CE 316 Geomatics - Chapter 9
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The unknown interger number of cycles of the reconstructed carrier phase between the satellites and the sensor at the beginning of the observation
When P-codes becomes encrypted and no longer availabe to the civilian users
Consists of a pair of stations for which simultaneous GPS data has been collected
The coarse/acquisition GPS code- a pseudo random (PRN) code transmitted at 1.023 MHz that repeats every milli-second. Each satellite has its own C/A code so that it can be uniquely identified and received seperately from other satellites transmitting on the same frequency
A list of positions or locations of a celestial object as a function of time
A particular fixed instant of time used as a reference point on a time scale
Indicates the positions of the satellites in view to an observer on the earths surface at a designated time and location
The minimum elevation angle used by the sensor to track GPS satellites
Occurs when solar flares cause irregular ionization of the ionosphere which in turn causes irregular refraction delays of a radio wave passing through the non-homogeneous medium
Geometric dilution of precision
The period during the 24-hour day when there are a sufficient number of satellites in view to meet the positioning requirement
The manner in which the GPS will be used in the field
Refers to a reflected signal that combines with the true signal resulting in a weaker position
PDOP-Position of Dilution of Precsion
A numerical indicator of the geometric strength of the satellite constellation in relation to positioning at a certain location
The independent reduction of observations made by a particular receiver using the pseudo-range with information broadcast from satellites
Computer processing of data after the observation session
Description of a survey job. There is no limit to the number of stations
Precise GPS code - usually encrypted and reserved for military applications
What frequency does the P-code run at and what is its repeating interval?
P-Code is a 10.23 MHz PRN code that repeats every week
PRN (Pseudo-random Noise) Number
A number assigned by the GPS system designers to a given set of pseudo-random codes. Typically a satellite will keep its PRN indefinitely. Used as a way of labeling satellites.
Measurements made using one of the pseudo-range codes on the GPS signal. They provide an unambiguous measure of the range to the satellite including the effects of the satellite and user clock biases.
Selective Availability (S.A.)
Used to describe the situation when the operators of the GPS system degrade the broadcast GPS message by dithering the clock or orbital information.
Coordinated Universal Time is a high precision atomic clock standard, with uniform and "leap" seconds. It is aligned with GMT
Three major componets of the GPS system are:
- The satellite system
- The control segment
- The user
The GPS receiver can compute the transmit time by knowing the _____ time a message was received.
The GPS reciever can compute the transmit time by knowing the exact time a message was recieved.
A GPS is message is assumed to travel at what speed?
The speed of light
Based on the GPS system for USA, answer the following questions:
A) What is the minimum number of satellites?
B) What is distance from the satellites to Earth?
Based on the GPS system for USA how many orbital planes are there, how many satellites follow each plane and at what degree are the satellites inclined with respect to the equator?
6 orbital planes with 4 satellites each @ 55˚ inclincation to the equatorial plane
What is the expected life of a satellite?
How many satellites are currently in orbit and healthy?
How many microwave signals are typically emitted from a GPS satellite? What are the names?
Two microwave signals, L1 and L2
What are the five primary signals from which observations are and used by surveyors and engineers?
Precise Position Systems (PPS) use the ______ signal to measure the ______ delay.
Precise Position Systems (PPS) use the L2 signal to measure the ionosphere delay.
Which code is unique to each satellite?
Which code is the basis for civilian GPS?
Ground stations around the world are used to...
- Monitor the health of satellites
- Upload orbital parameters to the satellite
Master control stations for the USA GPS are located where?
- Schriever Air Force Base, Colorado Springs
- Sunnyvale, California
How does single point positioning work?
- One receiver is used to determine the 3 dimensions position of a unknown point
- The measurements of the PRN (C/A or P) code are taken
- GPS receiver correlates the incoming PRN with a similar stable signal generated in the reciever
- ∆T∆c = pseudo-range between transmitting satellite and receiver
- Position is affeted by timing errors, dt
What are the sources of error associated with Single Point Positioning?
- Error in satellite position
- Satellite timing
- Receiver timing
- Site dependant (height of instrument, satellite geometry)
- Atmospheric conditions (ionosphere, troposphere)
What is differential GPS?
- GPS system that requires two or more receivers simultaneously receiving signals from the same satellites
- It is a form of corrected point positioning
- Uses short wavelength carrier phase measurements
Benefits of using differential measurement are:
- Elimination of errors in the satellite and receiver clock
- Elimination of erros in satellite ephemerides
- Minimization of atmospheric effects
Sources of error associated with differential GPS are:
- Receiver Noise
- Site dependent factors
- Not reading same satellites
- Distance between stations is too great
Precision in Differential GPS increases when?
Precision increases with a decrease in wavelength
The wavelengths of L1 and L2 are:
0.19m and 0.24m
In Differential GPS, L1 and L2 can provide how much precision?
Can provide cm to mm of precision
In Differential GPS, C/A and P code can provide how much precision?
Can provide decimeters to meters of precesion.
What must be considred when preparing for a GPS survey?
- Satellite availability
- Satellite azimuth
- Mask angle
What are the 5 types of GPS surveying methods?
- Static surveying
- Rapid Static surveying
- Reoccupation surveying
- Stop and go surveying
- Kinematic surveying
What is Static GPS Surveying?
- Most reliable surveying method
- Involves to multiple GPS stations
- Minimum 4 satellites for readings
- Classical method for long lines and highest accuracy
What is Rapid Static Surveying?
- One or more roving receivers occupying points for 1 - 20 minutes
- Reduced occupation time
- One or more fixed reference recievers
- Short observation time for baselines up to 15 kms
What is Relative Kinematic Positioning (RKP)?
- Dynamic system
- Used to provide relative survey control
- One or more receivers occupy points for severeal minutes
- Receiver must track a minimum of 4 identical satellites
Precise GPS RTK Positioning
Combination of differential GPS correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistcal tests- possibly with processing in real-time positioning (RTK)
Russian based satellite system
How many satellites does GLONASS have in its system?
At what altitude do they orbit?
Based on the GLONASS system for Russia how many orbital planes are there, how many satellites follow each plane and at what degree are the satellites inclined with respect to the equator?
Three orbital plane 120˚ apart, 8 satellites equally spaced 45˚ apart. Inclined 64.8˚ from the equatorial plane.
GLONASS references what geocentric coordinate system?
How is GLONASS controlled?
- Controlled by a system of laser and radar tracking stations
- System is based entirely on UTC and a hydrogen master clock.
What is the GALILEO?
- A GPS satellite with 26 SV's at an altitude of 23222 km
- 3 orbital planes at 56˚ inclination
- Low accuracy system open to all users
- High accuracy system for military use and paying customers
- Cost 3.4 Billion Euros
- China's GPS satellite launched in April 2007
- 35 SV's
- 5 SV's in geogyncronus orbit directly above equator
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