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GPS signals


Global Positioning System (GPS) satellites broadcast microwave signals to enable GPS receivers on or near the Earth's surface to determine location, velocity, and time. The GPS system itself is operated by the U.S. Department of Defense (DoD) for use by both the military and the general public.

GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages. The navigation messages include ephemeris data, used to calculate the position of each satellite in orbit, and information about the time and status of the entire satellite constellation, called the almanac.

There are 4 signals available for civilian use. In order of date of introduction, these are: L1 C/A, L2C, L5 and L1C. L1 C/A is also called the legacy signal and is broadcast by all satellites. The other signals are called modernized signals and not broadcast by all satellites. In addition, there are restricted signals, also broadcast to the general public, but whose encoding is secret and are intended to be used only by authorized parties. Nonetheless, some limited use of restricted signals can be made by civilians without access to the secret encoding details; this is called codeless and semi-codeless access, and is officially supported.

The interface to the User Segment (GPS receivers) is described in the Interface Control Documents (ICD). The format of civilian signals is described in the Interface Specification (IS) which is a subset of the ICD.

The GPS satellites (called space vehicles in the GPS interface specification documents) transmit simultaneously several ranging codes and navigation data using binary phase-shift keying (BPSK). Only a limited number of central frequencies are used; satellites using the same frequency are distinguished by using different ranging codes; in other words, GPS uses code division multiple access. The ranging codes are also called chipping codes (in reference to CDMA/DSSS), pseudorandom noise and pseudorandom binary sequences (in reference to the fact that it is predictable, but statistically it resembles noise).


GPS message format
Subframe no. Word no. Description
1 1–2 Telemetry and handover words
(TLM and HOW)
3–10 Satellite clock,
GPS time relationship
2–3 1–2 Telemetry and handover words
(TLM and HOW)
3–10 Ephemeris
(precise satellite orbit)
4–5 1–2 Telemetry and handover words
(TLM and HOW)
3–10 Almanac component
(satellite network synopsis,
error correction)
LNAV-L frames 4 and 5
Subframe no. Page no. Description
4 1–2, 6, 11–12,
16, 19–24
Reserved
2–5, 7–10 Almanac data for SV 25–32
13 Navigation message
correction table (NMCT)
14–15 Reserved for system use
17 Special messages
18 Ionospheric correction data
and UTC
25 A-S flags for SV 1–32,
health info. for SV 25–32
5 1–24 Almanac data for SV 1–24
25 Health info. for SV 1–24
almanac reference time
LNAV-U frames 4 and 5
Subframe no. Page no. Description
4 1, 6, 10–12,
16, 19–24
Reserved
2–5, 7–9 Almanac data for SV 89–95
13 Navigation message
correction table (NMCT)
14–15 Reserved for system use
17 Special messages
18 Ionospheric correction data
and UTC
25 A-S flags for PRN numbers 33–63,
health info. for SV 89–95
5 1–24 Almanac data for SV 65–88
25 Health info. for SV 65–88
almanac reference time
Message structure
(common fields)
Bits Information
1–8 Preamble
9–14 PRN of transmitting satellite
15–20 Message type ID
21–37 Truncated TOW count
38 Alert flag
277–300 Cyclic redundancy check
Message types
Type ID Description
10–11 Ephemeris and health
12, 31, 37 Almanac parameters
13–14, 34 Differential correction
15, 36 Text messages
30 Ionospheric and group delay correction
32 Earth orientation parameters
33 UTC parameters
35 GPS/GNSS time offset
Subframes
Subframe Bit count Description
Raw Encoded
1 9 52 Time of interval (TOI)
2 576 1,200 Time correction and
ephemeris data
3 250 548 Variable data
Subframe 3 pages
Page no. Description
1 UTC & IONO
2 GGTO & EOP
3 Reduced almanac
4 Midi almanac
5 Differential correction
6 Text
GPS Frequencies
Band Frequency
(MHz)
Phase Original usage Modernized usage
L1 1575.42
(10.23×154)
In-phase (I) Encrypted precision P(Y) code
Quadrature-
phase (Q)
Coarse/acquisition (C/A) code C/A, L1 Civilian (L1C), and
Military (M) code
L2 1227.60
(10.23×120)
In-phase (I) Encrypted precision P(Y) code
Quadrature-
phase (Q)
Unmodulated carrier L2 Civilian (L2C) code and
Military (M) code
L3 1381.05
(10.23×135)
Used by Nuclear Detonation (NUDET)
Detection System Payload (NDS);
signals nuclear detonations/
high-energy infrared events.
Used to enforce nuclear test
ban treaties.
L4 1379.913
(10.23×1214/9)
(No transmission) Being studied for additional
ionospheric correction
L5 1176.45
(10.23×115)
In-phase (I) (No transmission) Safety-of-Life (SoL) Data signal
Quadrature-
phase (Q)
Safety-of-Life (SoL) Pilot signal
Carrier freq.
deviation
Code phase (in chips)
0.0 0.5 (more phases) 1,022.0 1,022.5
−4,750 Hz ...
−4,250 Hz ...
(more
frequencies)
... ... ... ... ...
4,250 Hz ...
4,750 Hz ...

...
Wikipedia

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