Differential GPS

Surveying and other work that demands a high level of precision use Differential GPS (DGPS) to increase the position accuracy of a GPS receiver. A stationary receiver measures GPS timing errors and broadcasts correction information to other GPS units that are capable of receiving the DGPS signals. Consumer GPS receivers that support DGPS require a separate beacon receiver that connects to the GPS unit. Consumers can receive DGPS signals from free or commercial sources.
Unless you’re doing survey or other specialized work, you really don’t need DGPS capabilities. For consumer use, the increased accuracy of DGPS has mostly been replaced with WAAS.

Coast Guard DGPS
DGPS signals are freely broadcast by a series of U.S. Coast Guard stations in the United States. Whether you can receive these Coast Guard broadcasts depends on your location.
For more information on DGPS, including coverage maps, pay a visit to
www.navcen.uscg.gov/dgps/coverage/default.htm.

Commercial DGPS
DGPS services are offered commercially for the surveying market. You can rent or purchase electronic and radio equipment for gathering precise location information in a relatively small area.

What is WAAS?

Wide Area Augmentation System (WAAS) combines satellites and ground stations for position accuracy of better than three meters. Vertical accuracy is also improved to three to seven meters.

WAAS is a Federal Aviation Administration (FAA) system, so GPS can be used for airplane flight approaches. The system has a series of ground-reference stations throughout the United States. These monitor GPS satellite data and then send the data to two master stations — one on the west coast and the other on the east coast. These master stations create a GPS message that corrects for position inaccuracies caused by satellite orbital drift and atmospheric conditions. The corrected messages are sent to non-NAVSTAR satellites in stationary orbit over the equator. The satellites then broadcast the data to GPS receivers that are WAAS-enabled.

GPS units that support WAAS have a built-in receiver to process the WAAS signals. You don’t need more hardware. Some GPS receivers support turning WAAS on and off. If WAAS is on, battery life is shorter (although not as significantly as it is when using the backlight). In fact, on these models, you can’t use WAAS if the receiver’s battery-saver mode is activated. Whether you turn WAAS on or off depends on your needs. Unless you need a higher level of accuracy, you can leave WAAS turned off if your GPS receiver supports toggling it on and off. WAAS is ideally suited for aviation as well as for open land and marine use. The system may not, however, provide any benefits in areas where trees or mountains obstruct the view of the horizon. Under certain conditions — say, when weak WAAS satellite signals are being received or the GPS receiver is a long way from a ground station — accuracy can actually worsen when WAAS is enabled.
WAAS is only available in North America. Other governments are establishing similar systems that use the same format radio signals such as

• European Euro Geostationary Navigation Overlay Service (EGNOS)
• Japanese Multi-Functional Satellite Augmentation System (MSAS)

GPS receiver as altimeter

The elevation or altitude calculated by a GPS receiver from satellite data isn’t very accurate. Because of this, some GPS units have altimeters, which provide the elevation, ascent/descent rates, change in elevation over distance or time, and the change of barometric pressure over time. (The rough-and-ready rule is that if barometric pressure is falling, bad weather is on the way; if it’s rising, clear weather is coming.) Calibrated and used correctly, barometric altimeters can be accurate within 10 feet of the actual elevation. Knowing your altitude is useful if you have something to reference it to, such as a topographic map. Altimeters are useful for hiking or in the mountains. On GPS units with an electronic altimeter/barometer, calibrating the altimeter to ensure accuracy is important. To do so, visit a physical location with a known elevation and enter the elevation according to the directions in your user’s guide.

Airports are good places to calibrate your altimeter or get an initial base reading; their elevation is posted for pilots to calibrate their airplanes’ altimeters. If you’re relying on the altimeter/barometer for recreational use, I recommend calibrating it before you head out on a trip. Some GPS receivers have features that allow you to increase the accuracy of your location by using radio signals not associated with the GPS satellites. If you see that a GPS receiver supports WAAS or Differential GPS, it has the potential to provide you with more accurate location data.

GPS Receiver as an electronic compass

All GPS receivers can tell you which direction you’re heading — that is, as long as you’re moving. The minute you stop, the receiver stops acting as a compass. To address this limitation, some GPS receivers incorporate an electronic compass that doesn’t rely on the GPS satellites.

