4. GPS Satellites = Space Vehicles (SVs)
► Solar powered
► 3-4000 lbs each
► 10-parameter Almanacs approximate position
in space
► Input Signals:
Corrections from control stations
► Output Signals (2):
X,Y,Z and t data streams sent continuously from SVs
L1 channel: C/A Code (Coarse Acquistion) – civil use
L2 channel: P-Code (Precise) – military / special licensees only
GPS BASICS
5. Satellite Constellation
► 24-satellite constellation
(+3 backup=27)
► Elevation 12,000 mi
► 2 orbits/day (each)
► Six orbital planes:
55° inclination from equator
60° spacing about poles
4 SVs/plane
GPS BASICS
6. Air Force Ground Control
► Continuous time and position corrections sent to space vehicles from
ground control
Position corrections based on precise computer trajectory models
Time corrections based on Universal Coordinated Time (UTC)
► Time and position corrections re-transmitted from SVs to receivers
Time error <100ns at receiver after correction
Position error at receiver depends on which technology is used
► Master control station at Schriever AFB, CO (formerly Falcon AFB)
Control
Station
User
Corrections
(x,y,z,t)i
(x,y,z,t)i
+ Corrections
SVi
GPS BASICS
7. SV Data Structure
► 50Hz binary data sent in 300-bit packets (subframes)
► 5 subframes per frame, 25 frames per message
► Message restarts every 12.5 min
► Data is encrypted and modulated before transmission
► Each subframe contains parity bits for data corrections
Data frame:
1500 bits, 30 sec
1
2
Subframe:
300 bits, 6 sec
1 2 3 4 5
Clock
corrections
.
.
.
25
Precise (ephemeris)
orbital position data
SV system data
Complete
navigation
message:
25 frames,
12.5 min
GPS BASICS
8. SV Data Transmission
► SV data (position, time, system info, etc.) logical OR’d with PRN code, then used to
modulate high-freq. carrier
► PRN codes are unique signatures for each SV, one C/A and one P-code for each
► L1 = SPS signal (civil use), repeats every 1023 cycles
► L2 = PPS signal (military and special use only), repeats every seven days
SPS Carrier freq.
(uniform)
Pseudo-Random
Noise (PRN)
Data
@ 50Hz
PPS Carrier freq.
(uniform)
GPS BASICS
9. Code Phase Tracking
► Receiver slides ‘replica’ of PRN code in time and compares with SV signal until a match is found,
identifying SV
► Phase shift between signal and replica represents signal transit time (ti-T), ti=time on SV clock,
T=receiver time
Replica of SV PRN
from receiver almanac
Actual PRN
received from SV
GPS BASICS
Signal match
strength
10. Calculating Position
► The receiver position is
calculated by solving a set of
four Pythagorean equations:
(x1 - X)² + (y1 - Y)² + (z1 - Z)² = c²(t1 - T-d1)²
(x2 - X)² + (y2 - Y)² + (z2 - Z)² = c²(t2 - T-d2)²
(x3 - X)² + (y3 - Y)² + (z3 - Z)² = c²(t3 - T-d3)²
(x3 - X)² + (y3 - Y)² + (z4 - Z)² = c²(t4 - T-d4)²
Where:
► X,Y,Z and T are unknown position
and time at receiver
► (x,y,z)i are the four known satellite
positions
► di are the known differences in data
arrival time, from correction data
GPS BASICS
Receiver must calculate
actual position from best
fit between multiple
range calculations
Where am I?
11. Error Sources
SOURCE ERROR CONTRIBUTION
Ionospheric delays 10 m
Tropospheric delays 1 m
PRN Code Noise 1 m
SV Clock 1 m
SV Ephemeris Data 1 m
Pseudo-Range Noise 1 m
Receiver Noise 1 m
Multi-Path 0.5 m
TYPICAL ERROR WITH
BASIC GPS
15 m
GPS BASICS
Note: Selective Availabilty (SA) limited accuracy of SPS service to 100m until May 2000
12. 3. Differential GPS
► Reference station at a fixed, known location computes its location from SV signals and
computes error correction factors
► Correction factors are transmitted to remote receivers at radio frequency
► Usable range <30 km from reference station
► Reference receiver must be surveyed and located beforehand
► Coast Guard maintains ref. stations along most US coastlines
► Typical accuracy 1-5m
Reference station
at known location
Remote receiver
Correction factors
transmitted to remote
receiver via radio frequency
SV position data
received by
reference station
SV position data
received by
remote receiver Remote receiver
position modified by
correction factors
Correction factors
computed from
position errors
13. 4. Carrier Phase Tracking
► Reference receiver required, similar to DGPS
► Utilizes high frequency carrier waves instead of SV data and PRN code
► Remote position = reference position + difference in (x,y,z) derived from
difference in carrier cycle measurements
SV @ t1
SV @ t2
t2-t1 >15 min
Reference station
at known location
Remote receiver
Carrier waves
14. Carrier Phase Cycle Changes
► Example: Range from reference to remote receiver has changed by 10
cycles between t1 and t2
► Usable <30km from reference station
► Accuracy 4-10cm for fast static processing, 1-5cm for post-processing
► Must acquire signal while stationary for at least 15 minutes
► Good for mapping and surveying, impractical for real-time navigation
CARRIER PHASE TRACKING
10 cycles
Remote receiver
Reference receiver
Tagged cycles @ t1 Tagged cycles @ t2
19 cm
16. WAAS: Broadcast Corrections
Wide Area Augmentation System
► 2 geosynchronous satellites
► 2 main ground stations on east & west coast
► 25 ground substations
► Information broadcast with same data structure / same channel as GPS
► Must have a WAAS-capable receiver to use
► Accuracy <3m
► Developed by FAA for aircraft landings
Substations compute
local errors
Surveyed locations (25)
West coast East coast
Correction factors rebroadcast across
the US to be used by anyone
SV data
received at
substations
Orbiting GPS
satellites Geosynchronous WAAS satellites
Local errors transmitted to
main ground stations
Correction factors transmitted
to WAAS satellites
Correction factors computed at
main ground stations
17. Other Techniques
►Post Processing
Data saved and position computed later
►Data Links
Hard-wire connections between reference and
remote receivers
►Internet corrections
Correction factors available online for post
processing
18. IR Laser beams
rotate and fan out
6. Indoor GPS: Constellation 3Di
► Factory workspace filled with 3-D coordinate grid of IR light
► Receivers key into grid to determine position
► System eliminates the need for awkward, rigid fixtures and hard tooling for accurate alignment of
large parts
► Receivers can be mounted to parts, tools, fixtures, etc
► Accuracy 4-8ppm – i.e. 0.4-0.8mm over 100m range
► Implemented at Boeing Commercial Airplanes Manufacturing R&D
Factory workspace
Transmitters
Receiver mounted to tool
LED strobe
Each transmitter
rotates light beams
at a unique
frequency
TRANSMITTER
Azimuth computed from rotating beams
Elevation computed from LED pulses
19. 7. Accuracy Comparison & Applications
Technique Accuracy (2) Application
Basic GPS (SPS) 15 m Worldwide navigation
PPS* 10 m (restricted use)
DGPS 5 m
Navigation over territory
outside US
Carrier Phase
Tracking
5 cm Land Surveying
WAAS 3 m
Navigation over territory
inside US
LAAS** ? (under development)
Constellation 3Di 4-8ppm Factory tool positioning
* Military and special licensees only
** Local Area Augmentation System coming soon to an airport near you!
