The document discusses GPS signal structure and navigation messages. It explains that GPS signals contain ranging codes and navigation data to allow receivers to calculate travel time from satellites and satellite coordinates. The main signals, L1 and L2, are modified by coarse acquisition and precise codes. Navigation messages are transmitted at 50 Hz and contain data like GPS week numbers, date, and time to help receivers determine location. Anti-spoofing techniques generate encrypted codes to protect military receivers from interference.

1. GNSS Surveying, GE 205
Kutubuddin ANSARI
kutubuddin.ansari@ikc.edu.tr
Lecture 5, March 28, 2015
GPS Signal Structure

2. GPS Signal Requirements
• Method (code) to identify each satellite
• The location of the satellite or some
information on how to determine it
• Information regarding the amount of time
elapsed since the signal left the satellite
• Details on the satellite clock status

3. GPS Signals
1 = Peak amplitude (Û),
2 = Peak-to-peak amplitude (2Û),
3 = RMS amplitude (Û/√2),
4 = Wave period (not an amplitude)
In this simple wave equation
U is the oscillating variable,
A is the peak amplitude of the wave,
t is time,
K and b are arbitrary constants
representing time and displacement
offsets respectively.

4. Frequency
f is the frequency in Hertz ("Hz"),
meaning the number of flashes
per second.
T is the period in seconds ("s"),
meaning the number of seconds
per flash.
T and f are reciprocals

5. Binary number
In mathematics and digital electronics, a binary
number is a number expressed in the binary numeral
system or base-2 numeral system which represents
numeric values using two different symbols:
typically 0 and 1.
No.
2 49 1
2 24 0
2 12 0
2 6 0
2 3 1
2 1 1
110001
No.
2 50 0
2 25 1
2 12 0
2 6 0
2 3 1
2 1 1
110010

6. Binary number
110001 = 25
x 1+ 24
x 1+ 23
x 0 + 22
x 0 + 21
x 0 + 20
x 1
110010 = 25
x 1+ 24
x 1+ 23
x 0 + 22
x 0 + 21
x 1 + 20
x 0
Addition number
49
+50
99
Addition number
110001
+110010
1100011
No.
2 99 1
2 49 1
2 24 0
2 12 0
2 6 0
2 3 1
2 1 1

10. Ranging Code
• The GPS satellites continuously transmit navigation
signals in two frequencies in L (L1 & L2) band.
• Each Satellite send down exactly the same two radio
frequencies
L1 = 1575.42 Mhz
L2 = 1227.60 Mhz
• These signals contain ranging codes and navigation
data to allow the users to compute the travelling time
from satellite to receiver and the satellite coordinates
at any epoch.

11. • The main signal L- Band carrier wave are modified by
two ranging code
The clear/access or coarse/ acquisition (C/A) code
The private or precise (P) code
• The L1 frequency is transmitted twice, once with the
C/A code and once with P-code
•The L2 frequency is only encoded with P code
Ranging Code

12. C/A Code
• One code assigned to each GPS satellite
• A 1023 chip (bit) long binary sequence generated at a rate of 1.023 MHz
(1.023 million chips per second)
•The entire C/A code repeats itself every millisecond
•30 meter accuracy easily obtained with the C/A code
•The starting point for each code generated by each satellites is unique so
no two satellite have the same start point (or epoch)
•Available to all GPS users (not classified by military)
• A unique C/A code is assigned to each satellite and this number is used
to identify each satellite

13. P-Code
•The resolution of P-Code is ten times of the resolution of the C/A
code
•The P-code is similar to the C/A-code, but instead of a sequence
of 1023 chips, the chips counts runs to the millions (2 x1014
). As a
result, the complete sequence for the P-code taken 267 days to
complete.
•Only one P-code, satellite use different weeks from same code
and P-code repeats each week.
•Typically receivers need acquire the C/A code before switching
the P-code

14. How do you create codes?
o You use binary addition rules.
o 0+0=0
o 1+0=1
o 0+1=1
o 1+1=0
o GPS uses “shift registers.”
o The more shift registers you have, the more
complicated you can make your code.

15. How do you create codes?
Register 1 Register 2 Register 3 Code
1 1 1 -
Start with all 1’s in your shift registers
Add Register 1 and Register 3
The answer 0 goes into Register 1 and everything shifts to the
right.
Here is an example with 3 shift registers
For this example, 1+1 =0 ==> 0

16. How do you create codes?
Register 1 Register 2 Register 3 Code
1 1 1 -
0 1 1 1
Resulting in

17. Next 0+1=1
Register1 Register2 Register3 Code
1 1 1 -
0 1 1 1
1 0 1 1
How do you create codes?

18. After 2N
-1 steps (N is the number of registers), the
code repeats
Register1 Register2 Register3 Code
1 1 1 -
0 1 1 1
1 0 1 1
0 1 0 1
0 0 1 0
1 0 0 1
1 1 0 0
1 1 1 0
For 3 shift registers, the code repeats after 7 steps.

19. Real GPS
• Uses 10 shift registers.
• They add different registers to produce the codes for
different satellites.
• Satellite 1 uses 2 and 6.
• Satellite 2 uses 3 and 7, and so on.
• A 10-shift register code repeats after 210
-1, or 1023.

