CONTENTS

1. INTRODUCTION

2. HISTORY OF GPS

3. SATELLITE NAVIGATION CONSTELLATIONS

4. GPS ERROR

5. REFRENCE SYSTEM AND CO ORDINATE SYSTEM

6. STRUCTURE OF GPS SIGNAL

1
 1. INTRODUCTION

The Global Positioning System (GPS) is a satellite-based navigation and surveying system for
determination of precise position and time, using radio signals from the satellites, in real time or in
post-processing mode. GPS is being used all over the world for numerous navigational and
positioning applications, including navigation on land, in air and on sea, determining the precise
coordinates of important geographical features as an essential input to mapping and Geographical
Information System (GIS), along with its use for precise cadastral surveys, vehicle guidance in cities
and on highways using GPS-GIS integrated systems, earthquake and landslide monitoring, etc. In
India also, GPS is being used for numerous applications in diverse fields like aircraft and ship
navigation, surveying, geodetic control networks, crustal deformation studies, cadastral surveys,
creation of GIS databases, time service, etc., by various organisations.
The Navigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS)
developed by the U.S. Department of Defence (DoD) to replace the TRANSIT Navy Navigation
Satellite System (NNSS) by mid-90’s, is an all-weather high accuracy radio navigation and
positioning system which has revolutionised the fields of modern surveying, navigation and mapping.
For every day surveying, GPS has become a highly competitive technique to the terrestrial surveying
methods using theodolites and EDMs; whereas in geodetic fields, GPS is likely to replace most
techniques currently in use for determining precise horizontal positions of points more than few tens
of km apart.
The GPS, which consists of 24 satellites in near circular orbits at about 20,200 Km altitude, now
provides full coverage with signals from minimum 4 satellites available to the user, at any place on
the Earth. By receiving signals transmitted by minimum 4 satellites simultaneously, the observer can
determine his geometric position (latitude, longitude and height), Coordinated Universal Time (UTC)
and velocity vectors with higher accuracy, economy and in less time compared to any other technique
available today. GPS is primarily a navigation system for real-time positioning. However, with the
transformation from the ground-to-ground survey measurements to ground-to-space measurements
made possibly by GPS, this technique overcomes the numerous limitations of terrestrial surveying
methods, like the requirement of intervisibility of survey stations, dependability on weather,
difficulties in night observations, etc..

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 These advantages over the conventional techniques, and the economy of operations make GPS the
most promising surveying technique of the future. With the well-established high accuracy
achievable with GPS in positioning of points separated by few hundreds of meters to hundreds of km,
this unique surveying technique has found important applications in diverse fields.

2.History of GPS (Global Positioning System)

The Global Positioning System (GPS) is a U.S.-owned
satellite navigation system that provides users with
precise location, navigation, and timing services
globally. Its history spans over six decades, evolving
from military roots to becoming an essential civilian
utility.
Early Inspirations (1950s - 1960s)
 1957 - Sputnik Effect:
The Soviet launch of Sputnik 1 led U.S. scientists to
realize that satellites could assist in determining precise positions on Earth by analysing their radio
signals and the Doppler shift.
 1960- Transit System:
The U.S. Navy deployed the Transit system, the fiist operational satellite navigation system, used
primarily by submarines for accurate positioning.
Development of NAVSTAR GPS (1973 - 1993)
 1973- NAVSTAR Program:
The U.S. Department of Defence launched the NAVSTAR GPS program, intending to provide an all-
weather, day-and-night, global positioning system primarily for military use.
 1978- First GPS Satellite Launched:
The first Block I GPS satellite was launched, testing the concept of a space-based positioning system.
 1983 - Civilian Use Declared:
After the KAL 007 incident, President Ronald Reagan announced that GPS would be made available
for civilian purposes, enhancing safety and navigation.
 1993- Full Operational Capability:
A constellation of 24 satellites was completed, providing worldwide coverage and becoming fully
operational.
Modernization and Current Status (2000 - Present)
 2000 - Selective Availability Turned Off:
The intentional degradation of civilian GPS accuracy, known as Selective Availability, was turned
off, enabling greater precision.
 2000s - Modernization Efforts:
Newer satellites (Blocks IIR, IIR-M, IIF) were launched, improving accuracy, signal strength, and
reliability.

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 2020s
- GPS III Rollout:
The GPS III generation offers enhanced capabilities, including improved accuracy, advanced anti-
jamming features, and interoperability with other GNSS like Galileo and GLONASS.

