GPS

1. GPS
Toppy Huang

2. GPS System Architecture

3. What is GPS?
•
Global Positioning System
•
Built by U.S. Department of Defense
•
Fully operation since 1995 while 1
st
SAT launched in 1978
•
Two positioning services
•
SPS (Standard Positioning System) provides 100 m accuracy for civil users
•
PPS (Precise Positioning System) provides 17.8 m accuracy for US
military/government users
•
Provides Position, Velocity, and Time (PVT)

4. GPS Background
Why US built GPS?
•
Initially for military use on accurate targeting, location awareness, and monitor,
etc.
•
Cold War between US and USSR in 1970s
•
Arm race resulted in large competition for long-distance weapons,
like cruise missiles, PGM, and ICBM, etc.
•
Traditional inertial guidance system is no good for measuring long-
distance position.
•
Urgent demands for improving long-distance positioning accuracy
•
Vietnam War (1965 ~ 1975)
•
Traditional LORAN (LOngRange NAvigaton) system suffers from
electronic effects of weather and in particular atmospheric effects
related to sunrise and sunset.
•
Limited coverage due to radio capability

5. GPS Background
Why US built GPS? (Cont.)
•
1958~1967, Navy Navigation Satellite System (NNSS, Transit)
•
6 satellites running at 1100 km altitudes with 107 minute orbits
•
Long fixed and position update time
•
1974~1977, NavStar
•
Experiment on atomic clock stability and accuracy
•
1978~Now, GPS
•
1978, 1
st
SAT launched
•
1990, 21 SATs by BlockI & BlockII start operations
•
1994, Initial Operational Capability announced
•
1995, Full Operational Capability announced
•
1997~2004, Block IIR SATs launched for replacement
•
2005~, Block IIF SATs keep launching for improvement
•
GPS by US while GLONASS by USSR
GPSConstellationSta
tus

6. GPS Background
Why US open for civil use?
•
1983, Due to poor navigation system, an Korean Aircraft was shot down by
USSR missiles. 269 civilian deaths including 61 Americans.
•
1984, President Reagan open partial function for civil use, Standard Positioning
System (SPS).
•
1990, US DoD announced SA policy for degrading accuracy of SPS to 100~150
meters.
•
1998, Vice President Gore announced Block IIF SATs will increase two additional
frequencies for civil use.
•
2000, President Clinton stopped SA policy, accuracy of SPP is back to 10 ~15
meters.
•
2001, After the terror attack of Sept. 11th, US insisted there is no plan to use SA
policy again.

7. GPSSystem Architecture
•
Space Segment
•
Control Segment
•
User Segment

8. GPSSystem Architecture
Space Segment
•
Space Segment
•
24 operational Satellites + 4
active spares
•
6 orbital planes
•
12 hours orbits
•
20200 km altitudes
•
At least 4 SVs(Space Vehicles)
viewed at any time/places

9. GPSSystem Architecture
Control Segment(Cont.)
•
Monitor Station
•
Collects Smoothed Measurements(
勻化數據
) for each SVs, and send those to Master Control
Station
•
Weather information, ionosphere(
電離層
) data, and pseudo-range data, etc
•
SVs status information
•
Master Control Station
•
Computes orbital data, clock corrections, and ionosphere corrections, etc
•
Passes those Navigation Message to Ground Antenna
•
With orbital data modification, MCS monitors/diagnoses/adjusts all SVs operations
•
Ground Antenna
•
Upload Navigation Message to SVs every 8 hours
•
All SVs are exactly synchronized by MCS
•
MCS tells SVs exactly moving path

10. 3. Positioning
Pseudo range measurement
Carrier phase measurement

11. GPS
定位原理
地理座標系統
•
Position of SVs are presented in Earth-
Centered, Earth-Fixed X, Y, Z (ECEF XYZ)
coordinates.
•
The Z-axis points toward the
North Pole.
•
The X-axis is defined by the
intersection of the plane define
by the prime meridian (
子午
線
)and the equatorial plane(
赤道
面
).
•
The Y-axis completes a right
handed orthogonal system by a
plane 90
°
east of the X-axis and
its intersection with the equator.

