Gps-signal

GNSS Surveying, GE 205

Kutubuddin [email protected]

Lecture 5, March 28, 2015

GPS Signal Structure

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

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.

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

Binary numberIn 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 12 24 02 12 02 6 02 3 12 1 1

110001

No.2 50 02 25 12 12 02 6 02 3 12 1 1

110010

Binary number

110001 = 25 x 1+ 24 x 1+ 23 x 0 + 22 x 0 + 21 x 0 + 20 x 1110010 = 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 12 49 12 24 02 12 02 6 02 3 12 1 1

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 MhzL2 = 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.

• 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

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

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

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

complicated you can make your code.

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


Register 1

Register 2

Register 3

Code

1 1 1 -0 1 1 1

Resulting in

Next 0+1=1

Register1 Register2 Register3 Code1 1 1 -0 1 1 11 0 1 1


After 2N -1 steps (N is the number of registers), the code repeats

Register1

Register2

Register3

Code

1 1 1 -0 1 1 11 0 1 10 1 0 10 0 1 01 0 0 11 1 0 01 1 1 0

For 3 shift registers, the code repeats after 7 steps.

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.

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

10011110111010001001101111111110111100101101111011001101111101010100011111011000 11000100110011010000100000101111111000010000110101101011101011010011000001101000 01100010101011001000100100000110000011110000111010000011100100111011000000010110 01111000100101010111110101001111001011111011001010001011100001001110000111110111 01011101011011001111001001101011100100011011011111011001101011100001110101110001 10001111000001000111011011100010000011010011001001110000100010111000100100011011 11100011101010100110000000011001111001110101000010010001110010101010011100101101 11110011111110011010011101100111011001010010100110010101110111001110001101111001 10000010100011110011011110110011110100110111010011100110101010110100000101110001 11000111010110001111000100101001110101011000011000100011001010111001100001111100 00011111000100100011010001010001010010010001100001100100000110001100010100001101 10010110100110011000101101110011110010001010010100011110011101100001111101100101

This is the code for satellite 6

Real GPS

How do you compare codes?

100111101110100010011011111111101

000010001010011100001110010010001

Every time the numbers agree, add 1.

Every time the numbers disagree, subtract 1.

This example: 2 different satellites

100111101110100010011011111111111000010001010011100001110010010001

14 agree11disagree

Total score: 3

Not a perfect agreement


0110001010101100100010010000011000001111000011000101010110010001001000001100000111100001

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.


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.

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

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

Distance MeasuringTransmission Time

Receiver

Time delay

Satellite

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

The Navigation Message


FRAMESUB-FRAME

WORD

BITS

BIT

CODE


Signal Structure

L1 Carrier Wave 1575.42MHz

C/A Code 1.023MHz

Navigation Message 50Hz

Precise Code 10.23 MHz

Signal Structure

L2 Carrier Wave 1227.6MHz

Navigation Message 50 Hz

Precise Code 10.23 MHz

Signal Structure

• 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

GPS Week

Week beginning at 0000 GPS Time on

GPS Week Numberbroadcast by satellites

06 Jan 1980 013 Jan 1980 108 Aug 1999 102215 Aug 1999 102322 Aug 1999 029 Aug 1999 1

GPS Week + Day Of Week: 1627 1

Year + Day of Year: 2016 073

Year, Month, Day: 2016 03 14

GPS Week

Doy

Day of year is called doy Example 01 January 00131January 03101 February 03228February 059 31March 090 10 April 10031 December 365

Anti-Spoofing (A-S)

P-Code+W-Key

Y-CodePlans 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

GPS Signal Processing

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

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

Education

Gps-signal