GPS Test
10/24/2000
(NAME) (DATE/PERIOD)
DIRECTIONS: Answer the following questions on separate sheets of lined paper and then staple them to this paper. (100 points)
1. Name the governmental agency that is responsible for GPS and describe the various pieces of the system. (15 points)
The Air Force as a member of the Department of Defense is responsible for GPS. The System consists of 24 functioning and 4 backup satellites in space and a series of communication stations on the ground. Each satellite is equipped with an onboard atomic clock.
2. List the five logical steps that describe how GPS works. (10 points)
Here's how GPS works in
five logical steps:
1. The basis of GPS is
"triangulation" from satellites.
2. To "triangulate,"
a GPS receiver measures distance using the travel
time of radio signals.
3. To measure travel
time, GPS needs very accurate timing which it
achieves with some tricks.
4. Along with distance,
you need to know exactly where the satellites are
in space. High orbits and careful monitoring are the secret.
5. Finally you must
correct for any delays the signal experiences as it
travels through the atmosphere.
3. Describe how a single inexpensive GPS receiver is able to communicate with all the different GPS satellites in orbit. (10 points)
The pseudo-random code of each of the 24 satellites is stored in the GPS receiver memory. Since all satellites operate on the same frequency, the GPS receiver only needs to identify the code to know what satellite is sending each signal. The single frequency receiver is very inexpensive.
4. Describe in detail how the distance from a satellite to a handheld GPS receiver is calculated using pseudo-random code. (20 points)
In Review: Measuring Distance
1.Distance to a satellite is determined by measuring how long a
radio
signal takes to reach us from that satellite.
2.To make the measurement we assume that both the satellite and
our
receiver are generating the same pseudo-random codes at
exactly the
same time.
3.By comparing how late the satellite's pseudo-random code
appears
compared to our receiver's code, we determine how long it
took to
reach us.
4.Multiply that travel time by the speed of light and you've got
distance.
5. List the steps that help GPS units get perfect timing. (10 points)
In Review: Getting Perfect
Timing
1.Accurate timing is the key to measuring distance to
satellites.
2.Satellites are accurate because they have atomic clocks on
board.
3.Receiver clocks don't have to be too accurate because an extra
satellite
range measurement can remove errors.
6. What factors allow us to know the position of each GPS satellite in orbit so precisely? (10 points)
1.GPS satellites are so high up their
orbits are very predictable.
2.Minor variations in their orbits are
measured by the Department of
Defense.
3.The error information is sent to the
satellites, to be transmitted along
with the timing signals.
7. Describe the makeup of the ionosphere and troposphere and then describe what happens to GPS signals when they pass through the ionosphere and troposphere. (15 points)
The ionosphere is made up of
charged particles or ions. The
ionosphere deflects the GPS signal and allows it down. The troposphere is the atmospheric layer in
which all weather occurs. The
troposphere has little or no effect on the GPS signal.
8. Describe five situations in which GPS might prove useful to the public. (10 points)
1.
Navigation in any area of transportation
2.
Hiking
3.
Emergency Location
4.
Farming
5.
Surveying
6.
etc.