2.1 .Modernized GPS

Due to the vast civil applications of GPS technology during the past decade or so and due to the new
technologies used in the satellite and recives, the U.S government has decided to extend the
capabilities of GPS to give more benefits to the civil community. In addition to the existing GPS
signals, new signals will be transmitted by GPS satellite; see Figure 5. Moreover, this will increase
the robustness in the signals and improve the resistance to signal interference. This definitely will
lead to a better quality of service (QoS). The new signals added to the GPS (Fontana et al., 2001),
are: (i) a new L5 frequency in an aeronautical radio navigation service (ARNS) band with a signal
structure designed to improve aviation applications, (ii) C/A code to L2C carrier (L2 civil signal ),
and (iii) a new military (M) code on L1 and L2 frequency for the DoD has been added. It has the
potential to track signal even in poor conditions where the C/A code tracking on L1 would not be
possible. The new military code will be transmitted from the Block IIR-M and IIF satellites (Betz,
2002).
It is well known that the presence of dual frequency measurements (L1 and L2) has good advantages
to eliminate the effect of the ionosphere and enhance the ambiguity resolution especially for the high
precision measurements (Liu and Lachapelle, 2002). High-end civil dual frequency systems will be
based on L1 CA-code and the newly designed L2 C-code. In the coming few years the recives will
become more complex in order to allow tracking the new civil code on L2 and tracking the encrypted
P on L2 (A-S). The frequency of L5 is 1176.45MHz, with chipping rate of 10.23 MHz similar to P-
code. The high chipping rate of L5 code will provide high performance ranging capabilities and
better code measurement than L1 C/A code measurements (Dierendonck and Hegarty, 2000). L2 has
a better correlation protection with respect to L1 since it has a long code. This will be useful in severe
conditions where the GPS signals are weak such as navigation in urban, indoor, and forested areas.
The old codes and the new codes (Millitary and civil), on the L1, L2 and L5 need more advanced
modulation that better share existing frequency allocations with all signals by increasing spectral
separation, and hence conserve the spectrum. Consequently, binary offset carrier (BOC) is used for
the Military code modulations (Betz, 2002).

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 3.SATELLITE NAVIGATION CONSTELLATIONS

3.1.GLONASS
Glonass (pronounced Glo naas Russian, an acronym for' Global, 'naya Navtgatstonnaya
Sputnikovaya Sistema) has much m common With GPS m terms of Its system architecture, origin as
a military system, and even the terminology: C/A-code, P-code, Standard Positioning Service (SPS)
and Precise Positioning Service (PPS). The GLONASS SPS based on the C/A code was offered by
the Soviet Umon to the civil aviation community m 1988 at the height of glasnost. The P-code IS
intended to support the GLONASS PPS, of which little has been said m official pronouncements.
In the mid-1990s, GLONASS had looked very attractive to civil uses. The system was being
deployed smartly and approaching full constellation, and, best of all, its positioning accuracy was far
better than that available from GPS (remember Selective Availability?). Unfortunately, the system
declined and lost credibility With the user groups and receiver manufacture is. But there may now be
new Signs of hope. We offer a brief description below.
3.1.1. System Segments
Space segment: The constellation, consisting of 21 active satellites plus three on-orbit spares (see
Figure 3.5), can be described compactly as a Walker 24/3/2 (see defination below) with an
Inclination angle of 64.8 0 The orbital altitude is 19,100 km and period Il h 15 min. The ground tracks
repeat every eight days.

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A Walker T/P/F represents a constellation of T satellites in circular orbits Jn P orbital planes. The
orbital planes are spaced uniformly In right ascension of the ascending node (RAAN) [Section 4.3].
There are TYP satellites evenly distributed m each orbit. The relative phasing of the satellites
between adjacent planes is F in units of 360 0/ T. When a satellite m one plane IS crossing the equator
In northerly direction, the satellites nearest the equator In the adjacent planes are offset by F* 360 0/ T
(the more easterly satellite leading the more westerly satellite). As an aside, we note here that the
GPS constellation (Figure 4.14) with its uneven distribution of satellites m the orbital planes is not a
Walker constellation, circular orbits and uniform spacing of the orbital planes notwithstanding.
The Soviet Umon did things differently. They filled warehouses With prototype satellites designed
for one-year service life, and launched them three at a time, as necessary. The result IS that there are
100+ failed GLONASS satellites cluttering up mid-earth orbits ( about 25 for GPS).
Control Segment: Performs the same functions as the GPS CS. The monitor stations were
80 located on the Soviet territory, and have now been limited to Sites with ln Russia. The monitoring
consisted of laser tracking in addition to the RF measurements of satellite transmissions.
With a full constellation, GLONASS SPS has the potential for offering PVT estimates comparable in
accuracy with those available from GPS SPS of today.
User Segment: If it weren't for Javad, there wouldn't be any. (Indeed, Dr. Javad Ashjaee In different
guises has been the main source of GLONASS receives on the market since the early 1990s.)
3.1.2. Frequency Plan and Signal Structure
The earlier frequency allocations were: Gl (1598.0625—1607.0625 MHz) and G2 (1242.9375—
1249.9375 MHz). Recently, GLONASS has picked up additional spectral real estate G3, but the
plans for th1S frequency band have not been announced. (The GLONASS bands are often
represented as Ll and L2, but we'd refer to them as Gl and G2 to distinguish them from the GPS
bands,)
Like GPS, each satellite transmits three signals: On Gl, a C/A-like, 511-chip long PRN code repeated
with a perJ0d of I m s; and on both GI and G2, a 51. Ik-chip long PRN code With a period of I s. The
chipping rates of the GLONASS SPS and PPS signals are half those of GPS. But the navigation data
message IS transmitted at the same rate as GPS — 50 bps.
Unlike GPS, which uses CDMA signaling scheme (i.e., each satellite transmits a unique PRN code
on a common corner frequency), GLONASS employs frequency division multiple access (FDMA)
scheme: The same PRN is transmitted by each satellite, but at a different RF corner frequency. The
RF carries at Gl and G2 are channelized, the channel spacing being 9/16 or 0.5625 MHz at G I, and
7/16 or 0.4375 MHz at G2. The corner frequencies themselves are multiples of channel spacing.
At Gl, the center frequency for the kth channel IS fk - 1602+1<.0.5625 MHz k=-7 6
The 24 satellites get by With 14 channels by assigning the same channel to satellites on the opposite
sides of the earth (i.e., antipodal satellites). Difference m the carrier frequencies leads to low cross
correlations between the FDMA signals, making them orthogonal in the terminology of Section .
Why did the GLONASS Signal designs pick FDMA over CDMA? We can only speculate. A single
tone jammer can take out at most one satellite Signal an FDMA system but all

