Application of Satellite: GPS: GPS position location principles, satellite signal acquisition, GPS signal levels, Timing accuracy, GPS receiver operation, GPS navigation message.

1. Application of
Satellite GPS
Md. Humayun Kabir
Adjunct Faculty
Dept. of ETE, IIUC

2. ► GPS, or the Global Positioning System, is a global navigation satellite system that provides location, velocity and
time synchronization.
► It is a navigation system using satellites, a receiver and algorithms to synchronize location, velocity and time data
for air, sea and land travel.
► It has created the revolution in navigation and position location.
► It is mainly used in positioning, navigation, monitoring and surveying applications.
► GPS works in any weather conditions, anywhere in the world, 24 hours a day, with no subscription fees or setup
charges.
► The U.S. Department of Defense (USDOD) originally put the satellites into orbit for military use, but they were
made available for civilian use in the 1980s.
► GPS is everywhere. You can find GPS systems in your car, your smartphone and your watch. GPS helps you get
where you are going, from point A to point B.
Global Positioning System (GPS)

3. How GPS works
GPS satellites circle the Earth twice a day in a precise orbit. Each satellite transmits a unique signal and orbital parameters that
allow GPS devices to decode and compute the precise location of the satellite. GPS receivers use this information and trilateration
to calculate a user's exact location. Essentially, the GPS receiver measures the distance to each satellite by the amount of time it
takes to receive a transmitted signal. With distance measurements from a few more satellites, the receiver can determine a user's
position and display it electronically to measure your running route, map a golf course, find a way home or adventure anywhere.
To calculate your 2-D position (latitude and longitude) and track movement, a GPS receiver must be locked on to the signal of at
least 3 satellites. With 4 or more satellites in view, the receiver can determine your 3-D position (latitude, longitude and altitude).
Generally, a GPS receiver will track 8 or more satellites, but that depends on the time of day and where you are on the earth. Some
devices can do all of that from your wrist.
Once your position has been determined, the GPS unit can calculate other information, such as:
 Speed
 Bearing
 Track
 Trip distance
 Distance to destination
 Sunrise and sunset time
 And more

4. What are the three elements of GPS?
GPS is made up of three different components, called segments, that work together to provide location
information.
The three segments of GPS are:
Space (Satellites) — The satellites circling the Earth, transmitting signals to users on geographical position and
time of day.
Ground control — The Control Segment is made up of Earth-based monitor stations, master control stations and
ground antenna. Control activities include tracking and operating the satellites in space and monitoring
transmissions. There are monitoring stations on almost every continent in the world, including North and South
America, Africa, Europe, Asia and Australia.
User equipment — GPS receivers and transmitters including items like watches, smartphones and telematic
devices.

5. How does GPS technology work?
► GPS works through a technique called trilateration. Used to calculate location, velocity and elevation, trilateration collects
signals from satellites to output location information. It is often mistaken for triangulation, which is used to measure angles, not
distances.
► Satellites orbiting the earth send signals to be read and interpreted by a GPS device, situated on or near the earth’s surface. To
calculate location, a GPS device must be able to read the signal from at least four satellites.
► Each satellite in the network circles the earth twice a day, and each satellite sends a unique signal, orbital parameters and time.
At any given moment, a GPS device can read the signals from six or more satellites.
► A single satellite broadcasts a microwave signal which is picked up by a GPS device and used to calculate the distance from the
GPS device to the satellite. Since a GPS device only gives information about the distance from a satellite, a single satellite
cannot provide much location information. Satellites do not give off information about angles, so the location of a GPS device
could be anywhere on a sphere’s surface area.
► When a satellite sends a signal, it creates a circle with a radius measured from the GPS device to the satellite.
► When we add a second satellite, it creates a second circle, and the location is narrowed down to one of two points where the
circles intersect.
► With a third satellite, the device’s location can finally be determined, as the device is at the intersection of all three circles.
► That said, we live in a three-dimensional world, which means that each satellite produces a sphere, not a circle. The
intersection of three spheres produces two points of intersection, so the point nearest Earth is chosen.
Trilateration is a mathematical technique used by a global positioning system (GPS) device to determine user position, speed,
and elevation. By constantly receiving and analyzing radio signals from multiple GPS satellites and applying the geometry of circles,
spheres, and triangles, a GPS device can calculate the precise distance or range to each satellite being tracked.

