Post on 28-Nov-2014
PART -1
PRELIMINARY INVESTIGATION
1
1.1 INTRODUCTION
This report is a System analysis and design project, which is a study of global
positioning system software receiver Technology. In this project we studied how
gps receiver will works and processed the signal get desired location ,time and
position . We start with gps and its various components ,process and receiver
tracking system . Hence, this system makes it possible tracking the location of
things which consists gps receiver. This processes changes the signal to digits..
The process involves many models and theories that makes the gps successful.
Gps is used in large number of areas. For example mobile phone tracking vehicle
tracking system information providing using automated call defence uses, robotics,
etc. It facilitates the human computer interaction and also provides a way to
communicate with satellite communication.
The ultimate goal of the technology is to be able to produce a system that can
recognize with 100% accuracy the time and location . Even after years of research
in this area, the best gps software applications still cannot recognize location with
100% accuracy. Some applications are able to recognize over 95% position when
environment factors are constant.
Computer software that tracks the location of real world objects enable user to
have conversations with the satellite.
2
1.2 OBJECTIVE
To study global positioning system receiver and its various hardware
components and software used for this. . In this project our aim is to:
Working of gps receiver
Hardware components of gps
Software used for gps receiver
3
1.3 PROBLEM DEFINITION
Software GPS receivers can provide full access to base Band signal processing
inside the receiver channels. Thus It has become the key component when
investigating and Developing advanced GPS signal processing techniques.
In this presentation, a pure software gps receiver, developed in the plan group of
the university of Calgary, It consists of receivers that decode the signals from the
satellites.
The receiver performs following tasks:
Selecting one or more satellites
Acquiring GPS signals
Measuring and tracking
Recovering navigation data
4
1.4 WORKING OF GPS
For those who are unfamiliar with the term, GPS stands for Global Positioning
System, and is a way of locating a receiver in three dimensional space anywhere on
the Earth, and even in orbit about it.
GPS is arguably one of the most important inventions of our time, and has so many
different applications that many technologies and ways of working are continually
being improved in order to make the most of it.
To understand exactly why it is so useful and important, we should first look at
how GPS works. More importantly, looking at what technological achievements
have driven the development of this fascinating positioning system.
1.4.1 SIGNALS
In order for GPS to work, a network of satellites was placed into orbit around
planet Earth, each broadcasting a specific signal, much like a normal radio signal.
This signal can be received by a low cost, low technology aerial, even though the
signal is very weak.
Rather than carrying an actual radio or television program, the signals that are
broadcast by the satellites carry data that is passed from the aerial, decoded and
used by to the GPS software.
The information is specific enough that the GPS software can identify the satellite,
it’s location in space, and calculates the time that the signal took to travel from the
satellite to the GPS receiver.
5
Using different signals from different satellites, the GPS software is able to
calculate the position of the receiver. The principle is very similar to that which is
used in orienteering – if you can identify three places on your map, take a bearing
to where they are, and draw three lines on the map, then you will find out where
you are on the map.
The lines will intersect, and, depending on the accuracy of the bearings, the
triangle that they form where they intersect will approximate your position, within
a margin of error.
GPS software performs a similar kind of exercise, using the known positions of the
satellites in space, and measuring the time that the signal has taken to travel from
the satellite to Earth.
The result of the “trilateration” (the term used when distances are used instead of
bearings) of at least three satellites, assuming that the clocks are all synchronized
enables the software to calculate, within a margin of error, where the device is
located in terms of its latitude (East-West) and longitude (North-South) and
distance from the center of the Earth.
1.4.2 TIMING & CORRECTION
In a perfect world, the accuracy should be absolute, but there are many different
factors which prevent this. Principally, it is impossible to ensure that the clocks are
all synchronized.
Since the satellites each contain atomic clocks which are extremely accurate, and
certainly accurate with respect to each other, we can assume that most of the
problem lies with the clock inside the GPS unit itself.
6
Keeping the cost of the technology down to a minimum is a key part of the success
of any consumer device, and it is simply not possible to fit each GPS unit with an
atomic clock costing tens of thousands of dollars. Luckily, in creating the system,
the designers designed GPS to work whether the receiver’s clock is accurate or not.
There are a few solutions. However the solution that was chosen uses a fourth
satellite to provide a cross check in the trilateration process. Since trilateration
from three signals should pinpoint the location exactly, adding a fourth will move
that location; that is, it will not intersect with the calculated location.
This indicates to the GPS software that there is a discrepancy, and so it performs
an additional calculation to find a value that it can use to adjust all the signals so
that the four lines intersect.
Usually, this is as simple as subtracting a second (for example) from each of the
calculated travel times of the signals. Thus, the GPS software can also update its’
own internal clock; and means that not only do we have an accurate positioning
device, but also an atomic clock in the palm of our hands.
1.4.3 MAPPING
Knowing where the device is in space is one thing, but it is fairly useless
information without something to compare it with. Thus, the mapping part of any
GPS software is very important; it is how GPS works our possible routes, and
allows the user to plan trips in advance.