6
signals in a CDMA system.
3.1.3.Development Timetable
In mid-December 2005, the constellation consisted of 13 satellites, all of which were marked as
healthy. As announced well in advance, a launch on 25 December 2005 placed three new satellites m
orbit. The annual three-satellite launch late in the year has been a GLONASS ritual since 200(). (The
three satellites launched in 2000 and 2001 each were In service at the end of 2005, as were all
satellites launched between 2002 and 2004.)
According to Russian officials, the system IS planned to have 18 satellites In 2008 and full
operational capability with 24 satellites In 2010—2011 [Revnivykh (2005)]. New, longer life (seven
years) satellites called GLONASS-M are now being deployed and newer satellites called GLONASS-
K and GLONASS-MK are on the drawing boards.
Russian Presidential decrees galore over the past ten years not withstanding, GLONASS hasn't
received the necessary attention and resources to system it. With Galileo looking like a sure bet, IVs
unclear if a case can still be made for GLONASS.

3.2.Galileo
A JOINT Interval of the European Union (EU) and the European Space Agency (ESA), IS a GNSS
of considerable political, strategic, and economic importance to Europe. Galileo has the extraordinary
advantage of following In the path paved by GPS. A clean-slate start and newer technology would
make It a formidable competitor to GPS Ill, which it would precede. Galileo may also turn a profit.
The U.S. Government made no secret of its lack of enthusiasm for Galileo, seeing It at best as
unnecessary and at worst as a security threat. The international CIVIL user community, however,
likes the idea of competition between provides of free navigation services and welcomes the new
GNSS.
The political and business model adopted by Galileo IS different from that of GPS. GPS is a dual-use
system owned and operated by the U.S. Government and funded by the U.S. taxpayers. Galileo
would be a civil system funded by a public-private partnerships The primary role for implementing
and operating Galileo will go to a private consortium, which would seek revenues. Potential sources
of revenue are: fees for encrypted, value-added services, taxes on chipsets manufactured or sold in
Europe, and fees from equipment manufactures who would license the IP from the concesslonmre.
Unlike GPS, Galileo will have International participation and investment. The Canadian Space
Agency, an associate member of ESA, is a participant. EU has also signed agreements with China
and India that provide for investment and participation, though security and management Issues
remain to be worked out. Discussions with other countrys are said to be progress.

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Of course, the biggest difference between GPS and Galileo in 2005 is that GPS IS being used by
millions worldwide everyday while Galileo remains a gleam in the eyes of its planneis and backeis, a
system "yet to (seriously) disturb the heavens or the ether" [Last (2004)]. The brief account below IS
drawn mostly from Hein and Wallner (2005)].
3.2.1 System Segments
Space segment: Planned as a Walker 27/3/1 With inclination angle of 56 0 (see Figures 3.6 and 3.7).
The altitude IS 23,222 km (orbital radius: 29,994 km). There would be a spare satellite (also
transmitting) In each plane. The orbital period IS 14 his 4 mm, with ground track repeat every ten
days.
Ground segment: Planned to compose two control centers, five Tracking, Transmission and Control
(TT&C) Sites, ten uplink stations, 29 Galileo sensor stations, and an 'integrity processing facility:
User Segment: Civil uses would prefer to take advantage of all the Signals available for free, With
the usual calculation of incremental benefits veisus incremental receiver cost. The signals associated
with the GPS SPS (i.e., Ll C/A, L2C, and L5) and the Galileo Open Service (OS) (i.e., Ll, E5A, and
E5B, see below) will be used in different combinations for the different civil applications. We expect
an explosive growth In dual-mode receives when Galileo starts deploying operational satellites.
3.2.2.Services
Galileo plans to provide four types of navigation services and a search-and-rescue service. There will
be an Open Service, patterned after the GPS SPS, and three fee-based services, access to which
would be controlled via signal encryption. The fee-based services would offer higher performance In
the form of greater assurance on availability and integrity. These value-
83

