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GPS MEASUREMENTSUNIT-III
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
TRANSMITTING FREQUENCY
C/ A and P(Y) codes
P(Y) codes
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•Very accurate ,reference is atomic frequency standard
•The reference frequency is off by−4.567 × 10−3 Hz to
take the relativistic effect into consideration.
•Reference frequency used by the satellite is
10.229999995433 MHz
•Doppler frequency shift produced by the satellite
motion at L1 frequency is approximately ±5KHz
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•The C/ A and P(Y) code in the L1 are in quadrant
phase of each other and can be written as:
Where,
SL1 = signal at L1 frequency Ap = the amplitude of the P code P(t) =±Phase of P codeD(t) =±data codeØ = the initial phase Ac =the amplitude of the C/ A code C(t) =±phase of C/A code
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
There are basically two types of codes:-
•Coarse (or clear)/ acquisition (C/ A)
•Precision (P) codes
• Coarse / acquisition (C/ A)
•Bi-phase modulated with a chip rate of 1.023 MHz
•chip is about 977.5 ns (1/ 1.023 MHz) long
•C/ A code contains the main lobe and several sidelobes
Codes
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
• C/ A code is 1 ms long i.e 1,023 chips last 1 ms and
repeats after every ms
•Every satellite has its own C/A code
Data Format
1st row shows a C/ A code with 1,023 chips;
2nd row shows a navigation data bit;
5th row shows a page
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
Precision (P) codes
•Bi-phase modulated at 10.23 MHz
•Chip length is about 97.8 ns (1/ 10.23 MHz)
• It is generated from two pseudorandom noise (PRN)
codes with the same chip rate.
•One has 15,345,000 chips an other has 15,345,037
chips
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•There are no common factors between them i.ecode length
generated by these two codes is 23,017,555.5 (1.5 × 15,345,037)
seconds, which is slightly longer than 38 weeks.
•Navigation data rate carried by the P code through phase
modulation is at a 50 Hz rate
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
IONOSPHERIC DISPERSION EFFECTS
•The ionosphere generally region with altitude of
50 to 1000 km with free electrons
•GPS signals, propagating through an ionized
medium, are affected by the non-linear
dispersion characteristics of this medium
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
The change in path length due to the ionosphere is
Where,
ρ=Change in path lengthn=Index of Refraction
GPS codes are dependent upon the group inx of refraction
∴ ionospheric group delay is
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
& ionospheric phase delay is
Where, ng group index of refraction = vg / c.
np phase index of refraction = vp / c,
NT total no of electrons in electrons/m2
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
TROPOSPHERIC EFFECTS
•Troposphere is regions up to 80 km in altitude
•Troposphere is not dispersive for frequencies below30 GH
•Tropospherich Refraction may be conveniently separated
into 'dry' and 'wet' components.
•The dry component can be approximated by
DTC = 2.27 • 10-3 P0
P0 is thesurface pressure (in mbar)
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•The dry term contains approximately 90% of the total
zenith range error
•The wet component depends on the atmospheric
• Conditions all along the signal path Some of factors
effecting are the water vapour content, temperature,
altitude,the elevation angle of the signal path
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
RINEX format
•It is a internationally accepted data exchange format
& is in the standard ASCII format
•A RINEX file is a translation of the receiver.s own
compressed binary files.
•A draft version of the RINEX format was introduced in
1989 followed by a number of updates to
accommodate more data types
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•The current RINEX version 2.10 defines six
different RINEX files:-
(1)Observation data file,
(2)navigation message file
(3)meteorological file
(4)GLONASS navigation message file
(5)geostationary satellites
(6)satellite and receiver clock data file
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
The naming convention for RINEX files is “ssssdddf.yyt.”
•ssss-represent the station name
•ddd-represent the day of the year of first record
•f-represents the file sequence number within the day
•yy-represent the last two digits of the current year
•t-represent the file type
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
LINEAR COMBINATIONS OF OBSERVATIONS
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•Certain combinations of the basic code pseudo-range,
or carrier beat phase observables are required for different
type of application and the level of accuracy one
seeks from GPS,
•Many different differencing combinations are possible
1. first between receivers,
2. then between satellites,
3. lastly between epochs.
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
The notation we use for taking these differences is:-
•Δ denotes differences between two receivers
•▽ denotes differences between two satellite
•Δ denotes differences between two epochs
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
BETWEEN-EPOCH (DOPPLER) SINGLEDIFFERENCES
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•Frequency at GPS receiver is different than frequency transmitted by the satellite this is due to the
Doppler effect.
•The basic GPS Doppler observable is essentially the
change of phase between two epochs
•The corresponding between-epoch single difference (Doppler) equation is
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
We get,
Some techniques,
1. Intermittently Integrated Doppler (IID)
2. Consecutive Doppler Counts (CDC)
3. Continuously Integrated Doppler (CID)
Q.1. carrier phase Vs IID
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
BETWEEN-RECEIVER SINGLE DIFFERENCES
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•2 stations observing the same satellite, the
mathematical model for a between-receiver single
difference pseudo-range observable obtained
from P- or C/A-code
•It can be derived by differencing two simultaneous
pseudo-ranges as
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
We get,
•This greatly reduce the effects of errors associated
with the satellites:
•satellite clock errors,
•orbit errors and atmospheric delays
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
BETWEEN-SATELLITE SINGLE DIFFERENCES
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•Between -satellite single difference is formed by
differencing the observations of two satellites by
single receiver
•The mathematical model of the between-satellite
single range difference
We get
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
RECEIVER-TIME DOUBLE DIFFERENCES
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•It is the change from one epoch to the next, in the
between-receiver single difference for the same satellite.
The equation for the receiver-time double difference
carrier beat phase observable is
•This allows an easier editing of cycle slips
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
RECEIVER-SATELLITE DOUBLE DIFFERENCES
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
The receiver-satellite double difference can be
constructed either by:
• taking 2 between-receiver single difference observables,
involving the same pair of receivers but different satellites
• taking2 between-satellite single difference observables,
involving the same pair of satellites but different receivers
The two results, i.e., Δ▽(.) and ▽Δ(.), are identical.
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
•The receiver-satellite double difference observation
equation, for pseudo-range measurements is
Double differences remove, the effects of
•Errors due to misalignment between the 2receiver clocks
•Errors due to misalignment between the satellite clocksQ.2RECEIVER-SATELLITE TRIPLE DIFFERENCES
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
CYCLE SLIPS
If the satellite signal is blocked in any way, it can’t be
tracked anymore, When signal is lock again, the fractional
part of the measured phase would still be the same as if
tracking had been continuous.The integer cycle however is
discontinuous this is cycle slip
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Vrince Vimal,CSED BAHIR DAR UNIVERSITY
There are couple of approaches to dealing with this problem
•Manually
•Piecewise continuous polynomial
•Between-receivers, satellites, and time triple difference
•Q3. Clock errors