20. For example, here are the first 1000 numbers of the code
for satellite 1
00001000101001110000111001001000100001000101011000111101110010101101100111101011
00101100101001100111111011001111001001100110100011100010010001011000101101110000
00110110010001000101101000101001000000011111000110001011111011111100110111001011
01111000111111010100101000010101001110000110100111011000111101111100001111111111
01001001001001100111010101111100001000101101001111110000100110111100111000110110
10110110101000010110100101000101001000111001110001010010111010111010101000001011
01110011011001101000000000001110111011000110110101010110110001110001100110011111
01011111001110101010000011111100100101000000111010001111011010010110110000010010
01001100001101100001111011101110001101110110100111001000110101010000110110100101
11001011111111101100011100000011011100011000000100000000100000110101000101011110
11000111011010001100101011111001111010000000110111100110011101011110000011110110
01000100101011100000000100001010101001111101100111011011111100101111000100110101
Real GPS

21. 10011110111010001001101111111110111100101101111011001101111101010100011111011000
11000100110011010000100000101111111000010000110101101011101011010011000001101000
01100010101011001000100100000110000011110000111010000011100100111011000000010110
01111000100101010111110101001111001011111011001010001011100001001110000111110111
01011101011011001111001001101011100100011011011111011001101011100001110101110001
10001111000001000111011011100010000011010011001001110000100010111000100100011011
11100011101010100110000000011001111001110101000010010001110010101010011100101101
11110011111110011010011101100111011001010010100110010101110111001110001101111001
10000010100011110011011110110011110100110111010011100110101010110100000101110001
11000111010110001111000100101001110101011000011000100011001010111001100001111100
00011111000100100011010001010001010010010001100001100100000110001100010100001101
10010110100110011000101101110011110010001010010100011110011101100001111101100101
This is the code for satellite 6
Real GPS

22. How do you compare codes?
100111101110100010011011111111101
000010001010011100001110010010001
Every time the numbers agree, add 1.
Every time the numbers disagree, subtract 1.

23. This example: 2 different satellites
100111101110100010011011111111111
000010001010011100001110010010001
14 agree
11disagree
Total score: 3
Not a perfect agreement
How do you compare codes?

24. 01100010101011001000100100000110000011110000
11000101010110010001001000001100000111100001
01100010101011001000100100000110000011110000
11000101010110010001001000001100000111100001
Agreement is perfect
But if you recognize they are shifted by 1:
This example: same satellite codes, but shifted
Not so good - score of -3.
How do you compare codes?

25. Why are the codes shifted?
Distance (in meters) = Time Difference (in seconds) * 3 x108
m/sec
What is a typical Time Difference?
GPS satellites are ~20,000,000 meters above the Earth.
20,000,000/300,000,000~ 70 milliseconds
The shift gives the GPS receiver the time difference.

26. The GPS satellites transmit a very weak signal, about the same as the earth’s
inherent background radio noise. Both the GPS signal and the background
noise are random so that when we divide the signal up into time slices or
chips, the number of signal matches (X’s) will equal the number of non-
matches (0’s). If we slide the receiver’s pseudo-random code back and forth
until it lines up with the satellites, there will be more matches and we will
be able to distinguish the signal from the earth’s background noise.
Distance Measuring

27. The GPS receiver and satellite generate the same pseudo-random
code at exactly the same time. When the code arrives from the
satellite, the time difference is compared to the same code
generated by the receiver. This difference is multiplied by the speed
of light to determine the distance to the satellite.
Distance Measuring

28. Distance Measuring
Transmission Time
Receiver
Time delay
Satellite

29. It’s useful to have a computer to do these
comparisons, especially since you have to test a
lot of different shifts. Then you can plot how
good the agreement is as a function of shift.
Codes shifting

31. The Navigation Message

33. The Navigation Message
FRAME
SUB-FRAME
WORD
BITS
BIT
CODE

38. The Navigation Message

39. Signal Structure

40. L1 Carrier Wave 1575.42MHz
C/A Code 1.023MHz
Navigation Message 50Hz
Precise Code 10.23 MHz
Signal Structure

41. L2 Carrier Wave 1227.6MHz
Navigation Message 50 Hz
Precise Code 10.23 MHz
Signal Structure

48. • The GPS Week Number count began at approximately midnight
on the evening of 05 January 1980 / morning of 06 January
1980.
• Since that time, the count has been incremented by 1 each week,
and broadcast as part of the GPS message. The GPS Week
Number field is modulo 1024 (~19.6 years).
• This means that at the completion of week 1023, the GPS week
number rolled over to 0 on midnight GPS Time of the evening of
21 August 1999 / morning of 22 August 1999.
• Note that this corresponded to 23:59:47 UTC on 21 August 1999.
GPS Week

49. GPS Week
Week beginning
at 0000 GPS Time on
GPS Week Number
broadcast by satellites
06 Jan 1980 0
13 Jan 1980 1
08 Aug 1999 1022
15 Aug 1999 1023
22 Aug 1999 0
29 Aug 1999 1

50. GPS Week + Day Of Week: 1627 1
Year + Day of Year: 2016 073
Year, Month, Day: 2016 03 14
GPS Week

51. Doy
Day of year is called doy
Example
01 January 001
31January 031
01 February 032
28February 059
31March 090
10 April 100
31 December 365

52. Anti-Spoofing (A-S)
P-Code
+W-Key
Y-Code
Plans to phase out continuously on since January 31, 1994
To protect military receivers from hackers a “fake” Code
generation technique is called Anti-Spoofing (A-S)
•
• P-Code modulation on both L1 and L2

53. GPS Signal Processing

54. Jamming devices are radio frequency transmitters that
intentionally block, jam, or interfere with lawful
communications, such as cell phone calls, text messages,
GPS systems, and Wi-Fi networks.
Jammers can be built by people with basic technical
competence from readily available commercial
components and publicly available information
Jamming

55. o Low-level jamming can block detection, or induce
position errors.
o A 10 Watt battery-powered jammer…
* can cover hundreds of square miles
* cost: ~$50 in parts
* weight: ~1 lb
* volume: < 50 in3
in volume
Jamming