卫星定位理论与方法-第13次课-伪随机码测距原理

8 secs
With fast clock
5 secs wrong time
7 secs wrong time
Bad position because clock is off by one second
Third measurement (fast clocks)
Won’t go through the other two
4 5
Once GPS knows distance, it needs to know satellite's position Then correct for ionospheric and trospheric delays.
1
Trilateration from satellites is basis of system.
One of the Clever Ideas of GPS:
Use same code at receiver and satellite Synchronize satellites and receivers so they're generating same code at same time Then we look at the incoming code from the satellite and see how long ago our receiver generated the same code
1
Trilateration from satellites is basis of system.
4
Knowing where the satellites are
After all, they're 12,600 miles up High orbit Very stable orbits No atmospheric drag Survivability Earth coverage Monitored by Defense Department fly over US possession at least once a day DoD transmits corrections back to satellite Corrections transmitted from satellites to us
How Does GPS Work?
5 Steps to GPS
2
GPS measures distance from the satellites using speed of light.
3
To measure the distance GPS needs good clocks and a fourth SV
2 Satellite Ranging
Measuring the distance to a satellite Done by measuring travel time of radio signals
Speed-of-Light Measurement
Measure how long it takes the GPS signal to get to us Multiply that time by 186,000 miles/sec
1 Trilateration from satellites
By measuring distance from several satellites you can calculate your position
Trilateration
One measurement narrows down our position to the surface of a sphere
11,000 miles
We're somewhere on the surface of this sphere.
Trilateration
Second measurement narrows it down to intersection of two spheres
11,000 Miles
12,000 Miles
Accurate but expensive
Ground receivers just need consistent clocks
The secret is in an extra satellite measurement that adjusts the receiver clock
The ideal situation (accurate clocks)
你的位置是: 37o 23.323’ N 122o 02.162’ W
How Does GPS Work?
5 Steps to GPS
2
GPS measures distance from the satellites using speed of light.
3
To measure the distance GPS needs good clocks and a fourth SV
Pj1 ( X X j1 )2 (Y Y j1 )2 ( Z Z j1 )2 Pj 2 ( X X j 2 )2 (Y Y j 2 )2 ( Z Z j 2 )2 Pj 3 ( X X j 3 )2 (Y Y j 3 )2 ( Z Z j 3 )2
How Does GPS Work?
5 Steps to GPS
2
GPS measures distance from the satellites using speed of light.
3
To measure the distance GPS needs good clocks and a fourth SV
3
Accurate clocks necessary to measure travel time
Making sure both receiver and satellite are synchronized Whole system depends on very accurate clocks Satellites have atomic clocks
4 5
Once GPS knows distance, it needs to know satellite's position Then correct for ionospheric and trospheric delays.
1
Trilateration from satellites is basis of system.
5 Atmospheric Corrections
Too bad it's not operating in a vacuum Ionosphere is a band of charged particles Troposphere is our weather

由于地球本身就是一个球体，可以不需要观测第四颗 卫星，观测三颗卫星就可确定地面点位置（三球定位）
Intersection of two spheres is a circle
Trilateration
Third measurement narrows to just two points
Intersection of three spheres is only two points.
Trilateration
Leabharlann Baidu4 5
Once GPS knows distance, it needs to know satellite's position Then correct for ionospheric and trospheric delays.
1
Trilateration from satellites is basis of system.
5 sec (wrong)
7 sec (wrong)
Now third measurement won't go thru the other two.
9 sec (wrong)
How Does GPS Work?
5 Steps to GPS
2
GPS measures distance from the satellites using speed of light.
Trilateration
In practice 3 measurements are enough We can discard one point One point will be a ridiculous answer
We still need the 4th measurement because there are four dimensions to solve for (X,Y,Z and Time)
4 5
Once GPS knows distance, it needs to know satellite's position Then correct for ionospheric and trospheric delays.
1
Trilateration from satellites is basis of system.
In 2 dimensions for sake of drawing
This is where we really are
4 secs 6 secs
Adding a third measurement (accurate clocks)
4 secs
6 secs
Third measurement would go thru our position if correct
3
To measure the distance GPS needs good clocks and a fourth SV
4 5
Once GPS knows distance, it needs to know satellite's position Then correct for ionospheric and trospheric delays.
第12次课 伪随机码测距原理 测量与导航工程系 导航与定位教研室
陈明剑
GPS Positioning
e it ll te Sa
Sat ell ite
e it ll te Sa
Sa te ll it e
1. Known position of satellites 2. Range measurements from satellites Calculate position&time
Fourth measurement will decide between two points.
Fourth measurement will only go through one of the two points.
4 Ranges to resolve for Latitude, Longitude, Height & Time
Time (sec) x 186,000 = miles
If you've got good clocks, all you need to know is exactly when signal left satellite
How do we know when the signal left the satellite?
Status message
How Does GPS Work?
5 Steps to GPS
2
GPS measures distance from the satellites using speed of light.
3
To measure the distance GPS needs good clocks and a fourth SV
measure time difference between same part of code from satellite from ground receiver
Why a code?
Code lets many satellites operate on same frequency Code gives us a way to increase signal-tonoise ratio