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1. |
Q: |
Why does the GPS have two tiers of accuracy? |
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A: |
The GPS was developed to guide
intercontinental ballistic missiles and smart bombs. The U.S. government did not want it
used against its own people, so it implemented to tiers of accuracy. The most precise
positioning is reserved for the U.S. military and its allies. Everyone else is consigned
to less accurate positioning. |
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2. |
Q: |
What level of positioning accuracy is available
to civilians? |
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A: |
Civilians GPS receivers provide
position accuracy of between 15 and 100 meters (49.2 to 328 feet) in most cases. |
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3. |
Q: |
Does weather affect a GPS receiver's operation? |
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A: |
Weather does not affect the radio
signals sent from space to earth, so if on earth it is not too cold or too hot for the GPS
receiver to work, it will provide a position fix. Clouds, snow or rain definitely do not
affect a receiver's ability to pick up the satellites' radio signals. |
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4. |
Q: |
What does the term P code mean? |
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A: |
The term P code stands for the
Precise positioning signal broadcast by the GPS satellites. It provides the most accurate
positioning. The P codes are not affected by Selective Availability. |
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5. |
Q: |
What does the term CA code mean? |
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A: |
The term CA codes stands for the
Coarse Acquisition code broadcast by the satellites. It provides less accurate positioning
than the P codes and is affected by Selective Availability. |
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6. |
Q: |
Who has access to the P codes? Who has access to
the CA codes? |
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A: |
The P codes are accessible to the
U.S. military or their allies. The CA codes can be used by anybody. |
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7. |
Q: |
How many radio frequencies is the CA code
broadcast on? |
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A: |
The GPS satellites transmit radio
signals to earth at 1227.6 MHz (L2) and 1575.42 MHz (L1). The P codes are sent on both
frequencies. The CA codes are sent only on the L1 frequency. |
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8. |
Q: |
What does the acronym DGPS stand for? |
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A: |
Differential Global Positioning
System. |
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9. |
Q: |
What benefit does DGPS provide? |
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A: |
DGPS eliminates the inaccuracies
caused by Selective Availability. DGPS receivers provide much better accuracy than normal
civilian receivers. |
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10. |
Q: |
What is a GPS receiver doing during the time
known as Time to First Fix? |
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A: |
The first time a GPS receiver is
turned on, it must down-load and save the almanac of satellite positions. It can take up
to 12.5 minutes to down-load the almanac. It must be loaded when ever the receiver has not
be used for several months or it has been moved over 300 miles since the last fix. |
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11. |
Q: |
What happens to radio signals as they travel
through the ionosphere? |
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A: |
Radio signals slow down as they
travel through the ionosphere. The delay introduces error into a receiver's position
calculation. |
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12. |
Q: |
What purpose does the Ground Control Segment
serve? |
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A: |
The Ground Control Segment tracks
the satellites. It keeps their almanacs up-to-date, synchronizes their atomic clocks and
introduces Selective Availability. |
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13. |
Q: |
Why do the GPS satellites need atomic clocks? |
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A: |
Radio navigation requires that the
transmission of radio signals be synchronized. Atomic clocks provide the accuracy needed
to calculate a position. |
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14. |
Q: |
What is Selective Availability? |
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A: |
Selective Availability is random
error introduce by the U.S. Department of Defense into the CA codes. It makes the accuracy
of civilian receivers vary from 15 meters (49.2 feet) and approximately 100 meters (328
feet). |
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15. |
Q: |
A GPS receiver needs signals from how many
satellites to get a 3-D fix? |
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A: |
A GPS receiver must lock-on to four
satellites to get a 3-D position. Three satellites are used to calculate a horizontal
position plus altitude while the fourth satellite is used to synchronize the receiver's
clock with the satellites' atomic clocks. If the signals from only three satellites are
available, the receiver uses one to synchronize the time and the other two to calculate a
2-D position. |
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16. |
Q: |
Which 15th century explorer first realized
Columbus had discovered a new continent? How did he discover the truth? |
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A: |
In 1499, Amerigo Vespucci learned
that Columbus had discovered a new continent and not the West Indies. He learned the truth
when he sailed to the Brazilian coast and took a position measurement. In those days, anyone
could use an astrolabe to find their latitude, but finding the longitude was complex and
required extensive knowledge of the stars. Amerigo was one of the few people who had the
skill to measure longitude and when he did, he discovered that he was not where he was
suppose to be. |
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17. |
Q: |
Do GPS receivers work in a house? |
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A: |
GPS receivers do not work in a
house. The radio waves are block by objects like rocks, roofs, heavy foilage, etc. |
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18. |
A: |
Do GPS receivers work in a city with a tall
sky-line like New York? |
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Q: |
GPS receivers may or may not work in
a city with a high sky-line. It depends on the satellite positions and the proximity of
the tall buildings. For sure the buildings will block the satellite signals, so it is
highly likely that GPS receiver will not be able to see enough satellites to provide a
position fix. |
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