I. Global Positioning System (GPS) - A

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THE GPS

I.

Global Positioning System (GPS) - a U.S. space-based global navigation satellite

system(GNSS). It provides reliable positioning, navigation, and timing services to

worldwide users on a continuous basis in all weather, day and night, anywhere on or

near the Earth.

GPS is made up of three parts: between 24 and 32 satellites in Medium Earth Orbit,

four control and monitoring stations on Earth, and the actual navigation devices

users own. GPS satellites broadcast signals from space that GPS receivers use to

provide three-dimensional location (latitude, longitude, and altitude) plus the time.

The Official Name of GPS is Navigation System with Timing and Ranging Global

Positioning System (NAVSTAR-GPS). It was developed by the U. S. Department of

Defense, Ivan Getting, and the Massachusetts Institute of Technology (MIT).

Generations of GPS Satellites - There are five generations of the GPS satellites:

the Block I, Block II/IIA, Block IIR, Block IIR-M and Block IIF.

Block I satellites were used to test the principles of the system, and lessons learned

from those 11 satellites were incorporated into later blocks.

The BLOCK II satellites, space vehicle numbers (SVN) 13 through 21, are the first full

scale operational satellites developed by Rockwell International. Block II satellites

were designed to provide 14 days of operation without contact from the Control

Segment (CS). The Block IIs were launched from February 1989 through October1990.

The BLOCK IIA satellites, SVNs 22 through 40, are the second series of operational

satellites, also developed by Rockwell International. Block IIA satellites were

designed to provide 180 days of operation without contact from the CS. During the

180 day autonomy, degraded accuracy will be evident in the navigation message.

The Block IIAs were launched November 1990 through November 1997.

The BLOCK IIR satellites, SVNs 41 through 61, are the operational replenishment

satellites developed by Lockheed Martin and will carry the GPS well into the next

century. Block IIR satellites are designed to provide at least 14 days of operationwithout contact from the CS and up to 180 days of operation when operating in the

autonomous navigation (AUTONAV) mode. Full accuracy will be maintained using a

technique of ranging and communication between the Block IIR satellites. The

cross- link ranging will be used to estimate and update the parameters in the

navigation message of each Block IIR satellite without contact from the CS. The

design life of the Block IIR satellite is 7.8 years; each contains three Rb atomic

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clocks and have the SA and A-S capabilities. Launching of the Block IIRs began in

January 1997.

The BLOCK IIR-M satellites transmit a second civil signal L2C on the L2 frequency

and the military M signal on the L1 and L2 frequencies. SVN 49 also transmits on

the L5 frequency.

The Block IIF satellites will be the fifth generation of satellites and will be used for

operations and maintenance (O&M) replenishment.

GPS Constellation As of December 2009, the GPS constellation consists of 32

Block II/IIA/IIR/IIR-M satellites.

WGS84 - The new World Geodetic System is called WGS 84. It is currently the

reference system being used by the Global Positioning System. It is geocentric and

globally consistent within 1 m. Its parameters are:

Ellipsoidreference

Semi-majoraxis a

Semi-minor axis b Inverse flattening(1/f)

WGS 84 6,378,137.0 m 6,356,752.314

245 m298.257 223 563

II.

GPS Codes

1. The C/A code or Coarse/Acquisition Code is freely available to the public. It is

a 1,023 bit long pseudonoise code when transmitted at 1.023 megabits per

second (Mbit/s), repeats every millisecond. These sequences only match up,

or strongly correlate, when they are exactly aligned. Each satellite transmits

a unique PRN code, which does not correlate well with any other satellite's

PRN code. In other words, the PRN codes are highly orthogonal to one

another. This is a form ofCode Division Multiple Access (CDMA), which allows

the receiver to recognize multiple satellites on the same frequency.

2. The P(Y) code - The protected or precision code is modulated on both L1 and

L2 carrier signals and is usually reserved for military applications. The P-codeis a very long (about 1014 bits) sequence of pseudo-random binary biphase

modulations on the GPS carrier at a chipping rate of 10.23 MHz which does

not repeat itself for about 38 weeks. Each satellite uses a one-week segment

of this code which is unique to each satellite, and reset each week.

