GPS Flashcards
What the 3 segments of a GPS system?
- Space segment
- Control segment
- User segment
The amount of circular planes that the satellites orbit about?
6 dedicated circular planes
The minimum amount of satellites available to a user anywhere around the world?
6 guaranteed satellites available
The amount of time it takes for one satellite to orbit around earth?
12 hours
What does the control segment consist of?
The Master Control Station(MCS), and a group of five unmanned monitor stations(MS), and on MS co-located with the MCS
What is the purpose of a monitor station(MS)?
A monitor stations is simply a very accurate radio receiver, the function of which is to monitor the satellites within its area, collect range and other data and transmit it to the MCS for processing.
The amount of satellites one MS can monitor?
Up to 11
What is the function of the MCS?
To receive data from the MS’s, the information is used to predict the satellites orbit and timing errors. Corrections are then sent by one of four ground antennas(GA) located at each monitoring station, with the exception of Hawaii and the MCS, and then transmitted to the satellites.
The base principal GPS satellites use for position fixing?
GPS positioning is based on a principle called time of arrival, or TOA, ranging which utilises the transmission of radio signals and the accurate timing of transmission and arrival.
How do GPS obtain a fix using three satellites?
Accurate position fixing is achieved by using three distances from three different satellites. The system is referred to as rho-rho-rho, is based on the position of satellites in predictable orbits.
How does a GPS receiver, and a satellite determined range?
The signal transmitted by each satellite contains the exact time the message is sent and the GPS receiver, containing its own clock, compares the time of arrival of the signal with the time it was sent from the satellite. Since we know the speed of light waves, it’s just a simple matter of converting time to distance.
The initial range obtained using a GPS system is what type of range?
The range obtained will be pseudo range, since the calculation of the distance contains certain errors that must be corrected for later.
What is the circle of distance referred to when range is obtained from one satellite?
A surface of distance
The two frequency names of the continuous satellite transmissions
L1 and L2, in the L-band(UHF)
What is the L1 frequency, and the secondary message frequency?
1575.42 MHz, and 50 Hz.
What is the L2 frequency?
1227.6 MHz.
How is access to the L1 and L2 frequencies controlled?
By modulating the two frequencies with two pseudo-random codes called P (precise) and C/A (course acquisition).
Which L-band frequency is only available for military use?
L2
Does each satellite have its own generated unique P and C/A code?
Yes
The phenomenon, or error, of a satellite not being where it supposed to be is called?
Ephemeris error
Which segment of the GPS system monitors ephemeris error, and transmits the correction to the user segment?
The ground based stations
Ephemeris error information is passed on as?
Ephemerides
How often is clock drift corrected through data signals send to the the satellite as clock bias error?
Twice a day.
What is the clock error referred to on the receiver side?
receiver clock bias error
What what causes receiver clock bias error?
receiver clock bias error is a direct result of the difference in time between the satellite and receiver clocks
How is the difference between the time of the satellite clock and the receiver clock solved, that would allow for the actual position of the GPS be determined?
The difference is calculated by the data processor in the receiver using the pseudo range of a fourth satellite.
After what amount of satellite pseudo-range data will allow for the “cocked hat” position of 3 satellites to be an actual more accurate position using 3 satellites?
By obtaining the pseudo-range of a fourth satellite which will allow for the clock error to be resolved
What happens to electromagnetic waves once they enter the earths atmosphere?
They will slow down very slightly and a resultant tropospheric time delay becomes evident.
Signals below what angle will be automatically ignored due to tropospheric delay?
Signals below between 5 and 10 degrees, called the mask angle.
As the signals enter the ionosphere they are also bent slightly, the resultant deviation from a straight line also results in a time delay called?
Ionospheric delay
How is ionospheric delay resolved?
By using two frequencies instead of one (L1 and L2), because the time delay is inversely proportional to the square of the transmission frequency there will be a slightly different time delay in each frequency and a simple math formula is programmed into the P-code receivers to correct virtually all of the ionospheric delay.
