Basic Principles of GPS Operation

Question 1

Timeline of GPS Generation

Answer 1

• First generation: Block I satellites
• Second generation: Block II/IIA satellites
  *Third new generation: Block IIR/IIR-M satellites
  *Fourth new generation: Block IIF satellites
  *Fifth New Generation: Block III Satellites

Question 2

Which of the satellites under the GPS modernization is NOT available as of today?

Answer 2

Block IIA

Question 3

Which satellites constitutes the majority of the current GPS constellation?

Answer 3

Block IIF

Question 4

Current GPS Satellite Constellation

Answer 4

Block I
Block II
Block IIA
Block IIR
Block IIR-M
Block IIF
Block III

Question 5

Current GPS Satellite Constellation

Answer 5

Satellite plane inclination: Inclined at 55 degrees relative to the equator
Ground track for satellites having 28^ (blue) and 83^ (yellow) inclination

Question 6

Current GPS Satellite Constellation

Answer 6

Wide Area Augmentation System (WAAS)
- Developed by the Federal Aviation Administration (FAA)
- Air Navigation aid
- Provides GPS signal correction
- Enhances position
accuracy to less than 3
meters.

Question 7

The number of GPS Satellite

Answer 7

Distance calculation: Through microwave radio signal
Satellite requirements
 Theoretical need: 3 satellites
 Practical requirement: 4 satellites
 A fourth satellite: Accounts for receiver’s clock offset and
determines precise location

Question 8

Triangulation or Trilateration

Answer 8

Location Methods
* Triangulation: Works with angles
* Trilateration: Works with distances

GPS Receiver
* Measures distance via radio signal travel time
* Method used: Trilateration

Requirement for measurements
* GPS needs highly accurate timing

Mathematical need
* Ranges from our four satellites to determine exact position

Question 9

Distance between Satellite and GPS receiver

Answer 9

Distance = Speed of light x Time of Travel
- The speed of light is approximately 299,792.458 km/second (round 300,000)
- Time of travel = Time the signal was received - Time the signal sent

Question: If the signal took 0.5 seconds
to travel from the satellite to the GPS
receiver, how far is the receiver from the
satellite?

Question 10

Geometric Pinpoint 1

Answer 10

Distance measurement to a first satellite:

• Measured distance from a satellite: 11,000 miles.

Implication of Measurement
- Our potential location is pinpointed to:
* The surface of a sphere centered on the satellite
* Radius: 11,000 miles

Question 11

Geometric Pinpoint 2

Answer 11

Distance measurement to a second satellite: Measured distance: 12,000 miles.

Implication of second measurement:
- Positioned on two spheres:
- First Sphere: 11,000 miles from the first satellite.
- Second sphere: 12,000 miles from the second satellite.

Determining location
- Location pinpointed to: The circle of intersection between the two spheres 27

Question 12

Geometric Pinpoint 3

Answer 12

 Distance measurement to a third satellite
 Measured distance: 13,000 miles
 Implication of third measurement
 Positioned
 At two potential points of intersection where the 13,000-
mile sphere intersects with the circle from the first two
spheres’ intersection
 Determining plausible location
 One of the intersection points is often implausible:
 Typically, this point can be disregarded if it’s either too
far from Earth or moving at an impossible velocity.

Question 13

Geometric Pinpoint 4

Answer 13

 Precision with a fourth satellite
 To distinguish between the two potential
points
 Requires an observation from a fourth
satellite.
 Result of fourth observation
 Provides a unique solution to our
positioning problem

Question 14

Terms to know

Answer 14

NAVSTAR
Block Satellite
Constellation
Triangulation