Gps

Task 1

Objective: Understanding the principle of GPS Technology.

Theory:

The Global Positioning System (GPS) is and earth orbiting-satellite based navigation system. GPS is an

operational system, providing users worldwide with twenty-four hour a day precise position in three

dimensions and precise time traceable to global time standards. GPS is operated by the United States Air

Force under the direction of the Department of Defense (DoD) and was designed for, and remains under

the control of, the United States military. While there are now many thousands of commercial and

recreational civil user’s worldwide, DoD control still impacts many aspects of GPS planning, operation,

and use. Primarily designed as a land, marine, and aviation navigation system, GPS applications have

expanded to include surveying, space navigation, automatic vehicle monitoring, emergency services

dispatching, mapping, and geographic information system geo referencing. Because the dissemination of

precise time is an integral part of GPS, a large community of precise time, time interval, and frequency

standard users has come to depend on GPS as a primary source of control traceable through the United

States Naval Observatory to global time and frequency standards.

History of GPS :

Developed in the 1960s, the Navy Transit satellite navigation system still provides some service as a two-

dimensional (horizontal) positioning system. Good (200 meter) Transit positioning requires knowledge of

the user altitude as well as a model of user dynamics during the fix, a process of integrating satellite signal

Doppler shifts (the change in received signal frequency caused by the changing range) during the fly-over

of the satellite. Another Navy system, based on the Timation satellites carried stable clocks (quartz,

rubidium, and cesium) over the course of the program in the 1960s and 70s and was the precursor to the

precise time capabilities of GPS (Easton 1978). GPS began in 1973 as a test program using ground-based

transmitters at the U. S. Army

Proving Ground at Yuma, Arizona, later augmented with early versions of GPS satellites first launched in

1978. During the 1980s, GPS, although not yet fully operational and requiring careful planning for missions

during times of satellite availability, was increasingly used by both military and civilian agencies. Land,

air, and sea navigation, precise positioning, carrier phase survey techniques, and precise time and frequency

dissemination were all accomplished to a limited extent during the initial phases of GPS deployment

(Klepczynski 1983). By 1989 ten development satellites, termed Block I satellites, had been successfully

launched. By 1990, 43 laboratories requiring precise time were using GPS to synchronize their atomic

clocks (Clements 1990). By 1994, 24 Block II and IIA operational GPS space vehicles (SVs) had been

launched. The Block IIA SVs can store up to 14 days of uploaded data in case contact is lost with ground

stations and can operate for 180 days with degraded navigation receiver performance. The next generation

of space vehicles, the Block IIR SV s will incorporate changes to include the capability of maintaining

precise time keeping without Control Segment uploads for periods of up to 210 days by exchanging data

between GPS SVs (Rawicz, Epstein, and Rajan 1992).

In December of 1993, GPS reached Initial Operational Capability, with a minimum of 24 satellites in orbit.

On July 17, 1995 the Air Force announced that GPS had met all requirements for Full Operational

Capability with 24 Block II SVs in orbit. With over 50 companies supplying a selection of over 275 GPS

receivers to a global market, the well-established user community of navigators, surveyors, geologists,

geodesists, time and frequency users, and many thousands of recreational user has come to accept GPS as

a viable military and civilian system.

Civil and Military GPS:

While controlled and maintained by the DoD, the GPS user community has a large civil component. In the

1977 National Plan for Navigation, published by the U. S. Department of Transportation (DoT), the

NAVSTAR GPS user community was planned to include 27,000 military receivers. While the potential for

a civil-sector user base was recognized, the document did not include plans for a civil GPS service (U.S.

DoT 19773-14; 3-15). A decade later the Federal Radio navigation Plan (FRP) (U.S. DoD and DoT 1986)

stated that GPS would be available to civil users, worldwide, on a continuous basis but with accuracy

limited to 100 meters (95 percent).

