• Standard Positioning Service
  • Two MHz bandwith, 50 bps on frequency L₁.
  • Chip rate 1.023 MHz, sequence length is one millisecond.
  • Full 3 dimensional solution requires four satellites.
  • Elevation mask angle 15 degrees.
  • Commercial receivers limited to 1000 kts, 60,000 ft.
• Precise Positioning Service
  • Twenty MHz bandwidth.
  • P-code on L₁ and L₂.
  • Chip rate 10.23 MHz, repeats in 267 days. Sequence is reset each week.

• Block I: protoype (11)
• Block II: operational constellation (24)
• Block IIR: replenishment
• Block IIF: follow-on

Block IIR:

• UHF links between satellites every 15 minutes; autonomous navigation and ranging keeps constellation correct up to 180 days without ground updates.
• On-orbit subsystem reprogramming.

Block IIF:

• 15 year design life
• Autonomous operations up to 60 days
• 4710 pounds, body is 74 inches x 66 inches x 56 inches
• 1510 watts EOL
• Uplink at 1783.74 MHz, 1 or 2 kbps
• Downlink at 2227.5 MHz, up to 32 kbps
• Ground reprogrammable telemetry format

Commercial sales estimated at more than $40 billion by 2004.

Correction data broadcast by fixed stations.

• FAA - Wide Area Augmentation System (WAAS)
• USCG - Coastal Navigation

Your government is lying to you (and everyone else).

• Dither - Fluctuate clock; rapid rate.
• Epsilon - Incorrect ephemeris; slow rate.

Anti-Spoof (A-S)

• Encryption of P-codes on L₁ and L₂; encrypted P-code is called Y-code.
• Encryption and overlay on Y-code on L₂, replaces P-code.
• Resists locking to mimicked GPS signals.
• Degrades L₂ phase observable.
• Came on-line in early 1994.

Anti-Jam (A-J)

• Nulling antennas (element phase shifting; amplitude weighing)
• Aiding (Intertial Nav)
• Signal Processing (full receiver state; all signal states)

Defined frequencies for GPS:

• L₁ at 1575.42 MHz for C/A, P-code, navigation
• L₂ at 1227.60 MHz for P-code, navigation
• L₃ at 1381.05 MHz
• L₄ at 1379.913 MHz for nuclear burst detection reporting
• L₅ at 1176.45 MHz for nuclear burst detection reporting

Mode	Circular Error Probable (50%)	2 distance root mean square (95%)	Time
PPS	8m	21m	100 nanosecond
SA = 0 (off)	13m	30m
Current SA	42m	100m	167 nanosecond

A GPS navigation message is transmitted on L₁ at 50 bits per second. It contains:

• System time
• Clock correction parameters
• ionospheric delay model parameters
• vehicle ephemeris
• vehicle health

Here are a few hypothetical attacks on the GPS service.

GPS spoof

Pseudolite. Overcome Receiver Autonomous Integrity Monitor (RAIM), which needs signals from at least six satellites.

GPS jamming

L band is spread spectrum at 1575 MHz with known PN codes.

DGPS spoof

Incorrect "corrections".

DGPS jamming

Block VHF updates at opportune moments.

Some future GPS Enhancements:

• SA turned off?
• Additional frequency (L₄) for isotropic measurement
• Localized GPS denial
• Rockwell, Hughes, Lockheed/Martin NAVWAR ($3 million-plus)