GPS: How it Works

• GPS receivers measure the distance between themselves and at least three satellites to determine position.
• The receiver calculates its position as the intersection of spheres, using satellite distances as radii and satellites as centers.

GPS Signal Components

• Each GPS satellite transmits a signal with a unique digital code for identification.
• The signal includes a navigation message with satellite orbit data ("ephemeris data"), GPS time, almanac data, and coefficients for atmospheric correction.
• The receiver calculates atmospheric effects on signal transmission.

Time and Distance Calculation

• Satellites transmit the instant of signal transmission in GPS time.
• Receivers synchronize to GPS time to calculate the difference between transmission and reception.
• The distance, or "pseudorange," is calculated by multiplying the time difference by the speed of the signal (186,000 miles per second).
• Atomic clocks could offer absolute accuracy, but are too expensive for GPS receivers.
• Crystal oscillators synchronized with satellite clocks achieve necessary precision instead.

Position Fixing

• Absolute distance accuracy would allow latitude and longitude to be fixed using two satellites.
• GPS acquires pseudorange from a third satellite to adjust for inaccuracies.
• Pseudoranges adjust until Lines of Position (LOPs) converge.
• The receiver solves simultaneous equations with pseudoranges from observed satellites to estimate position.
• The receiver accounts for its velocity by comparing satellite signal frequencies to an internally generated reference signal.
• The Doppler effect is used to compute velocity relative to satellites.
• Recalculation leads to a final position fix.

GPS Basics

• GPS is a radionavigation system.
• It was designed and operated by the U.S. Department of Defense (DOD).
• It is available to both military and civilian users.
• It was designed in 1973 and initial test satellites were launched in 1978.
• Reliable worldwide latitude and longitude determination was possible by 1991.
• The 24th satellite was launched in 1994 to complete the system.
• Full operational capability was declared on July 17, 1995.

GPS Satellites and Signals

• The GPS constellation consists of 21 active and 3 "spare" satellites.
• Satellites orbit at 10,900 nautical miles and a 55° inclination to the equator.
• Each satellite completes an orbit in 12 hours, repeating the ground pattern daily, but appearing ~4 minutes earlier each day.
• Satellite orbits require periodic updates tracked by monitoring stations.
• Data is recomputed and transmitted back to satellites.
• Satellites transmit on two frequencies: L1 (1575.42 MHz) and L2 (1227.6 MHz).
• Nonmilitary users process only L1 with a Coarse/Acquisition (C/A) code.
• Military receivers can use signals on both frequencies.
• New signals are being added including Civilian L2C, L5, and a new civilian L1.
• The satellite altitude and orbital patterns mean at least six satellites are always observable from the ground.
• Signals from three satellites are sufficient for latitude and longitude; a fourth is needed for altitude determination.
• Receiving additional signals increases accuracy.
• Satellites transmit on the same frequencies, but each has identifying information.

GPS Applications

• GPS is used in maritime, air, rail, bus, private cars, and wilderness hiking.
• It can be used to show a user's location, or the location data can be transmitted to a central location for tracking.
• It can be used to monitor movements of vehicles and wild animals.
• Advanced applications with computer processing achieves centimeter accuracy for land surveying.
• Some cellular telephones have integrated GPS capabilities for emergency services.

GPS Accuracy

• GPS provides two levels of fix accuracy.
• SPS (Standard Positioning Service) is available to all users.
• Without Selective Availability, it provides horizontal accuracy of ~13 meters for 95% of fixes.
• Vertical accuracy is 22 meters or less, but may be of less interest to boaters.
• PPS (Precision Positioning Service) is available for U.S. military and authorized users, as well as a limited number of nonmilitary users, offering higher accuracy and encrypted data.
• Differential signals improve the accuracy.

GPS and Chart Considerations

• GPS should be used with up-to-date nautical charts labeled with NAD-83 or WGS-84 datum systems.
• Older charts based on NAD-27 may result in position errors of up to 160 meters.
• Receivers may have correction factors for older chart datums.
• GPS location information may be more accurate than the chart being used.
• First surveys were less accurate than current GPS.
• GPS positions are shown to a degree of precision beyond the scale of chart markings.
• 6 feet of the third decimal places has little practical meaning or navigation.