8
Figure 3.7 Galileo constellation (based on an ESA image).
added services would also translate reliability into some form of legal guarantees, a subject clearly
outside our realm.
• Open Service (OS): Accessible to all without user fees; like GPS SPS, but clamed to be better.
• Commercial Service (CS): Fee-based service offering assured level of performance, Including
service availability.
• Safety-of-Life Service (SOL): Fee-based service aimed at transport applications with high
integrity: authentication of Signal, certification and guarantee of service, to comply with the
requirements of the International Civil Avlatlon Organization (ICAO) and International Maritime
Organisation (IMO).
• Public Regulated Services (PIS): Fee-based service Intended for government agencies (law
enforcement, national security, and emergency services) and for military applications (Galileo-guided
mumtion?). Controlled access With high lntegrity and availability, and interference-resistant signals.
• Search and rescue service (SAR): To support the 'search' part of search-and-rescue task; 98%
probability of detecting a distress Signal within ten Minutes and 100 m accuracy; data downloaded to
ground segment over dedicated UHF channels, with an acknowledgment to distress beacon.
3.2.3.Frequency Plan and Signal Structure
Galileo navigation signals are to be transmitted in four frequency bands : E5A, E5B, E6, and E2-Ll-
El. We'll refer to the last as Ll for short. All satellites will share the same frequency bands and utilize
code division multiple access (CDMA) technique
84
Each Galileo satellite IS planned to transmit SIX navigation signals With navigation data: LIE LIP,
E6C, E6P, E5A, and E5B. Four of these signals (LIF, E6C, E5A, and E5B) will also have data-free
versions transmitted in phase quadrature. The power spectral densities of these signals are shown m
Figure 3.8. Note that this plot coveys frequencies beyond the allocated band, but the out-of-band
transmissions will conform to the ITU regulations.
Some combination of the Galileo signals will be available for each of the services listed above.
Access to the full capabilities of a Signal will be controlled by encryption:

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 • Codes and data encrypted (for CS and PIS): LIP, E6C, E6P
• Selected data fields encrypted (for CS): LI_F, E5B
• No encryption: E5A
3.2.4.Development Timetable
Galileo's Implementation is planned to proceed in three phases:
• Development and validation (2003—2005), to include building and deployment of two prototype
satellites, called Galileo System Testbed Satellites, for in-orbit validation, and deployment of the fiist
four operational satellites
• Deployment of space and ground segments (2006—2007) Commercial operation (starting in
2008).
The Galileo Joint Authority (GJU) was established in 2003 to manage the program until the end of its
development phase In 2006. EC and ESA are the founding membeis of the GJU. Other organizations
and countries perhaps even the United States, may acqulre a stake in it. A private sector
concesslonaire, a consortmm of leading European aerospace and telecommunications companies,
would take over from the GJU in 2006.

4.GPS ERRORS
GPS errors are a combination of noise, bias, blunders. PS measurements are potentially subject to
numerous sources of error in addition to clock bias. Among these are uncertainties in the satellite
orbits (known as satellite ephemeris errors), errors due to atmospheric conditions (signal velocity
depends on time of day, season, and angular direction through the atmosphere), receiver errors (due
to such influences as electrical noise and signal matching errors), and multipath errors (reflection of a
portion of the transmitted signal from objects not in the straight-line path between the satellite and
receiver (Lillesand and Keiffer, 2004). 1.1.1. Noise Errors: Noise errors are the combined effect of
PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter). Noise and
bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used
in the position solution (Dana, 1997). 1.1.2. Bias Errors Bias errors result from Selective Availability
and other factors. Selective Availability (SA) is the intentional degradation of the SPS signals by a
time varying bias. It is controlled by the DOD to limit accuracy for non-U. S. military and
government users (Dana, 1997). Other Bias Error sources are discussed in the later part of the
module. 1.1.3. Blunders Blunders can result in errors of hundreds of kilometers. Control segment
mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers.
User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of
meters. Receiver errors from software or hardware failures can cause blunder errors of any size.
(Dana, 1997)