Operation
Like with an old-fashioned compass, you can stand still and see which direction your GPS receiver is pointing toward. The only difference is that you see a digital display onscreen instead of a floating needle. On some GPS receivers, you need to hold the unit flat and level for the compass to work correctly. Other models have a three-axis compass that allows the receiver to be tilted.
Paying attention to these factors can improve the performance and convenience of an electronic compass:

• Magnetic fields: Metal objects, cars, and other electronic devices reduce the accuracy of any electronic or magnetic compass.
• Battery life: Using an electronic compass can impact battery life. Some GPS receivers have settings that turn off the compass or only use it when the receiver can’t determine a direction from satellite data.

Calibration
Electronic compasses need to be calibrated whenever you change batteries. If your GPS unit has an electronic compass, follow your user guide’s instructions to calibrate it. Usually, this requires being outside, holding the GPS unit flat and level, and slowly turning in a circle twice.

GPS Built-in maps

Every GPS receiver has an information page that shows waypoints and tracks. The page is a simple map that plots travel and locations. It doesn’t show roads, geographic features, or man-made structures. Some GPS receivers have maps that show roads, rivers, cities, and other features on their screens. You can zoom in and out to show different levels of detail. The two types of map receivers are

• Basemap: These GPS units have a basemap loaded into read-only memory that contains roads, highways, water bodies, cities, airports, railroads, and interstate exits. Basemap GPS receivers aren’t expandable, and you can’t load more detailed maps to the unit to supplement the existing basemap.
• Uploadable map: More detailed maps can be added to this type of unit (in either internal memory or an external memory card). You can install road maps, topographic maps, and nautical charts. Many of these maps also have built-in databases, so your GPS receiver can display restaurants, gas stations, or attractions near a certain location.

GPS receivers that display maps use proprietary map data from the manufacturer. That means you can’t load another manufacturer’s or software company’s maps into a GPS receiver. However, clever hackers reverse-engineered Garmin’s map format. Programs on the Internet can create and upload your own maps to Garmin GPS receivers; GPSmapper is popular.
A handheld GPS receiver’s screen is only several inches across. The limitations of such a small display certainly don’t make the devices replacements for traditional paper maps.

GPS Receiver Alarms

A GPS receiver alarm can transmit a tone or display a message when you approach a location that you specify. This feature can be especially useful when you’re trying to find a place and visibility is limited by darkness or inclement weather — or you’re busy doing something else and aren’t looking at the GPS receiver screen.

Display and output of a GPS receiver

GPS receivers have three choices for information display or data output:

• Monochrome LCD screen: Most GPS receivers have a monochrome liquid crystal display (LCD) screen.
• Color screen: These are especially useful for displaying maps. Color screens usually have shorter battery lives than monochrome ones.
• No screen: Some GPS receivers only transmit data through an expansion slot or a cable; a receiver with a cable is often called a mouse GPS receiver because it resembles a computer mouse. All GPS data can be sent to the laptop and processed there with mapping software.

Most GPS receivers that have screens can output data to a PC or PDA. A GPS receiver’s screen size depends on the receiver’s size. Smaller, lighter models have small screens; larger units sport bigger screens. Generally, a bigger screen is easier to read. Different models of GPS receiver also have different pixel resolutions; the higher the screen resolution, the more crisp the display will be. For night use, all screens can be backlit.

What Information that can be Obtained from GPS Receivers?

GPS receivers provide your location and other useful information:

• Time: A GPS receiver receives time information from atomic clocks, so it’s much more accurate than your wristwatch.
• Location: GPS provides your location in three dimensions:
• • Latitude (x coordinate)
• • Longitude (y coordinate)
• • Elevation
• The vertical (elevation) accuracy of consumer GPS receivers isn’t that great. It can be within 15 meters, 95 percent of time. Some GPS units incorporate more accurate barometric altimeters for better elevation information.
• Your location can be displayed in a number of coordinate systems, such as
• Latitude/longitude
• Universal Transverse Mercator (UTM)
• Speed: When you’re moving, a GPS receiver displays your speed.
• Direction of travel: A GPS receiver can display your direction of travel if you’re moving. If you’re stationary, the unit can’t use satellite signals to determine which direction you’re facing. Some GPS units have electronic compasses that show the direction the receiver is pointed whether you’re moving or standing still.
• Stored locations: You can store locations where you’ve been or want to go with a GPS receiver. These location positions are waypoints. Waypoints are important because a GPS unit can supply you with directions and information on how to get to a waypoint. A collection of waypoints that plots a course of travel is a route, which can also be stored. GPS receivers also store tracks (which are like an electronic collection of breadcrumb trails that show where you’re been).
• Cumulative data: A GPS receiver can also keep track of information such as the total distance traveled, average speed, maximum speed, minimum speed, elapsed time, and time to arrival at a specified location. All this information is displayed on different pages of the GPS receiver’s display screen. One page shows satellite status, another page displays a map, another displays trip data, and so on. With buttons on the receiver, you can scroll to an information page to view the data that you’re interested in seeing.

GPS errors

A number of conditions can reduce the accuracy of a GPS receiver. From a top-down perspective (from orbit down to ground level), the possible sources of trouble look like this:

• Ephemeris errors: Ephemeris errors occur when the satellite doesn’t correctly transmit its exact position in orbit.
• Ionosphere conditions: The ionosphere starts at about 43–50 miles above the Earth and continues for hundreds of miles. Satellite signals traveling through the ionosphere are slowed down because of plasma (a low density gas). Although GPS receivers attempt account for this delay, unexpected plasma activity can cause calculation errors.
• Troposphere conditions: The troposphere is the lowest region in the Earth’s atmosphere and goes from ground level up to about 11 miles. Variations in temperature, pressure, and humidity all can cause variations in how fast radio waves travel, resulting in relatively small accuracy errors.
• Timing errors: Because placing an atomic clock in every GPS receiver is impractical, timing errors from the receiver’s less-precise clock can cause slight position inaccuracies.
• Multipath errors: When a satellite signal bounces off a hard surface (such as a building or canyon wall) before it reaches the receiver, a delay in the travel time occurs, which causes an inaccurate distance calculation.
• Poor satellite coverage: When a significant part of the sky is blocked, your GPS unit has difficulty receiving satellite data. Unfortunately, you can’t say that if 50 percent (or some other percentage) of the sky is blocked, you’ll have poor satellite reception; this is because the GPS satellites are constantly moving in orbit. A satellite that provides a good signal one day may provide a poor signal at the exact same location on another day because its position has changed and is now being blocked by a tree. The more open sky you have, the better your chances of not having satellite signals blocked. Building interiors, streets surrounded by tall buildings, dense tree canopies, canyons, and mountainous areas are typical problem areas.

If satellite coverage is poor, try moving to a different location to see whether you get any improvement.

How accurate is a GPS receiver?

According to the government and GPS receiver manufacturers, expect your GPS unit to be accurate within 49 feet (that’s 15 meters for metric-savvy folks). If your GPS reports that you’re at a certain location, you can be reasonably sure that you’re within 49 feet of that exact set of coordinates.
GPS receivers tell you how accurate your position is. Based on the quality of
the satellite signals that the unit receives, the screen displays the estimated
accuracy in feet or meters. Accuracy depends on

• Receiver location
• Obstructions that block satellite signals

Even if you’re not a U.S. government or military GPS user, you can get more accuracy by using a GPS receiver that supports corrected location data.
Corrected information is broadcast over radio signals that come from either

• Non-GPS satellites
• Ground-based beacons

Two common sources of more accurate location data are

• Differential GPS (DGPS)
• Wide Area Augmentation System (WAAS)

Although survey-grade GPS receivers can provide accuracy of less than two centimeters, they are very specialized and expensive, require a lot of training, and aren’t very portable. Their accuracy is achieved with DGPS and postprocessing collected data to reduce location errors. The average GPS user doesn’t need this level of precision.

Clouds, rain, snow, and weather don’t reduce the strength of GPS signals enough to reduce accuracy. The only way that weather can weaken signals is when a significant amount of rain or snow accumulates on the GPS receiver antenna or on an overhead tree canopy.

