GPS Details Flashcards
GPS Signal Speed
Speed of light:
299,792.458 km/second
About 300,000 km/second
Distance = Speed (velocity) × Time
Time = Distance / Speed (velocity)
GPS Signal Structure
Transmits a microwave radio signal with the following components:
- Two sine waves
- Two digital codes
- A navigation message
GPS Signal Structure
The carriers and the codes: The distance from the user’s receiver to the GPS satellites.
The navigation message: The coordinates (location) of the satellites.
Controlled by highly accurate atomic clocks
GPS Signal Structure - Wavelength
(A) A sinusoidal wave
(B) a digital code
GPS Carrier Frequencies
L1 Carrier Frequency
- Generated at 1,575.42 MHz
- Wavelength of 19 cm
- Modulated with C/A-code, P-code, navigation message.
L2 Carrier Frequency
- Generated at 1,227.60 MHz
- Wavelength of 24 cm
- Modulated with P-code and the navigation message
GPS Carrier Frequencies
All GPS satellites transmit the SAME L1 & L2 carrier frequencies.
Dual frequencies (L1,L2): Correct ionospheric delay
GPS Digital Codes
Each code: (a) binary streams and (b) random signals
Types: C/A-code, P-code, and Y-code
Two levels of GPS positioning and timing services by the DoD
The Precise Positioning Service (PPS)
The Standard Positioning Service (SPS)
C/A-code: Coarse Acquisition code
A civilian GPS code with 1,023 binary digits at 1.023 Mbps on the
L1 carrier
A unique C/A-code, making it identifiable
Simpler and available to all, but less precise than P-code
P-code: Precision (also known as precise or protected)
code
A military GPS code on L1 & L2 carriers with 10.23
Mbps speed
A unique weekly P-code, e.g., PRN 20 for the twentieth
week.
Used for anti-spoofing with encryption in precise
positioning
Y-code
Anti-spoofing: P-code with an encrypted W-code
Accessible only to users with specialized equipment, like
military receivers
GPS Navigation Message
- Data: Added to L1 & L2 frequencies with 37,500 bits in 25 frames
of 1,500 bits each
Transmitted at 50 kbps, taking 12.5 minutes
GPS Navigation Message
Navigation message include:
Satellites coordinates over time
Satellite health status
Satellite clock correction
Almanac (orbit and clock details)
Atmospheric data
GPS Modernization
Modernized versions (Blocks IIR-M and later) aim for:
Better accuracy
Signal availability
System integrity
GPS Modernization
Additions:
L2 frequency got C/A-code with Block IIR-M.
Two M-codes added to L1 & L2 in Block IIR-M.
Block IIF introduced a third civil signal (L5).
Block III generation extends GPS operations to 2030.
Upgrades:
GPS ground control improved.
Satellites monitored from at least two stations.
Currently, 16 monitor stations: 6 by Air Force, 10 by NG
GPS Receivers
Availability & Price
In 1980, the GPS receiver, Magellan NAV 100, cost $2,900
Now, over 500 GPS receivers from 70+ companies available
GPS Receivers
GPS receiver types by capability - Continued
3. Single-frequency Code and Carrier Receiver: Provides raw
C/A pseudoranges, L1 carrier-phase, and navigation message
4. Dual-frequency Receiver: Gives all GPS signal components,
including both L1, L2 carriers and codes. Most expensive and
accurate
Time System in GPS
GPS signals
Governed by atomic satellite clocks
Used for time synchronization
Time Systems in GPS
UTC (Universal Time Coordinated)
The global standard for regulating clocks
An accurate atomic time system aligned with Earth’s rotation
Leap seconds
Adjust civil time, compensating for Earth’s slowing rotation
Does not exceed 0.9 seconds
Time Systems in GPS
UTC (Universal Time Coordinated)
Maintained by the U.S. Naval Observatory
An atomic time scale based on International atomic time
GPS time corresponds directly to UTC.
Pseudorange Measurement
Distance between a GPS receiver and satellite is determined using
P-code or C/A-code.
Known as “code-phase measurement” for positioning
Pseudorange Measurement
Distance Computation:
The satellite sends a PRN code, and the receiver creates a matching replica.
When the receiver picks up the transmitted code, it compares it with the replica to determine signal travel time.
Distance is calculated as signal travel time
times the speed of light.
Code-phase Idea Behind
A GPS receiver matches its PRN code with the satellite’s signal
to determine signal travel time.
It adjusts its code until it aligns with the satellite, signifying the
travel duration.
Pseudorange Measurement Issue
Due to synchronization errors and biases, the measured distance is termed “pseudorange” instead of range.
Carrier-Phase Measurement
- Distance via carrier phases
- Combines full and fractional cycles from both receiver and satellite, multiplied by carrier wavelenght.
- More accurate than pseusorange due to the L1 frequency’s smaller 19 cm wavelenght.
Carrier phase Measurement
This method counts carrier cycles between the satellite and
receiver.
The challenge is the carrier frequency’s uniformity, making every
cycle appear identical.
Carrier phase vs Code phase
The carrier frequency’s higher rate, over 1 GHz, is 1,000 times
faster than the pseudo random code’s 1 MHz, making it more
accurate due to closer pulses.
The 1.57 GHz GPS signal has a 19-24.4 cm wavelength at light
speed, making the carrier signal more precise than the pseudo
random code alone.
Cycle Slips
A discontinuity or a jump in GPS carrier-phase
measurements
Due to signal loss from obstacles, interference, or
receiver issues
Corrected by the Triple Difference Observable
Linear Combination of GPS Observation
GPS measurements have errors and biases from satellite,
receiver, atmospheric refraction and satellite geometric effects.
Combining GPS observables can mitigate these.
Linear Combinations of GPS Observation
Between-receiver single difference
Between-satellite single difference
Double difference
Triple difference