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ftp://tycho.usno.navy.mil/pub/gps/gpsb2.txthttp://en.wikipedia.org/wiki/Global_Positioning_Systemhttp://en.wikipedia.org/wiki/Flatteninghttp://en.wikipedia.org/wiki/Bitshttp://en.wikipedia.org/wiki/Megabits_per_secondhttp://en.wikipedia.org/wiki/Megabits_per_secondhttp://en.wikipedia.org/wiki/Millisecondhttp://en.wikipedia.org/wiki/Correlatehttp://en.wikipedia.org/wiki/Orthogonalhttp://en.wikipedia.org/wiki/Code_Division_Multiple_Accessftp://tycho.usno.navy.mil/pub/gps/gpsb2.txthttp://en.wikipedia.org/wiki/Global_Positioning_Systemhttp://en.wikipedia.org/wiki/Flatteninghttp://en.wikipedia.org/wiki/Bitshttp://en.wikipedia.org/wiki/Megabits_per_secondhttp://en.wikipedia.org/wiki/Megabits_per_secondhttp://en.wikipedia.org/wiki/Millisecondhttp://en.wikipedia.org/wiki/Correlatehttp://en.wikipedia.org/wiki/Orthogonalhttp://en.wikipedia.org/wiki/Code_Division_Multiple_Access


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3. The (Y) code is a special form of P code used to protect against false

transmissions; special hardware, available only to the U.S. government, must

be used to decrypt the P(Y) code.

GPS Frequencies

1. L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code

and encrypted precision P(Y) code, plus the new L1C on future Block III

satellites.

2. L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and

newer satellites.

3. L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection

System Payload (NDS) to signal detection of nuclear detonations and other

high-energy infrared events. Used to enforce nuclear test ban treaties.

4. L4 (1379.913 MHz): Being studied for additional ionospheric correction.

5. L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal.

This frequency falls into an internationally protected range for aeronautical

navigation, promising little or no interference under all circumstances. The

first Block IIF satellite that would provide this signal is set to be launched in

2009

Satellite Navigation Message - It is made up of three major components. The

first part contains the GPS date and time, plus the satellite's status and an

indication of its health. The second part contains orbital information called

ephemeris data and allows the receiver to calculate the position of the satellite. Thethird part, called the almanac, contains information and status concerning all the

satellites; their locations and PRN numbers.

Satellite Ephemeris - The description of the satellite orbits and clock correction

parameters variable over time used for positioning and baseline computations. The

ephemeris may be broadcast (projected ahead into time and subject to selective

availability) or precise (post-fitted).

III.

GPS Measurements

Pseudorange is the measure of the apparent propagation time from the satellite to

the receiver antenna, expressed as a distance. The apparent propagation time is

determined from the time shift required to align a replica of the GPS code

generated in the receiver with the received GPS code. The time shift is the

difference between the time of signal reception (measured in the receiver time

frame) and the time of emission measured in the satellite time frame). Pseudorange

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is obtained by multiplying the apparent signal-propagation time by the speed of

light. Pseudorange differs from the actual range by the amount that the satellite

and receiver clocks are offset, by propagation delays, and other errors including

those introduced by selective availability.

Pseudo-ranging with Carrier phase Measurements - The period of the carrierfrequency times the speed of light gives the wave length, which is about 0.19

meters for the L1 carrier. With a 1% of wave length accuracy in detecting the

leading edge, this component of pseudorange error might be as low as 2

millimeters. This compares to 3 meters for the C/A code and 0.3 meters for the P

code. However, this 2 millimeter accuracy requires measuring the total phase, that

is the total number of wave lengths plus the fractional wavelength. This requires

specially equipped receivers.

IV.

GPS accuracy is affected by a number of factors, including satellite positions, noise

in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise

can create an error between 1 to 10 meters and results from static or interference

from something near the receiver or something on the same frequency. Clouds and

other atmospheric phenomena, and objects such a mountains or buildings between

the satellite and the receiver can also produce error, sometimes up to 30 meters.

The most accurate determination of position occurs when the satellite and receiver

have a clear view of each other and no other objects interfere.

Sources of GPS errors

1. Ionosphere and troposphere disturbances: These cause the satellite signal to

slow down as it passes through the atmosphere. However the GPS system

has a built in model that accounts for an average amount of these

disturbances.

2. Signal reflection or Multipath distortion: Here the signal hits and is reflected

off objects like tall buildings, rocks etc. This causes the signal to be delayed

before it reaches the receiver.

3. Ephemeris errors: Ephemeris errors are also known as orbital errors. These

are errors in the satellites reported position against its actual position.

4. Clock errors: The built in clock of the GPS receiver is not as accurate as the

atomic clocks of the satellites and the slight timing errors leads to

corresponding errors in calculations.

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5. Visibility of Satellites: The more the number of satellites a GPS receiver can

lock with, the better its accuracy. Buildings, rocks and mountains, dense

foliage, electronic interference, in short everything that comes in the line of

sight cause position errors and sometimes make it unable to take any reading

at all. GPS receivers do not work indoors, underwater and underground.

6. Satellite Shading: For the signals to work properly the satellites have to be

placed at wide angles from each other. Poor geometry resulting from tight

grouping can result in signal interference.

