3. GPS

Question 1

Dilution of Precision. Are Satellite more precise when closer together or further apart?

Answer 1

Better GPS accuracy is obtained by points that are well separated. When different satellites have different precision values, the overall precision is diluted. The time difference between the signals is greater when they are further apart because they are at greater angles relative to eachother.

Precision is important in position, time, horizontal, vertical.

Question 2

Coarse Acquisition Code (C/A Code)

Answer 2

Transmitted on L1
Unique to each satellite

Question 3

Precision Code (P Code)

Answer 3

Transmitted on L1 and L2
Unique to each satellite
Authorized by US DoD

Question 4

What are the 2 GPS satellite codes?

Answer 4

C/A Code
P Code

Question 5

Selective Availability

Answer 5

Decrages positional accuracy up to 100m for C/A code. Can be adjusted at any time by the US DoD.

Question 6

Ionospheric Porpagation Errors

Answer 6

Ionosphere disperses the signal transmission causing errors of 5-15m during the day and 1m at night.

Question 7

Tropospheric Propagation Errors

Answer 7

Lower portion of the earth’s atmostphere. Not frequency dependant. Error of 2.5 to 15m.

We can reduce this down to 0.5m based on standard atmospheric models to correct the error.

Question 8

Multi Path Errors

Answer 8

Reflection of satellite signals causing difference path lengths.

Errors of 10m or more.

Dominant source of error for DGPS.

Question 9

Ephemeris Data Errors

Answer 9

Defines the orbit of the satellite.

There are errors in the predicted orbit due to the gravitational sources, solar wind, atmospheric friction.

Causes error around 2m.

Question 10

Onboard Clock Error

Answer 10

Error in the onboard clock. 1m error.

Question 11

Receiver Clock Error

Answer 11

Receiver’s clock is not accurate so it receives a time correction.

Question 12

What is Differential GPS (DGPS)

Answer 12

Enhances GPS through the use of differential corrections to the basic satellite measurements. The corrections are sent via radio tower.

Question 13

Wide Area Augmentation System (WAAS)

Answer 13

Similar to DGPS except corrections are made by ground-based reference stations and sent via satellite instead of radio.

Question 14

Local Area Augmentation System (LAAS)

Answer 14

Local reference receivers located around the airport sent to a central location which formulates a correction message sent via VHF.

Question 15

2 types of users for GPS

Answer 15

Authorized - by US DoD - use P Code

Non-Authroized - anyone can use C/A Code

Question 16

How is Ionospheric delay accounted for?

Answer 16

Ionospheric interference is affected by the signal frequency. L1 and L2 are at different frequencies so by receiving both, the receiver can perform a dual frequency comparison to compensate for ionospheric delay.

Question 17

Trimble GPS

Answer 17

Is our military GPS system. Has L1/L2 antenna and uses crypto.

Question 18

ECPINS stands for?

Answer 18

Electronic Chart Precise Integrated Navigation System

Question 19

Spread Spectrum Transmission

Answer 19

Different information is sent over different frequencies bands or time domains in the same signal.

Question 20

Code Division Multiple Access

Answer 20

Signals can be encoded and sent in the same frequency band, then decoded to separate them.

Question 21

What are 5 types of error for GPS systems?

Answer 21

Tropospheric error
Mltipath errors
Ephemeris Data Errors
Clock Errors
Ionospheric

Question 22

Ephemeris Data Errors

Answer 22

The data being transmitted by the satellite is incorrect.