In these radio navigation documents position accuracy is usually specified as a two standard deviation (95

percent) radial error or 2drms (2 distance root mean squared) uncertainty estimate. For GPS the 95 percent

probability and 2drms accuracy are equivalent (DoD and DoT 1995, A-2). The 1985 Comprehensive Global

Positioning System User Policy defined both a military, encrypted, Precise Positioning Service and a "lower

level of accuracy" Standard Positioning Service (U.S. DoD and DoT 1986, B-32).

Standard Positioning Service:

The Standard Positioning Service (SPS) is defined in the most recent FRP as: the standard specified level

of positioning and timing accuracy that is available, without restrictions, to any user on a continuous

worldwide basis. The accuracy of this service will be established by the DOD and DOT based on U. S.

security interests. SPS provides a predictable positioning accuracy of 100 meters (95 percent) horizontally

and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340 nanoseconds (95

percent).

Precise Positioning Service:

The FRP defines the Precise Positioning Service (PPS) as: the most accurate direct positioning, velocity,

and timing information continuously available, worldwide, from the basic GPS. This service is limited to

users specifically authorized by the U.S. P(Y)-code capable military user equipment provides a predictable

positioning accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters (95 percent) vertically

and time transfer accuracy to UTC within 200 nanoseconds (95 percent) (DoD and DoT 1995, A-36). By

the time the 1992 FRP was published, the projected 1995 estimate of 53,000 civil users of GPS exceeded

the projected number of military users estimated at 19,000 (U.S. DoD and DoT 1993, 3-41). Civil users

now constitute the majority of GPS users. The 1994 FRP estimates the current total number of GPS users

at over 500,000 in the United States alone (U. S. DoD and DoT 1995, 3-7).

GPS Segments:

The Global positioning System (GPS) comprises three segments:

1. The Space segment (all function satellites)

2. The Control segment (all ground station involved in the monitoring of the system: master control

station, monitoring stations & ground control)

3. The User segment (all civil and military GPS users)

Space Segment:

The Space Segment is designed to consist of 24 satellites orbiting the earth at approximately 20200Km

every 12 hours. At time of writing there are 26 operational satellites orbiting the earth. The space segment

is so designed that there will be a minimum of 2 to 3 satellite visible above a 15deg cut off angle at any

point of the earth's surface at any time. Each GPS satellite has several very accurate atomic clocks on board.

The clocks operate at a fundamental frequency of 10.23MHz. This is used to generate the signals that are

broadcast from the satellite.

Control Segment:

The Control Segment consists of one master control station, 5 monitor stations and 4 ground antennas

distributed among 5 locations roughly on the earth equator. The Control Segment tracks the GPS satellites,

updates their orbiting position and calibrates and synchronizes their clocks.

A further important function is to determine the orbit of each satellite and predict its path for the following

24 hours. This information is uploaded to each satellite and subsequently broadcast from it. This enables

the GPS receiver to know where each satellite can be expected to be found. The satellite signals are read at

Ascension, Diego, Garcia & Kwajalein. The measurements are then sent to the master control station in

Colorado Springs where they are processed to determine any errors in each satellite. The information is

then sent back to the four monitoring stations equipped with ground antennas and uploaded to the satellites.

User Segment:

The User Segment comprises of anyone using a GPS receiver to receive the GPS signal and determine their

position and / or time. Typical applications within the user segment are land navigation for hikers, vehicle

location, surveying, marine, navigation, aerial navigation, machine control etc.

Information Transmitted by the satellites?

The following is the key information transmitted by the satellites constellation on either a continuous or

periodical basis.

1. Satellite Health

2. Ephemeredes

3. Constellation Almanac

4. Time

5. Ranging Signals

6. Atmospheric Correctional Data

The Ephemeredes describe the detailed orbital characteristics of the satellite from which it is transmitted.

Simply this is the satellite's mechanism for describing where it is.

The satellites Almanac describe the course orbital data for all satellites in the constellation. Simply this data

describes where all the satellites are, roughly, allowing the receiver to know where to look, roughly, for a

satellite.