In fact, it is often the mapping data which elevates the price of the GPS solution; it
must be accurate and updated reasonably frequently. There are, however, several
kinds of map, and each is intended for different users, with different needs.
7
Road users, for example, require that their mapping data contains accurate
information about the road network in the region that they will be traveling in, but
will not require detailed information about the lie of the land – they do not really
worry about the height of hills and so forth.
On the other hand, hiking GPS users might wish to have a detailed map of the
terrain, rivers, hills and so forth, and perhaps tracks and trails, but not roads. They
might also like to adorn their map with specific icons of things that they find along
the way and that they wish to keep a record of – not to mention waypoints;
locations to make for on their general route.
Finally, marine users need very specific information relating to the sea bed,
navigable channels, and other pieces of maritime data that enables them to navigate
safely. Of course, the sea itself is reasonably featureless, but underneath quite some
detail is needed to be sure that the boat will not become grounded.
Fishermen also use marine GPS to locate themselves and track the movement of
shoals of fish both in real time, and to predict where they will be the next day. The
advent of GPS fixing has also meant that co-operative fishing has become much
easier, where there are several boats all relaying their locations to each other while
they locate the best fishing waters.
Special kinds of marine GPS known as fish finders also combine several functions
in one to help fishermen. A fish finder comprises GPS and also sonar, along with
advanced tracking functions and storage for various kinds of fishing and maritime
information.
8
SYSTEM ANALYSIS OF GPS RECEIVER
Part-11
9
2. REQUIREMENTS OF GPS
2.1 HARDWARE COMPONENTS
Antenna
RF Board
RF Front End
RF/IF down-conversion board (with FPGA)
DSP Board
DSP
2.2 SOFTWARE COMPONENTS
Firmware
RF Board FPGA
DSP Board FPGA
S/W
Signal Processing S/W
Navigation S/W
10
2.1 HARDWARE COMPONENTS
GPS SIGNAL RECEIVER
2.1.1 ANTENNA
The GPS antenna combines a planar antenna and a frequency converter, which
translates the high-frequency phase-modulated spread spectrum signal of the GPS
system to an intermediate frequency. This way a standard coaxial cable (e.g.
RG58) can be used for the connection with the GPS clock and a distance of up to
300 meters (with RG58) or even 700 meters (with a low-loss cable type like
RG213) between receiver and antenna is possible without additional amplifier.
11
Ambient temperature: -40 ... 65°C Warranty: Three-Year Warranty RoHS-
Status of the product: This product is fully RoHS compliant WEEE status of the
product: This product is handled as a B2B category product. In order to secure a
WEEE compliant waste disposal it has to be returned to the manufacturer. Any
transportation expenses for returning this product.
2.1.2 RF BOARD
RF board stands for Radio Frequency Printed Circuit Boards. The frequency
for RF board is normally between 300MHz ~ 3GHz, or much bigger, so normally
FR4 board cannot meet the requirements, so we need to use special material to
achieve the high frequency and we named this kind of boards as RF boards. RF
board is excellent in high frequency performance due to its low dielectric tolerance
and loss of material.
RF board is ideal for applications with higher operating frequency requirements.
Right now, we normally use following material The fabricate process is similar
like FR4, but the copper plating is more complex than FR4, because material
characteristics, it’s much harder to metalize the
through hole (copper plating), and other process is complex than FR4, so need
unique handling method and experienced workers from the computer fans,
squeaking chairs, or heavy breathing. e.g., creative sound cards, intel sound cards,
acer sound card, philips sound cards.
2.1.3 RF FRONT:
In a radio receiver circuit, the RF front end is a generic term for all the circuitry
between the antenna and the first intermediate frequency (IF) stage. It consists of
12
all the components in the receiver that process the signal at the original incoming
radio frequency (RF), before it is converted to a lower intermediate frequency (IF).
In microwave and satellite receivers it is often called the low-noise block (LNB) or
low-noise down converter (LND) and is often located at the antenna, so that the
signal from the antenna can be transferred to the rest of the receiver at the more
easily handled intermediate frequency.
For most super-heterodyne architectures, the RF front end consists of:
An impedance matching circuit to match the input impedance of the receiver
with the antenna, so the maximum power is transferred from the antenna;
A 'gentle' band-pass filter (BPF) to reduce input noise and image frequency
response;
An RF amplifier, often called the low-noise amplifier (LNA). Its primary
responsibility is to increase the sensitivity of the receiver by amplifying
weak signals without contaminating them with noise, so they are above the
noise level in succeeding stages. It must have a very low noise figure (NF).
The mixer, which mixes the incoming signal with the signal from a local
oscillator (LO) to convert the signal to the intermediate frequency (IF).
2.1.4 RF/IF DOWN CONVERSION:
The LBC-4000 L-Band IF to 70 MHz IF (140 MHz optional) indoor converter is a
1RU 19-inch chassis with two front panel accessible up converter or down
converter modules. It contains two diode “OR-ed” internal power supplies, for
increased reliability and microprocessor-based Monitor & Control (M&C)
functions. The LBC-4000 up converter module translates a 70 MHz IF input signal
(140 MHz optional) up to a user selected frequency at L-Band (950 to 2000 MHz).
