GPS Study Guide
9. What is P code?
"Precise" code
12. What do monuments and graticules allow us to do
Allow us to determine the position of any object we choose on the surface of the Earth The surface of the Earth is studded with monuments - carefully positioned metal or stone markers - whose coordinates are known quite accuractely. These lie in the "numerical graticule" which we all agree forms the basis for geographic position. Measurements in the units of the graticule and based on the positions of the monuments allow us to determine the position of any object we choose on the surface of the Earth.
2. What kind of information does an almanac have?
Almanac information is broadcast to provide close, but not precise, satellite position information. The almanac for all satellites is broadcast from each satellite
20. What is an almanac?
An almanac is a description of the predicted positions of heavenly bodies
21. What is a GPS base station?
An antenna-receiver configuration is called a base station by placing a GPS receiver antenna and place it precisely at that known point T - a point that has been surveyed by exacting means and whose true position is known to within a centimeter
22. What is an error vector?
An entity with both magnitude and direction that represents the amount and direction by which the reading (GPS fix) missed the true point
13. What is the minimum constellation of satellites in the GPS?
24 which includes 3 spares The United States GPS design calls for a total of at least 24 and up to 32 solar-powered satellite radio transmitters, forming a constellation such that several are "visible" from any point on Earth at any given time. The minimum "constellation" of 24 includes 3 "spares". The satellites are also referred to as Space Vehicles or SVs.
2. How many satellites are necessary to get a good 3-D spatial fix?
4 satellites Distances to several satellites are calculated from measurements of the times it takes for radio waves to reach from the satellites, whose positions are known precisely, to the receiver antenna.
26. How different is WGS84 from NAD27 on the Earth's surface?
About 10 meters on the Earth's surface NAD27 was based on parameters determined by Clarke in 1866
15. What is the length of time that a GPS executes an orbit of the Earth?
12 hours NAVSTAR satellites are neither polar nor equatorial, but slice the Earth's latitudes at about 55 degrees, executing a single revolution every 12 hours. Although each satellite is in a 12 hour orbit, an observer on Earth will see it rise and set about 4 minutes earlier each day. There are 4 or 5 satellites in slots in each of six distinct orbital planes (labeled A, B, C, D, E, and F) set 60 degrees apart. The orbits are almost exactly circular.
19. Is the error of two different receivers that are about 200 miles from one another similar? Explain.
19. If two GPS receiver antennas are close, and use the same satellites, they will perceive almost the same errors. For any given point at any given time, the Error will be almost exactly the same for both receivers. Close is about 500 km or 300 miles
18. How are the GPS satellites monitored and how often?
5 ground based stations, twice a day per each station the satellite passes over Both their tracks and their innards are monitored by five ground-based stations. Each satellite passes over at least one monitoring station twice a day. Information developed by the monitoring station is transmitted back to the satellite, which in turn rebroadcasts it to GPS receivers. Subjects of a satellites broadcasts are the health of the satellites electronics, how the track of the satellite varies from what is expected, the current almanac for all the satellites and other info. Ground based stations are located at: Near the equator, located on Ascension Island in the South Atlantic At Diego Garcia in the Indian Ocean On Kwajalein Atoll in the Pacific In Hawaii also in the Pacific At Cape Canaveral, Florida And the master control station (MCS) at Schriever (formerly Falcon) Air Force Base near Colorado Springs, Colorado Other ground-based stations exist, primarily for uploading information to the satellites
25. How much error is there in the differential fix for a rover that is 21 km from the base station? What is the rule of thumb you used to calculate it?
7 cm (21km / 3 = 7cm) The formula for the amount of error you might expect with differentially corrected data is dependent on the distance between the base station antenna and the rover antenna. A rule of thumb is that the fix will be in error by one additional centimeter for each three kilometers between the two antennas. This relationship is approximately linear: 300 km would produce error of about a meter
9. How accurate is a single GPS position?
7 meters A single position derived by reading coordinates from a GPS receiver and finding the corresponding position on a map is usually accurate with within 7 meters or so More complex procedures are involved in collecting data accurate enough for a GIS.
13. Explain clock errors.
A GPS receiver determines its' fix by determining how long it takes a signal to get from the satellite to the receiver antenna. This requires that the atomic clocks in the satellites be synchronized. Even a small amount of difference in the clocks between the satellites and the GPS receiver can make a huge difference in distance measurements, becaue the GPS signal travels at about 300,000,000 meters per second.
2. What is a base station?
A GPS receiver that is set at a precisely known geographic point.
5. What is trilateration?
A mathematical process that measures the distances related to a triangle. A method to determine the position of a point by knowing the distance from that point to two other known objects, in this case, satellites.
8. What is 2dRMS?
A measure of accuracy based on two standard deviations of a normal distribution where RMS means root-mean-square. Ninety-five percent of the fixes will be within a circle with this radius.
23. What is autonomous GPS?
A single GPS unit operating by itself
5. Does the GPS assign absolute or relative coordinates?
Absolute GPS gives people an easy method for both assigning and using absolute coordinates
11. Where on Earth can't GPS be used?
Anywhere not having a direct view of a substantial portion of the sky, such as: • Inside buildings • Underground • In very severe precipitation • Under heavy tree canopy • Around strong radio transmissions • In "urban canyons" amongst tall buildings • Near to powerful radio transmitter antennas GPS satellites transmit have very short lengths - about 20 cm. This short length is good for measuring because it follows a very straight path** but on the flip side, short waves do not penetrate matter very well so the transmitter and the receiver must not have much solid matter between them or the waves are blocked, as light waves are easily blocked. **Longer radio waves such as AM and FM band may bend considerably
14. What is the altitude of the GPS satellites and what is the benefit of such an altitude?
At a "middle altitude" of about 11,000 nautical miles (NM) OR roughly 20,400 kilometers (km) OR 12,700 statute miles (sm) above the Earth's surface This middle altitude puts them considerably above the standard orbital height of the space shuttle, most other satellites, and the enormous amount of space junk that has accumulated. They are also well above Earth's air, where they are safe from the effects of atmospheric drag. When GPS satellites "die" they are sent to orbits about 600 miles farther out. They are considerably below the geostationary satellites (about 19299 nm), which are usually used for communications and sending TV, telephone, and other signals back to Earth-based fixed antennas. The geostationary satellites have to be above the equator to stay in synchronization with Earth's rotation. And they have to be synchronized with Earth's rotation; otherwise you couldn't point satellite dishes at them.
4. What is the range of vertical error as compared to horizontal error?
Because of the geometry of the satellites, vertical error will almost always exceed horizontal error on or near the surface of the Earth. The fact that all satellites are necessarily above the fix being taken generally means that vertical error will be 1.5 to 2.5 as great as horizontal error
1. Explain the concept behind differential correction.
By setting a base station at a precisely known geographic point, it can analyze and record errors in the GPS signals it receives. Because the base station knows exactly where its' antenna is, it can assess the GPS signals that try to tell it that it is somewhere else. These signal errors will be almost equivalent to the signal errors affecting other GPS receivers in the local area, so the accuracy of locations calculated by those other receivers may be improved, dramatically, by information supplied by the base station.
7. What is CEP?
Circular Error Probable is a measure of accuracy of GPS fixes. It is the radius of a circule expressed in linear units (such as meters). For a given situation, 50% of the fixes will fall within the circle and 50% will fall outside the circle.
8. What is C/A code?
Coarse/Acquisition code The satellites broadcast only "bits" of information: zeros and ones. For most civilian use, this transmission, and the ability to make meaning out of it, is called the "C/A code" The word "Coarse" is in contrast to another code used by the satellites: the "P" or "Precise" code. The term "Acquisition" refers to the capability that allows both civilian and military receivers to acquire the approximate position of the receiver antenna. The C/A code is a sequence of 1023 bits which is repeated every one-thousandth of a second.
21. What is constellation vacillation?
Constellation vacillation happens when trying to collect data while moving. As the antenna moves along the path to different positions, the receiver may choose different constellations of satellites as it tries to pick the best of satellites to calculate positions due to signal obstruction caused by tree canopy, buildings, overpasses or tunnels. Because of the various errors, which differ from satellite to satellite, a position fix reported by the receiver using a new constellation may be different from where it would have been had the previous constellation been retained. Thus, the position fixes may not follow a clean line, but may jump from side to side of the true path. When a receiver is set up at a single point, you can try to optimize the view of the satellites by staying away from obstructions. While moving, you have little opportunity to pick the points at which the receiver calculates a position.
11. What is DOP? Is high DOP good?
DOP is Dilution of Precision. It is a number which is a measure of the quality you might expect from a position measurement of the GPS system based solely on the geometric arrangement of the satellites and the receiver being used for the measurement. High DOP values are not good as high (poor) DOP values can magnify other errors
1. Explain the analogy to GPS described in Figure 2.1.
Distances - determined by the measurement of time - forms the foundations of GPS Both the GPS and the soundhouse/ship example use sound to determine distance. They both use more than one source (soundhouses or satellites) to determine a more precise location using intersections of circles or trilateration.
7. What characteristics does the civilian carrier phase have?
Each satellite actually broadcasts on at least two frequencies, only one of which is for civilian use. The civilian carrier frequency is 1575.42 megahertz (1575.42 million cycles per second) By comparison, FM radio signals are about 100 megahertz. GPS radio waves cycle about 15 times as often, and are, therefore, one-fifteenth as long: about 20 centimeters from wavetop to wavetop The modulation of the wave is pretty dull, even when compared to "golden oldies" radio stations.
3. What is ephemeris information?
Each satellite broadcasts ephemeris information that applies to that satellite only, that provides up-to-the-minute corrections. The satellites are not completely predictable in their orbits because of such forces as gravitational pull from the Sun and Moon, the solar wind, and various other small factors. Therefore the satellites are carefully monitored by ground stations and told their positions; each satellite then rebroadcasts this information to GPS receivers.
12. How does the receiver listen to several satellites at once?
Each satellite has its own distinctive PRN code. Satellite numbers are the PRN numbers which is the principle way satellites are identified. Most receivers have several channels that are tuned to receive the civilian GPs frequency. Although all channels are tuned to the same frequency, a single channel can track a GPS satellite by locking onto its PRN code. In expensive receivers with several or many channels, each channel is assigned full time to tracking a single satellite. Older receivers "time share" a channel - flipping it between satellites, as you might flip between channels on a TV trying to keep track of two programs at once.
17. What type of clock does a GPS satellite have? How good is it?
Each satellite has on board 4 atomic clocks (either cesium or rubidium) that keep time to within a billionth of a second or so, allowing users on the ground to determine the current time to within about 40 billionths of a second (40 nanoseconds).
5. How often does a satellite send a signal?
Each satellite sends a signal continuously, like a radio station that broadcasts 24 hours a day
16. Explain atmospheric errors.
Error that is caused by changes in signal speed as the signal travels different parameters of atmosphere The signal travels through a virtual vacuum of "empty space" for most of the journey from satellite to receiver antenna. When it gets to the Earth's atmosphere, the speed drops very slightly -by an amount that varies somewhat randomly. Since the calculation of the range to the satellite depends on the speed of the signal, a change in signal speed implies an error in distance, which produces an error in position finding. Half of the mass of the Earth's atomosphere is within 3.5 miles of seal level. Virtually all of it is within 100 miles of the surface. So the signal gets to go the speed limit for electromagnetic radiation for more than 12,000 of the more than 12,600 miles of the trip.
9. What is the 2dRMS of a single position fix?
For a NAVSTAR GPS, a number of experiments suggest that 50% of the latitude and longitude fixes you obtain with a single receiver operating by itself (i.e. autonomously) will lie within 12 meters of the true point. Fifty percent of the altitude fixes will lie within 21 meters. The 2DRMS radius is 30 meters horizontally and 32 meters vertically. These numbers assume that selective availability is off.
3. What is the definition of error?
For the logging of a given point, error is defined as the distance between what your GPS receiver records as the position of the antenna and the true position of the antenna.
11. What is over-determined position finding?
Four satellites are required for a 3D fix but if your receiver has access to five or more at a given time, each set of four of the satellites available will provide a different opinion on the position of the point being sought. A compromise agreement based on all the satellites' input is probably better than the position indicated by any one set of four. The GPS receiver may be set to collect data this way.
2. What is GLONASS?
GLObal Navigation Satellite System The navigation system used by the Russians, GLObal Navigation Satellite System. A more general and recent acronym is GNSS which stands for Global Navigation Satellite Systems
16. What is the civilian GPS signal called?
GPS L! The radio frequency used for the Civillian GPS signal is called GPS L1 and is at 1,575.42 megahertz (MHz)
27. How many datums are there?
Hundreds - many countries have their own When a GPS receiver gives you a latitude and longitude you must know the datum that is the basis for the numbers. NAVSTAR GPS is based fundamentally of WGS84 but lots of GPS units can calculate and display coordinates in many other datums
18. What is an ideal satellite arrangement for 4 satellites to get the best GDOP?
Ideally, you would like to have the four satellites at the vertices of a regular tetrahedron (a 3 -sided pyramid with all four surfaces equilateral triangles) and the receiver antenna at the center. This would provide the greatest geometric strength but of course it is impossible for GPS because three of the satellites would be below the horizon. The best alternative would be when one satellite is directly overhead and the three others are close to the horizon, spaced on a circle, separated from each other by 120 degrees. Simply put, you get the best DOP when the satellites are spread out; the more satellites clump together the worse the DOP value. In summary, DOP is a measure of the extent to which satellite geometry exacerbates the other errors that may occur in the measurement.
7. How is the GPS like a digitizer?
If one considers the Earth's surface as the digitizing tablet and the GPS receiver antenna as the crosshairs of the puck, one could consider the ground-based portion of a GPS system and a digitizer analogous.
27. What is an elevation mask?
If the base and rover are widely separated the rover might see a satellite that the base cannot view. For example: if the base and rover use the same elevation mask (say 10 degrees) the rover might see "Satellite B" while the base would not. A measure of the minimum elevation angle, above the horizon, above which a GPS satellite must be located before the signals from the satellite will be used to compute a GPS location solution. Satellites below the elevation angle are considered unusable. The elevation mask angle is used to prevent the GPS receiver from computing position solutions using satellites which are likely to be obscured by buildings or mountains.
6. Explain the concept of finding X in 2 dimensions as shown in Figures 2.4 and 2.5.
In 2 dimensions the representation of the earth is a flat circle. The distances from the point X to the 2 satellites (a and b) are the radii of two intersecting circles which have the points a and b as their centers. The position of point X lies on one of the two common points of the two intersecting circles. One of the points will be unlikely, leaving the other point to be determined as point X The points a and b represent two of the GPS satellites at a precise instant of time and point X represents the position of the GPS antenna at that same precise moment. The 0.1 second that it takes for the signals from the satellites to reach the receiver is compensated for in the design of the system.
7. Explain the concept of trilateration in 3 dimensions.
In 3 dimensions, the representation of the earth is a spheroid. Same concept as trilateration in 2D except one is finding the intersection of 3 spheres rather than the 2 circles used in 2D. The third point or satellite, cannot reside in the same plane formed by points a, b, and X Theoretically, 3 satellites, creating 3 circles could be used but in reality a 4th satellite is needed
10. How low can you get error by averaging if time is not an issue?
In general, the more fixes you take and the more time you spend, the better your average will be. If you are prepared to take data at one point for several weeks to several months your error will get down to approximately 1 to 2 meters, basically due to the law of large numbers. This will not be a practical way to reduce error in most applications.
8. Why does a 3-D fix require a 4th satellite?
In reality, a GPS receiver needs 4 satellites to calculate a position as the receiver depends on the satellite clocks to set itself correctly. This is because the clock in a GPS receiver is not as good as the four clocks in each satellite.
24. How does differential GPS use the error vector from the base station to correct the rover error?
It allows us to correct the readings of other GPS receivers in the area that are collecting fixes at unknown points. If two GPS receiver antennas are close, and use the same satellites, they will perceive almost the same errors. For any given point at any given time, the Error will be almost exaxtly the same for both receivers. The true value can be closely approximated by effectively subtracting out the error. Mathematical equations can be used to cancel out most of the error in a GPS position found by an antenna that is close to another antenna which is over a known point. Close is about 500 km or 300 miles E = O - T (Error = the reading O - position of T) t = o - E (true value t = any nearby reading - Error)
30. What is postprocessing GPS?
More correctly called post-mission-processing GPS The data from both the base and the rover are brought together later in a computer and the appropriate correction is applied to each fix created by the rover. For mapping and GIS activities, this approach yields better results. You can get more accurate fixes through post processing, but it requires a few more time consuming steps
25. Why might you put an almanac on a PC?
It is useful for planning a data collection session. It is not necessary in order to use the data you have collected. For example, when used with a Pathfinder Office module called QuickPlan, it can tell you how many satellites are available at any time during the day, and what DOP values you can expect. Further, copying an almanac to the PC is illustrative of the communication process, so we do it for practice A NAVSTAR almanac provides a description of approximately where the satellites are at any given moment in time. Collected and retained by the GPS reciver, the almanac tells the receiver which satellites it will be able to see during a data collection session. That is, the almanac helps in the initial phases of outdoor data collection. So the receiver automatically collects an almanac every time it is turned on, if it can. To collect an almanac, the receiver needs to be picking up the signal from at least one satellite for about 15 minutes. Alamanacs are available from the satellites, all day, every day, and usually have validity for uyp to 3 months. Of course is a new satellite is put up, the almanac becomes less useful immediately.
31. Is differential correction worth it?
It's up to the user to determine the scope and accuracy of the project and make that decision. The resulting data still contains error. Some of the inaccuracy has been taken out, but the results are by no means exactly correct.
3. How do we formally keep track of space?
Latitude / Longitude graticule Formally, we usually delineate geographical space in two dimensions on the Earth's surface with the latitude-longitude graticule, or with some other system based on that graticule
28. What are the advantages and disadvantages of latitude-longitude? Of projected numbers
Latitude and longitude numbers of a given datum provide an exact method of referencing any given single point but they are difficult to calculate with and multiple points suffer from distortion problems when plotted. Projected coordinate numbers are easy to calculate with but in general, misplace points with respect to other points - thus producing errors in distances, sizes, shapes and/or directions
4. What are absolute versus relative coordinates?
Latitude-Longitude provides absolute coordinates Describing a feature or area based on features around it yields relative coordinates, for example Relative coordinates are an informal, more intuitive way to organize space based on the features in that space.
16. What is satellite geometry?
The positions of the satellites in the sky at the time a given position on the ground is sought.
17. If two satellites are at a 90 degree angle from the receiver is the margin of error lower or higher than if it was a more obtuse angle? Explain.
Margin of error is lower if the satellites are at 90 degrees and increases with a more obtuse angle. For each distance from point sought to a satellite, there is an error distance (delta). If this error distance is plus or minus, then the error distance is 2 times delta If the satellite is at a more obtuse angle (greater than 90 degrees) the distance from the point sought to the satellite is greater, therefore the error distance is greater as well. Note: Satellite geometry can make a big difference in the quality of the position you calculate. In actual GPS measurements, of course, it is a volume rather than an area that surrounds the point being sought, but the same general principles apply.
17. With expensive GPS receivers how can ionospheric error by calculated out?
More sophisticated GPS equipment considers both the frequencies transmitted by each satellite. Since the ionosphere affects the different frequencies differently a correction can be calculated. Significant changes in signal speed occur throughout the atmosphere, but the primary contributions to error come from the ionosphere, which contains charged particles under the influence of the Earth's magnetic field and from the troposphere - the dense part of the atomosphere that we breathe, that rains on us, and that demonstrates large variations in pressure and depth.
1. What is NAVSTAR?
NAVigation System with Time and Ranging informally known as "Navigation Star" A constellation of 24 to 32 satellites orbiting the Earth, broadcasting data that allows users on or near the Earth to determine their spatial positions. The more general term used in the U.S. is "Global Positioning System" or GPS. In the Western world GPS usually implies NAVSTAR and the Kennedy book used the terms interchangeably.
14. Should you collect data when 4 satellites are in view? Why?
No, data collection with fewer than five satellites in view in pretty iffy In general, the more satellites in view, the better the accuracy of the calculated position as the GPS receiver uses the "best" set of four satellites to calculate a given point. "Best" is based on a DOP value and the more satellites there are in view, the more combinations of four can be considered for the "best" combination
15. What is over-determined position finding? Is it better than normal position finding?
Over-determined position can be used when more than four satellites are available, which means the receiver can use more than four satellites to determine a position. Since a set of four satellites can determine a position, a set of five** could determine five positions and the "average" of these positions could be considered better than one alone. Presumably, over-determined position finding results in a more accurate result. **by considering satellites 1234, 1235, 1245, 1345, 2345 A more complex statistical method is used to provide the most accurate position and will not be discussed in this text
19. What is PDOP?
Position DOP - generally considered the best single indicator of geometric strength - is a combination of HDOP and VDOP (actually the square root of the sume of squares of HDOP and VDOP) The overall DOP number is made up of several "sub-DOPs" • HDOP (Horizontal DOP) is a combination of NDOP (North DOP) and EDOP (East DOP) • VDOP is Vertical DOP • PDOP (Position DOP) • TDOP is Time DOP • GDOP "Geometric" DOP is a combination of PDOP and TDOP
6. What is the difference between positions and locations?
Position refers to the coordinates that specify where one is (position is absolute) Location referes to where one is with respect to objects around them. (location is relative) Location can be determined by combining map and/or GIS data
3. What do NAVSTAR SVs emit rather than sound?
Radio waves The waves that are used to measure the distance are electromagnetic radiation (EM).
5. What is random error?
Random errors are deviations from a "true" value that follow no predictable pattern. Generally, random errors are those cause by factors we cannot measure or control
29. What is RDGPS?
Real-Time Differential GPS Correcting errors by the differential correction method implies that the base station - the receiver at the known point - can communicate with the roving receiver(s). In practice this happens either: After the readings from the receivers are loaded into a computer, which we call post-mission differential correction OR At the time the readings are being taken, called Real-time Differential GPS (RDGPS) In RDGPS, a radio link is set up between the base station and the rover. As soon as the calculation for a given point is completed by the receivers, a rover uses the correction signal broadcast by the base station to adjust its opinion of where the point is
22. What is Selective Availability?
SA is a policy employed by the military for years that deliberately corrupted the GPS signals so that an autonomous GPS could not assure accuracy of better than 100 meters. This policy was terminated on 2 May 2000. Current users of autonomous receivers may know their locations within about 5 meters.
24. What is a SSF file?
SSf is a file extension that means Standard Storage Format Raw data files constitute intermediate steps in the construction of the SSf files
13. In Chicago, it is somewhat more important that there is a good view of the sky to the south for good reception. Why?
Satellites are in oblique orbits and their tracks give the north and south poles a wide berth. In looking at satellite tracks over a 6 hour period, there are a dearth of satellite tracks above the 45 degree parallel in the northern hemisphere. The statement about reception being better toward the south applies only to the middle and upper latitudes in the northern hemisphere. There is still good GPS coverage - all the way to the North Pole More about selective availability, Chap 3, p. 83 Error deliberately introduced by the GPs managers in the C/A broadcast to diminish the accuracy of GPS recievers. Sometimes the satellites lied about their positions. Sometimes they lied about when they sent the code. The government guaranteed that 95% of the time a fix would be within 100 meters of the true position.
31. What is the SPCS and how is it different from latitude-longitude and UTM
State Plane Coordinate System All U.S. states have State Plane Coordinate Systems developed by the U.S. Coast and Geodetic Survey, originally in the 1930's. Each state operates pretty independently and the coordinate system(s) used in one state are not applicable to another. The system or systems (sometimes more than one is used) used are based on different projections depending on whether that state in mostly north-south or east-west. Units are either feet or meters. Zone boundaries frequently follow county boundaries. SPCS have a scale error maximum of 1 unit in 10,000 The possible error with the UTM coordinate system may be larger: 1 in 2,500 Note: Text uses equipment by Trimble Navigation, Ltd and Magellan, Inc because their units work well, are quite accurate, have programs of educational discounts.
19. What type of information about the GPS satellites is kept track of on the ground?
Subjects of a satellites broadcasts are the health of the satellites electronics how the track of the satellite varies from what is expected the current almanac for all the satellites and other info.
6. What is systematic error?
Systematic errors are deviations from a "true" value that do follow a predictable pattern. Systematic errors are those we can account for, measure, and perhaps, correct for. Errors p. 132-133 GPS readings tent to cluster around the true location. We can use the fact that large numbers of random errors tend to be self-cancelling. That is, the average position (if you take the means of many latititudes, of many longitudes, of many altitudes) will be much closer to the true value than the typical single measurement
10. What is PRN?
The C/A code is called a Pseudo-random noise code - the term "noise" coming from the idea that an aural version of it would greatly resemble static one might hear on a radio
11. How does the receiver use 0s and 1s to determine the range from the satellite to the receiver?
The PRN code is not random; a given satellite uses a computer program to generate a particular code. The GPS receiver uses a copy of the same program to generate identical code. The satellite and the receiver begin the generation of the code at exactly the same moment in time. The receiver can determine its' range from the satellite by comparing the two PRN sequences (the one it receives and the one it generates). The receiver first determines how much the satellite signal is delayed in time, and then, since it knows the speed of radio waves, it can calculate how far apart the two antennas are in space
30. What is northing?
The distance along the surface of a "bumpless" Earth, in meters, from the EQUATOR. The easting is more complicated to explain since it depends on the zone and a coordinate system that allows the number to always be positive. The representation of an object in the UTM coordinate system, when that system is based on WGS84 is a northing and easting The representation of an object in the UTM coordinate system when that system is based on NAD27 would yield a different value for northing and easting.
21. What is a GPS receiver and what are its features?
The handheld portion of a GPS system, a basic satellite receiver consists of: An antenna (whose position the receiver reports) Electronics to receive the satellite signals A microcomputer to process the data and to record position values Controls to provide user input to the receiver A screen to display information Elaborate units have computer memory to store position data points and the velocity of the antenna. Information can be uploaded to a computer and then installed in a GIS software database. An additional feature over the basic GPS unit is the capability to receive data from and transmit data to other GPS receivers - a technique called "real-time GPS" that may be used to considerably increase the accuracy of position finding.
23. What must you consider when collecting data on a bike?
The line of sight requirement. The antenna needs to be positioned far enough away from your body or above it or the receiver will consistently lose its lock on one satellite or another. One solution is to affix the antenna to a pole mounted to the bike and place the receiver in a backpack worn by the cyclist.
13. To what is the quality of a position fix related?
The number of satellites in view Their geometry, or arrangement in the sky Prior to the end of 1993, GPS has less than the full complement of 24 satellites operating. In earlier years, there were periods during the day when there were not enough satellites in view from a particular point on the ground to provide a position fix.
1. How does a GPS receiver know where the satellites are?
The position of a satellite at any time may be predicted with a hight degree of accuracy by a few mathematical equations. A satellite orbiting the Earth may be modeled by formulas contained in the memory of the microcomputer in the receiver. When the formulas are applied to bodies at the high altitudes of the GPS satellites, where they are free from atmospheric drag, the formulas are relatively simple and can predict the position of the satellite quite accurately.
14. Explain ephemeris errors.
The receiver expects each satellite to be at a certain place at a particular given time. Every hour or so, in its' data message, the satellite tells the receiver where it is predicted to be at time "t" hence. If this ephermeris prediction in incorrect - the satellite isn't where it is expected to be, even by just a meter or two - then the measurement of the range from the receiver antenna to the satellite will be incorrect.
23. What is a rover receiver?
The rover antenna moves and records the GPS fixes and the base station antenna remains in the the same location.
4. How is the clock in a receiver kept accurately on GPS time?
The short answer is that the receiver clock is reset to GPS time by the satellites each time a position is found. Such resetting is necessary because, while the receiver clock is very consistent over short periods of time, it tends to drift over longer periods. If you don't use the receiver for a week or two, you may notice a difference of several seconds between the time the receiver displays and true time. The clocks in the satellites keep time to about a tenth of a billionth of a second (a tenth of a nanosecond) The receiver clock obviously costs a lot less than the four $50,000 clocks in each satellite hence the discrepancy in accuracy. If you consider "time" as the fourth dimension and accept that it takes one satellite to fix each dimension, then it is clear that four satellites, working in concert, can set the clock and provide a 3D spatial position. One might presume that only three satellites are required for a 3D fix. But given that the receiver has only an approximate idea of what time it is, what must be calculated is a 4D fix. So four satellites are required. It is not correct to say that three satellites are used for the 3D fix and the fourth sets the receiver clock. Rather, all of the satellites operate in concert to find the true "position" of a receiver antenna that may move in space (relative to the Earth) and does move in time.
6. What is the difference between carrier and modulation?
The signal consists of 2 parts - a carrier and modulation A carrier is on all the time. Modulation of the carrier is the voice or music one hears while listening to a radio station The carrier produces silence (or a low hiss) whereas if your radio is tuned to a frequency on which no nearby station is broadcasting you will hear static.
24. What is vital concerning datums, projections, and coordinate systems, when combining GPS data with existing GIS data?
They must all be identical It is vital that when GPS data is to be combined with existing GIS or map information, the following items must be identical: • datum designation • projection designation • coordinate system designation • measurement units that are used
15. Explain receiver errors.
These errors result from a number of factors related to receiver design, cost and quality. Some receivers, for example, cannot exactly measure and compute the distance to each satellite simultaneously. If a GPS receiver can track up to six satellites at a single moment, then it is a six-channel receiver. In any receiver, the computer must work with a fixed number of digits and is therefore subject to calculation errors. Perfection in position calculation by computer is not possible because computers cannot do arithmetic on fractions exactly.