This data is broadcast to the User Segment so that it can be stored and employed for initial satellite

acquisition and for visibility prediction.

Satellites are identified by:

1. Space Vehicle Number (SVN), and

2. Pseudo Random Noise number (PRN)

The Space Vehic1e number indicates the chronological order in which the satellites were launched. Most

GPS Receivers employ the PRN to identify which satellite they are observing.

How does it Work?

Some Principles to Acknowledge

There are 24 operational satellites

They orbit the earth approximately every 12 hours

They are positioned in six (6) orbital planes

Therefore there will usually be something in the order of 6 to 8 satellites visible above the horizon at any

point in the world and at any time of the day.

1. Each satellite emits information relating to its position, relative to the earth and timing information.

This timing information is derived from extremely accurate atomic clocks (cesium or rubidium) that

are synchronized to all other satellite clocks and to the ground control stations.

2. GPS Receivers are equipped with quartz clocks that are synchronized to GPS time via the data

transmitted from the constellation.

3. Timing is the basis of location computation.

4. The satellite radiates coded signals that are received by the user’s GPS receiver.

5. The computation in its simplest form is triangulation. Space Based Triangulation.

Producing Locations:

The determination of position is a simple as the following:

1. A signal is transmitted from a satellite containing the Time of Departure of the signal.

2. The signal is received by the GPS Receiver and the Time of Arrival is registered.

3. We know that Radio waves (the signal) travel at the Speed of Light.

4. We know where the satellite is from the information radiated from the satellite.

Therefore, we can determine the distance from our receiver to a particular satellite. This allows the

construction of a hemisphere, whose centre is the satellite and whose radius is the calculated distance from

a particular satellite to our receiver.

When this process is repeated for another satellite that is in view, then the two hemispheres with cut through

each other. Repeating this process again with a third satellite and the intersection of the three hemispheres

will form a point, which is where your receiver is located.

This all seems a bit top heavy, but remember that the satellites are constantly transmitting information and

the receiver is usually capable of producing a location result up to 10 times every second.

The accepted rule for most receivers is that the receiver must continuously track a minimum of four (4)

satellites to produce a location that contains a latitude, longitude and altitude. Of course most receivers

available today will track many more satellites than four (4).

This is important, as mentioned previously, the constellation operates on 12 hour orbits, therefore the

constellation that is visible (being used by the receiver) is always changing, and hence the receiver needs

to be looking for new satellites as the current in use satellites begin to disappear from view.

Task 2

Objectives:

Understanding the principle of GPS Satellite, generation of L1 carrier frequency, operation

of GPS Receiver and establishing the link between the GPS Satellite and GPS Trainer.

GPS Satellite Block diagram:

On board the Satellite have four atomic clocks. The following time pulses and frequencies required for day-

to-day operation are derived from the resonant frequency of one of the atomic clocks shown in figure:

1. The 50Hz data pulse.

2. The C/A code pulse (Coarse/Acquisition code, PRN-Code, coarse reception code at a frequency of

1023 MHz), which modulates the data using an exclusive or operation (this spreads the data over a 1

MHz bandwidth).

3. The frequency of the civil L1 carrier (1575.42MHz) The data modulated by the C/A code modulates

the L1 carrier in turn by using Bi-Phase-Shift-Keying (BPSK). With every change in the modulated

data there is a 180 deg change in the L1 carrier phase.

This is the simplest technique employed by GPS receivers to instantaneously give a position and height and

/ or accurate time to a user. The accuracy obtained is a better than 100m (usually around the 30-50m mark)

for civilian users and 5-15m for military users.

Task 3

Objective:

Understanding the shape of Earth and Measurement of latitude, longitude.

Earth Shape:

A significant problem when using the GPS system is that there are very many coordinate systems

worldwide. As a result, the position measured a calculated by the GPS system does not always coincide

with one's supposed position.

