GNSS (25-05-18) Flashcards
(30 cards)
What is the US GPS system called?
NAVSTAR GPS
What is the Russian GPS system called?
GLONASS
What is the European GPS system called?
GALILEO
What is the Chinese GPS system called?
BEIDOU
What are the advantages of GPS?
Worldwide
Very accurate
24hr
4D monitoring (3D plus time)
Space based
What is the principle of operation with GPS?
The satellite and the A/C have clocks which are synced.
At a predetermined time a pulse is transmitted by the satellite and the A/C can calculate the distance from the satellite.
What number of GPS satellites provides a 3D fix?
4
What number of GPS satellites provide a position 2D fix?
3
What errors are encountered with a GPS system?
Propagation speed changing through the ionosphere
Errors in the accuracy of the transmission schedule (Einstein relatively)
Errors in the accuracy of the clocks (more likely in the A/C)
What are the 3 components of a GPS system?
Space segment
Control segment (ground facilities that track GPS sats, monitor Tx, perform analysis and send commands and data)
User segment
What frequency band do GPS satellites broadcast on?
UHF
What is the L1 signal of GPS?
1575.42MHz
contains C/A code: course acquisition
Contains P code: precision (friendly military)
What is the L2 code for GPS?
1227.6MHz
contains P code for owners military
How are each individual satellites signals differentiated between?
With a unique Pseudo Random Noise by binary phase shift keying
What allows GPS systems to be easily jammable?
Their signal is very weak
What is sent in the navigation message of a satellite in GPS?
Almanac - predicted satellite position
Ephemeris - correction data for error of satellite position
Conversion factors for GPS to UTC time
ionospheric model (every 12 hours)
satellite clock error
Satellite status
How long is a satellite navigation message valid for?
4 hours
How is a nav message from GPS sent and how long does it take for first position fix?
it is sent in 25 subframes each of which is sent in 30 seconds
in total taking 12.5mins
the sky search take 2.5mins
total time for first position fix is 15mins
What does the control segment of GPS consist of?
The master control station
12 command and control groudn antenna
16 monitoring stations
What is the main tasks of the control segment of GPS?
Manage space segment performance
nav data upload
monitoring satellites
How is error in the user clock bias accounted for?
All satellite position ranges are reduced by the same amount until they intersect
due to the error being the same in all systems.
How does a satellite provide a 3D fix in terms of latitude and longitude?
Calculates A/C position using Cartesian coordinates from semi minor and major axes.
This is then input into the WGS84 model to give lat, long and alt
What can geometric dilution of position (GDOP) be subdivided into?
PDOP - position
HDOP - horizontal
VDOP - vertical
TDOP - time
What is selective availability?
Where the atomic clock of a GPS satellite can be dithered to purposefully degrade accuracy
(turned off by presidential decree in May 2000)
