GPS Details

Question 1

GPS Signal Speed

Answer 1

 Speed of light:
 299,792.458 km/second
 About 300,000 km/second

Distance = Speed (velocity) × Time
Time = Distance / Speed (velocity)

Question 2

GPS Signal Structure

Answer 2

Transmits a microwave radio signal with the following components:

• Two sine waves
• Two digital codes
• A navigation message

Question 3

GPS Signal Structure

Answer 3

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

Question 4

GPS Signal Structure - Wavelength

Answer 4

(A) A sinusoidal wave
(B) a digital code

Question 5

GPS Carrier Frequencies

Answer 5

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

Question 6

GPS Carrier Frequencies

Answer 6

All GPS satellites transmit the SAME L1 & L2 carrier frequencies.

Dual frequencies (L1,L2): Correct ionospheric delay

Question 7

GPS Digital Codes

Answer 7

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)

Question 8

C/A-code: Coarse Acquisition code

Answer 8

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

Question 9

P-code: Precision (also known as precise or protected)
code

Answer 9

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

Question 10

Y-code

Answer 10

 Anti-spoofing: P-code with an encrypted W-code
 Accessible only to users with specialized equipment, like
military receivers

Question 11

GPS Navigation Message

Answer 11

• 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

Question 12

GPS Navigation Message

Answer 12

Navigation message include:
 Satellites coordinates over time
 Satellite health status
 Satellite clock correction
 Almanac (orbit and clock details)
 Atmospheric data

Question 13

GPS Modernization

Answer 13

Modernized versions (Blocks IIR-M and later) aim for:
 Better accuracy
 Signal availability
 System integrity

Question 14

GPS Modernization

Answer 14

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

Question 15

GPS Receivers

Answer 15

Availability & Price
 In 1980, the GPS receiver, Magellan NAV 100, cost $2,900
 Now, over 500 GPS receivers from 70+ companies available

Question 16

GPS Receivers

Answer 16

 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

Question 17

Time System in GPS

Answer 17

GPS signals
 Governed by atomic satellite clocks
 Used for time synchronization

Question 18

Time Systems in GPS

Answer 18

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

Question 19

Time Systems in GPS

Answer 19

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.

Question 20

Pseudorange Measurement

Answer 20

 Distance between a GPS receiver and satellite is determined using
P-code or C/A-code.
 Known as “code-phase measurement” for positioning

Question 21

Pseudorange Measurement

Answer 21

 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.

Question 22

Code-phase Idea Behind

Answer 22

 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.

Question 23

Pseudorange Measurement Issue

Answer 23

Due to synchronization errors and biases, the measured distance is termed “pseudorange” instead of range.

Question 24

Carrier-Phase Measurement

Answer 24

• 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.

Question 24

Carrier phase Measurement

Answer 24

 This method counts carrier cycles between the satellite and
receiver.
 The challenge is the carrier frequency’s uniformity, making every
cycle appear identical.

Question 25

Carrier phase vs Code phase

Answer 25

 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.

Question 26

Cycle Slips

Answer 26

 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

Question 27

Linear Combination of GPS Observation

Answer 27

 GPS measurements have errors and biases from satellite,
receiver, atmospheric refraction and satellite geometric effects.
 Combining GPS observables can mitigate these.

Question 28

Linear Combinations of GPS Observation

Answer 28

 Between-receiver single difference
 Between-satellite single difference
 Double difference
 Triple difference