13
The L-Band output can drive the input of the Comtech EF Data MBT-4000 block
up converter or other RF equipment with an L-Band input.The LBC-4000 down
converter module translates an L-Band (950 to 2000 MHz) IF input signal down to
a user selected frequency in the 70 MHz (140 MHz optional) IF band. The LBC-
4000 can be locked to an internal reference or an external 5 or 10 MHz reference
signal. The LBC-4000 is an excellent choice forinterfacing legacy 70 or 140 MHz
equipment to quad-band or tri-band block converters.
2.1.5 DSP BOARD:
DSP boards or digital signal processor computer boards are central to the
implementation of high-performance industrial systems. They collect and process
digital data from many sources, and distribute the results to other elements of the
system. There are three main sources of data in a real system: signals (in and out
from the DSP processor), messages to communicate with system controllers, and
messages to communicate with other DSP boards. Important features of DSP
boards include a fast processor and good communication channels as DSP boards
need to collect and distribute data from/to many different sources.
Computer backplane or bus choices for DSP boards include PCI, ISA or EISA,
PCMCIA, PC/104, Mac PCI, SUN Sbus, PMC bus, PXI bus, Multi bus, STD bus,
VME bus, VXI or MXI bus, and DT-connect I and II interface. PCI is a local bus
system designed for high-end computer systems. ISA is a standard for I/O buses
that was set back in 1984 when IBM was the standard. PCMCIA devices (PC
Cards) are credit-card-sized peripherals predominantly used in laptop computers.
PC/104 gets its name from the desktop personal computers designed by IBM
(PCs), and from the number of pins used to connect the cards together (104). Mac
PCI is a local bus standard developed by the Intel Corporation. Designed by Sun
14
in 1989, the SBus board was the standard I/O inter-connect for Sun computers,
which typically run under the Solaris or SunOS flavor of the UNIX operating
system. The PMC Bus is actually a form factor, not a bus -- it is electrically the
same as the PCI Bus, but the shape of the card and the bus connectors are
different. PXI is a superset of Compact PCI and adds timing and triggering
functions, imposes requirements for documenting environmental tests, and
establishes a standard Windows-based software framework. STD bus is often
referred to as the "Blue Collar Bus" because of its rugged design and small size,
the STD Bus was originally designed for factory and industrial environments. It
uses 16-bit architecture. VME bus is a 32-bit bus used in industrial, commercial
and military applications. Motorola developed the VME standard, with others, in
the late 1970s. DT-connect I and II is Data Translation's DT-Connect Interface.
Important processor or DSP performance specifications to consider for DSP boards
include number of processors, clock speed, floating point performance, integer
performance, operations, maximum addressable memory, and operating
temperature. General features and options to consider when looking for DSP
boards include real-time clock, interrupt controller, memory management unit,
dual port memory, and direct memory access. Communications options include
serial I/O ports, parallel I/O ports, on board A/D converter, and on board D/A
converter. Some DSP boards can accept daughter boards and some DSP boards
are daughter boards. An important environmental parameter to consider when
searching for DSP boards is the operating temperature.
2.1.6 DSP
Digital signal processing algorithms typically require a large number of
mathematical operations to be performed quickly and repetitively on a set of data.
15
Signals (perhaps from audio or video sensors) are constantly converted from
analog to digital, manipulated digitally, and then converted again to analog form,
as diagrammed below. Many DSP applications have constraints on latency; that is,
for the system to work, the DSP operation must be completed within some fixed
time, and deferred (or batch) processing is not viable A simple digital processing
system
Most general-purpose microprocessors and operating systems can execute DSP
algorithms successfully, but are not suitable for use in portable devices such as
mobile phones and PDAs because of power supply and space constraints. A
specialized digital signal processor, however, will tend to provide a lower-cost
solution, with better performance, lower latency, and no requirements for
specialized cooling or large batteries.
The architecture of a digital signal processor is optimized specifically for digital
signal processing. Most also support some of the features as an applications
processor or microcontroller, since signal processing is rarely the only task of a
system. Some useful features for optimizing DSP algorithms are outlined below.
2.2 SOFTWARE COMPONENTS
2.2.1 FIRMWARE:
Firmware is software that is embedded in hardware. You can update your firmware
in most GPS receivers. Firmware is the software that controls how hardware works
and responds to inputs. It’s called firmware instead of software because users
generally aren’t supposed to play around with it. But you’re not just any old user,
16
are you? Almost all electronic hardware contains some form of firmware. A
television remote control contains firmware that controls what signals are sent via
IR depending on what button is pressed. A cell phone contains a lot of firmware
controlling cell access, phone books, security etc
A GPS contains a lot of firmware controlling many of the key functions of the
device
Reception of satellite data
Decoding of positional information
Processing of data
Conversion of data into different formats
Interpretation and display of information
External communication with devices
Storing and managing route/waypoint data
2.2.2 RFPGA:
The FPGA (Field-Programmable Gate Array)implementation of an adaptive filter
for narrow band interference excision in Global Positioning Systems is described.
