link to your GPS and display your position on a map. It does not complete data transfer such as waypoints and routes. It has the ability to import data from Excel and text files, but an intermediate program will be needed to get the data from the GPS unit to the file for import. A bonus with Streets and Trips is that also comes with Pocket Maps that allow you to save maps to a Pocket PC.
Street Atlas allows you the same position display with a GPS. In addition to that function, it also allows you to plan a route and provides turn by turn instructions as you navigate. Of course, to be of any use it needs to be installed on a laptop. Unlike Streets and Trips, you must purchase the pocket mapping software separately. Streets and Trips runs about $40 and Street Atlas about $50. I have a copy of Streets and Trips that I got for $20 after a mail in rebate. Both programs are excellent and selection boils down to personal preference more than anything else.
Delorme also makes a topographic program, Topo USA (http://www.delorme.com/topousa/default.asp). This program runs about $99 and includes streets as well topographic contour maps. The topographic maps are at a 1:100,000 resolution. More detailed 1:24,000 maps, more suitable for hiking, are available on a state by state basis. Your GPS will interact in the same manner as Street Atlas.
A more cost effective way to go for a topographic program is National Geographic’s Backroads Explorer (maps.nationalgeographic.com/topo/backroads.cfm) that costs around $40. Although it lacks some of the features of Delorme’s software, it does include street maps as well as topographic information. Backroads Explorer also allows direct connection with a GPS for transfer of waypoints, routes, and tracks. It lacks the 3D topographic mapping the Delorme product offers, but it does have shaded relief to better define contours on the maps displayed.
For those that are into creating their own maps there is a program, called 3DEM (http://www.visualizationsoftware.com/3dem.htmlhat allows you load elevation data and generate a 3D terrain image. The terrain image can be overlaid with maps to form a 3D map. The primary data used to generate the terrain models are from radar topography. NASA scanned about 95% of the earth’s surface during shuttle missions and there is a wealth of sources available where you can download the data. These models can also accept data from your receiver so that you can see a track of where you’ve been in a 3D representation.
There is lots of additional software available on the market, too many to discuss them all here. A lot of programs offer various ways of linking with a GPS directly or are a great way to search and print maps before you leave on that next trip. There are also a number of utilities which can format data so that it can work with a number of programs.
GPS receivers are innovative and fun gadgets that can really be a powerful tool for navigation. Every day people come up with more interesting uses for the receiver. From handheld to vehicle mounted receivers, these devices are changing the way the man moves through the world. Have fun and I’ll see you on the road!