17-08-2010, 05:49 PM
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Why do we need GPS?
Trying to figure out where you are is probable manâ„¢s oldest pastime.
Finally US Dept of Defense decided to form a worldwide positioning system.
Also known as NAVSTAR ( Navigation Satellite Timing and Ranging Global positioning system) provides instantaneous position, velocity and time information.
Components of the GPS
Space segment
control segment
user segment
Space Segment:
24 GPS space vehicles(SVs).
Satellites orbit the earth in 12 hrs.
6 orbital planes inclined at 55 degrees with the equator.
This constellation provides 5 to 8 SVs from any point on the earth.
Control Segment:
The control segment comprises of 5 stations.
They measure the distances of the overhead satellites every 1.5 seconds and send the corrected data to Master control.
Here the satellite orbit, clock performance and health of the satellite are determined and determines whether repositioning is required.
This information is sent to the three uplink stations
User Segment:
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
User Segment:
There are two services SPS and PPS
The Standard Positioning Service
SPS- is position accuracy based on GPS measurements on single L1 frequency C/A code
C/A ( coarse /acquisition or clear/access) GPs code sequence of 1023 pseudo random bi phase modulation on L1 freq
The Precise Position Service
PPS is the highest level of dynamic positioning based on the dual freq P-code
The P-code is a very long pseudo-random bi phase modulation on the GPS carrier which does not repeat for 267 days
Only authorized users, this consists of SPS signal plus the P code on L1 and L2 and carrier phase measurement on L2
Cross Correlation
Anti- spoofing denies the P code by mixing with a W-code to produce Y code which can be decoded only by user having a key.
What about SPS users?
They use cross correlation which uses the fact that the y code are the same on both frequencies
By correlating the 2 incoming y codes on L1 and L2 the difference in time can be ascertained
This delay is added to L1 and results in the pseudorange which contain the same info as the actual P code on L2
GPS Satellite Signal:
L1 freq. (1575.42 Mhz) carries the SPS code and the navigation message.
L2 freq. (1227.60 Mhz) used to measure ionosphere delays by PPS receivers
3 binary code shift L1 and/or L2 carrier phase
The C/A code
The P code
The Navigation message which is a 50 Hz signal consisting of GPs satellite orbits . Clock correction and other system parameters
How does the GPS work?
Requirements
Triangulation from satellite
Distance measurement through travel time of radio signals
Very accurate timing required
To measure distance the location of the satellite should also be known
Finally delays have to be corrected
Triangulation
Position is calculated from distance measurement
Mathematically we need four satellites but three are sufficient by rejecting the ridiculous answer
Measuring Distance
Distance to a satellite is determined by measuring how long a radio signal takes to reach us from the satellite
Assuming the satellite and receiver clocks are sync. The delay of the code in the receiver multiplied by the speed of light gives us the distance
Getting Perfect timing
If the clocks are perfect sync the satellite range will intersect at a single point.
But if imperfect the four satellite will not intersect at the same point.
The receiver looks for a common correction that will make all the satellite intersect at the same point
Error Sources
95% due to hardware ,environment and atmosphere
Intentional signal degradation
Selective availability
Anti spoofing
Selective Availabity
Two components
Dither :
manipulation of the satellite clock freq
Epsilon:
errors imposed within the ephemeris data sent in the broadcast message
Anti spoofing
Here the P code is made un gettable by converting it into the Y code.
This problem is over come by cross correlation
DGPS
Errors in one position are similar to a local area
High performance GPS receiver at a known location.
Computes errors in the satellite info
Transmit this info in RTCM-SC 104 format to the remote GPS
Requirements for a DGPS
Reference station:
Transmitter
Operates in the 300khz range
DGPS correction receiver
Serial RTCM-SC 104 format
GPS receiver
Data Links
Land Links
MF,LF,UHF/VHF freq used
Radiolocations,local FM, cellular telephones and marine radio beacons
Satellite links
DGPS corrections on the L band of geostaionary satellites
Corrections are determined from a network of reference Base stations which are monitored by control centers like OmniSTAR and skyFix
RTCM-SC 104 format
DGPS operators must follow the RTCM-SC 104 format
64 messages in which 21 are defined
Type 1 contains pseudo ranges and range corrections,issue of data ephemeris (IODE)and user differential range error(URDE)
The IODE allows the mobile station to identify the satellite navigation used by the reference station.
UDRE is the differential error determined by the mobile station
DGPS
DGPS gives accuracy of 3-5 meters,while GPS gives accuracy of around 15-20 mts
Removes the problem associated with SA.
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