1. Global Positioning System
GPS
Global Positioning System
Diana Cooksey, Montana State University, LRES Department

2. Overview What is GPS & how does it work? Satellites Radio signals
Almanacs
Timing

3. What is GPS? Satellites orbiting the earth
Positioning, navigation and timing
Operates 24 hrs/day
Used for any application requiring location information
GPS is a satellite-based system, operated and maintained by the U.S. Department of Defense (DoD), that provides accurate location and timing information to people worldwide. The system transmits radio signals that can be used by GPS receivers to calculate position, velocity and time anywhere on earth, any time of day or night, in any kind of weather.
The NAVSTAR GPS concept was developed in the early 1970's to meet the U.S. military’s need for improved navigation and positioning. The first satellite was launched in 1978 and Full Operational Capability (FOC) was achieved in April 1995.
The Global Positioning System is a National resource and an international utility for positioning, navigation and timing.

4. GPS Constellations United States European Union Russia
NAVSTAR GPS (Navigation Satellite Timing & Ranging system); 28 satellites
European Union
Galileo; 30 satellites
Russia
Global Navigation Satellite System (GLONASS); 24 satellites (10 healthy)
GLONASS is a Russian satellite navigation system which now consists of 12 healthy satellites.
Galileo is Europe's contribution to the next generation Global Navigation Satellite System (GNSS). Unlike GPS, which is funded by the public sector and operated by the U.S. Air Force, Galileo will be a civil‑controlled system that draws on both public and private sectors for funding. The service will be free at the point of use, but a range of chargeable services with additional features will also be offered. These additional features would include improved reception, accuracy and availability. Design of the Galileo system is being finalized and the delivery of initial services is targeted for 2008.

5. GPS Segments Space Control User
The GPS consists of 3 segments: space, control and user.
The space segment is the satellite constellation. The first Block I satellite was launched in early The 1986 Challenger disaster slowed the GPS constellation development. In February 1989 the first Delta 2 launch took place. The constellation is now fully operational and consists of 24 or more satellites.
The control segment is operated by the U.S. Department of Defense (DoD) which tracks and maintains the satellites. The Department of Transportation (DoT) now has management responsibility, along with DoD.
The user segment consists of both military and civilian users. Military uses of GPS include navigation, reconnaissance, and missile guidance systems. Civilian use of GPS developed at the same time as military use, and has expanded far beyond anyone's original expectations.

6. Space Segment: GPS Satellites
Power
Sun-seeking solar panels
Nicad batteries
Timing
4 atomic clocks
The GPS satellites weigh about 900 kg and are about 5 meters wide with the solar panels fully extended. They are built to last about 7.5 years, but many have outlasted their original estimated life-span. The solar panels provide primary power; secondary power is provided by Nicad batteries. On board each satellite are four highly accurate atomic clocks.
As of May 2003, there are 28 usable satellites in place.

7. Satellite Orbits
Orbit the earth at approx. 20,200 km (11,000 nautical miles)
Satellites complete an orbit in approximately 12 hours
There are four satellites in each orbit plane, and each plane is inclined 55 degrees relative to the equatorial plane (the satellite path crosses the equator at a 55 degree angle). The high altitude insures that satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag. It also insures satellite coverage over large areas.
GPS satellites orbit around the earth, in contrast to TV satellites which are in geostationary orbits (they rotate with the earth). The GPS satellites cross over any point on the earth approximately twice per day.