The algorithm implemented is a delayed LMS(Least Mean Squares) adaptive
algorithm improved by incorporating a leakage factor, rounding and constant
resetting of the filter weights. This was necessary as the original adaptive
algorithm had stability problems : the filter weights did not remain fixed, and
tended to drift until they overflowed, causing the filter response todegrade. Each
model was first tested in Simulink,implemented in VHDL (Verilog Hardware
Description Language) and then downloaded to an FPGA board for final testing.
Experimental measurements of anti-jammargins were obtained Single channel
adaptive filtering techniques have been shown to be an effective technique for
17
mitigating multiple narrowband interferences to GPS systems. Since they can
beseamlessly inserted between the existing GPS antenna and receiver. they offer a
cost effective solution that involves minimum system disruption. However to
become a fully practical solution the size and power demands of their hardware
implementation should be minimised. FPGAs (Field-Programmable Gate Arrays)
offer the potential forachieving the goals of small size, weight and power
consumption and in this paper the implementation of an adaptive filter using an
FPGA device is described.In Section 2 an experimental system, termed mini-
GISMO, is described and an overview of the system architecture is presented. The
use of interpolation and decimation filters within the FPGA is also described.The
main adaptive algorithm implemented is the delayed LMS (Least Mean Squares)
adaptive algorithm (Haykin, 2002). As discussed in Section 3 this algorithm is well
suited to FPGA implementations. However, particularlyin the presence of strong
interferences, the originaladaptive algorithm had stability problems, as on
convergence, the filter weights did notremain fixed, and tended to drift until they
overflowed,causing the filter response to degrade. In Section 4 it is shown that
incorporating a leakage term and rounding instead of truncating resulted inthe
weights remaining near the optimal values. However, this solution introduced
memory effects, which produceda second null when the interference frequency was
changed. Resetting the weights every second removed this problem and appeared
to have the least stability effects, as a short pulse in the output every second didn’t
cause any undesirable results in this algorithm. Also, the bit allocations were
optimised to reduce the quantization error. By reducing the quantisation noise
power a smaller leakage factor is required to stabilise the adaptive algorithm
resulting in a slower drift of the weight.
18
2.2.3 DIGITAL SIGNAL
Digital signal processing has traditionally been done using enhanced
microprocessors. While the high volume of generic product provides a low cost
solution, the performance falls seriously short for many applications. Until
recently, the only alternatives were to develop custom hardware (typically board
level or ASIC designs), buy expensive fixed function processors (eg. an FFT chip),
or use an array of microprocessor.
Signal processing:
The antenna preamplifier of a GPS receiver generally converts the incoming signal
to a signal of a lower frequency. This INTERMEDIATE FREQUENCY is
obtained by mixing the incoming signal with a pure sinusoidal signal generated by
the local oscillator (the quartz "clock"). The frequency of this BEAT
FREQUENCY is the difference between the original (doppler-shifted) received
carrier frequency and the local oscillator. The intermediate or beat frequency is
then processed by the signal tracking.
19
2.2.4 NEVIGATIONAL SIGNAL PROCEESING
Digital signal processing is the processing of digitised discrete time sampled
signals. Processing is done by general-purpose computers or by digital circuits
such as ASICs, field-programmable gate arrays or specialized digital signal
processors (DSP chips). Typical arithmetical operations include fixed-point and
floating-point, real-valued and complex-valued, multiplication and addition. Other
typical operations supported by the hardware are circular buffers and look-up
tables. Examples of algorithms are the Fast Fourier transform (FFT), finite impulse
response (FIR) filter, Infinite impulse response (IIR) filter, and adaptive filters
such as the Wiener and Kalman filters.
Statistical signal processing — analyzing and extracting information from signals
and noise based on their stochastic properties
Audio signal processing — for electrical signals representing sound, such as
speech or music
Speech signal processing — for processing and interpreting spoken words
Image processing — in digital cameras, computers, and various imaging
systems
Video processing — for interpreting moving pictures
Array processing — for processing signals from arrays of sensors
Time-frequency signal processing — for processing non-stationary signals
Filtering — used in many fields to process signals
20
2.3 SOFTWARE BASED RECEIVER
Global Navigation Satellite System has become a necessity tool for navigation and
positioning in both civilian and military field and applications. Global Positioning
System (GPS) is a satellite-based navigation system. It is based on the computation
of range from the receiver to multiple satellites by multiplying the time delay that a
GPS signal needs to travel from the satellites to the receiver by velocity of light.
GPS has already been used widely both in civilian and military community for
positioning, navigation, timing and other position related applications. The system
has already proved its reliability, availability and good accuracy for many
applications. Due to this nature, in future, other countries like Europe are going to
launch new satellite-based navigation system called Galileo. There is also a
proposal to launch Quasi Zenith Satellite System for navigation in Japan. It is
necessary to simulate and analyze new signal structures for the development of
new satellite-based navigation systems. In the research community, many
researchers come out with new ideas and algorithms for better accuracy of GPS by
mitigating or minimizing various types of errors and effects like multipath.