In order to understand how the GPS system functions, it is necessary to take a look at the basics of the

science that deals with the surveying and mapping of the Earth surface, geodesy. Without this basic

knowledge, it is difficult to understand why with a good portable GPS receiver the right combination has

to be selected from more than 100 different map reference systems. If an incorrect choice is made, a position

can be out by several hundred meters.

Different Earth Shapes like:

1. Geoids

2. Spheroid

3. Worldwide reference ellipsoid WGS-84

Format of latitudes and longitudes:

Where a numeric latitude or longitude is given, the two digits immediately to the left of the decimal point

are whole minutes, to the right are decimals of minutes, and the remaining digits to the left of the whole

minutes are whole degrees. Eg. 4533.35 is 45 degrees and 33.35 minutes. ".35" of a minute is exactly 21

seconds. Eg. 16708.033 is 167 degrees and 8.033 minutes. “.033” of a minute is about 2 seconds.

Observations:

S. No. Latitude Longitude City Country

Conclusion:

Task 4

Objective:

Understanding the principle of PRN code in GPS and principal of autocorrelation in GPS.

PRN codes:

This section describes the principle of the PRN code that is used and its use for GPS.

PRN stands for Pseudo Random Noise. In normal language it means consists of a long series of bits (0’s

and 1’s). At first sight there doesn’t seem to be a regular pattern in the bits. But there is! The codes-patterns

used for GPS repeat themselves after the 1023rd bit. These codes can be easily made with very few digital

elements. For the 1023 bit pattern 10 shifting registers and some digital adders are needed. In general with

n shifting registers a series of 2n -1 bits can be generated. For n = 10 this will become 1024 (= 210) - 1 =

1023 bits. The codes are generated with a speed of 1.023 MHz (or 1023000 bits per second). An example

with four shifting elements is given in the picture below.

The GPS satellites broadcast the PRN codes mixed (see picture below) with the other GPS information,

like orbital (also called ephemeris) and clock-parameters, but also parameters concerning the other

satellites. By mixing the PRN-code with the 50 Hz data the total signal is spread out over a broad part of

the spectrum. This technique is called spread spectrum. This section won't go very deep in this complex

matter, but the result is that the signal power is very low, even beneath the noise floor. In other words: it

has become very hard to distinguish the signal from noise (that is always present on signals).

When the GPS signals are received by the user of GPS, the PRN-code and GPS data have to be separated.

This is done by again mixing the received signal with a locally generated PRN-code. This must be the same

PRN-code which has been generated in the satellite. It is important that equal parts of the code are mixed

with each other.

Therefore the code generated in the receiver must be shifted in time until the two codes are exactly

synchronous. In this special case when the receiver ‘locks’ (also referred as full correlation) the two codes

can block .each other out and the GPS-data remains and can be further processed. This method is called

dispreading.

Every satellite has its own unique PRN-code so that the GPS receiver can distinguish the signals from

various satellites. GPS receiver is able to generate 32 PRN-codes. Until now so many satellites have not

been launched.

When the GPS receiver has to start up it doesn't know which GPS signal is from which satellite. Therefore

it tries to lock with the 32 known PRN-codes one by one. If one code locks then the information of one

satellite can be decoded. This information also contains data about other satellites and the rest can soon be

received too.

The main reason for using PRN codes in the GPS system is that the PRN code enlarges the unambiguous

measurement range. One must keep in mind that after 1023 bits the code is repeated. It is case that GPS-

receiver is aware it is ‘looking’ at the right code and not at its predecessor or successor. Looking at the

wrong code gives a navigation error of 300 km (corresponding to the code length of 1 millisecond).

Autocorrelation:

The ideal GPS receiver would have an infinitely wide receiver BW which would allow the receiver to

capture 100% of the GPS spread spectrum signal. The normalized autocorrelation function for an infinitely

wide BW is generally illustrated as shown in figure below.

The auto-correlation peak is maintained by continually adjusting the locally generated code for peak

correlator output. The unlimited BW provides a sharp correlation peak and steep early/later slope which

facilitates accurate error correction for the codelock- loop (also called Delay Lock Loop).