8. Satellite Signals Radio signals, 2 frequencies Two levels of service
Standard Positioning Service (SPS)
Precise Positioning Service (PPS)
The radio signals travel at the speed of light: 300,000 km per second (186,000 miles per second). It takes 6/100ths of a second for a GPS satellite signal to reach earth. These signals are transmitted at a very low wattage (about 300‑350 watts in the microwave spectrum).
C/A code (Coarse Acquisition code) is available to civilians as the Standard Positioning Service (SPS). Before SA was turned off, SPS provided a predictable positioning accuracy of 100 meters horizontally, 156 meters vertically, and time transfer acuracy to UTC within 340 nanoseconds (95 percent). SPS now provides average horizontal accuracy of <= 13 meters 95% of the time and average vertical accuracy of <= 22 meters 95% of the time.
The Precise Positioning Service (PPS), available only to the military (and other authorized users), provides higher accuracy via the P code.
(Anti‑spoofing (AS) guards against fake transmissions of satellite data by encrypting the P‑code to form the Y‑code)

9. Satellite Signals Radio signals contain Unique pseudorandom code
Ephemeris
Clock behavior and clock corrections
System time
Status messages
Almanac
Each satellite transmits a radio signal containing its unique pseudorandom (appears to be random but is not) code. This code identifies the satellite and distinguishes it from other satellites. The signal also contains the precise location of the satellite (ephemeris data), its clock behavior and clock corrections, system time (highly accurate because of the atomic clock on-board the satellites) and status messages (usually referring to satellite health). In addition, an almanac is also provided which gives the approximate location data for each active satellite. The almanac is automatically downloaded from the satellites to the GPS receiver when the receiver is operating outside. It takes about 12 minutes to receive an almanac. The almanac data can be transferred to the office computer and used to display a graphic showing the locations of all satellites. This information can also be used to predict satellite availability for a specific mapping time and date.

10. Satellite Signals Require a direct line to GPS receivers
Cannot penetrate water, soil, walls or other obstacles
Trees, buildings, bridges, mountain ranges, your hand (over the receiver antenna) or your body can all block the satellite signals. Heavy forest canopy causes interference, making it difficult to compute positions. In canyons (and "urban canyons" in cities) GPS signals are blocked by mountains or buildings.

11. Satellite Almanac Sent along with position and timing messages
Prediction of all satellite orbits
Needed to run satellite availability software
Valid for about 30 days
The almanac has information about the orbits of all 24 satellites. A GPS receiver uses the almanac (for quick acquisition of satellite positions), along with satellite data messages, to precisely establish the position of each satellite it is tracking. Satellite availability software uses the almanac to make graphs of satellite locations overhead and to calculate the best times to survey in a particular area. GPS receivers automatically collect a new almanac each time they are turned on for more than about 15 minutes. It is important to use an up‑to‑date almanac when viewing satellite availability during mission planning. Almanac data are valid for about 30 days, but a new almanac should be transferred to satellite availability software as frequently as possible.

12. Control Segment: US DoD Monitoring
Colorado Springs
Hawaii
Kwajalein
Ascension
The control segment consists of five Monitor Stations (Hawaii, Kwajalein [West Pacific], Ascension Island [South Atlantic], Diego Garcia [Indian Ocean], Colorado Springs), three Ground Antennas (Ascension Island, Diego Garcia, Kwajalein) and a Master Control Station (MCS) located at Schriever Air Force Base in Colorado.
The monitor stations track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite's navigation message. Updated information is transmitted to each satellite via the Ground Antennas.
The GPS satellites send 1) satellite data messages (position and timing), 2) an almanac, and 3) orbital corrections they receive from the Master Control Station. The GPS receiver uses all this information to compute positions.
Diego Garcia
Orbits precisely measured
Discrepancies between predicted orbits (almanac) and actual orbits transmitted back to the satellites

13. User Segment
The user segment consists of receivers that provide positioning, velocity and precise timing to users worldwide.
Civilian applications of GPS exist in almost every field, from surveying to transportation to natural resource management to agriculture. The civilian community is a powerful political force in influencing GPS policy decisions.
90% of the data created in the world today has some type of geographic component, and civilian users outnumber military users worldwide. Examples of civilian GPS applications include 1) GPS on a helicopter to identify the location of victims in search and rescue operations, 2) GPS on a tractor/combine linked to a yield monitor to generate yield maps (precision farming), 3) GPS used for aircraft navigation or to mark where rangeland weeds have been sprayed, 4) GPS used for recreational sailing navigation, 5) Emergency services response system: a combined GPS/GIS system used to dispatch emergency vehicles and find the quickest route to a destination (GPS is also being used for pizza delivery systems!), 6) GPS to help a backpacker navigate in the woods.