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The analysis of errors computed using the Global Positioning System is important for understanding
how GPS works, and for knowing what magnitude errors should be expected. The Global Positioning
System makes corrections for receiver clock errors and other effects but there are still residual errors
which are not corrected. The term user equivalent range error (UERE) refers to the error of a
component in the distance from receiver to a satellite. These UERE errors are given as ± errors
thereby implying that they are unbiased or zero mean errors. These UERE errors are therefore used in
computing standard deviations. The standard deviation of the error in receiver position, σrc, is
computed by multiplying PDOP (Position Dilution of Precision) by σR, the standard deviation of the
user equivalent range errors. σR is computed by taking the square root of the sum of the squares of
the individual component standard deviations. PDOP is computed as a function of receiver and
satellite positions. User equivalent range errors (UERE) are shown in the Table 1.1. There is also a
numerical error with an estimated value, σnum, of about 1 meter. The standard deviations, σR, for the
course/acquisition and precise codes are also shown in the table. These standard deviations are
computed by taking the square root of the sum of the squares of the individual components (i.e., RSS
for root sum squares). To get the standard deviation of receiver position estimate, these range errors
must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical
error. Electronics errors are one of several accuracydegrading effects outlined in the table above.
When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to
about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy. However,
the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are
on average accurate to about 5 meters (16 ft) horizontally. σR for the C/A code is given by:

The standard deviation of the error in estimated receiver position σrc, again for the C/A code is given
by:

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The error diagram in the Fig. 1.2 shows the inter relationship of indicated receiver position, true
receiver position, and the intersection of the four sphere surfaces.

Fig. 1.2. Geometric Error Diagram Showing Typical Relation of Indicated Receiver Position,
Intersection of Sphere Surfaces, and True Receiver Position in Terms of Pseudo range Errors, PDOP,
and Numerical Errors.

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13
The GPS signals sent from the SVs are subject to a variety of error sources before they are processed
into a position and time solution in the receiver. As with most systems these error sources take the
form of zero-bias noise, bias errors, and blunders. 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:
4.1. Selective Availability:
Selective availability is the single largest source of C/A-code error. Y-code capable GPS receivers
can remove SA with knowledge of the SA algorithm. SA takes the form of a slowly varying range
error for each SV. SA introduces the largest bias errors in the Standard Positioning System
accounting for most of the 100-meter (95 percent) error in the SPS (Dana, 1997).
4.2.Clock and Ephemeris Errors:
Ephemeris errors occur when the satellite doesn’t correctly transmit its exact position in orbit. Clock
and ephemeris data sets represent the difference between the SV clock and GPS time and permit the
estimation of SV position at the time of transmission of the tracked codes. A GPS parameter, the
User Range Accuracy (URA), is a range error estimate indicative of the “maximum value anticipated
during each sub frame fit interval with uniform SA levels invoked” (Anon 1995, 35). The URA is
transmitted as an integer power of two. Although the URA is not specified as a definite indicator of
SA error magnitude, for a Block II SV affected by SA, a URA of 32 meters is common (Dana, 1997).
4.3.Ionospheric Delays:
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. (Dana, 1997) A major source of bias error is the delay of the GPS carrier signals as
they pass through the layer of charged ions and free electrons known as the ionosphere. (Dana, 1997).
Varying in density and thickness as it rises and falls (50 to 500 kilometre’s) due to solar pressure and
geomagnetic effects, the ionosphere can delay the GPS signals by as much as 300 nanoseconds (100
meters) (Klobuchar 1982). The diurnal (24-hour) changes in the ionosphere cause the largest
variations in delay. At night the delay is at a minimum and the thinner and higher night time
ionosphere is more easily modelled than the less dense and thicker layer during the day. The signals
from SVs at low elevation angles with respect to the local horizon experience the largest delays as the
signal passes through more ionosphere than if the SV were directly overhead. Using the P-code, or
special codeless (signal-squaring) techniques, the delay through the ionosphere can be computed by a
receiver capable of measuring the phase delay difference between the code carried on the L1 and L2
signals (Dana, 1997). These dual frequency methods result in a substantial reduction of the
ionospheric bias, making it possible to transfer sub nanosecond clock offset measurements over
thousands of kilometres (Dunn and others 1993, 174). For a single frequency (L1) C/A-code receiver
the ionospheric delay can be estimated from the ionospheric delay model broadcast by the SVs. The
Master Control station calculates the parameters for delay using a cosine model that computes delay
for a given local time-of day and the elevation angle for the path from the receiver to an SV. Some
users compute an 6 ionospheric delay estimate from their own models. Using the broadcast model
under normal conditions removes about half of the error (Fees and Stephens 1987) leaving a residual
error of around 60-90 nanoseconds during the day and 10 to 20 nanoseconds at night (Knight and
Rhoades 1987). Signals from SVs at high elevation angles experience smaller delays, but use of the