Selective Availability (SA)

• When GPS was introduced for civilian use, the DOD downgraded its accuracy through Selective Availability (SA).
• SA was set to zero in May 2000.
• In 2007, the recommendation to permanently preclude SA capability was accepted, for many critical services.
• However, GPS signals are subject to local interference that can degrade accuracy.

GPS Integrity

• GPS has no inherent way to alert users to malfunctioning satellites.
• Receivers calculate fixes from three healthy satellites.
• Aviation and marine receivers use Receiver Autonomous Integrity Monitoring (RAIM).
• Differential GPS signals and Wide Area Augmentation System (WAAS) receivers notify users of erroneous satellite transmission.

GPS Receivers

• Portable and installed GPS receivers are available.
• Some portable units are handheld with an integral antenna.
• Others have the option to be mounted for external antenna use, and draw power from vessel.
• Handheld GPS receivers are a valuable component of the "abandon-ship bag."

GPS Receiver Capabilities

• Some installed GPS units have integrated signal acquisition, computing, and display.
• Others have signal acquisition and computing equipment in a separate antenna unit.
• These units can operate multiple displays.
• They can operate multiple displays from a single antenna.
• Position information transmits to other devices on a data network.
• A full-function receiver displays the vessel's geographic position.
• Displays include course and speed over ground (COG and SOG), computed and smoothed.
• Courses and bearings are shown as true or magnetic, as well as the option to manually input degrees of varation.
• GPS compasses use two or three antennas to derive heading data.
• Internal memory can store coordinates of WAYPOINTS for future use.
• ROUTES can be preplanned using waypoints.
• Most sets can store multiple routes and reverse the direction of a route.
• Transfers from one leg to the next on a route can be automatic or manual, some with alarms.
• Internal computing capabilities determine distance and direction between waypoints.
• The computed track data assists with cross-track error, and helps the pilot to bring the user back on track.
• Receivers can display how long it will take to reach a certain point and give an ETA based on combining speed, distance and time.
• A "man overboard" function saves the vessel's position when activated.
• GPS signals include precise time information displayed as local zone time or UTC.
• GPS Time is not the same as correct UTC. UTC is adjusted to compensate for slowing of the earth's rotation, and the GPS time is about 14 seconds ahead of UTC.
• Some GPS receivers have a correction for time difference, and some show a slight lag between calculation and display.
• Some units display sunrise and sunset times or tidal information.
• Receivers may store/display monochrome or multicolored chart representations.
• The icon on the chart indicates the direction.
• Many can be programmed to stand "anchor watch," in which the receiver sounds an alarm if the vessel drifts outside the set limit.

GPS Antennas

• External GPS antennas are small.
• They range between 1 to 3 inches in diameter, and 2 to 15 inches high.
• Antennas must be mounted on the vessels exterior with a 360 view.
• Mounting should be high enough to provide a clear view without pitch and roll causing the signal to degrade.
• A separate radar pulses is also necessary to avoid GPS antenna in being in signal.

Differential GPS (DGPS)

• Civilian GPS fix accuracy degrades due to limitations within the C/A code.
• Differential GPS (DGPS) offsets this source of error through refinement.
• The Maritime DGPS network by the U.S. Coast Guard facilitates maritime navigation requirements beyond standard civilian GPS operation.
• The civilian mode of GPS can be obtained alone with or without the activation of SA.
• The system is enabled with a series of fixed reference stations that contain precise geographical coordinates in its parameters.
• To generate a compensating correction for all signals in sight, each known satellite is measured and compared with a system signal to assess position.
• Data corrections transmit within a broadcast range of 75 to 700 miles.
• Although DGPS was initially planned as a system for major ports, the signals of its transmitters now offer coverage for all coastal waters.
• Also included in this are the Great Lakes, plus Alaska, Hawaii and Puerto Rico out to 200 nautical miles offshore.

WAAS Differential Corrections

• The Department of Transportation has developed the Wide Area Augmentation System (WAAS).
• As opposed to DGPS, there is no separate receiver (and antenna) needed.
• When in remote locations from DGPS transmitters, WAAS position accuracy can be superior to what is achieved using the low-frequency Coast Guard signals.
• There are marine GPS receivers available that use WAS corrections in lieu of traditional marine broadcasts.
• WAAS corrections are unavailable for open ocean waters; European EGNOS are available in many foreign countries.