14. How Does GPS Work? Calculating a Position
GPS receiver calculates its position by measuring the distance to satellites (satellite ranging)
How does GPS work? GPS satellites are constantly transmitting signals that contain orbit data and timing information. Receivers pick up those signals and use the information to compute positions.
Note: Receivers don’t send signals back to satellites, contrary to what many people think.
The distance measurement calculated by a GPS receiver is to as a range or “pseudorange.”
In order to compute a position, we START by measuring the distance between the receiver and the satellites. The satellites are the known points; the GPS receiver on the ground is the unknown point. The range (actually pseudorange: estimate of range) is measured as elapsed transit time.

15. Measuring Distance to Satellites
1. Measure time for signal to travel from satellite to receiver
2. Speed of light x travel time = distance
Distance measurements to 4 satellites are required to compute a 3-D position (latitude, longitude and altitude)
Since radio waves travel at the speed of light, we can multiply the travel time of the GPS signal by 300,000 kilometers per second (186,000 miles per second) to get the distance between the GPS satellite and the receiver. Once we have the distance measurements it's basically a problem of geometry: if we know where the 4 satellites are and how far we are from each satellite, we can compute our location through trilateration.
Note: It takes about .06 seconds for a GPS radio signal to reach Earth.

16. Measuring Satellite Signal Travel Time
How do we find the exact time the signal left the satellite?
Synchronized codes
In order to measure the travel time of the satellite signal, we have to know when the signal left the satellite AND when the signal reached the receiver. Presumably our receiver "knows" when it receives a signal, but how does it know when the signal left the satellite?
GPS satellites generate a complicated set of digital codes. These codes are complicated enough that they can be compared easily and unambiguously (look at the diagram of the code shown on the slide in red). They are "pseudo‑random" sequences that actually repeat every millisecond. The trick is that the GPS satellites and our receivers are synchronized so they're generating the same code at exactly the same time.
So, when a GPS receiver receives codes from a satellite, it looks back to see how long ago it (the receiver) generated the same code. The time difference is how long the signal took to get from the satellite to the receiver. In other words, the receiver compares how "late" the received satellite code is, compared to the code generated by the receiver itself.

17. One measurement narrows down our position to the surface of a sphere
12,000 miles is the radius of a sphere centered on the satellite. Our position could be anywhere on the surface of that sphere.

18. A second measurement narrows down our position to the intersection of two spheres
The intersection of two spheres is a circle. Now we know that our position is somewhere on that circle.

19. A third measurement narrows down our position to just two points
The three spheres intersect at only two points. Usually we can discard one of the two points because one point might be nowhere near the earth or it might be moving at a ridiculous speed. The computers in GPS receivers have various techniques for distinguishing the correct point from the incorrect one. But there is a reason we need a fourth measurement...

20. Correcting for Timing Offset
The first three measurements narrow down our position
A fourth measurement is needed to correct for timing offset (difference in synchronization between satellite and receiver clocks)
Satellites use highly accurate atomic clocks
Receivers use accurate quartz clocks
Timing offset refers to the difference in synchronization between the satellite clock and the receiver clock. Atomic clocks are far too expensive to put in GPS receivers, so a correction must be applied to compensate for the difference between the satellite and receiver clocks.
Remember that establishing the travel time of the GPS signal is the first step in calculating the distance between the satellite and receiver (travel time x speed of light = distance), and determining the receiver's position (using distance measurements from four satellites).
The next four slides show graphically how the timing offset correction works.

