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
Intersection of two spheres is a circle
Trilateration
Third measurement narrows to just two points
Intersection of three spheres is only two points.
Trilateration
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.
你的位置是: 37o 23.323’ N k?
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
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
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
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.
measure time difference between same part of code from satellite from ground receiver
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
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)
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?
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
te li l 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
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