Global Positioning System (GPS)

Question 1

How does GPS accurately determine a person’s location?

Answer 1

Multiple satellite take measurements and the intersection points are eliminated to get one possible location.

Question 2

How to get satellite location?

Answer 2

Each GPS transmit its location and time stamp to receivers on earth. The message contain the time the message was sent, the satellite’s precise orbit, and the general system health and rough orbits of all satellites

Question 3

What are the two unknown to get user location?

Answer 3

• Location of satellite

- Travel time

Question 4

What does the ground control system continuously do?

Answer 4

• Track GPS satellites
• Estimate clock and orbit
• Keep GPS time
• Upload data that describes clock & orbit for each

Question 5

What does the ground control system infrequently do?

Answer 5

• Command small maneuvers to maintain orbits
• Command small clock corrections
• Command major relocations to compensate for any sv failures

Question 6

Why use L Band?

Answer 6

L band waves are able to penetrate through various types of weather conditions and vegetation. The range is 1-2GHz which does not require beam antennae and less ionospheric delays.

Question 7

What are the components to calculate travel time?

Answer 7

1. Known transmission time
2. Known satellite location
3. Speed of radio wave
4. Time of arrival

Question 8

What is the velocity estimation with GPS Receiver?

Answer 8

The user’s velocity will change the apparent frequency by the extra number of wavelengths they are going through each second. If the user is going towards the satellite, the received frequency would increase by the user’s speed in wavelengths/seconds.

Question 9

What is Enhanced Cell-ID (E-CID)?

Answer 9

There are directional antennas at the base station. Accuracy of 100-200m

Question 10

What is uplink time difference of arrival (U-TDOA)?

Answer 10

Uses location measurements units installed in base stations to calculate location using triangulation.
- Works with any wireless phone and has accuracy of 20-40m

Question 11

What are the advantages of A-GPS Based Positioning?

Answer 11

• Reduces the search space
• Better clock synchronization
• Location is known almost instantly
• Decent estimates even indoors

Question 12

What are the assistance types of A-GPS based positioning?

Answer 12

Mobile Station Based (MSB) - Mobile station helps in acquiring GPS satellite information quickly.
Mobile Station Assisted (MSA) - Mobile server helps calculation of position by using GPS receiver signal information.