POSITIONING BY SATELLITE: THE BASICS

POSITIONING BY SATELLITE:

THE BASICS

SGU 4823

SATELLITE NAVIGATION

1- Satellite Positioning

2- Satellite Positioning

3- Satellite Positioning

3- Satellite Positioning

(Satellite & Receivers Errors)

R1 SV1 R2 SV2 A SV3 R3

Ambiguities in Positioning

To Determine Your Coordinate

You Need To Know

• Range from Satellite to Receiver

• Coordinate of the Satellites

Range

Receiver (X,Y,Z)

3- Satellite Range Determine Position of

Receiver A

GPS SERVICES

The NAVSTAR system is guaranteed by

the Department of Defense to provide

two sets of services.

• Standard Positioning Service (SPS)

• Precise Positioning Service (PPS)

Standard Positioning Service (SPS)

• Standard level of positioning and timing accuracy. • Available without qualification or restrictions.

• Accuracy is established by the U.S.D.O.D. • Horizontal positioning:

• + 100 meters with 95% probability.

• + 300 meters with 99.99% probability. • Vertical positioning:

•+ 156 meters with 95% probability.

Precise Positioning Service (PPS)

• Limited to authorized users only.

• U.S. and allied Federal Governments; Authorized Foreign and Military users; and Eligible Civil Users.

• PPS information is encrypted by Anti-Spoofing. • Horizontal Positioning:

• + 22 meters with 95% probability. • Vertical Positioning:

GPS Satellite Signals

• SVs transmit two microwave carrier signals.

• L1 frequency (1575.42 MHz)

carries the navigation message and the SPS code signals.

• L2 frequency (1227.60 MHz)

Original GPS Signal is Sinus Shaped

• Sinus shaped signal cannot be used to

determine position under real-time mode.

• The Signal needs to be modulated.

• Modulated by Pseudo Random Noise (PRN)

code

Two Types of PRN Codes

• C/A code - coarse/acquisition

– can be accessed by the general public (SPS). – modulate the L₁ signal only.

• P code - precision

– recerved for authorised users only (PPS). – modulate both the L₁ and L₂ signal.

C/A Code

• Sequence of 1023 binary digit.

• 10011000011110011100101010……0011

• Repeated every 1 millisec- 1023 mil bit/sec.

• 1 bit = 1 micro sec

• Phase length of C/A code = 300 meter

P Code

• More accurate than C/A code.

• Phase length is 30 meter.

• Repeated every 266 days !.

• Very long sequence (7 days)

How Satellite - Receiver Range is

Measured

• Code Phase Tracking

• Carrier Phase Tracking

Code Phase Tracking

• GPS receiver produces replicas of the C/A and/or P (Y)-Code.

• Each PRN code pre-determined, unique series of bits.

• The receiver produces the code sequence for a specific SV with code generator.

• Receivers either store a complete set of

precomputed code chips in memory, or using shift register.

C/A Code Generator

• The C/A code generator produces a different 1023 chip sequence for each phase tap setting. • Shift register: shifted in time by slewing the clock that controls the shift registers.

• Memory: lookup scheme the required code chips are retrieved from memory.

C/A Code Phase Assignments

• The C/A code generator repeats the same 1023-chip PRN-code sequence every

millisecond.

• PRN codes are defined for 32 satellite identification numbers.

Range is Measured By Signal

Delay Time Of Arrival (TOA)

• The receiver slides a replica of the code in time until there is correlation with the SV code.

If the receiver applies a different PRN

code to an SV signal there is no

When the receiver uses the same code as the

SV and the codes begin to line up, some

signal power is detected.

No correlation.

As the SV and receiver codes line up

completely, the spread-spectrum carrier signal

is de-spread and full signal power is

A GPS receiver uses the detected signal power

in the correlated signal to align the C/A code in

the receiver with the code in the SV signal.

Full Correlation

.

Time Delay

How Satellite and Receiver Range is

Computed

Range = Time Delay * Speed of Light = 0.06 saat x 299 792 458 m/s

[XYZ]₁

[XYZ]₂

[XYZ]₃

[XYZ]₄

Carrier Phase Tracking

• L1 and/or L2 carrier signals are used in carrier phase tracking.

• L1 carrier cycles have a wavelength of 19 centimeters.

• Carrier Phase tracking can provide ranging

Time Delay is Not Measured

• Tracking carrier phase signals provides no time of transmission information.

• The carrier signals, while modulated with time tagged binary codes, carry no time-tags that

distinguish one cycle from another.

• The measurements used in carrier phase tracking are differences in carrier phase cycles and fractions of cycles over time.

Range

How Many Cycles ? Receiver (X,Y,Z)

Ambiguity Resolution

• To determine the number of carrier phase

cycles between satellite and receiver.

• L

Signal wavelength = 19 cm

• Range = (19 cm * Number of Cycles ) +

Fraction of Cycles observed by Receiver.

Efficiency of the Ambiguity

Resolution Algorithm Will

Determine …..

• Types of Kinematic Survey - Real Time ?,

Stop & Go ?, Antaenae Swap ?

• Cycle Slip Problem

How To Determine the Satellite

Coordinate

i.e. the satellite orbits needs to be

known……..

Orbit Determination

GPS satellite orbit information is

important since the position of the

GPS stations are obtained with

respect to GPS satellite positions.

Two Ways

• Broadcast Ephemerides

• Precise Ephemerides

Broadcast Ephemerides

• Broadcasted from GPS satellites.

• The accuracy of the orbits is limited.

• OK for Navigation Purposes.

ORBIT

PRN number for data ... 2 Issue of ephemeris data ... 224

Semi-Major Axis (meters) ... 2.65603E+07 C(ic) (rad) ... 1.88127E-07

C(is) (rad) ... -1.00583E-07 C(rc) (meters) ... 321.656 C(rs) (meters) ... 87.6875 C(uc) (rad) ... 4.36418E-06 C(us) (rad) ... 2.70829E-06

Mean motion difference (rad/sec) ... 5.04521E-09 Eccentricity (dimensionless) ... 0.0139305 Rate of inclination angle (rad/sec) .. 4.11089E-10 Inclination angle @ ref. time (rad) .. 0.950462 Mean Anomaly at reference time (rad) . -2.62555 Corrected Mean Motion (rad/sec) ... 0.000145859 Computed Mean Motion (rad/sec) ... 0.000145854 Argument of perigee (rad) ... -2.56865

Rate of right ascension (rad/sec) .... -8.43857E-09 Right ascension @ ref time (rad) ... 1.75048 Sqrt (1 - e^2) ... 0.999903

Sqr root semi-major axis, (m^1/2) .... 5153.67

CLOCK:

PRN number for data ... 2 Week number... ... 797 Predicted user range accuracy 32 Health of satellite ... 0

L1 - L2 Correction term ... 9.31323E-10

Issue of clock data ... 224 Time of clock data ... 240704 Clock offset ...

-0.000158074

Clock drift ... -2.50111E-12

Rate of clock drift ... 0

• Precise ephemerides obtained by tracking the satellites.

• Example

• GPS orbit determination by International GPS Service for Geodynamics(IGS).

• GPS satellite orbits computed independently by using data from IGS stations.

• The precise orbits are derived using 24 hour data segments.

The U.S. Department of Transportation's Civil GPS Service has designated NOAA to be the federal agency responsible for providing accurate and timely

Global Positioning System (GPS) satellite ephemerides to the general public.

U.S. DEPARTMENT OF COMMERCE