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Chapter 2 – GPS Signals


									         Chapter 2 – GPS Signals

GPS Signal Structure

How Measurements can be made

Receiver Capabilities and Limitations

Measurement Errors and Biases
           2.1a GPS Signal Structure
Traditional GPS Satellites
– Blocks II, IIA, and IIR

Modernized GPS Satellites
– Blocks IIR-M & Subsequent SV

Microwave Radio Signal
–   2 Carrier Frequencies (or sine waves)
–   Modulated by two digital (ranging) codes
–   Navigation Message
–   Additional digital codes for Modernized SV
                         2.1b GPS Signal Structure
              (Traditional GPS Satellites & Ionospheric Delay)
L1 Carrier Phase (Civilian & Military) all SV
– 19cm wave length @ SOL generated at 1575.42 MHz
– C/A (ranging) Code Modulation & P-code Modulation (binary bi-phase)
    • Short stream of 1,023 binary digits – Repeating every ms – Unique to each SV
    • Chipping rate of 1.023Mbps (ie 1 micro-second at 300 meters)
    • Relatively less precise than P-code yet less complex and available to all users

L2 Carrier Phase (Military) all SV
– 24.4cm wave length @ SOL generated at 1227.60 MHz
– P-Code (ranging) Modulation (unique PRN contains C/A and P-code)
    •   Very Long sequence of binary digits – Repeats 266 days in 38 1 week segments
    •   10x faster than C/A code – Chipping rate of 10.23 Mbps – Initialize Sat/Sun
    •   Anti-Spoofing January 31, 1994 p-code encrypted with w-code = y-code
    •   More precise than C/A-code yet more complex and only available to Military
               2.1c GPS Signal Structure
Modernized GPS Satellites (2 additional codes)
– L2 Civil-Moderate (L2 CM) & L2 Civil-Long (L2 CL)
   •   Both codes transmit at a Chipping rate of 511.5Kbps
   •   L2CM has a Stream of 10,230 chips - Length of 20ms
   •   L2CL has a Stream of 767,250 chips - Length of 1.5 s
   •   L2CL is 75% longer than the L2CM code

   • Only 3 Block IIR-M SV – so advantage has yet to be seen
        – Increased Civilian accuracy
                 2.1d GPS Signal Structure
GPS Navigation Message Signal
–   Modulated on L1 - L2 Carrier & L2CM Code only (ground control)
–   50 bps binary data stream added to P(Y) and C/A codes,
–   Then modulated on the L1 and L2 carriers
–   Complex 25 frames – 1,500 bits each – (37,500 bits) 5 sub (300 bits each)
–   750 seconds or 12.5 minutes to be transmitted

Navigation Message contains:
–   Coordinates of the GPS Satellites as a function of time
–   Satellite clock correction model parameters
–   Satellite health status
–   Satellite almanac (not complete until the “HOW” is received)
–   Atmospheric (ionospheric) correction model parameters
                 2.2a GPS Modernization
Next 20 yrs a combined Block IIR-M and GPS III SV’s
Modernizations Aims
–   Provide signal redundancy
–   Improve Positioning Accuracy
–   Signal Availability
–   Systems Integrity
Five-Phase Modernization (improve 80m to better the 2m)
–   1) Improvement in Operational software (orbital tracking)
–   2) Better orbital modeling
–   3) (Sept 2005) Addition of 6 new Monitor Stations ( 5 new in future)
–   4) Test the backup facility of the MCS
–   5) follow-up on the modeling upgrade
                  2.2b GPS Modernization
Addition of new Civilian codes            (1st Sept 25, 2005)
– L2-CM & L2-CL on L2 frequency add to Block IIR-M SV
   •   Improved reference resistance & Enhanced Tracking & full L2 access
   •   Better Positioning performance indoors and in Forest areas
   •   Civilians can correct for Ionospheric delay errors
   •   Better SV tracking & enough modern SV’s expect 2m or better data

Addition of new Military codes
– 2 new M-codes on both L1 & L2 add to Block IIR-M SV
                    2.2c GPS Modernization
Safety-of-Life Civil Aviation Code
– L2 frequency ground radar interference
– L5 is 3rd Civil code at 1,176.45MHz on L1 and L2
    • More robust with higher power level
    • 2 PRN ranging codes (I5-code & Q5 code) chipping at 10.23 Mbps – length 1ms
    • Navigation Message data transmit at 100 symbols per second on I5 code only
– Added to future Block IIF Satellites

