13-15 Flashcards
Entire scope of satellite system is referred to
Global Navigation Satellite Systems (GNSS)
Advantages of GNSS
Precise timing and positioning
Low cost
Can be operated anytime in any condition
Global positioning satellites system is based on ….
signals transmitted from satellites whose positions are known
What does GPS surveying use?
Trilateration
What is the reference datum of GPS?
World Geodetic System (WGS84)
What is WGS84?
Earth-centered Earth-fixed coordinate system, that uses GRS80 ellipsoid and compatible to International Terrestrial Reference System
Explain Earth-Centered Earth-Fixed !
Origin: earth’s center of mass, defined for whole world
XYZ AXIS
GPS can be divided into 3 segments? Explain all of them!
Space segment - 24 satellites and 6 orbital planes. These set and orbital provide 24-hr satellite at latitudes of 80 N and 80 S. Satellites fly in medium earth orbit at an altitude 20,200 km above the earth. Each satellite circle the earth twice a day.
Ground/control segment - monitor the signals and track the positions of satellites to make precise the predictions of satellite orbits and correction parameters. This information is uploaded to the satellites to predict satellite positions and systematic errors.
User segment
GPS Signal (2 aspects)
Frequencies - fundamental frequency of the atomic clock, which is 10.23 MHz (L1=154 f, L2=120f, L5=115f, L5 belongs to the GPS modern area plan)
Ranging codes - C/A code: the basis for civilian P-code: for military and authorized New signals= L2C: for civilian users 2023 M code: a new military signal 2022 L1C: for civilian users 2020
Navigation message - will be managed onto C/A and P(Y) ranging codes. Types of information:
a. Date and time and satellite status
b. Broadcast ephemeris (information of satellites orbital parameters, clock correction, age of data, etc)
c. The almanac
What is broadcast ephemeris
Contains information of satellite orbital parameters, clock correction coefficient, accuracy, age of data, etc
Anti-spoofing effect
Fake gps signal like a hacker. So stop false signals by encrypting P code (military code) with a secret W code to derive in the Y code
Errors in GPS
Clock biases
Tropospheric and Ionospheric refraction
Satellite ephemeris error
Multipath error
Geometry of observed satellites
Skyplot
Explain Trophospheric and ionospheric refraction (error in GPS)
Ions depends on the space weather. It would affect Pseudorange and carrier cycle measurements.
Explain satellite ephemeris errors (error in GPS)
Broadcasting ephemerides is not very precise so the updated post-survey ephemerides is necessary
Explain multipath error (error in GPS)
When the signal reaches the receiver after one or more reflections (mantul di tempat lain dulu). Error because it might interfere other signals.
