Eclipses.

Considering two solid opaque objects, Moon and Earth, arranged according to the picture and illuminated by a large source of light Sun. Earth is Eliminated by the rays from Sun which reach it, but owing to the presence of Moon between Sun and Earth, shadows are obtained on parts of Earth. By drawing the extreme rays from Sun it can seen that the region a on Earth receives no light, and this region, known as the umbra, is therefore in total darkness. The region on Earth between a, b, and a, d receives light from only part of Sun, and is hence in partial shadow; such a region is known as a penumbra. Beyond b to c, and beyond d to e, the region is illuminated by rays from the whole of Sun, and is therefore bright. 

Illustrates the eclipse of the sun

At some stage in their movement round the sun, S, the moon M comes between it and the earth E. A total eclipse is observed at the region a on the earth, and a partial eclipse is observed at the regions b and d. Observers at c and e on the earth receive rays from the whole of the sun, so that no eclipse is seen. The sun’s appearance at different points on the earth is illustrated by small circles below the main figure.

                The moon is not a Iuminous object; it merely reflects light from the sun. An eclipse of the moon happens when the earth comes between it and the sun, since the moon is then in the shadow of the earth and is no longer illuminated by the sun.

Summary:

Eclipses of the sun are due to the shadow formed when the moon passes between the sun and earth. An umbra is a region of total shadow; a penumbra is a region of partial shadow.    

REFERENCE
M. Nelkon M.Sc.(Lond.), A.K.C

GPS Signal Errors

Measurement Errors

            Range uncertainty (Due to clock/Atmospheric error)

Errors in position

            Intersection area with error

Errors in GPS Signal

            Orbit error

            Ionospheric error 

            Topospheric error

            Receiver noise

            Multipath

            Jamming

System and Receiver errors

Clock error

Transmission delay

Imprecision in algorithm for range decoding

Multi path errors

Causes of Range Uncertainty

Ionospheric effects                          3 meter

Atmospheric effects                        0.5 meter

Satellite/system errors                      2 meters

Receiver errors                               0.5 meter

Multipath                                         depends

Total Range Error                           6 meters

TOTAL Positional Error                 10 meters

GPS accuracy of GPS depend on

            Dependent errors

                        Noise

                        Multipath

                        Receiver hardware limitation

            Independent errors

                        Satellite clock errors

                        Error in broadcast ephemeris data

                        Signal propagation delay due to Ionosphere and Troposphere

Differential GPS

The differential correction techniques used to enhance the quality of location data gathered GPS receivers.

Differential correction can be applied in

Real-time directly from the field

When postprocessing data in the office

Combining both methods provides flexibility during data collection and improves data integrity

L2 Carrier

This is a carrier frequency

The L1 carrier is modulated by both the C/A and P codes

L1 carrier cycles have a wavelength of 19 cm.

Transmit timing information by the GPS satellites

L2 Carrier

This is a carrier frequency

L2 carrier is only modulated by the P code

L1 carrier cycles have a wavelength of 24.4 cm

Transmit timing information by the GPS satellites

GPS Satellite signal to Positioning

When a GPS Satellite is rotating around the earth it will transmit continuously satellite signal to the earth.

Measure arrival time of GPS signals from several satellites simultaneously

To determine a position on earth, the receiver has to calculate its range (distance) to several satellites.

Decode the GPS signal and figure out the signal propagation

Decode the navigation message and convert it into satellite positions.

Use at least 4 pseudoranges acquired at the same time from 4 different satellites to compute a position

GPS Satellite distance or range

There are two types to calculate distance or range to satellites:

             Code Ranging by Code Matching

             Carrier Phase Ranging by Shift measurements

Code Ranging by Code Matching

Code rang or pseudo range is the distance between satellite and the receiver.

Code range calculated by (D_i) = C_L X ∆_t (C_L=Speed of the light, ∆_t=Signal travel time)

Synchronized the satellite and receiver to generate same frequency (Code) same time.

P code is more accurate than the C/A code

Carrier Phase Ranging by Shift measurements

Measure the wavelengths from satellite to receiver to calculate range or distance.

The wavelengths of the carrier waves are very short, L1 - 19cm and L2 -24cm.

More accurate than P code distance calculation. 

Carrier phase can be measured to millimeters.

L1 and L2 Atomic clock The atomic clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time Atomic standards clocks are using in monitoring stations.  Stability of at least 10-13 sec

GPS Signal

          The signal leaves from GPS satellite antenna with a combination of the three components:

                - Carrier wave

                - Ranging codes

                - Navigation Message

       GPS Satellites transmit frequencies L1=1575.42 MHz and L2=1227.6 MHz modulated with two types of code, P-code and C/A code and with navigation message. Fundamental frequency is 10.23 MHz and can be modify to different frequency. This P-Code and C/A code come with two carrier frequency L1 and L2.

Modernization of GPS Signal 

GPS codes and carrier phase

P(Precise) Code

P-code PRN code is 6.1871 × 1012 bits long

The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code

Each satellite repeats its portion of the P code every 7 days

P code is unique for each satellite

P-code was modulated with the W-code to generate the Y-code

Users can’t access the P Code and Y code on both L1 and L2 without an encryption key. 

P-Code modulates both the L1 and L2 carrier phases

C/A Code

The C/A Code (Coarse Acquisition) modulates the L1 carrier phase.

The C/A code repeats every 1023 bits (one millisecond)

Navigation Message

Navigation Message modulates the L1-C/A code signal

The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits.

The Navigation Message corrects Clock corrections, and other system parameters.

Satellites in the Orbit

Global Positioning System is used for

Positioning and Navigation

To take very accurate time

Ranging or Distant measurement

GPS Segments;

        GPS need three factors to work 

Space Segment

The space segment consists of the satellites themselves

Altitude of 20,000km

32 operational satellites

Four to twelve GPS satellites visible from any point on the earth

There are six orbital planes with at least four satellites in each plane

Period of the satellite 11hrs and 56mins to orbit the earth

Generate and Transmit code, carrier phases and navigation message

Control Segment

There are five major monitoring/tracking stations spaced around the globe and one Master Control Station located in Colorado Springs, Colorado

Each station sends information to the Control Station which facilitate control station to control them and predict their orbits

Monitor the clock errors

Each stations monitor and check up the satellites twice a day

The monitoring stations transmitted all the tracking data to the master control station

User Segment

The user requires a GPS receiver in order to receive the transmissions data from the satellites.

GPS receiver calculates the location (Longitude and latitude) based on signals from the satellites.

The users consist of military and civilians.

Users will get the accurate time

Number of applications to use (GIS, Traffic and transport, Security service, Disaster managements)