However, it is quite difficult to implement the user developed algorithms in the
current hardware-based GPS receivers. The hardware-based GPS receivers contain
ASICs that provide the least user flexibility. Thus, it is necessary to have Software-
based GPS receivers, at least in the research community for easy and quick
implementation, simulation and analysis of algorithms, parameters and threshold
values. Since, the CPU processing power is increasing with reduced cost, it is now
possible to build real-time software-based GPS receivers at least for static or low
dynamic environments. As predicted by Moor’s Law, the CPU power is increasing
and we hope that this trend will continue in future as well and hence, it will be
21
possible to develop real-time all environment software-based GPS receivers. In this
paper, we briefly introduce the architecture of a SGR, signal processing technique
and give some examples of simulation using SGR.
2.3.1 SOFTWARE-BASED GPS RECEIVER ARCHITECTURE
The architecture of a conventional GPS receiver is shown in Figure 1. It consists of
RF front-end and signal processor that are all built upon IC chips. The outputs of
the signal processor are either displayed directly on the receiver display unit or fed
to a PC for further processing or integration with other devices. Since, the signal
processing is all done inside the hardware chips, users have limited access to
change the parameters or install new algorithms. Figure 2 shows architecture of a
software-based GPS receiver (SGR). It consists of a RF front-end device, which is
still a hardware component. The rest of the signal processing is done using high
level programming language like C/C++, Matlab etc. If we compare Figure a and
Figure b, the only difference we see is the replacement of hardware components by
software tools for signal processing. We still need RF front-end since the present
capacity of CPU is still not able to process the signal directly from the antenna at
1.5GHz. Figure c shows the merits and demerits of using hardware-based and
software-based receiver. A hardware-based receiver is fastest in signal processing
however, it has the least level of flexibility, where as a software-based receiver has
the highest level of flexibility but is the slowest in processing speed. There are
products using FPGA-based receivers which is the compromise between the two.
22
Figure a and b
23
2.3.2 GPS SIGNAL PROCESSING
L1 band GPS signal is transmitted at 1.5 Ghz and since the receiver cannot process
the signal directly at this frequency, the RF front-end device down converts from
1.5Ghz to a much lower frequency of about 4Mhz. This frequency is called
Intermediate Frequency (IF). During this conversion process, the signal is also
digitized (A/D conversion) at 1bit, 2bit or higher rate and sampled at some
frequency, e.g. 16Mhz. We use the down-converted signal for further processing.
The first task of signal processing is to identify the visible satellites by finding the
satellite code phase and Doppler frequency. The code phase provides the beginning
of C/A code Since the satellites are moving all the time (and probably the receiver
may also move) wealways have some Doppler frequency. The rough estimation
process of code phase and Doppler frequency is called acquisition. Basically, for
acquisition, we generate C/A code for the satellite and modulate with the carrier
wave. This receiver generated signal is then correlated with incoming signal and
the correlation value is evaluated to make decision whether a satellite visible. If we
think that the satellite is visible, then the code phase value and Doppler frequencies
noted. Once, we complete acquisition successfully, we know the satellites that are
24
visible at that time. In the next step, we track the visible satellites continuously for
fine tuning of the code phase and Doppler frequency. This process is called
tracking. The tracking process removes the C/A code and carrier wave from the
GPS signal and hence the remaining signal represents navigation data and some
noise. Thus, from navigation output, we can extract navigation data parameters
which are necessary to compute pseudo range from the receiver to satellite. Figure
(a) shows raw GPS data collected from antenna and down converted to IF. This
data just looks like noise and no information can be known unless we perform
acquisition and tracking on the data. This is due to the fact that the GPS signal
level is below the noise level or the signal is weaker than the noise. Figure (b)
shows the result of acquisition from raw data shown in Figure (a). The acquisition
output shows the code phase(beginning point of C/A code) and Doppler frequency.
Figure (c) shows tracking results. The tracking result extracts navigation data bits
as shown in which are simply the sequence.
25
2.3.3 SGR AS RESEARCH AND SIMULATION TOOL
We mentioned earlier that SGR has much flexibility compared to conventional
receiver. We will discuss and give some examples how these flexibilities of SGR
are used to extract information that are otherwise not possible in conventional GPS
receiver.
26
some of the fundamental parameters of signal processing in SGR. IF frequency and
sampling frequency are fixed for a particular front-end device. By changing these
two values, we can use the same software tool for different types of frontend
27
device that acquire GPS signal from the antenna. Below we will discuss some of
the flexibilities point by point.
2.3.4 WEAK SIGNAL PROCESSING
The Doppler frequency search step, code period acquisition integration time, noise
bandwidth code period tracking integration time depends on the signal
quality. If the signal level is normal, we can use 1000Hz Doppler frequency step
and 1ms code period integration time for acquisition.
However, if the signal is weak, and then we need Figure a: Basic parameters that
can be changed by a user in SGR for various types of signal processing and
simulation to reduce the Doppler frequency search step and increase the code
period integration time in acquisition. For example, if we integrate raw data for
3ms for acquisition then we need to reduce the Doppler frequency search step to
300Hz. This will increase processing speed but help us in detecting weak signals.