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broadcast model under abnormal conditions can occasionally introduce more error than that caused
by the actual delay (Dana, 1997).
4.4.Tropospheric Delays:
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. GPS signal delays through the
troposphere, the layer of atmosphere usually associated with changes in weather (from ground level
up to 8 to 13 kilometre’s), are subject to local conditions and are difficult to model. GPS does not
broadcast a tropospheric correction model but several such models have been developed. Some
receivers make a limited model available that computes tropospheric delay from receiver height and
SV elevation angle using nominal atmospheric parameters. (Dana, 1997). Because accurate
tropospheric delay models (Turner and others 1986) require local pressure, temperature and humidity
(PTH) data as well as receiver height and elevation angle to the SV, these models are difficult to
apply in real-time situations. The errors introduced by an unmodeled troposphere may be as much as
100 nanoseconds at low elevation angles (less than 5 degrees), but are more typically in the 30-
nanosecond range (Knight and Rhoades 1987). Residuals after application of a simple, no-PTH,
model (Gupta 1980) are in the 10-nanosecond range.
4.5.Multipath:
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. Multipath interference, caused by local reflections of the GPS signal that mix with the
desired signal, slowly introduces varying bias errors of one to two nanoseconds for navigation
receivers aboard aircraft in flight. For land-based systems, local conditions and exact antenna
placement can result in errors of up to 150 nanoseconds. (Dana, 1997). Nominal errors for land-based
receivers are in the 30-nanosecond range (Braasch, 1995). Careful attention to antenna placement,
antenna design, the use of choke rings, and the use of materials that absorb GPS radio-frequency
signals can mitigate much of the potential multipath 7 interference, but these measures must be
carefully designed to allow for the different multipath reflections from the constantly changing SV
elevations and azimuths. In many applications it is difficult or impossible to completely eliminate
multipath errors. (Dana, 1997)
4.6.GPS Signal Noise
Propagation of the GPS signals from the SV to the receiver introduces noise from galactic sources,
ionospheric scintillations, and cross correlation from other GPS SV signals that results in small noise
(zero bias) errors in the three-nanosecond range. (Dana, 1997)
4.7.Receiver Noise and Delays
Receiver noise can introduce two to three nanoseconds of zero bias noise in the timing
measurements of a GPS receiver. Delays within a receiver can be calibrated by the manufacturer, but
if receiver delays change with temperature or change differently between channels of a multi-channel
receiver, timing bias errors can result. Antenna cable delays must be recomputed or calibrated if
cable lengths change or cables of different materials are used. (Dana, 1997). There have been reports
of cable delays being both temperature and signal strength dependent (Lewandowski, Petit and

15
Thomas 1991, 5). Manufacturers can provide cable delays for the equipment they supply (Dana,
1997).
4.8. Receiver Oscillator Errors
While precise time standards at the Control and Space Segments of GPS are designed to keep user
clock requirements to a minimum, receiver oscillators must provide enough stability to ensure that
they can be rated properly by GPS receiver software and that they provide a low noise timing
reference. This is sometimes difficult to accomplish in high dynamic environments or when the
receiver internal temperatures cannot be controlled or compensated for (Dana, 1997).
4.9.SV clock errors
The uncorrected by Control Segment can result in one-meter errors in position. (Dana, 1997)
4.10. Geometric Dilution of Precision
Geometric Dilution of Precision (GDOP) is a measurement of the sensitivity of a receiver position or
time estimate to changes in the geometric relationship between the receiver position and the positions
of all of the SVs used to form the position or time estimate. If the 8 SVs used for a navigation
solution were all in about the same place in the sky, directly above a receiver position, for instance,
the position solution for height would be less sensitive to pseudo-range changes than would the
poorly defined (diluted) solution for horizontal position. If the SVs were distributed around the field
of view of the receiver, horizontal and vertical positioning would be more equally sensitive to
pseudo-range changes. GDOP is a dimensionless multiplier that can be used to estimate the effect of
pseudo-range errors on a complete position and time solution. The single GDOP parameter is the
square root of the sum of the diagonal terms of the covariance matrix that is formed from the inverse
of the matrix of directional derivatives for each of the SV positions and pseudo-ranges used in the
position solution. For a specified receiver position and a set of SVs, GDOP can be separated into
threedimensional position (PDOP) or spherical (SDOP) dilution, two-dimensional horizontal
(HDOP), or one dimensional vertical (VDOP) or time (TDOP) estimates. These separate components
of GDOP are formed from covariance terms and so are not independent of each other. A high TDOP
(time dilution of precision) in a navigation receiver will eventually influence position errors as
erroneous receiver clock bias estimates are used to correct pseudo range measurements. The
computation of geometric dilution of precision involves many numerical equations. Computations
were provided to show how PDOP was used and how it affected the receiver position error standard
deviation. When visible GPS satellites are close together in the sky (i.e., small angular separation),
the DOP values are high; when far apart, the DOP values are low. Conceptually, satellites that are
close together cannot provide as much information as satellites that are widely separated. Low DOP
values represent a better GPS positional accuracy due to the wider angular separation between the
satellites used to calculate GPS receiver position. HDOP, VDOP, PDOP and TDOP are respectively
Horizontal, Vertical, Position (3-D) and Time Dilution of Precision. (Dana, 1997)
4.11.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

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sky you have, the better the chances of not having 9 satellite signals blocked. Building interiors,
streets surrounded by tall buildings, dense tree canopies, canyons, and mountainous areas are typical
problem areas (McNamara, 2004