Understanding Selective Availability (SA)

The average GPS user didn’t always have 15-meter accuracy. In the 1970s, studies showed that the less-accurate C/A-code for nonmilitary use, was more accurate than the U.S. government intended. Originally thought to provide accuracy within 100 meters, experiments showed that C/A accuracy was in the range of 20–30 meters. To reduce the accuracy of C/A-code, the U.S. government developed Selective Availability (SA). SA adds errors to NAVSTAR satellite data and prevents consumer GPS receivers from providing an extremely precise location fix.

Selective Availability was temporarily turned off in 1990 during the Persian Gulf War. There weren’t enough U.S. and allies military P-code GPS receivers, so the Coalition troops used civilian GPS receivers. The Gulf War was the first use of GPS in large-scale combat operations. On May 2, 2000, SA was turned off permanently. Overnight, the accuracy of civilian GPS users went from 100 meters to 15 meters. Turning off SA on a global scale was directly related to the U.S. military’s ability to degrade the C/A-code on a regional basis. For example, during the invasion of Afghanistan, the American military jammed GPS signals in Afghanistan to prevent the Taliban from using consumer receivers in operations against American forces.

Understanding GPS Receiver types

GPS receivers generally fall into five categories.

Consumer models
Consumers can buy practical GPS receivers in sporting goods stores. They’re easy to use and are mostly targeted for recreational and other uses that don’t require a high level of location precision. The Big Three manufacturers in the consumer GPS market are Garmin (www.garmin.com), Magellan (www. magellangps.com), and Lowrance (www.lowrance.com). Consumer GPS receivers are reasonably priced, from less than $100 to $400 in the U.S. This book emphasizes the features of and how to use the consumer-oriented, handheld types.
When you buy a consumer receiver, opt for a 12-channel GPS model over an older 8-channel model:

• 12-channel parallel receivers: These acquire satellites faster and operate better under foliage and tree-canopy cover.
• 8-channel receivers: These are slow when acquiring satellite signals and have difficulty operating even under light tree cover. Don’t consider purchasing an 8-channel receiver. Even if you were given one for free, it’s like running the latest version of Windows on a 386 computer.

U.S. military/government models
GPS units that receive P-code and Y-code are available only to the government. These portable units are Precision Lightweight GPS Receivers (PLGRs — or, more fondly, pluggers). First-generation PLGRs were big and boxy and provided accuracy within four meters. The newest precise GPS receivers are DAGRs (Defense Advanced Global Positioning System Receivers) and are smaller, more accurate, and have mapping features like consumer GPS units. For the specifications of U.S. military GPS receivers, including pictures of different units, visit http://army-gps.robins.af.mil.

Mapping/resource models
These portable receivers collect location points and line and area data that can be input into a Geographic Information System (GIS). They are more precise than consumer models, can store more data, and are much more expensive.

Survey models
These are used mostly for surveying land, where you may need accuracy down to the centimeter for legal or practical purposes. These units are extremely precise and store a large amount of data. They tend to be large, complex to use, and very expensive.

Commercial transportation models
These GPS receivers, not designed to be handheld, are installed in aircraft, ships, boats, trucks, and cars. They provide navigation information appropriate to the mode of transportation. These receivers may be part of an Automated Position Reporting System (APRS) that sends the vehicle’s location to a monitoring facility.
Trimble Navigation (www.trimble.com) is one of the biggest players in the nonconsumer GPS receiver market. If you’re interested in discovering how commercial and higher-end GPS units work and the features they support, check out the Trimble Web site.

Satellite data

GPS units receive two types of data from the NAVSTAR satellites.

Almanac
Almanac data contains the approximate positions of the satellites. The data is constantly being transmitted and is stored in the GPS receiver’s memory.

Ephemeris
Ephemeris data has the precise positions of the satellites. To get an accurate location fix, the receiver has to know how far away a satellite is. The GPS receiver calculates the distance to the satellite by using signals from the satellite.
Using the formula Distance = Velocity x Time, a GPS receiver calculates the satellite’s distance. A radio signal travels at the speed of light (186,000 miles per second). The GPS receiver needs to know how long the radio signal takes to travel from the satellite to the receiver in order to figure the distance. Both the satellite and the GPS receiver generate an identical pseudo-random code sequence. When the GPS receiver receives this transmitted code, it determines how much the code needs to be shifted (using the Doppler-shift principle) for the two code sequences to match. The shift is multiplied by the speed of light to determine the distance from the satellite to the receiver.