28
Also, we need to increase the integration time in tracking loop. This type of signal
processing by changing the parameter values is not possible in conventional GPS
receiver. Figure b shows an example for increase in integration time from 1ms to
3ms. When the integration time is 1ms, the correlation peak is not clear enough to
make a decision for satellite visibility. But, when the integration time is increased
to 3ms, we can see a very clear correlation peak and we can make a decision that a
particular satellite is now detected. Figure b (a) Signal acquisition using 1ms
integration time. The result is not so clear with multiple peaks. (b) Signal
acquisition using 3ms integration time with the same data as in (a). Now, the
correlation peak is quite clear and a decision can be made regarding visibility of
satellite.
2.3.5 MULTIPATH MITIGATION TECHNIQUE
In spite of continuing improvements in GPS receivers and antenna technology,
multipath signal has remained a major source of error in GPS positioning. In order
to minimize the error due to multipath, we need to understand the multipath
behaviour and corresponding signal characteristics. In order to understand the
effect of multipath we can analyze the signal by using various types of correlators
(narrow, wide etc) by defining chip delay (listed in Figure a) between early and
late chips. We can compute the correlation peak for every code period. A
correlation peak will appear as a perfect triangle.
29
There has been no effect from multipath. Due to multipath, the two sides of the
triangle will be neither symmetrical nor straight lines. The shape and amplitude of
the triangle is deformed by the amount of multipath and some other noise. Thus by
analysing the correlation peak (triangular shape) we can estimate the amount of
multipathand hence develop a technique to minimize or mitigate the multipath. In
this regard, we are conducting research using left hand and right hand circular
polarized GPS antenna to analyze how the reflected signal (which accounts
formultipath) affects a correlation peak.
Figure c shows a correlation peak obtained by processing a raw GPS signal.
Correlation peak computed from raw GPS signal for 0.5 chip delay. The peak
shape is not a perfect triangular due to effect from multipath
2.3.6 REMOTE SENSING USING GPS SIGNAL:
Recently, GPS signals have been used for remote sensing purpose. GPS signals are
transmitted at 1.2Ghz and 1.5Ghz in two different bands. This is similar to
microwave remote sensing. GPS signals are transmitted with right hand circular
polarization. When, this signal is reflected by some object the polarization may
change from right hand to left hand and vice versa. Thus by observing the reflected
signal together with two different types of antennas with right hand and left hand
polarization, we can predict the object type that reflects the GPS signal. Using this
30
technique, soil moisture and wind velocity has been estimated. In order to conduct
this type of analysis, we need software-based receiver so that we can process the
received signal with different parameter values using our own algorithms. The
reflected signals are much weaker than direct signal and hence a conventional
receiver cannot be used. Also, we need to compute many intermediate values like
shape of the correlation peak and it s amplitude rather than the position of the GPS
antenna itself. This is possible only in software-based receivers. Besides these
analysis and simulation listed above, we need software-based receiver for
analyzing noise and interference (jamming), simulate new codes, limitation of
navigation data length and many other things. In current GPS signal, the navigation
data length is limited to20ms. This impose a restriction on data integration beyond
20ms during the tracking process. However, for tracking very weak signal, we do
need to integrate longer data period. Thus we need to see what will happen if we
change the navigation data length from 20ms to something else in our new design.
On the other hand we can also have a data less component of the signal in one of
the phases of the signal which is now implemented in new forthcoming GPS
signals. This assists the receiver in processing weak signals and hence make the
receiver capable of indoor positioning. All these can be simulated if we have
software-based receivers. In SGR, we can generate different types of signals for
interference analysis. This will help us how different types of signal with different
level of strength affect GPS signal processing. For example, we can simulate the
effect of a TV signal on GPS or we can analyze the effect of other GNSS signals
on GPS or vice versa.
31
2.4 FLOW CHART OF GPS WORKING
32
TEST PLATFORM FOR RECIEVER
33
2.4 WORKING MODEL OF GPS
34
35
2.6 DATA FLOW DIAGRAM FOR GPS
2.6.1 CONTEXT LEVEL DIAGRAM FOR GPS
36
A handheld gps system gets location data from satellites and the final destination
from the user. The system then directs the user to their destination.