6. Here is an illustration of satellite ranging:
As a device moves, the radius (distance to the satellite) changes. When the radius changes, new spheres are produced, giving us a
new position. We can use that data, combined with the time from the satellite, to determine velocity, calculate the distance to our
destination and the ETA.

7. Working Principle of the GPS:
Principle of Operation
The basic principle of operation of the GPS is that the location of any point can be determined if its distance is known from four objects or points with
known positions. Theoretically, if the distance of a point is known from one object, then it lies anywhere on a sphere with the object as the centre
having a radius equal to the distance between the point and the object [Figure 13.15 (a)]. If the distance of the point is known from two objects,
then it lies on the circle formed by the intersection of two such spheres [Figure 13.15 (b)].
The distance from the third object helps in knowing that the point is located at any of the two positions where the three spheres intersect [Figure
13.15 (c)]. The information from the fourth object reveals the exact position where it is located, that is at the point where the four spheres intersect.

8. In the GPS, the position of any receiver is determined by calculating its distance from four satellites. This distance is
referred to as the ‘Pseudorange’. The information from three satellites is sufficient for calculating the longitude and
the latitude positions; however, information from the fourth satellite is necessary for altitude calculations. Hence, if
the receiver is located on Earth, then its position can be determined on the basis of information of its distance from
three satellites. For airborne receivers the distance from the fourth satellite is also needed.
In any case, GPS receivers calculate their position on the basis of information received from four satellites, as this
helps to improve accuracy and provide precise altitude information. The GPS is also a source of accurate time, time
interval and frequency information anywhere in the world with unprecedented precision.
The GPS uses a system of coordinates called WGS-84, which stands for World Geodetic System 1984. It produces
maps having a common reference frame for latitude and longitude lines. The system uses time reference from the
US Naval Observatory in Washington DC in order to synchronize all timing elements of the system.

9. Satellite Signal Acquisition
Acquisition:
► The purpose of acquisition is to determine visible satellites and coarse values of carrier frequency and code phase of the satellite signal.
► The satellites are differentiated by the 32 different PRN sequences.
► The code phase is the time alignment of the PRN (pseudorandom noise) code in the current block of data.
► The code phase in useful to be able to generate a local PRN code that is perfectly aligned with the incoming code.
► The third parameter is the carrier frequency which corresponds to the IF.
► The IF should be known from the L1 carrier frequency of 1575.42 MHz and from the mixer in the down-converter.
► Take Doppler effect into consideration.
Signal Acquisition:
► It is a process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values
that can be manipulated by a computer.
► Data acquisition systems, abbreviated by the acronyms DAS or DAQ, typically convert analog waveforms into digital values for processing.
► The components of data acquisition systems include: Sensors converts physical parameters to electrical signals.
► Signal conditioning circuitry converts sensor signals into a form that can be converted to digital values.
► Analog-to-digital converters convert conditioned sensor signals to digital values.
► Learn more in: Real-Time ECG-Based Biometric Authentication System
Satellite acquisition system: Satellite acquisition system acquires the desired satellite by either moving the antenna manually around the
expected position of the satellite or by programming the antenna to perform a scan around the anticipated position of the satellite.
Acquisition and Tracking Acquisition is the process of locking onto a satellite’s C/A code and P code. A receiver acquires all available satellites when it
is first powered up, then acquires additional satellites as they become available and continues tracking them until they become unavailable. Tracking
is a planned or intended horizontal path of travel with respect to the Earth rather than the air or water. The track is expressed in degrees from 0°
clockwise through 360° (true, magnetic, or grid).

