GPS Flashcards
What is DGPS? Could also be asked as describe GPS differentiation
Differential GPS. DGPS is a system that makes GPS more accurate by using additional ground-based stations to correct any errors in the GPS signals. They send their corrections over VHF to DPGS receivers which automatically applies the corrections. Increases accuracy from around 15m to within 3m. Can work up to 50 miles offshore.
Explain and describe what is meant by DOP?
Dilution of precision. This is with regard to the angle of cut of the position lines from received from the satellites. A good DOP has a lower number (1-3) and tight angle of cut and bad DOP has a higher number (6-8) and large angle of cut.
Explain HDOP
Horizontal dilution of precision refers to horizontal variables (on the surface of the earth) latitude and longitude and is used by mariners.
How would you put a satellite derived position (SDP) on a chart.
Check if the chart is WGS84, if not then check for corrections on chart
What does GPS provide?
Provides a mean of navigation by satellites which relies on accurate time info to determine ranges
What’s NAVSTAR GPS stand for?
Navigation satellites with time and ranging Global Positioning System.
What’s the datum for GPS?
WGS-84
Name 5 sources of error
- Satellite orbital (position) error 1-5m
- Satellite atomic clock error 0-2m
- Ionosphere error delay 0-30m
- Tropospheric delay (weather, temp, pressure etc) 0-30m
- Multi path error 0-10m
- Receiver noise error 0-10m
- Receiver clock error
- Dilution of Precision (DoP)
What does the space segment consist of?
24 active satellites, orbiting around 20,200km altitude. Each satellite transmits data on two coded frequencies (one for civilian- and one for military use), containing its own orbital position, identity and accurate time data, as well as other the position of the other satellites.
What happens if a control system was ever disabled?
Then pre stored messages are available in each satellite with a 14 day prediction.
After 14 days the accuracy exceeds 400m and are therefore no longer suitable for navigation.
What are the control segments and where would you find some?
Monitor stations that track and monitor the satellites position and their atomic clock.
You can find a few along the equator, their HQ is in Colorado Springs, US.
What is the user segment?
GPS receiver and operator. The receivers job is to generate a code for comparison, determine the doppler shift / time difference and create a pseudo-range. Multi channel receivers can measure distance from 4 or more satellites and can therefore provide almost instantaneous readings for position.
What is the principle behind GPS position fixing?
A satellite will generate a signal. This signal contains the satellites identity, orbital position (as well as the position of the other satellites in the constellation), very accurate time stamp and time correction. The radio wave is sent out at the speed of light. At the same time, a GPS receiver will generate the identical distinctive code for comparison upon receiving the satellites signal. When the receiver receives the code it will create a common time and calculate any differences in the received code and generated code. Any error in the timing will be due to the Doppler Shift Principle. Our receiver will then determine a pseudo range (therefore a position circle) from the satellite. In order to fix a position in the 3D plane, we use the signals from at least three satellites with a fourth satellite accounting for accuracy of time and any position uncertainty
What is the doppler shift principle? (Doppler effect)
The apparent change of frequency of a wave due to relative motion between the source of a wave and the observer
How can we use the doppler shift principle?
If we know the wavelength that is emitted, then any change in that frequency when the wave is returned can be measured off as speed