2.6.2 LEVEL 1 DFD
37
SYSTEM DESIGN ANALYSIS OF GNSS
SOFTWARE
PART -3
38
3.1 INTERFACE DESIGN OF GNSS
Spider makes use of Microsoft SQL Database Server Desktop Engine to manage
the Configuration, operational parameters settings of GPS receivers and other
external devices in the stations. Multiple software modules and components in the
system can then access the data from the database simultaneously and this open
architecture allows the user customization and flexibility for
The software interface for generating different types of data file in different
formats Moreover the Spider server and the Microsoft SQL Database run
automatically and continuously as a Windows service, so the software modules and
database can be automatically launched once the computer server is started and the
whole system can run in normal operating condition according to predefined
operational parameters even if Windows is not logged or the Spider software
39
interface is closed. In case of a total power failure, the software will restart as the
computer reboots in order to provide full reliability. At the start of the system, the
network operator will define the basic GPS station parameters such as station
name, coordinates, GPS receiver & antenna model, antenna height offset, data rate
of GPS receiver as well as other external sensors, data communication ports,
automatic data polling intervals and the storage path in the computer server. The
Spider server will automatically be linked to each GPS reference station during a
predefined interval (e.g. every 10 minutes, 30 minutes, 1 hour, etc) via different
communication strategies as previously discussed and will download the data files
stored in the memory of GPS reference station receivers. Besides, the Spider server
will then convert the raw data source to produce various data files in different data
rates, data file lengths and data formats such as Leica MDB proprietary format,
RINEX format and GPS Hatanaka compressed format to finally store into different
user assigned locations in the computer server. In case of having GPS reference
station receivers and Spider server connected by PSTN or wireless dial-up
communication, the Spider server can be automatically disconnected the
communication line once the data files are completely downloaded and thus save
the communication costs. Furthermore, if the communication link is not stable and
fails to complete the data downloading, the software will automatically re-
download that missing data files in the next downloading interval.
40
In addition, the network operator is able to define a FTP server or a Web server
address, so Spider server will transmit raw data, RINEX files, and other associated
files such as quality checking files and event log files immediately when available
or at specific time intervals to one or multiple FTP or Web servers for an easy
access by the GPS users community. Different files can be pushed to different FTP
servers. The users can share and distribute these data files by the Internet.
41
3.2 SYSTEM MANAGEMENT CONSOLE
The console management control of the whole GPS reference station network is
illustrated by the network operator has a full operational status view of each GPS
reference station of the whole network through the system management console
interface. It displays the entire network operation status including connection
status, receiver operation status such as power level and memory status of the GPS
receiver, data logging and real-time data broadcasting status, satellite tracking
status such as the number of tracking satellites on L1 and L2, signal to- noise ratio,
azimuth and elevation angles of each satellite; and external meteorology and tilt
sensor data stat In case of abnormal behaviors happening in GPS
In case of abnormal behaviors happening in GPS reference station such as
communication failure, receiver’s power low, low memory space, data logging
42
failure or RTK data output failure, then the color of the corresponding functions
icons will change and error message will be displayed in order to clearly notify the
network operator.
The network operator has a full operational status view of each GPS reference
station of the whole network through the system management console interface. It
43
displays the entire network operation status including connection status, receiver
operation status such as power
3.3 INTEGRITY MONITORING AND ERROR REPORTING
MECHANISM
A Spider server produces quality check report file automatically every time it
completes the GPS raw data files downloading and the quality control procedure
checks the completeness and consistency of all data downloaded, monitors the
various communication links and the operation status of the entire system. These
quality check report files can also be automatically forwarded to another server or
a web platform for any operators’ inspection.
In order to be more efficient and faster response in tackling system and data quality
problems, the network operator can define a range of inspection criteria and
tolerance values. The diagram 5 shows the checking criteria and tolerance values
defined as:
− GPS receiver’s related issues:
− Receiver’s power voltage, free memory space and internal temperature
− Receiver start up failure
− Receiver data logging status
− Receiver data downloading status
− Communication link related issues:
− Communication between Spider server and GPS receivers
− Upload / Download status
− FTP data forwarding status
− Event and alarm sending status
− Data quality related issues:
44
In some countries, there is also an e-mail’s service to pager forwarding, so this
alarm e-mail message can also be shown on the operators’ pager for immediate
problem solving. Moreover, the network operator can make use of “Command line
processing” advanced feature to launch automatically another application script
which will launch for instance a SMS messaging program. In addition, this
command line processing feature can also be used for performing an integrity
monitoring by running other applications such the Teqc +QC developed by
UNAVCO. The new release of the Leica processing software SKI-Pro can also be
invoked automatically for computing the GPS baselines in a network adjustment.
By this way a comparison can be made immediately against the known values of
the baselines coordinates for a continuously and automatically station stability
checking which is definitively mandatory for such services.
3.4 REMOTE CONTROL & SECURITY
The software is designed in a modern Server / Client architecture and provides a
remote GUI client interface that can be installed on any remote computer.
Therefore, using Internet TCP/IP networks or dial-up connections, the network
operator with this GUI client interface can connect from anywhere to a Spider
server which has a fixed IP address assigned. The network operator can remotely
monitor the entire GPS network performances and also configure and control a
Spider server anywhere in the world which control all connected
As an Administrator, the client logged has full control over the Spider server and
the GPS receivers. He can start and stop the various operations, create and change
the configurations set parameters and modes, etc. This access right is usually only
45
granted to network supervisors and operators. However, if the client has only the
Viewer privileges, he can only inspect the system and receiver status but not
control the operation of the software and configuration parameters setting.