5.REFERENCE SYSTEMS
Coordinate System
The definition of reference coordinate system is crucial for the description of satellite motion, the
modeling of observable and the interpretation of results. Reference coordinate system in satellite
geodesy is global and geocentric by nature since satellite motion refers to the center of mass of the
earth (Seeber, 2003; Hofmann-Wellenhof et al., 2001). In satellite geodesy, two reference systems
are required: (a) space-fixed, inertial reference system for the description of satellite motion, and (b)
earth-fixed, terrestrial reference system for the positions of the observation stations and for the
description of results from satellite geodesy. The positioning with using GNSS depends mainly on
knowing the satellite coordinates. The position of the receiver is calculated with respect to the instant
position of the satellite. By considering the range vector relation between satellite and receiver, the
coordinate of the satellite and receiver should be expressed in the same coordinate system. In satellite
geodesy, the two systems are used and the transformation parameters between the space fixed and
earth fixed are well known and used directly in the GNSS receiver and post processing software to
compute the position of the receivers in the earth fixed system. Terrestrial reference system is defined
by convention with three axes, where Z-axis coincides with the earth rotation axis as defined by the
Conventional International Origin (CIO). The Xaxis is associated with the mean Greenwich
meridian, and the Y-axis is orthogonal to both Z and X axes and it completes the right-handed
coordinate system, Fig. 9. One example of the terrestrial reference system is the WGS84. GPS has
used the WGS84 as a reference system (Leick, 2003), and with WGS84 associated a geocentric
equipotential ellipsoid of revolution. 17 The basic idea, in geodesy, behind using the reference
ellipsoids is that they fit the real shape of the earth. Another example of terrestrial reference frame is
the International Terrestrial Reference Frame (ITRF), which is established by Central Bureau of the
International Earth Rotation Service (IERS). The ITRF is regularly updated and is more accurate than
WGS84, but the difference between WGS84 and ITRF is now in the order of a few centimeters. This
difference is mainly due to the difference between the reference stations used by each system when it
is realized. Both systems are geocentric and the transformation parameters between them are
regularly published by IERS. The representation of position in geocentric Cartesian coordinates (X,
Y and Z) has less significance in navigation. Hence, the ellipsoidal representation (longitude, latitude
and height above the ellipsoid) are more commonly use for coordinate representation.

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The relation between Cartesian coordinate (X, Y, Z) and ellipsoidal coordinates (ϕ, λ, and h) is well
known by using the following formulas:

Here, a, b are the semi axes of the ellipsoid. The Cartesian coordinate of WGS84 is called also ECEF
(Earth Centered Earth-Fixed) coordinate system. As mentioned above, the realization of the reference
frame depends on the coordinates of ground reference stations. The Galileo Terrestrial Reference
Frame (GTRF) is expected to be similar to ITRF, but will be based on the coordinates of the Galileo
ground stations. The differences between WGS84, ITRF and the GTRF are expected to be in the
order of a few centimeters. The two coordinate systems are compatible, and the accuracy obtained is
good enough for most of the applications including navigation. For high precise measurements and
for centmetric accuracy between the various systems, the transformation parameters are expected to
be published by the geodetic service providers such as IERS. Glonass uses the PZ90 as a reference
coordinate system which is basically a ECEF system. The transformation parameters between PZ90
and WGS84 is published by IERS (Leick, 2003). 5.2 Time Reference Frame There are many time
reference systems used and they are based on various periodic processes such as the earth rotation.
The major types of these systems are shown in Table 1. The conversion between time systems is
accomplished by well known formulas. In GNSS (e.g GPS), instead of the dynamic time system
itself, the atomic time system serves as reference. Glonass satellite clock is moved according to UTC

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(SU). The Galileo System Time (GST) will be a continuous coordinate time scale steered towards the
International Atomic Time (ITA) with an offset of less then 33 nanoseconds. The GST limits,
expressed as a time offset relative to ITA, 19 should be 50 nanoseconds for 95 percent of the time
over any yearly time interval. The difference between GST and ITA and between GST and UTC shall
be broadcast to the users using the signal-in-space of each Galileo service. The Galileo ground
segment will monitor the offset of the GST with respect to the GPS system time and eventually
broadcast the offset to users.

6.STUCTURE OF GPS SIGNAL

The overall of mentioned signals (Modernized GPS, Galileo and Glonass signals), make up the
GNSS signals. Each satellite system has specific signal characteristics, but each system attempts to
be compatible with the others in order to prevent the interferences and attenuation between the
signals. It is important to consider that the processing of all signals should be performed using the
same receiver, thus a complex receiver design is supposed to be designed and built. As mentioned
above, The GNSS frequency plan shall respect the radio-regulations as they are discussed and agreed
on at ITU forums. The available spectrum which can be used for the development of Radio-
Navigation Satellite Systems (RNSS) is shown in Figure 7.