Covering GPS ground stations

Ground stations are the control segment of GPS. Five unmanned ground stations around the Earth monitor the satellites. Information from the stations is sent to a master control station — the Consolidated Space Operations Center (CSOC) at Schriever Air Force Base in Colorado — where the data is processed to determine each satellite’s ephemeris and timing errors.

An ephemeris is a list of the predicted positions of astronomical bodies such as the planets or the Moon. Ephemerides (the plural of ephemeris) have been around for thousands of years because of their importance in celestial navigation. Ephemerides are compiled to track the positions of the numerous satellites orbiting the earth.
The processed data is sent to the satellites once daily with ground antennas located around the world. This is kind of like syncing a personal digital assistant (PDA) with your personal computer to ensure that all the data is in sync between the two devices. Because the satellites have small built-in rockets, the CSOC can control them to ensure that they stay in a correct orbit.

Understanding GPS Signals

GPS satellites transmit two types of radio signals: C/A-code and P-code.
Briefly, here are the uses and differences of these two types of signals.

Coarse Acquisition (C/A-code)
Coarse Acquisition (C/A-code) is the type of signal that consumer GPS units receive. C/A-code is sent on the L1 band at a frequency of 1575.42 MHz. C/A broadcasts are known as the Standard Positioning Service (SPS). C/A-code is less accurate than P-code (see the following section) and is easier for U.S. military forces to jam and spoof (broadcast false signals to make a receiver think it’s somewhere else when it’s really not). The advantage of C/A-code is that it’s quicker to use for acquiring satellites and getting an initial position fix. Some military P-code receivers first track on the C/A-code and then switch over to P-code.

Precision (P-code)
P-code provides highly precise location information. P-code is difficult to jam and spoof. The U.S. military is the primary user of P-code transmissions, and it uses an encrypted form of the data (Y-code) so only special receivers can access the information. The P-code signal is broadcast on the L2 band at 1227.6 MHz.

P-code broadcasts are known as the Precise Positioning Service (PPS).

The GPS satellites

In GPS jargon, a satellite is the space segment. A constellation of 24 GPS satellites (21 operational and 3 spares) orbits about 12,000 miles above the Earth. The satellites zoom through the heavens at around 7,000 miles per hour. It takes about 12 hours for a satellite to completely orbit the Earth, passing over the exact same spot approximately every 24 hours. The satellites are positioned where a GPS receiver can receive signals from at least six of the satellites at any time, at any location on the Earth (if nothing obstructs the signals).
A satellite has three key pieces of hardware:

• Computer: This onboard computer controls its flight and other functions.
• Atomic clock: This keeps accurate time within three nanoseconds (around three-billionths of a second).
• Radio transmitter: This sends signals to Earth.

GPS satellites don’t just help you stay found. All GPS satellites since 1980 carry NUDET sensors. No, this isn’t some high-tech pornography-detection system. NUDET is an acronym for NUclear DETonation; GPS satellites have sensors to detect nuclear-weapon explosions, assess the threat of nuclear attack, and help evaluate nuclear strike damage. The solar-powered GPS satellites have a limited life span (around 10 years). When they start to fail, spares are activated or new satellites are sent into orbit to replace the old ones. This gives the government a chance to upgrade the GPS system by putting hardware with new features into space.

A short history of GPS

Military, government, and civilian users all over the world rely on GPS for navigation and location positioning, but radio signals have been used for navigation purposes since the 1920s. LORAN (Long Range Aid to Navigation), a position-finding system that measured the time difference of arriving radio signals, was developed during World War II.

The first step to GPS came way back in 1957 when the Russians launched Sputnik, the first satellite to orbit the Earth. Sputnik used a radio transmitter to broadcast telemetry information. Scientists at the Johns Hopkins Applied Physics Lab discovered that the Doppler shift phenomenon applied to the spacecraft — and almost unwittingly struck gold.