3.5 REAL-TIME DATA BROADCASTING:
The software supports both RTK and DGPS data broadcasting on each networked
GPS reference station to be used by RTK and GIS GPS rover users. The real-time
data stream can be broadcasted in various formats such as the compacted Leica
proprietary, RTCM v2.x CMR and CMR+ through different communication
solutions including radio, GSM, ISDN and PSTN network and also Internet. The
real-time data can be either transmitted directly from the GPS reference stations in
the field or it can be routed back and centralized in the Spider server to be re-
distributed to the GPS rover users. Other sophisticated data distribution facilities
such as Access Servers, web application services and charging mechanisms can
complete the solution. A list of common use communication devices including
various brands of radio modems, GSM terminals and fax modems are already
defined in the device interface and the network operator can select the suitable one
and configure two streams of real-time data of any networked reference station in
different formats and output rates via different communication devices
simultaneously. This is a flexible solution to meet different users’ needs and area
coverage. It can also work in time-slicing mode for different real-time data streams
broadcasting of different GPS reference stations in different divided time intervals
by using the same radio frequency channel without signal interference or jamming
problem
46
To broadcast any real-time data over the Internet, the data stream from each GPS
reference station needs to be routed to the Spider server and converted and
forwarded to a unique static IP address and port number of a web server which has
a permanent Internet connection. The real-time data stream is continuously
available on Internet. Multiple rover users can connected to this Internet Web
server by using a Pocket PC with a CDMA or GPRS PCMCIA modem, and then
access the real-time data stream from the specific IP address simultaneously.
People can receive real time data corrections from any GPS reference station
located in the world for real-time positioning where the wireless CDMA or GPRS
signals are available without any geographical distance restriction on corrections
transmission. The success of achieving high precision real-time positioning over
long baseline length is however still dependent on the resolving integer ambiguity
algorithms implemented on the GPS rover receivers.
According to the result of a RTK field test done in Beijing PRC in August 2003 by
using the SR530 GPS receiver, which accessed real-time data stream from two
GPS reference stations in Beijing via Internet, the horizontal accuracy for a short
baseline of around 10 km was on the 2 cm level (1 sigma); and for the long
baseline test of around 55 km was on the 4 Comment [M1]
47
APPENDIX
HISTORY
The GPS System was created and realized by the
American Department of Defense (DOD) and was
originally based on and run with 24 satellites (21
satellites being required and 3 satellites as replacement).
48
Nowadays, about 30 active satellites orbit the earth in a
distance of 20200 km. GPS satellites transmit signals
which enable the exact location of a GPS receiver, if it is
positioned on the surface of the earth, in the earth
atmosphere or in a low orbit. GPS is being used in
aviation, nautical navigation and for the orientation
ashore. Further it is used in land surveying and other
applications where the determination of the exact position
is required. The GPS signal can be used without a fee by
any person in posession of a GPS receiver. The only
prerequisite is an unobstructed view of the satellites (or
rather of the sky).
The correct name of the system is NAVSTAR
(Navigation System for Timing and Ranging), but
commonly it is referred to as GPS (Global Positioning
System).
49
APPLICATION OF GPS
Navigation: GPS allows soldiers to find
objectives, even in the dark or in unfamiliar
territory, and to coordinate troop and supply
movement. In the United States armed forces,
commanders use the Commanders Digital
Assistant and lower ranks use the Soldier Digital
Assistan
Target tracking: Various military weapons
systems use GPS to track potential ground and air
targets before flagging them as hostile These
weapon systems pass target coordinates to
precision-guided munitions to allow them to
engage targets accurately. Military aircraft,
particularly in air-to-ground roles, use GPS to find
targets (for example, gun camera video .
Missile and projectile guidance: GPS allows
accurate targeting of various military weapons
including ICBMs, cruise missileprecision-guided
munitions. Artillery project
Rescue: Downed pilots can be located faster if
their position is known.
Reconnaissance: Patrol movement can be
50
managed more closely.
GPS satellites carry a set of nuclear detonation
detectors consisting of an optical sensor (Y-
sensor), an X-ray sensor, a dosimeter, and an
electromagnetic pulse (EMP) sensor (W-sensor),
that form a major portion of the United States
Nuclear Detonation.
CONCLUSION
In this project we analysis basic concept that are used in
global positioning system. We performed the analysis gps
receiver using Gnss software . There are following things
that can be concluded from the study:-
Gps receiver can be implement into two ways:
Hardware based software receiver
Software based receiver
Gps receiver used bpsk digital modulation
technique for satellite signal
It uses wass system with two parameters
(corrected gps parameters ionospheric
parameters).
It uses six sec. time to alarm
It uses Inegrity for Montioring performed
51
with in avionics in the RAIM.
MMR(mulitimode receiver )receive basic
gps and wass.
Mathematic Model of Low Cost :This
project allows us to differentiate between the
accuracy that can be achieved by Appling
different models.
Gps receiver used different model for
different system.
BIBLIOGRAPHY
1. Principles of digital communication by Taub
and Shilling.
2. Sins integrated system for vehicle tracking by
Cao Fu Xiang, Law
3. GPS Made Easy by Lawrence Letham
4. GPS for Land Surveyors by Jan Van Sickle
5. GPS Navigation Guide by Jack W. Peters
52
6. Modeling and simulating GNSS signal structures
and receiver by jon olafur winkel
53