Block III Satellites – takes us to the year 2030
– Improved autonomous GPS positioning accuracy
– RTK user will be able to resolve ambiguity instantaneously
– Centimeter-Level accuracy in real time (possibilities)
            2.3a GPS Receiver Types
1980 over $200,000  2006 $100.00 to $15,000
First analog generation  new digital generation
– Bulky and heavy  smaller and lighter
– New programmable software receivers not hardware
– More flexible and cost effective
Single Frequency Receivers
– Access the L1 Frequency only
Dual Frequency Receivers
– Access both the L1 and L2 Frequency
Number of Channels
– Multi channel, currently most 9-12 channels
Special Features
– Raw data input, Cost, Ease of use, Power consumption, size & weight
                      2.3b GPS Receiver Types
Single Frequency Code Receivers
 –   Measures the pseudorange with C/A-code only
 –   No other measurements are available
 –   Least expensive & Least accurate
 –   Mostly recreation type GPS
Single Frequency Carrier-smoothed Code Receivers
 – Measures the pseudorange with C/A-code
 – Can use higher-resolution Carrier frequency internally (improve code)
Single Frequency code and Carrier Receivers
 – Output raw C/A-code pseudoranges
 – L1 carrier-phase measurements & navigation message
 – In addition to all above capabilities
Dual Frequency Receivers
 – Most sophisticated and most expensive
 – Can output all GPS signal components (L1 and L2 Carriers, C/A code, P-code)
 – Anti-spoofing has blocked P-code and L2 carrier output
      • Work around techniques (Z-Tracking & Cross-correlation
      • Work around not needed with Block IIR-M
                     2.4a Time Systems
GPS Signal is Controlled by Accurate Timing Devices
Receiver & Satellite Clocks synchronization

Worldwide Time Systems
– Coordinate Universal Time (UTC) on Atomic Time Scale
   • Based on International Atomic Time (TAI) [Laboratories Worldwide]
– For Surveying & Navigation -- UTC with Leap Second – UT1
   • Compensate for Earth rotation – not Atomic time – within 0.9s of UTC
   • June 30 or December 31 ( As of May 2006 TAI is ahead 33s of UTC)
                    2.4b Time Systems
GPS Time
Used for Referencing, or tagging, the GPS Signal
Based on Time Clocks at the Monitor Stations &
Based on Time Clocks onboard GPS Satellites
No Leap Seconds introduced on GPS Time
GPS Time is continuous time
GPS Time was set equal to UTC January 6, 1980
By January 1, 2006 GPS time was ahead 14s
GPS Time difference is given in the Navigation message
Both Satellite & Receiver Clocks are offset from GPS time [Errors]
GPS week number (number of weeks since Jan. 6, 1980)
            2.5 Pseudorange Measurements
Pseudorange: (biases & synchronization errors contaminate signal - thus pseudorange)
 – “Measure of the range, or distance, between the GPS receiver and the
   GPS satellite” (antenna center –to- antenna center)
Measurements range to determine position
 – P(Y) code or C/A code can be used to measure pseudorange
Synchronization (an assumption of signal perfection)
 – Transmitted code vs receiver replica computes travel time
 – Travel time X the speed of light = range between SV & Rec
      • (299,729,458 meters/second) or (186,282.397 miles/second) in a vacuum

Civilian code
 – Originally civil code was 300m 10x lower than Pcode
 – Receiver technology has improved accuracy almost the same
        2.6 Carrier Phase Measurements
Ranging with Carrier Phase instead of Pseudoranging
– Range = sum of the total number of full cycles plus fractional cycles at
  the receiver and the satellite, multiplied by the carrier wavelength (L1 =
– Carrier phase measurements are more accurate than code
Carrier cycles & Carrier Wave length
The sinusoidal concern (all cycles look the same)
– Initial integer cycle ambiguity (ambiguity bias)
Tracking Phase changes after power up
– If there is NO Cycle slip Error or signal loss
Differential processing for high accuracy (real or post)
– PPP – Precise Point Positioning for stand-alone receivers
            2.7 Doppler Measurements
Change in Signal Pitch, Doppler Effect or Frequency shift
– Difference in frequency of the acoustic signal received (GPS
  receiver) compared to when it left the source (SV)

Result of Relative Motion
– Calculate satellite velocity and receiver velocity
– Might not be accurate enough for some applications
                          2.8 Cycle Slips
A Jump, in the GPS Carrier Phase Measurements
Cycle slips due to Temporary signal loss
–   Obstruction of GPS Satellite signal
–   Buildings, bridges, trees and other objects
–   Noisy or weak signal
–   Radio interference or severe ionospheric disturbances
–   High receiver dynamics or Receiver malfunction
Size of the Cycle slip
– Brief, several seconds or more
– One cycle or millions of cycles
Avoiding large errors by detecting Cycle slips
– Triple difference observable
– Visual inspection
– Zero baseline test for receiver malfunctions (two receivers)
                    2.9a Linear Combinations of GPS
GPS Signals - Corrupted by many Errors & Biases
– Errors that are difficult to model fully decrease accuracy
– Receiver within close proximity show same errors
Groups of Errors and Biases removed by linear combination
– 1) Satellite Related
   • Measurement difference between 2 GPS Receivers (DGPS)
   • Subtracting observable difference is known as “Double difference”
   • Triple difference – differencing 2 double difference, will remove cycle
                             2.9a Linear Combinations of GPS
Groups of Errors and Biases removed by linear combination
– 2) Receiver Related
     • Measurement difference between 2 signals of the same receiver
     • Subtracting observable difference is known as “Double difference”

– 3) Atmospheric errors and biases
     • Measurement difference between 2 GPS Receivers

– 4) Dual-frequency data (ionospheric delay is inversely proportional to the square of the carrier phase)
     • Ionosphere-free linear combination (Use of L1 & L2 carrier phase)
     • Wide-lane observable

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