12. Positioning
•
Both “Pseudo Range” and “Carrier Phase” to
get the distancebetween each satellite and
the device.
•
And then use the broadcast ephemeris
calculate the location (x1, y1, z1; x2,y2,z2).
•
ρ
i
(j) = r
i
(j) + c t
e
j=1,2,3,4
•
ρ
i
(j) =[(x(j)-x
i
)
2
+(y(j)-y
i
)
2
+(z(j)-z
i
)
2
)]
1/2
+ c t
e
•
j=1,2,3,4

13. Pseudorange
(DLL-Delay lock loop for code
tracking)
•
Reference time: TLM words
•
The resulting distances are not only related to the
distance between the receiver antenna and the satellites,
but also to an imperfect alignment to the receiver's time
scale to the GPS time scale.
τ = Tb -Ta
= (τb-△ts) -(τa-△tr)
= (τb-τa) + △tr -△ts
cτ = c [ (τb-τa) + △tr-△ts+ △ta]
ρ = R + c △tr-c △ts+ c △ta
ρ= Pseudo-range
R = true range
= [(x-x(j))
2
+ (y-y(j))
2
+ (z-z(j))
2
]
1/2

14. Carrier Phase Tracking
(PLL for carrier phase tracking)
•
The wavelength of the carrier waves are very short
compared to the C/A and P code chip lengths.
•
L1=154*10.23MHz=1575.42MHz, λ=19cm
•
L2=120*10.23MHz=1227.60MHz,λ=24cm

15. GPS
定位原理
Pseudo-range Measurement
•
Time clock in GPS receiver and
SVs are “synchronized”.
•
Time of Arrival(TOA)
•
距離
(D) =
速率
(V) x
旅行時間
(T)
•
衛星距離
=
光速
x
訊號延遲時間

16. GPS
定位原理
3D
空間定位
•
A sphere with center (x
0
, y
0
, z
0
) and radius ris the set of all points
(x, y, z) such that
•
The GPS receiver could be located at the intersection of three
spheres, one around each satellite, with a radius equal to the
pseudo-range between the satellite and the receiver.
Spher
e

17. GPS
定位原理
3D
空間定位
(Cont.)
•
Given that 3 positions of SVs are (X1,Y1,Z1),
(X2,Y2,Z2), and (X3,Y3,Z3)
•
Get the pseudo-range to these 3 SVs, say RS1,
RS2, and RS3
•
Then, the position of GPS receiver (Ux,Uy,Uz) is
done by :
X1,Y1,Z1
X2,Y2,Z2
X3,Y3,Z3
Ux,Uy,Uz
RS3
RS2RS1
3~1,)()()(
222
 iwhereUzZiUyYiUxXiRSi
X
Y
Z

18. GPS
定位原理
Why we need 4
th
SV ?
•
4
th
SV is used for clock correction
•
In fact, Atomic clock in SVs is much
more precise than the clock in GPS
receivers.
•
Then, actual distance to the SV is called Rt :
•
Thus, 4 unknown variables need 4 equations :
X1,Y1,Z1
X2,Y2,Z2
X3,Y3,Z3
Ux,Uy,Uz, Cb
RS3
RS2
RS1
RS4
X4,Y4,Z4
biasclockuserCbwhereCbRsRt  ,
4~1,)()()(
222
 iwhereUzZiUyYiUxXiCRS bi
X
Y
Z

19. GPS
定位原理
3D
定位
/ 2D
定位
/ Velocity
•
With 3 SVs tracked, we only get 2D position•
3 unknown variables (Ux, Uy, Cb) with 3 equations
•
With 4 SVs tracked, we’ll get 3D position
•
4 unknown variables (Ux, Uy, Uz, Cb) with 4 equations
•
With 4 SVs tracked, the Delta-range measurement is used to
get 3D velocity
•
Doppler is measured to provide the relative velocity between the
receiver and SVs.
•
More precise pseudo-range measurement could be achieved by
Doppler measurement.