A ionospheric delay fix can only be made with receivers that can?
Receive both L1 and L2 frequencies, the civil receivers using C/A code, can only receive the L1 frequency.
C/A code receivers, that only receive L1 frequency, can correct ionospheric error by 50% using what method?
Satellite transmitted data that allows the receiver to mathematically model the current ionospheric behaviour.
Define multi-path interference?
This is caused by the reflection of satellite signals from object close to the receiver; in the case of an aeroplane form the wings and fuselage.
Position lines whose angles are close together will not create as clearly defined a fix as one where larger angles exist, is called?
Dilution of precision (DOP), which is a geometric error.
Clock biased error is combined with geometric error. If the angles of inclination of the position lines were smaller the error, or cocked hat, would be greater. This is?
Geometric dilution of precision(GDOP)
The PDOP scale?
form 1 to 6, where 1 is perfect and 6 is the minimum acceptable.
The more satellites the better the?
Geometry precision that can be achieved
PDOP is a combination of?
Vertical dilution of precision (VDOP)
Horizontal dilution of precision (HDOP)
The number of satellites required for 3D navigation?
4
The number of satellites required for 2D navigation?
3
GPS required accuracy?
100 metres or better 95% of occasions
300 metres or better 99.9% of occasions
What are two systems used to improve reliability of service?
RAIM, receiver autonomous integrity monitoring
GPS integrity broadcast
Receiver autonomous integrity monitoring requires a minimum of?
5 satellites
Parallel offset can be used by a GPS receiver to?
Establish way points parallel to an air traffic service route to remain clear of controlled airspace
Doppler shift in GPS transmission is used to?
Calculate the groundspeed
In relation to the NAVSTAR/GPS satellite navigation system, ‘Search the Sky’ is a:
procedure that starts after switching on a receiver if there is no stored satellite data available
In NAVSTAR/GPS the PRN codes are used to:
differentiate between satellites
The geometric shape of the reference system for the satellite navigation system NAVSTAR/GPS, defined as WGS 84, is:
an ellipsoid
Are GPS transmissions continuous yes or no?
Yes
What does the secondary 50 Hz navigation message provide the receiver during the continuous L1 transmission?
Information on satellite orbits, clock corrections and other data.
Which satellite signal is only modulated by the P-code and is restricted to military use?
L2 signal
Which satellite signal is modulated by both C/A-code and P-code but only allows civil aircraft access to the C/A code?
The L1 signal
The US DoD introduced deliberate errors called dithering, to which transmission pseudo-random code?
The C/A-code
The satellite clock bias error is received by the GPS user via the?
The secondary 50 Hz Navigation message
The timing system used by GPS?
The timing system used by GPS is called GPS time and differs marginally from UTC.
The difference in time between the satellite and the receiver is called?
Receiver clock bias error
If the satellite clock is out of sync with GPS master located at the MCS, this called?
Satellite clock bias error
Why does tropospheric delay increase when a satellite is at a low angle to the horizon?
Because the waves have to travel through more of the atmosphere at a low angle, resulting in a farther delay
Can ionospheric delay corrections be made for C/A code users, using the L1 and L2 signal method?
No, it is restricted for the P-code military users only
How can C/A-code users correct ionospheric delay by up to 50% without using the combined L1 and L2 method?
By the satellite transmission which includes data that allows the receiver to mathematically model the current ionospheric behaviour.
What would be the ideal geometry of satellites?
One satellite is directly overhead the receiver and two or three others located between 15 and 75 degrees.
What is the time taken to receive the complete Navigation Message (complete set of data) from all satellites?
12.5 minutes (= 30 seconds per data frame)
What is almanac data?
Almanac data includes a set of parameters for each GPS satellite that can be used to calculate its approximate location in orbit.
How can the difference between satellite clock bias error and receiver clock bias error be calculated, and make the cocked hat type position a more accurate fix?
Using the pseudo-range from a fourth satellite