21. Note: The explanation of correcting for timing offset will be shown in two dimensions for illustration. Remember that in reality it takes three measurements to locate a point in three dimensions.
In an ideal situation there would be no timing error. Let's say we're 4 seconds from satellite A and 6 seconds from satellite B: our position is where the 2 circles intersect.

22. with no timing error: if we have accurate clocks, and we add a third measurement, all three circles intersect at the correct point, because the circles represent the true ranges from the three satellites.

23. If the receiver clock is one second fast (it's ahead one second from the satellite clock) the receiver will "think" the distance from satellite A is 5 seconds and the distance from satellite B is 7 seconds. And it "thinks" our position is where the two dotted circles intersect.

24. But with inaccurate clocks, the circles cannot intersect: there is no point that can be 5 seconds from A, 7 seconds from B and 9 seconds from C.
When the receiver gets a series of measurements that cannot intersect at a single point, it finds the adjustment to all measurements that lets the ranges go through one point. In this example, subtracting 1 second from all three measurements makes the circles intersect at a point.
So, by adding one extra measurement we can cancel out any consistent clock error the receiver might have. Remember that in 3 dimensions this means we really need 4 measurements to cancel out any error.

25. 5 Things to Take Away 3 GPS segments
Satellites transmit radio signals containing
Unique pseudorandom code
Ephemeris
Clock behavior and clock corrections
System time
Status messages
Almanac
Formula for satellite ranging (D = t ∙ v)
4 satellites to compute an accurate 3-D position (the 4th measurement is needed to correct for timing offset)
We are not the only country with a GPS system

26. Overview How accurate is GPS? Error sources Differential correction
Accuracy levels

27. GPS Error Atmospheric effects Multipath Satellite geometry
Measurement noise (receiver error)
Ephemeris data
Satellite clock drift
Selective availability (SA)
The accuracy of GPS is determined by the sum of several sources of error.
The travel time of GPS satellite signals can be altered by the ionosphere and the troposphere.
Multipath error occurs when GPS satellite signals bounce off other objects before reaching the receiver antenna.
Satellite geometry affects the quality of GPS positions computed by the receiver.
Measurement noise is distortion of the signal by electrical interference (occurring at the receiver antenna) or receiver error. Measurement noise is also called receiver error or receiver noise.
Orbital position errors may be present in the ephemeris data. The ephemeris for a particular satellite is simply a list of the satellite's positions as a function of time. It tells where the satellite will be and when.
Small variations in the satellite atomic clocks can translate to large position errors: a clock error of 1 nanosecond translates to 1 foot or .3 meters user error on the ground.
Selective availability (SA) was intentional scrambling of the GPS satellite signals by the U.S. government. SA was turned off at midnight on May 1, 2000.
Some of these error sources will be discussed in more detail in the following slides.

28. Ionospheric & Tropospheric Refraction
The travel time of GPS satellite signals can be altered by the ionosphere and the troposphere.
The ionosphere and troposphere both refract GPS signals, causing the speed of the GPS signal to be different from speed of a GPS signal in space.
The troposphere is the lower part of the earth's atmosphere where temperature decreases with an increase in altitude. It can be < 9 km thick over the poles and >16 km thick over the equator. The presence of neutral atoms and molecules in the troposphere affects electromagnetic signal propagation.
In the ionosphere, which ranges from about 50 km above the earth's surface to about 1,000 km or more, ionizing radiation (principally from solar ultraviolet & x-ray emissions) causes electrons to exist in sufficient quantities to affect radio-wave propagation.

29. Multipath
Multipath is the phenomenon whereby a signal arrives at a receiver's antenna by way of two or more different paths. The difference in pathlengths causes the signals to interfere with each other at the antenna and to contribute an error to the pseudorange observable. This phenomenon is similar to a "ghost" or double image on a TV set. Multipath is usually noted when operating near large reflecting obstacles such as buildings and fences, but signals can also reflect off the ground and roofs.