10. GPS Signal Structure
The GPS signal contains three different types of information, namely the pseudorandom code, ephemeris data and almanac data.
The pseudorandom code (PRN code) is an ID (identity) code that identifies which satellite is transmitting information and is used
for ‘pseudorange’ calculations. Each satellite transmits a unique PRN code. Ephemeris data contains information about health of the
satellite, current date and time. Almanac data tells the GPS receiver where each satellite should be at any time during the day. It
also contains information on clock corrections and atmospheric data parameters. All this information is transmitted at two
microwave carrier frequencies, referred to as L1 (1575.42 MHz) and L2 (1227.60 MHz). It should be mentioned here that all
satellites transmit on the same carrier frequencies, however different codes are transmitted by each satellite. This enables GPS
receivers to identify which satellite is transmitting the signal. The signals are transmitted using the code division multiple access
(CDMA) technique.
Pseudorandom codes (PRN codes) are long digital codes generated using special algorithms, such that they do not repeat within
the time interval range of interest. GPS satellites transmit two types of codes, namely the coarse acquisition (C/A code) and the
precision code (P code). C/A code is an unencrypted civilian code while the P code is an encrypted military code. During military
operations, the P code is further encrypted, known as the Y code, to make it more secure. This feature is referred to as
‘antispoofing’. Presently, the C/A code is transmitted at the L1 carrier frequency and the P code is transmitted at both L1 and L2
carrier frequencies. In other words, the L1 signal is modulated by both the C/A code and the P code and the L2 signal by the P
code only. The codes are transmitted using the BPSK (binary phase shift keying) digital modulation technique, where the carrier
phase changes by 180◦ when the code changes from 1 to 0 or 0 to 1.
The C/A code comprises 1023 bits at a bit rate of 1.023 Mbps. The code thus repeats itself in every millisecond. The C/A code is
available to all users. GPS receivers using this code are a part of standard positioning system (SPS). The P code is a stream of 2.35
× 1014 bits having a modulation rate of 10.23 Mbps. The code repeats itself after 266 days. The code is divided into 38 codes,
each 7 days long. Out of the 38 codes, 32 codes are assigned to various satellites and the rest of the six codes are reserved for
other uses. Hence, each satellite transmits a unique one-week code. The code is initiated every Saturday/Sunday midnight
crossing. Precise positioning systems (PPS), used for military applications, use this code.

11. Other than these codes, the satellite signals also contain a navigation message comprising the ephemeris and almanac data. This
provides coordinate information of GPS satellites as a function of time, satellite health status, satellite clock correction, satellite
almanac and atmospheric data. The navigation message is transmitted at a bit rate of 50 kbps using BPSK technique. It comprises
25 frames of 1500 bits each (a total of 37 500 bits). Figure 13.16 shows the structure of the GPS satellite signal.

12. GPS Receiver
There exists only one-way transmission from satellite to users in GPS system. Hence, the individual user does not need the
transmitter, but only a GPS receiver. It is mainly used to find the accurate location of an object. It performs this task by using the
signals received from satellites.
The block diagram of GPS receiver is shown in below figure.
The function of each block present in GPS receiver is mentioned below.
Receiving Antenna receives the satellite signals. It is mainly, a circularly polarized antenna.
Low Noise Amplifier (LNA) amplifies the weak received signal
Down converter converts the frequency of received signal to an Intermediate Frequency (IF) signal.
IF Amplifier amplifies the Intermediate Frequency (IF) signal.
ADC performs the conversion of analog signal, which is obtained from IF amplifier to digital. Assume, the sampling & quantization
blocks are also present in ADC (Analog to Digital Converter).
DSP (Digital Signal Processor) generates the C/A code.
Microprocessor performs the calculation of position and provides the timing signals in order to control the operation of other
digital blocks. It sends the useful information to Display unit in order to display it on the screen.