In reality, a GPS receiver would need a brick wall band pass filter with a BW of at least ten times the code

C/A code chipping rate to be capable of capturing> 99% of the GPS spread spectrum signal. For most GPS

receiver this is generally not practical to achieve.

Task 5

Objective:

Understanding the principle of Geometry of the Satellite and importance of PDOP, HDOP,

and VDOP.

DOP (Dilution of Precision):

The accuracy with which a position can be determined using GPS in navigation mode depends, on the one

hand, on the accuracy of the individual pseudo-range measurements and on the other, on the geometrical

configuration of the satellites used. This is expressed in a quality, which in navigation literature is termed

DOP

(Dilution of Precision).

There are several DOP designations in current use:

GDOP: Geometrical DOP (position in 3-D space, incl. time deviation in the solution).

PDOP: Position DOP (position in 3-D).

HDOP: Horizontal DOP (position on a plane).

VDOP: Vertical DOP (height only).

The accuracy of any measurement is proportionately dependent on the DOP value.

This means that if the DOP value doubles the error in determining a position increases by a factor of two.

Satellite Geometry and PDOP :

PDOP can be interpreted as a reciprocal value of the volume of a tetrahedron, formed by the positions of

the satellites and user, as shown in figure. The best geometrical situation occurs when the volume is at a

maximum and PDOP at minimum. PDOP played an important part in the planning of measurement projects

during the early year of GPS, as the limited deployment of satellites frequently produced phases when

satellite constellations were geometrically very unfavorable. Satellite deployment today is so good that

PDOP and GDOP values rarely exceed 3.

It is therefore unnecessary to plan measurements based on PDOP values, or to evaluate the degree of

accuracy attainable as a result, particularly as different PDOP values can arise over the course of a few

minutes. In the case of kinematic applications and rapid recording processes, unfavorable geometrical

situation that are short lived in nature can occur in isolated case.

The relevant PDOP values should therefore be included as evaluation criteria when assessing critical results.

PDOP values can be shown with all planning and evaluation programmes supplied by us in figure below.

Observations:

S. NO. PDOP HDOP VDOP

Task 6

Objective:

Observe the complete GPS Environment

Now no more theory. Up to experiment no. 7 you have studied all the basics of GPS. Now in this experiment

we are analyzing the entire GPS system. You have to take all the readings not only the lab but also in the

campus, if possible in the city or state or country or in the entire world.

Procedure:

Following steps has to be perform while doing the experiments.

Step 1: Please go through the manual before performing any practical.

Step 2: Install the software from the CD i.e. Open WinZip from the CD and run the setup file. If you don't

have WinZip then please install WinZip from the CD itself.

Step 3: Connect mains cord to the trainer ST2276. Don't switch on the system now.

Step 4: Connect serial cable to the port which is available on the trainer. Connect another end of the cable

to PC serial port (COM1, COM2, COM3 etc.).

Step 5: Connect the patch antenna to SMA (subminiature) connector of the ST2276 trainer.

Step 6: Place the antenna in the open space i.e. Place the antenna outside the window.

Step 7: Switch on the trainer ST2276.

Step 8: Precaution, don't touch the antenna during the on condition.

Step 9: Open software from start / program file/GPS Diag. Now click on option like

COM1, if it is not possible to detect then check your PC com port. If your PC com port is COM2 then click

COM2 in the software. As soon as you click on any of these com port according to your PC the software

will start displaying some signals. After this click on stop button, now go to the observation table for noting

down the values.

Step 10: Take at least four reading by placing the antenna at four different locations. But switch off the

power during placing the antenna on different location.

Observation:

In this observation table you have to take readings like UTC Time, UTC Date, Sats Used, Latitude,

Longitude, Speed, Altitude, Quality, PDOP, HDOP, VDOP, PRN, Elev, Az, SNR, Used?, & also readings

from the received data.

At least you have to take three to four readings on different different places. Please use blank sheet for the

observation and make at least four columns for four different places.

Result & Conclusion: