Answers for Chapter 4 About GPS Receivers
1. Q: What is the most important part of a GPS receiver?
A: The antenna is the most important part of a GPS receiver. If the receiver cannot detect the radio signals from the satellites, it will not be able to calculate a position and it will be useless.
2. Q: What is the horizontal accuracy of a civilian receiver?
A: The horizontal accuracy of a civilian receiver varies between 15 and 100 meters (49.2 and 328 feet) if the receiver is locked onto four satellites positioned in a good geometry.
3. Q: What is the vertical accuracy of a civilian receiver?
A: The vertical accuracy of a civilian receiver varies between 100 and 156 meters (328 and 512 feet) if the receiver is locked onto four satellites positioned in a good geometry.
4. Q: Why does the accuracy of a civilian receiver randomly vary?
A: Civilian receivers are affected by Selective Availability which is the random error introduced into the CA codes to deliberately make civilian receivers less accurate than military receivers. If Selective Availability is ever eliminated, the accuracy of civilian receivers would not vary randomly.
5. Q: What is satellite geometry?
A: Satellite geometry refers to the positions of the satellites in relation to the receiver. A receiver's accuracy is affected by satellite geometry. The satellite geometry that provide the highest accuracy is when one satellite is directly overhead while the others are evenly spread around the horizon. A poor geometry is when the satellites are close together because they do not offer distinct positions for triangulation.
6. Q: What does the term DOP refer to?
A: DOP stands for Dilution of Precision and is a measurement of how satellite geometry affects the accuracy of the position calculation. A low position DOP means an accurate position calculation can be made. If the position DOP is too high, the receiver cannot provide an accurate position.
7. Q: What is a multipath signal?
A: A multipath signal is a radio wave that enters the receiver from more than one source. If a signal  enters the receiver directly from the satellite and also as a reflected signal from a nearby object, it is multipath.
8. Q: How does a multipath signal affect a receiver's accuracy?
A: Most receivers cannot tell the difference between a direct signal and a reflected signal, so it cannot tell which one to use in the position calculation. If it uses the reflected signal in the calculation, position accuracy decreases.
9. Q: Is the altitude provide by a GPS receiver accurate enough to use in navigation?
A: Not really. The best-case accuracy of the receiver's altitude is 100 meters (328 feet). Most topological maps have topological divisions of 6 to 15 meters (20 to 50 feet). An altimeter is a useful tool if its accuracy is closer to the accuracy provided by the map.
10. Q: What does "DGPS ready" mean.
A: "DGPS ready" means that the receiver can understand RTCM correction data. Most receivers require the purchase of a separate beacon and antenna to pick-up the RTCM correction radio signals. The correction information is sent from the beacon to the receiver which uses it to correct its position calculation.
11. Q: What is a map datum?
A: All features on a map are reference to the same point. That point is called the datum. Maps drawn using the same datum, have a unique coordinate for every point on the map. If the same area is mapped using different map datum points, a specific mountain may have one coordinate using one datum and a completely different coordinate using the other datum.
12. Q: All datum points on earth?
A: No. The World Geodetic System 1984 (WGS 84) is based on the GPS satellite constellation. Most of the other datums are land based and each country generally has maps based on their own datum point.
13. Q: Is a datum the same as a coordinate grid?
A: No it is not. The datum is the reference for the coordinate grid. The grid provides the unique number for every point on the map. The datum is the reference point from which all the coordinate grid numbers start.
14. Q: What is a coordinate grid?
A: A coordinate grid is the lines drawn on a map that uniquely identify every point on the map. The lines are usually drawn east to west and north to south and form squares on the map.
15. Q: What are the two most common grids?
A: Universal Transverse Mercator (UTM) and Latitude/Longitude.
16. Q: Is it necessary to buy a receiver that can interface to a computer?
A: It depends on your use. If you do not use a moving map on a computer or down load waypoints from map programs, you do not need a computer interface. If you use map databases on CDROM or moving maps, a receiver with a computer interface is necessary.
17. Q: What is a channel?
A: A channel is the part of the receiver's electronics that locks onto a satellite's signal and delivers the satellites radio signal to the processor inside the receiver that calculates position.
18. Q: Is a multichannel receiver better than a single channel receiver?
A: It depends on your use. Multichannel receivers are good in dynamic situations where either the user or the surroundings are in motion. A single channel receiver works fine in a static situation where you can stand still while the receiver updates its position.
19. Q: What is a waypoint?
A: A waypoint is simply a coordinate of a location. Receivers are designed to remember waypoints for use in navigation. The user can supply a name for each waypoint which is stored in the receiver's memory and makes it easier to remember the location.
20. Q: What is the Goto function?
A: GPS navigation is based on Going to a waypoint. The Goto function allows you to select a waypoint and the receiver steers you to that location. The Goto function always indicates the direct route from your current position to the waypoint.
21. Q: What is a route?
A: A route is an ordered list of waypoints. A receiver can direct you from the first waypoint on the list, to the second waypoint, then the third waypoint, etc. The receiver automatically calculates the distance and direction between each waypoint in the route. The user selects which waypoints are part of the route and their order.
22. Q: Can a GPS receiver replace a compass?
A: Not really. A compass senses the earth's magnetic field to indicate direction. A GPS receiver cannot detect the magnetic field. It calculates direction of travel by comparing current position with the last measured position. If the receiver is not moving, it cannot calculate direction; whereas, a compass can still indicate direction when its user is standing still.
23. Q: What is a bearing?
A: The compass direction between your current position and any destination.
24. Q: Can a GPS receiver compensate for declination?
A: Most receivers automatically compensate for declination in all bearing calculations. The receiver has a table of declination corrections for any place in the world stored in memory. Once the receiver locks onto the satellites and determines its position, it can look up the declination for the area and make declination corrections.
25. Q: What is the magnetic north setting on a receiver?
A: Generally, bearings are referenced to either the north pole or the magnetic pole. Setting the receiver to the magnetic north mode makes the receiver report direction with respect to the magnetic pole, which means the bearings reported by the receiver correspond to a compass.
26. Q: When should a receiver be set to the magnetic north mode?
A: When ever the bearing reported by the receiver needs to correspond to the bearing shown on the compass, the receiver should be set to the magnetic north mode.
27. Q: When should a receiver be set to the true north mode?
A: Whenever the bearing reported by the receiver needs to match the bearings measured from a map, the receiver should be set to the true north mode.
28. Q: Do the north-south grid lines on a map always point to the north pole (true north)?
A: No. There is usually some difference between the grid north-south lines and the earth's pole-to-pole lines; however, the difference is usually so slight that the receiver can be set to the true north mode and it will be accurate enough for navigation.
29. Q: What is the best unit of distance to use with the UTM grid.
A: The UTM grid is based on the meter, so the best unit of distance would be the meter or kilometer. A receiver reports distances in meters when the data format is set to metric.

 

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