20. GPS Receiver
Understanding how the GPS Receiver deals signal collection,
Acquisition and tracking.

21. GPS transmitter (Satellites)
•
Each GPS satellite transmits a microwave radio signal
composed of two carrier frequencies
•
L1=154*10.23MHz
=1575.42MHz, λ=19cm
•
L2=120*10.23MHz
=1227.60MHz,λ=24cm
•
Embedded into each carrier frequency are two PRN
codes:
•
C/A code: 1.023MHz
•
P code: 10.23MHz

22. Simplified GPS Receiver Block Diagram

23. NAV Message Content and Format Overview
•
Almanac
•
Information about the status of
the satellites and approximate
orbital information.
•
Used to calculate which satellites
are currently visible. (acquire)
•
Many newer GPS receivers are
able to acquire the satellites
without waiting for the almanac.
•
Ephemeris
•
Precise information about the
orbit of each satellite.
•
Used to calculate the location of
a satellite. (xi,yi,zi)
300bits
1500bits/30sec
*25 frame
=12.5mins

24. GPS Receiver
•
GPS RF front-end collects digitized IF data
•
DSP
•
Acquisition and Tracking
•
Parameters influent acquisition
•
Precorrelationbandwidth (front-end BW)
•
Analog-to-digital conversion (ADC)
•
Sampling frequency
•
Predetectionintegration time
•
Carrier Tracking was performed using a FLL-assisted-PLL

25. Block Diagram of GPS Acquisition
Process
•
Code Correlation method
•
The receiver generated (respond)
signals are continuously shifted to
detect the maximal correlation
between the received and
generated code sequences.
•
If the satellite and receiver clocks
were errorless, this time shift
would be equal to the travel time
(τ) of the code sequence from the
satellite to the receiver.

26. GPS Test

27. GPS Test
•
C/N
•
Doppler Shift
•
Sensitivity
•
Acquisition sensitivity
•
Tracking sensitivity
•
TTFF (Time to First Fix)
•
Cold Start
•
Warm Start
•
Hot Start
•
Position Accuracy

28. GPS Test-TTFF
Data Type Cold StartWarm StartHot Start
PreviousPositionX O O
Time X O O
Almanac X O O
Ephemeris X X O
–Cold start: Receiver does not have time or position information, no valid
ephemeris (or almanac) data (typical TTFF 30-50 sec, maybe up to several
minutes)
–Warm start: Last position and approximate time known, valid almanac, no valid
ephemeris data (typical TTFF 30-40 sec)
–Hot start: Time and last position known, valid almanac and ephemeris (typical
TTFF 1-5 sec)
• Need to specify satellite power level when testing TTFF

29. GPS Test-Sensitivity
•
minimum= -174dBm/Hz + C/No
minimum
+ NF
receiver
•
Sensitivity
•
Acquisition sensitivity
•
Tracking sensitivity
•
Minimum level of signal that allows GPS receiver to acquire or track the GPS
signal (may also be specified in terms of C/No)
•
Acquisition sensitivity: minimum level to successfully perform TTFF under cold start
(typically around -140 to -150 dBm)
•
Tracking sensitivity: minimum level to maintain location fix once it has been attained
(typically -150 to -160 dBm)
•
Test requires multi-satellite GPS signal with valid navigational messages for
TTFF, and real-time satellite power control to reduce power levels to test
sensitivity

30. Sample Test Plan
Test Item Suggested Spec.
1Minimum C/N C/N >= 38dB
2DopplerShift C/N>=38dB
3
Acquisition
time
Hot start TTFF<= 1 sec
4 Cold start TTFF< =45 sec
5 Cold start TTFF@
sensitivity level
< =100 sec
6TrackingSensitivity <=-154dBm,
7AcquisitionSensitivity >=-145dBm
8PositionAccuracy <=10m

31. SiRFStar IV

32. GPSmax input power
•
best performance is obtained when the signal levels are between -125
dBmand -117 dBm. These received signal levels are determined by :
•
GPS satellite transmit power
•
GPS satellite elevation and azimuth
•
Free space path loss
•
Extraneous path loss such as rain
•
Partial or total path blockage such as foliage or building
•
Multipath caused by signal reflection
•
GPS antenna
•
Signal path after the GPS antenna

33. Test sample of C/N v.s. Input Level
•
CN is proportionate to input Level.
53.5
49.1
44.3
39.38
34.37
29.51
24.68
20.12
0
10
20
30
40
50
60
C/N v.s. Input Level
Test data