A down-to-earth, painless example of the Doppler shift principle is when you stand on a sidewalk and a police car speeds by in hot pursuit of a stolen motorcycle. The pitch of the police siren increases as the car approaches you and then drops sharply as it moves away. American scientists figured out that if they knew the satellite’s precise orbital position, they could accurately locate their exact position on Earth by listening to the pinging sounds and measuring the satellite’s radio signal Doppler shift. Satellites offered some possibilities for a navigation and positioning system, and the U.S. Department of Defense (DoD) explored the concept. By the 1960s, several rudimentary satellitepositioning systems existed. The U.S. Army, Navy, and Air Force were all working on independent versions of radio navigation systems that could provide accurate positioning and allweather, 24-hour coverage. In 1973, the Air Force was selected as the lead organization to consolidate all the military satellite navigation efforts into a single program.

This evolved into the NAVSTAR (Navigation Satellite Timing and Ranging) Global Positioning System, which is the official name for the United States’ GPS program. The U.S. military wasn’t just interested in GPS for navigation. A satellite location system can be used for weapons-system targeting. Smart weapons such as the Tomahawk cruise missile use GPS in their precision guidance systems. GPS, combined with contour-matching radar and digital image-matching optics, makes a Tomahawk an extremely accurate weapon. The possibility of an enemy using GPS against the United States is one reason why civilian GPS receivers are less accurate than their restricteduse military counterparts.

The first NAVSTAR satellite was launched in 1974 to test the concept. By the mid-1980s, more satellites were put in orbit to make the system functional. In 1994, the planned full constellation of 24 satellites was in place. Soon, the military declared the system completely operational. The program has been wildly successful and is still funded through the U.S. DoD.

What Is GPS?

GPS stands for Global Positioning System. A special radio receiver measures the distance from your location to satellites that orbit the earth broadcasting radio signals. GPS can pinpoint your position anywhere in the world. Pretty cool, huh? Aside from buying the receiver, the system is free for anyone. You can purchase an inexpensive GPS receiver, pop some batteries in it, turn it on, and presto! Your location appears on the screen. No map, compass, sextant, nor sundial is required. Just like magic. It’s not really magic, though, but has evolved from some great practical applications of science that have come together over the last 50 years.
Other satellite Global Positioning Systems are either in orbit or planned, but this book uses the term GPS for the Global Positioning System operated by the United States government.

Digital Orthophoto Quadrangle (DOQ)

Digital Orthophoto Quadrangle (DOQ) data consists of a computer-generated image of an aerial photograph. The image is corrected so that camera tilt and terrain relief don’t affect the accuracy. DOQs combine the image characteristics of a photograph with the geometric qualities of a map. The USGS has DOQs available for the entire United States. Most are grayscale, infrared photos; there are higher-resolution color photos for a few large U.S. metropolitan areas.

A booming business provides high-resolution, color aerial photographs to individuals, government agencies, corporations, and educational and nonprofit organizations. Companies like AirphotoUSA (www.airphotousa.com), Keyhole (www.keyhole.com) and DeLorme’s TopoBird subsidiary (www.topobird.com) provide imagery with quality and resolution that’s close to what was only available to intelligence agencies. If you want aerial photographs for business or government purposes, check these commercial sources.

Who is Mr. Sid?

This question is more accurately stated as what is MrSID? MrSID is the Multi-Resolution Seamless Image Database. It’s a file format used for distributing large images over networks, originally developed by a company called LizardTech. Graphics in MrSID format are compressed with a lossless compression algorithm (a method of compressing data that guarantees the original data can be restored exactly) designed to produce relatively small, high-resolution images. The file format is perfect for aerial and satellite images that have large file sizes, and the government is increasingly using it for distributing data. (The Library of Congress is even using it for electronic versions of paper documents.) A number of free viewers support MrSID; use Google to find download sites. (One of my favorites is IrfanView, which is available at www.irfanview.com.)

Digital Raster Graphics (DRG)

Digital Raster Graphics (DRG) data is a scanned image of a USGS topographic map. These digital maps are available for free on the Internet or are sold commercially in collections on CDs or DVDs.
These digital maps are scanned at 250 dpi (dots per inch) and stored in a TIFF file format, using embedded GeoTIFF (geographic information) tags for location data.
You can view the map by itself or both the map and its location data. Use one of the following methods:

• View the map by opening the DRG file with any current graphics program that supports large TIFF files.
• Use the DRG file with a mapping program that supports GeoTIFF to view the map and access its location data.

For more technical details about USGS digital map data, check out the agency’s product Web site:
mapping.usgs.gov/products.html