30. Satellite Geometry Geometric Dilution of Precision (GDOP)
GDOP can magnify or lessen other GPS errors
Wider angles  better measurements
Components of GDOP
HDOP; H=horizontal  lat/long
VDOP; V=vertical  altitude
TDOP; T=time  clock offset
PDOP values
<=4 excellent
acceptable
>=9 poor
Satellite geometry can affect the accuracy of GPS positioning. GDOP is a measure of the quality of the satellite configuration and refers to where the satellites are in relation to one another.
More notes on satellite geometry for those who are interested in the difference between DOPs:
HDOP (Horizontal Dilution of Precision) refers to horizontal measurements (lat, lon).
VDOP (Vertical Dilution of Precision) refers to altitude.
TDOP (Time Dilution of Precision) refers to clock offset.
GDOP refers to three position coordinates plus clock offset in the solution. It is a measure of the quality of a geometric constellation for position and time solutions. GDOP2 = PDOP2 + TDOP2

31. Dilution of Precision (DOP)
The ideal orientation of four or more satellites is to have one satellite directly overhead and the other three evenly spaced above the receiver.
In the diagram at the right, all the satellites are clustered together in one quadrant of the sky. This would result in a poor DOP value.
A low DOP value represents a good satellite configuration, whereas a higher value represents a poor satellite configuration. The DOP changes with time as the satellites move along their orbits.

32. Ephemeris Data A satellite’s positions as a function of time
Each satellite broadcasts its individual ephemeris
Can contain orbital position errors
The ephemeris for a particular satellite is simply a list of the satellite's positions as a function of time. It tells where the satellite will be and when.
Each satellite broadcasts an individual ephemeris that is updated continuously. The ephemeris contains satellite locations that have been computed from orbit measurements, along with corrections that are transmitted to the satellites by the DoD.
The ephemeris is used by receivers, along with almanac data, to establish precisely the position of each satellite being tracked. The ephemeris message contains orbit data for an individual satellite, whereas the almanac contains orbit data for all the GPS satellites.
Remember that the almanac is a prediction of the orbits of all satellites and can be obtained from any GPS satellite. A GPS receiver automatically collects an almanac each time it is in operation for about 12.5 minutes. The almanac is also used for quick acquisition of satellite positions by the receiver.
Orbital position errors may be present in the ephemeris data, causing errors in the positions a GPS receiver calculates.

33. Selective Availability (SA)
The accuracy of GPS signals was intentionally degraded by the DoD
SA was the largest component of GPS error
SA was turned off on May 1, 2000
SA, implemented for national defense reasons, introduced artificial clock and ephemeris errors. This caused position errors up to 70 or even 100 meters. (SA was activated on July 4, 1991). On May 1, 2000, President Clinton announced that SA would be discontinued. According to the President, "the decision to discontinue Selective Availability is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide. This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world."
New technologies developed by the military enable the U.S. to degrade the GPS signal on a regional basis for national security purposes, making the worldwide degradation unnecessary. According to the White House Office of the Press Secretary, "GPS users worldwide would not be affected by regional, security-motivated GPS degradations, and businesses reliant on GPS could continue to operate at peak efficiency."

34. GPS Error Budget
Ionosphere meters (0.4)
Troposphere meters (0.2)
Ephemeris data meters (0)
Satellite clock drift meters (0)
Multipath meters (0.6)
Measurement noise meters (0.3)
Selective availability meters
Total ~ 10 meters
These numbers indicate possible error in meters. Numbers in parenthesis indicate typical error per satellite. Remember that satellite geometry, an additional source of GPS error, can magnify or reduce the effects of other GPS errors.
How can we cancel out some of this error and achieve better accuracy with GPS receivers? The solution is explained in the next slide.