13. GPS Navigation Message
Every satellite receives from the ground antennas the navigation data which is sent back to the users through the
navigation message. The Navigation Message provides all the necessary information to allow the user to perform the
positioning service. It includes the Ephemeris parameters, needed to compute the satellite coordinates with enough
accuracy, the Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions, the
Service Parameters with satellite health information (used to identify the navigation data set), Ionospheric
parameters model needed for single frequency receivers, and the Almanacs, allowing the computation of the
position of ”all satellites in the constellation”, with a reduced accuracy (1 - 2 km of 1-sigma error), which is needed
for the acquisition of the signal by the receiver. The ephemeris and clocks parameters are usually updated every two
hours, while the almanac is updated at least every six days.
Besides the "legacy" L1 C/A navigation message, four additional new messages have been introduced by the so
called GPS modernisation: L2-CNAV, CNAV-2, L5-CNAV and MNAV. The "legacy" message and the first three of the
modernised GPS are civil messages, while the MNAV is a military message. In modernised GPS, the same type of
contents as the legacy navigation message (NAV) is transmitted but at higher rate and with improved robustness.
The messages L2-CNAV, L5-CNAV and MNAV have a similar structure and (modernised) data format. The new format
allows more flexibility, better control and improved content. Furthermore, the MNAV includes new improvements for
the security and robustness of the military message. The CNAV-2 is modulated onto L1CD, sharing the same band as
the "legacy" navigation message.

15. This is the primary vehicle for communicating the NAV message to GPS receivers. The NAV message is also known
as the GPS message. It includes some of the information the receivers need to determine positions. Today, there
are several NAV messages being broadcast by GPS satellites, but we will look at the oldest of them first. The legacy
NAV (NAV) message continues to be one of the mainstays on which GPS relies. The NAV code is broadcast at a low
frequency of 50 Hz on both the L1 and the L2 GPS carriers. It carries information about the location of the GPS
satellites called the ephemeris and data used in both time conversions and offsets called clock corrections. Both
GPS satellites and receivers have clocks on board. It also communicates the health of the satellites on orbit and
information about the ionosphere. The ionosphere, along with the troposphere, is a layer of atmosphere through
which the GPS signals must travel to get to the user. It includes data called almanacs that provide a GPS receiver
with enough little snippets of ephemeris information to calculate the coordinates of all the satellites in the
constellation with an approximate accuracy of a couple of kilometers. The Navigation code, or message, is the
vehicle for telling the GPS receivers some of the most important things they need to know. Here are some of the
parameters of its design.
The entire Navigation message, the Master Frame, contains 25 frames. Each frame is 1500 bits long and is divided
into five subframes. Each subframe contains 10 words and each word is comprised of 30 bits. Therefore, the entire
Navigation message contains 37,500 bits and at a rate of 50 bits-per-second takes 12½ minutes to broadcast and
to receive on a completely cold start. In other words, getting the whole thing is not instantaneous. It does take a
bit of time for the receiver to update its Navigation Message.

16. In the five sub-frames of the legacy Navigation Message. TLM stands for telemetry. HOW stands for handover word.
Over on the right-hand side in the illustration, you see the clock correction, GPS satellite health, et cetera, in sub
frame one. Two and three are devoted to the ephemeris. In four and five, you see ionosphere, and then PRN
(Pseudo Random Noise) satellite numbers and almanac are mentioned. The PRN term is used because the GPS
signals that the receiver uses for positioning appear to be random noise, but in fact, the signal is pseudo (false)
random noise because in truth, the signals are very carefully designed and consistent. They are not noise at all.
They just seem to be irregular. The PRN numbers 25 to 32 in sub-frame number four mean that satellite's almanacs
from number 25 to number 32 are to be found there. In subframe five the PRNs from 1 to 24, those satellites have
their almanacs, in other words, a little bit of their ephemerides. You might wonder why they are there. This is that
identification system. In other words, when a receiver acquires the Navigation Message from one satellite -
embedded in that message - there's a bit of information, just a bit, that will tell the receiver where it can find the
rest of the entire constellation in the sky. This helps it acquire the additional satellites after it's got the first one.
That's what the satellite almanac does.
The essential point here is that this message is the fundamental vehicle for the satellite to communicate important
information to the receiver. After the receiver has acquired the signal from that satellite, the NAV message tells the
receiver where the satellite is. The ephemeris is the satellite coordinate system. It tells the receiver where the
satellite is at an instant of time. The clock correction is one of the ways that the satellite can tell the receiver what
time it is on-board the satellite. The ionosphere is that information that will allow the receiver to make some
atmospheric corrections on the signal it receives from a particular satellite.

17. Assignment Topic:
Timing accuracy of GPS