Principles of Cartography
Course Code: Geog 2003
Instructor: Dr. M. M. Yagoub
Exercise 5:
Global Positioning System (GPS)
List and define the main segments of GPS
1. Space Segment: The
Space Segment of the system consists of the GPS satellites (send radio signals
from space )
2. Control Segment: The Control Segment
consists of a system of tracking stations located around the world
3. User Segment: The GPS User Segment
consists of the GPS receivers and the user community
Discuss in details the Space Segment
The
U.S. Department of Defense (DOD)
has a network of 27 (NAVigation
Satellite Time and Ranging) satellites (32 in the future), at an altitude of
20,200 km orbit, placed on six orbital planes, inclined 60o to the
equator and are spaced every 60o in longitude, the orbital period is
about 12 hours. An observer at any point on the earths surface can receive
radio signal from at least four of the satellites at any time of the day or
night
First
launched in 1978 and become fully operational on 1993 ($12
Billion)
List and define the GPS Positioning
Services
Precise Positioning Service
(PPS): Authorized users with
cryptographic equipment and keys and specially equipped receivers approved by
the U. S. Government, can use the PPS. PPS Predictable Accuracy: 22 meter Horizontal accuracy, 27.7 meter vertical accuracy, and 100 nanosecond time accuracy
Standard Positioning
Service (SPS):
Civil users world-wide use the SPS without charge or restrictions. Most
receivers are capable of receiving and using the SPS signal. The SPS accuracy
is intentionally degraded by the DOD by the use of Selective Availability (SA). SPS Predictable Accuracy:
100 meter horizontal accuracy , 156 meter vertical accuracy, and 340 nanoseconds time accuracy
Discuss how GPS Works
The GPS receiver measures its distance
(pseudo-range) from the satellite based on time the signal takes to
arrive at location and the speed of light (3X108 m/s or
186,000 miles/s) - distance = time X speed
The satellite itself send enough information
to calculate its current location relative to the receiver. By acquiring the
same information from three
satellites we can compute the position based on the intersection of the three
spheres
Signals cannot penetrate wall, water, soil,
or other obstacles. Thus GPS cannot be used for underground positioning or
underwater navigation, in cities with many tall buildings, and forests with
dense cover
Discuss the Factors Affecting GPS Accuracy
Atmospheric errors:
Radio
signals travel at different velocities through the atmosphere depending on time
of day or night, the season of the year, the angel of the signal through the
atmosphere, and so on. (Ionosphere, Troposhere)
Multipath:
Reflected signals from buildings, cliffs, etc.
Ephemeris
(position of the satellite in space)
Clock errors from the satellite
Position and number of satellites in sky: 4 required for 3D (horiz. and vertical), 3
for 2D (no elevation). Ideally, 3 every 120° horizon. with 20°
elev., 1 directly above
blockage
(of satellite signal) by foliage, buildings, etc. (unobstructed views of the
satellites for vertical angles of greater than 20o above the
horizon)
Receiver errors: Imperfection of the receiving equipment in matching the satellite
signals
List GPS Receiver Characteristics
Processing speed & channel capacity (# of satellite data streams simultaneously
processed) (time must be accurately measured to nanosecond = 10-9 seconds)
Storage capability: internal & PCM/CIA cards
Codes it can process (L1, L2; code, carrier
phase, etc.)
Antenna type and remote connection support
Interface capabilities (RTCM, NMEA)
The
above factors affect on cost
List
some GPS Applications
GPS can keep track of travel distance, compute speed, record travel time and other valuable data
Surveying: Location of control point (Bench marks)
Photogrammetry:Aerial survey control points
Routes and navigation: flights direction, car
positioning, Hiking, biking, and marine navigation
GIS applications: Digital data (x,y,z) and attribute. Software
is readily available to convert GPS data to GIS formats
