GNSS receiver technology and application review

time:2021-03-27
ar:haomingHu

# Special for examination review , For reference only

1.GPS GNSS A term is used to explain

（1）
GPS：Global Positioning System
GNSS：Global Navigation Satellite System
GNSS ： In the United States GPS、 Russia's Glonass、 Europe's Galileo、 China's [ Beidou navigation system ]

（2）GPS characteristic

• Realize global continuous navigation and positioning service all day
• High positioning accuracy
• The observation time is short
• There is no intervisibility between stations
• The instrument is easy to operate
• GPS More adaptable , The requirements for the environment are very small

2. Meridian instrument

english ：transit
Positioning principle ： Using the Doppler effect

3.GPS structure

• The space constellation part ：24 star 、6 Two orbital planes 、 dip angle 55°、 The satellite is about 20200Km
• Ground control system ： System master station （1 individual ）、 Ground monitoring stations （5 individual ）、 Ground stations （3 individual ） 【 The main control station 、 Monitoring stations 、 Injection stations 】
• User equipment part ：

4. Beidou development

（1） Beidou generation RDSS：RDSS(Radio Determination Satellite Service), Satellite radio measurement service . The user needs to respond to the service beam and send the observation data back to the central station （ Simply put, it's an application ） In order to position , therefore RDSS It's an active service

（2） Beidou II RNSS：RNSS(Radio Navigation Satellite System), Satellite radio navigation system .GPS The system is typical RNSS, It's passive . Beidou II mainly developed passive location system .

（3） Beidou ： Precise timing 、 Short message communication 、 High positioning accuracy 、 With compatibility

5. MATLAB Basic knowledge

（1） Basic commands
（2） Predefined variables
（3） Common functions
（4） Vector creation

​ Create array ( Row vector )a=[1 3 pi 3+5i] Vector elements with spaces or English comma “,” Separate .

​ b=1:10 b=1 2 3 4 5 6 7 8 9 10

​ x= linspace(1,2,5) x=1.0000 1.2500 1.5000 1.7500 2.0000

（5） Matrix creation function ones( Generate all 1 matrix )、zeros ( Generate all 0 matrix )
A： >>ones(3) Create whole 1 Of 3x3 Array
B：>>ones(3,4) Create whole 1 Of 3x4 Array
Seek the square array A The inverse matrix of can call functions inv(A).

（6） Select statement

if and switch Add... After the end end

（7） mapping
B： Three dimensional network diagram （mesh） And surface graphs （surf）

（8） function Output parameter use [ ] Input formal parameters with （ ）

6. Three laws of Kepler

（1） The first law ： All the planets orbit the sun in different elliptical orbits , The sun is at a focal point in these ellipses .（ The relative perigee position of a satellite in orbit , It's a function of time ）
（2） The second law ： The geocentric path of the satellite , The distance vector between the earth's center of mass and the satellite's center of mass , The area swept is equal in the same time .

（3） The ratio of the third power of the long half axis of the elliptical orbits of all planets to the second power of the revolution period is equal .
（6） Satellite orbit 6 Parameters

7.GPS Satellite signal

Satellite signal ：GPS Modulated waves sent by satellites to users for positioning and navigation

l Structural components ：3 Two signal components —— carrier 、 Pseudo code 、 Data code

l The clock is basically f frequency （f0）：10.23MHz（ Atomic clocks on the planet frequency ）

Two carriers ：
The reason why UHF band is chosen as carrier ：

• UHF signals travel in the form of direct waves , It helps to weaken the influence of ionospheric refraction on the signal

• Use carrier to modulate PN code signal , The carrier frequency is required to be much higher than the pseudo code bandwidth

• In order to make effective use of spectrum resources , And reduce interference between radios

• High Doppler shift can be obtained by satellite motion , It is helpful to measure the driving speed of users （fd、frequency_step）

L1 There are C/A code 、P code 、 Data code L2 There is only P Code and data code
L1 The signal on the carrier （ a key ）
C/A code ：1.023MHz navigation message：50Hz P code 10.23MHz

Signal expression
L2 The signal on the carrier

navigation message：50Hz P code 10.23MHz

Signal expression ：

8. Pseudo random noise

（1）m Sequence （ Deduce a cycle by yourself ）
（2）C/A code ： By two 10 Stage feedback shift register combination produces （T = 1ms） The digital rate is 1.023 Mb/s
​ Good autocorrelation ;
​ Excellent cross-correlation characteristics , It can be used in CDMA system ;
​ Short size , Easy to search and capture ;

Navigation message structure ： frame 、 Subframe 、bit The transmission rate of the message is 50b/s, So broadcasting a message requires 30s, The duration of a subframe message is 6s.

9. Satellite signal acquisition

（1） Receiver structure ( To understand the role of each part ）

Receiver processing flow ： The antenna —> RF front end —> Baseband signal processing —> Positioning and navigation solution

Capture ： For a specific satellite signal, it should be within the carrier frequency shift range and C/A Two dimensional search within the code phase range , To get the closest carrier frequency and C/A Code phase

track ： Accurately determine the carrier frequency and C/A Code phase , And track the changes

analysis ： Recover baseband signal from spread spectrum signal D(t) The process of

demodulation ： Extracting information from a modulated radio signal

（2） Serial capture ：

Serial capture principle （ Reference resources ）：

The receiver generates local PN code locally , And know the phase of the generated PN code . Input the system's GPS If signal and local NCO The carrier generators multiply each other to get two mutually orthogonal components , The locally generated PN code is sampled at a certain rate and multiplied by the two orthogonal components obtained in the previous step , They are divided into I Luhe Q Road signals .C/A Code modulation and P The code branch is on the orthogonal branch , The assumption is I Access Rd . Of course, due to the introduction of phase difference in the process of signal transmission , When demodulating I Branches are not necessarily modulated I Access Rd ,Q So is the bypass , The two do not necessarily correspond to each other , So in order to determine whether a received signal is detected , It is necessary to study two branch signals at the same time . The integration after correlation is to obtain the summation result of the length values of all relevant data , The square operation is to get the power of the signal . Finally, add the power of the two branches , Only when the frequency and phase of local PN code and local carrier are the same as if signal , The final power is very high , Otherwise, the result is close to zero . According to this conclusion, considering the interference of noise , In the actual design, a decision threshold should be set , When the sum of the power of the two signals is greater than the set threshold, it is determined that the acquisition is successful , Go into the tracking process , Otherwise, continue to scan other frequencies or phases .
The process


Formula derivation （ Focus on understanding ）
（3）Costas Carrier tracking method of environmental sanitation （ Focus on understanding the block diagram and formula derivation ）

deduction ：