6.1.SIGNAL PROCESSING AND RECEIVER DESIGN

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The main function of the signal processor in the receiver is the reconstruction of the carriers and
extraction of codes and navigation messages. After this stage the receiver performs the Doppler shift
measurement by comparing the received signal by a reference signal generated by the receiver. Due
to the motion of satellite, the received signal is Doppler shifted. The code ranges are determined in
the delay lock loop (DLL) by using code correlation. The correlation technique provides all
components of bimodulated signals. The correlation technique is performed between the generated
reference signal and the received one (Hofmann-Wellenhof et al., 2001). The signals are shifted with
respect to time so that they are optimally matched based on mathematical correlation. Currently some
geodetic type receivers are available on the market tracking GPS and Glonass satellites
simultaneously on both frequencies, in particular the Ashtech Z18 receiver and the TPS (Topcon
Positioning Systems) Legacy receivers. The future GNSS receiver could be designed to track the
different GNSS signals and could be of many types: • The first type could process all GNSS signals
GPS L1, L2, L5 and Galileo OS, CS using L1, E5 and E6 and also Glonass L1 and L2. • The second
type uses free signal and codes, GPS L1 and L2C and Galileo OS, on L1 and E5. • The third type
uses L1 and E5. 15 • Forth type uses GPS L1 and L2 (which are already in the market (Ries et al.,
2002). • Fifth type uses GPS and Glonass signals (which are already exist), (Leick, 2003). The most
common receiver types are Intermediate Frequency receiver (IF) and the software defined radio
receiver (SDR). In the RF front-end receiver the signal is down converted to an intermediate
frequency and then sampled, but SDR uses direct digitization, or bandpass sampling. Details on
GNSS receiver design could be found in (Schmid et al., 2004; Julien et al., 2004a). The main
components of RF-FE combined GNSS receiver are shown in Figure 8. After sampling and analog to
digital conversion (ADC) of the received signal, the receiver performs parallel despreading. The
received base-band signal is multiplied in parallel with the spreading codes of all visible satellites.
The received signal of each satellite is multiplied in parallel with different code delay offsets. These
products are then accumulated to compute the cross-correlation function.

Because BOC signals are used in Galileo, supplementary measures are necessary due to the multiple
correlation peaks of the auto-correlation function. Carrier tracking is performed using a phase-locked
or frequency-locked loop (PLL or FLL). Coherent correlation combined with differential or non-
coherent correlation can be done for the pilot and the data channel (Schmid et al., 2004). Multiple

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signals will be available at L1 within the next few years (Hein et al., 2004). Galileo will use a
different modulation scheme for its signals such as BOC and QPSK, while 16 GPS uses binary phase
shift keying (BPSK) modulation for the open signals at L1 and L2. The L5 signal that will appear
with the Block IIF satellites in 2006, will have quadrature phase shift keying (QPSK). The binary
offset carrier (BOC) modulation scheme of Galileo provides better multipath and receiver noise
performance compared to the GPS binary phase shift keying (BPSK) modulation. More complex
techniques are already developed for tracking BOC signal, such as bump jump and BPSK-like.

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 7.CONCLUTION
The Global Positioning System (GPS) stands as one of the most transformative technological
developments of the modern era, offering precise positioning, navigation, and timing services across
the globe. Originally developed by the U.S. Department of Defense in the 1970s for military use,
GPS has since evolved into a cornerstone of civilian and commercial navigation worldwide.
Built upon a constellation of satellites orbiting Earth, GPS works by transmitting signals that allow
receivers to calculate their exact location using trilateration. Alongside the U.S. GPS, other satellite
navigation systems such as Russia’s GLONASS, Europe’s Galileo, and China’s BeiDou contribute to
the broader Global Navigation Satellite System (GNSS), improving global coverage and reliability.
Despite its precision, GPS is subject to various sources of error, including signal delay due to the
ionosphere, multipath interference, and satellite clock inaccuracies. However, these errors are
mitigated through correction techniques and the use of augmentation systems. The accuracy and
consistency of GPS measurements depend on standardized reference systems like WGS 84 (World
Geodetic System 1984) and coordinate systems that define positions on the Earth's surface.
Structurally, GPS is composed of three main segments: the space segment (satellite constellation),
the control segment (ground stations that monitor and manage satellites), and the user segment
(devices and receivers that utilize the signals). Each part plays a vital role in ensuring the system
functions efficiently and accurately.
In conclusion, GPS integrates complex scientific principles and cutting-edge technology to provide
global positioning services that are now indispensable in daily life. With ongoing improvements in
satellite design, signal precision, and integration with other GNSS systems, GPS will continue to play
a crucial role in navigation, science, communication, and global connectivity.

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 REFFRENCE
 GLOBAL POSITION SYSTEM:Signals,Mesurements,and Performance
Second edition:by Pratap Misra and Per Enge
 https://www.princeton.edu/~alaink/Orf467F07/GNSS.pdf
 https://dspmuranchi.ac.in/pdf/Blog/GPS%20ERRORS.pdf

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