Global Positioning System

NAVIGATION SATELLITES

NAVSTAR Global Positioning System (GPS)

Twenty four plus satellites (21 navigation, 3 spares) in four orbital planes at 11,000 mile orbits. Run by US Air Force, Falcon Air Force Base, Colorado, with 1 master and 4 monitor stations.

Defined Services:

Standard Positioning Service
Precise Positioning Service

Satellite Vehicle types:

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.

Differential GPS

Broadcast by fixed stations of GPS correction data.

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

Global Navigation Satellite System (GLONASS)

Twenty four plus satellites in three orbital planes in 11,870 mile orbits
Run by Russian military.
Similar accuracy to GPS.
Same PRN on different L-band frequencies.
Navigation messages use Soviet-standard UTC and earth reference coordinates.

NAVSTAR GLOBAL POSITIONING SYSTEM (GPS)

Coarse Acquisition (C/A)

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

Twenty MHz bandwidth.
P-code on L₁ and L₂.
Chip rate 10.23 MHz, repeats in 267 days. Sequence is reset each week.

Selective Availability (SA)

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

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

Affects the code and carrier phase measurements on L₁ and L₂.

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

L₁ at 1575.42 MHz for C/A, P-code, navigation
L₂ at 1227.60 MHz for P-code, navigation

Accuracy

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

Navigation message

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

Hypothetical Attacks

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.

Future GPS Enhancements

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