35. Differential Correction
GPS receiver on the ground in a known location (base station)
Acts as a static reference point
Transmits error correction messages to other GPS receivers in the local area (real-time)
Differential correction can be done on computer after GPS data are collected (post-processed)
Differential correction reduces the effects of some GPS errors. It cannot correct for multipath or receiver error because it counteracts only errors that are common to both reference and roving receivers.
With real‑time differential correction the base receiver computes timing errors and transmits error correction messages to other GPS receivers in the local area.
With post‑processed differential correction, the base receiver computes timing errors and stores the error data to a file. The rover file is later processed using differential correction software which uses error data from the base file to correct the rover data. The base and rover receivers have to "see" the same set of satellites at the same time, so the base file has to start before the rover file starts and end after the rover file ends.

36. The base and roving receivers collect GPS data at the same time from the same satellites. The base station is normally set up to track all satellites in view, insuring that it will "see" the 4 satellites that the roving receiver uses to compute positions.
With real-time differential GPS (DGPS) the corrections are transmitted from the base to the rover via radio link, so differential correction in real-time is instantaneously applied. Real-time DGPS is necessary when navigating where high accuracy required. Real-time DGPS is useful when relocating features that have been previously mapped.
With post-processed DGPS, the rover and base files are processed on computer by the differential correction software. Corrections are applied to the rover file during processing.
The GPS receiver on the cow tracks its movement to determine grazing patterns.

37. How accurate is GPS?
Recreational and mapping grade m
C/A code
Autonomous
Recreational and mapping grade m
With differential correction
Submeter mapping grade cm to 1 m
C/A code & carrier
Survey grade cm
Dual frequency
Advanced survey methods
GPS accuracy depends on many factors including type of equipment, time of observation, and position of satellites.
With autonomous data collection, no differential method is applied. The largest source of error with uncorrected positions is atmospheric delay.
Recreational and mapping grade receivers using real-time or post-processed differential correction can achieve from 1-5 meter accuracy. Real-time correction is less accurate than postprocessing due to different ephemeris at the base and rover (because of distance between the two), frequency of the output message (latency) – depends on transmission rate of the signal (baud rate) and correction output rate (5 sec to 30 sec), and datum issues – errors occur when reference stations broadcast corrections using a datum other than WGS-84 (datum error is usually small but can be as great as 5 to 10 m in some areas). These factors do not degrade the accuracy of your data beyond 5 meters at the default age limit. The errors they cause are more evident with a submeter mapping system.
Carrier phase receivers measure the distance from the receiver to the satellites by counting the number of waves that carry the C/A code signal; some receivers use what is called "carrier‑smoothed code" to increase the accuracy of the C/A code. The carrier wave is a much finer measuring tool than the superimposed code (19 cm vs m), so it yields more accurate satellite ranges.
Trimble GeoXTs are capable of “post-processed carrier phase differential.” In this mode, the receiver measures the code, but also measures the number of carrier waves between the satellite and the receiver. This method requires more rigorous data collection techniques: an accurate antenna height, a clear view of the sky, 5 satellites in view, rover within 50km of the base station, a base station collecting synchronized measurements over a surveyed-in coordinate, and lock on satellites must be maintained to achieve a minimum “block” of data of at least 10 minutes. With post-processed carrier phase differential, greater accuracies are obtained with longer occupation times. Accuracies are submeter in the horizontal after postprocessing.
Dual frequency survey grade receivers can provide cm or even mm accuracy using network surveys or real‑time kinematic (RTK) methods.

38. Six Main Sources of GPS Error
Atmospheric effects
Multipath effects
Satellite geometry
Measurement noise
Ephemeris data
Satellite clock drift
Here are the six main sources of GPS error (SA is no longer a factor).
Which two of the 6 GPS errors cannot be corrected by differential correction?
Hint: Only errors common to the base and rover can be corrected.

39. Things to Take Away
6 major sources of error affect the accuracy of GPS positions
Atmospheric error  largest source
Previously  SA
Almanac and ephemeris data are different
Differential correction increases accuracy