User equivalent range errors (UERE) are shown below:

Source Effect (m) Source Effect (m)

Signal arrival C/A 3Satellite clock

errors2

Signal arrival P(Y) 0.3 Multipathdistortion

1

Ionospheric

effects5

Tropospheric

effects0.5

Ephemeris errors 2.5

Selective Availability (SA) - a feature in GPS that adds intentional, time varying

errors of up to 100 meters (328 ft) to the publicly available navigation signals. This

was intended to deny an enemy the use of civilian GPS receivers for precision

weapon guidance. It was disabled by the Clinton Administration on May 1, 2000. It is

also called Intentional Degradation.

Anti Spoofing - It is the encryption of the P-code signal transforming it to Y-code

which is unavailable to civilian users.

V.

GPS Point Positioning - coordinates of the antenna position at an unknown point

are sought with respect to the WGS84 reference frame. In this method, the knownpositions of the tracked GPS satellites (the position of a satellite can be computed

from ephemerides) are being used to determine the position of unknown point using

single GPS receiver by a method similar to the method of resection used in plane

table surveying

GPS Relative Positioning - Computation of the relative difference in position

between two points by the process of differencing simultaneous reconstructed5


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carrier phase measurements at both sites. The technique allows cancellation of all

errors which are common to both observers, such as clock errors, orbit errors, and

propagation delays. This cancellation effect provides for determination of the

relative position with much greater precision than that to which a single position

(pseudorange solution) can be determined. It is also called Phase difference

processing.

GPS Surveying Methods

1. Static GPS surveying typically uses a network or multiple baseline approach

for positioning. It may consist of multiple receivers, multiple baselines,

multiple observational redundancies and multiple sessions. A least squares

adjustment of the observations is required. This method provides the highest

accuracy achievable and requires the longest observation times from less

than an hour to five hours or longer.

2. Pseudo-static GPS - Also known as pseudo-kinematic and repeat occupation,

this relative positioning technique relies upon two or more simultaneous

observations at a point pair, separated by some time interval (typically 60

minutes or more), in order to solve the integer bias terms from the change in

satellite geometry occurring between the repeat observations.

3. Rapid Static Surveying - GPS surveying technique utilizing multiple

observables (dual-frequency carrier phase, C/A and P codes) to resolve

integer ambiguities with shortened observation periods. The method mayalso be used for observations with the receiver in motion known as on-the-fly

ambiguity resolution. It is also called Fast Ambiguity Resolution and Fast

Static Surveying

4. Kinematic Surveying- Observations while a receiver is in motion. In surveying

applications, kinematic refers to uninterrupted carrier-phase measurements

following successful solution of the integer ambiguities. This can be

accomplished in a continuous mode where the receiver remains in motion for

precise positioning of a vehicle, or in an intermittent mode where data isrecorded only after a receiver is brought to a stationary point, and the

observations while in motion are tracked as a way to maintain the integer

ambiguities.

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5. Real-Time Kinematic (RTK) Surveying - RTK is a process where GPS signal

corrections are transmitted in real time from a reference receiver at a known

location to one or more remote rover receivers. In this case, the base station

is located at a known surveyed location, often a benchmark, and the mobile

units can then produce a highly accurate map by taking fixes relative to that

point. It is also called Carrier-Phase Enhancement GPS or CPGPS.

6. Differential GPS (DGPS) It is a single point code positioning with

pseudorange corrections applied from simultaneous observations at a known

position. One to ten meter accuracy is typical. DGPS requires that a GPS

receiver be set up on a precisely known location. This GPS receiver is the

base or reference station. The base station receiver calculates its position

based on satellite signals and compares this location to the known location.

The difference is applied to the GPS data recorded by the second GPS

receiver, which is known as the roving receiver. The corrected informationcan be applied to data from the roving receiver in real time in the field using

radio signals or through postprocessing after data capture using special

processing software.

VI. References:

ftp://tycho.usno.navy.mil/pub/gps/gpsb2.txt

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http://www.rbf.com/cgcc/glossary.htm

http://www.gisdevelopment.net/technology/gps/techgp0030.htm

http://www.survequip.com/what-is-rtk/

http://en.wikipedia.org/wiki/World_Geodetic_System#Updates_and_new_standards

http://en.wikipedia.org/wiki/Introduction_to_the_Global_Positioning_System

http://www.esri.com/news/arcuser/0103/differential1of2.html

http://www.roseindia.net/technology/gps/sources-of-GPSe-error.shtml

http://www.kowoma.de/en/gps/errors.htm

http://www.directionsmag.com/article.php?article_id=228

http://en.wikipedia.org/wiki/GPS_signals

http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/311.htm#navmeg

http://onlinemanuals.txdot.gov/txdotmanuals/ess/gps_static_surveying.htm

http://www.maps-gps-info.com/gps-accuracy.html

http://www.navcen.uscg.gov/navinfo/Gps/ActiveNanu.aspx

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I. Global Positioning System (GPS) - A

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