4. How fast does EM travel?
They move a lot faster than sound. Regardless of frequency, in a vacuum EM moves at about 299,792.5 km/s which is roughly 186,282 statute miles per second.
12. List the types and average amounts of error that contribute to the error budget.
Typical error sources and values for receivers of the Pathfinder class are: Satellite clocks <1 meter Ephermeris error < 1 meter Receiver error <2 meters Ionosheric <2 meters Tropospheric <2 meters Multipath depends on the situation and the receiver, may be large These values correspond to averages of many readings rather than the error that might be expected from a single reading. Although experimentation shows that the more fixes you record the better the data become, the increase in accuracy created by taking a large number of fixes really cannot be counted upon. The existence of systematic errors that might be present because of particular atmospheric conditions (or for certain, when selective availability was active) makes the law of large numbers inapplicable. There are better ways to get really accurate data. And more accurate data is almost always required when using GPS for GIS purposes.
29. Explain UTM.
Universal Transverse Mercator is a coordinate system developed based on a series of 60 transverse Mercator projections. These projections are further subdivided into areas, called "zones" covering 6 degrees of longitude and usually 8 degrees of latitude. A coordinate system is imposed on the resulting projection such that the numbers are always positive "to the right and up
28. What should the elevation mask for a rover be? Why?
Usually base stations are set up with an elevation mask value of 10 degrees. A good rule of thumb is to increase the elevation mask for the rover by one degree for every 1—kilometers (60 miles) it is from the base station. So a setting of 15 degrees works well for the rover in general if it is within 500 km of the base station. Fifteen degrees is also good for avoiding difficulties due to terrain and the elevation angle reduces errors that tend to occur when signals come from satellites low on the horizon, since those signals must pass through more atmosphere. In any event the rover elevation mask must be set high enough so that there is no possibility the rover will record data from a satellite that the base is not recording
25. What is the world's most widely accepted datum? What is it similar to?
WGS84 NAD83 The most recent, widely accepted datum is the WGS84 or World Geodetic System of 1984 based on the GRS80 ellipsoid. In the coterminous states of the U.S. this datum is said to be virtually identical (within millimeters) with NAD83, or North American Datum of 1983. GRS80 stands for Geodetic Reference System 1980
18. What can be done about tropospheric error?
We are pretty much stuck with tropospheric errors especially using the moderately inexpensive code-based equipment available to civilians. This will change when a second civilian signal is added to the system. Need to read this portion of the text
22. What is it a good idea to collect points by time rather than distance?
When collecting points by distance, spurious points caused by errors will be recorded if the spurious point is more than the distance interval you specified to record measurements. You are more likely to get a ragged path with many spikes in it by using distance as the logging interval. If you record points at a set time interval, there is a better chance that a spurious point may be ignored.
20. What is a PDOP mask and how might you use this value practically in a survey?
When setting up a GPS receiver you set up the maximum allowable PDOP. PDOP is the most imn portant single DOP to consider. On one screen you set the PDOP value prior to collecting data. During data collection, on another screen you get a report on the value of PDOP of the constellation the receiver was using. The PDOP mask says: "If the PDOP is too high, then don't calculate any positions." The recommended PDOP mask value settings are 1 to 4 Great 5 or 6 Okay 7 or 8 Marginal greater than 8 Unacceptable
10. What is the rule of thumb for the difference between horizontal and vertical GPS accuracy?
horizontal x 1.5 = vertical Variances in the horizontal accuracy should be multiplied by 1.5 (and perhaps as much as 3.0) to get an estimate of vertical accuracy. The accuracy of the third dimension of GPS data is not as great as the horizontal accuracies.
8. What level of accuracy can the GPS provide if money is no object?
less than 1 centimeter horizontal Horizontal real-world accuracies of less than 1 centimeter can be provided with expensive, survey grade equipment. The accuracy provided by survey grade equipment is not typically used for general GIS spatial analysis and mapmaking activities.
12. Can you do anything about bad DOP values?
• DOP values can be monitored during data collection and the data logger can mask out data with excessive DOP values • DOP values, which can be predicted considerably in advance for any given location, can perhaps be reduced by selecting appropriate times to collect data • Differential correction cannot eradicate errors created by inappropriate DOP values
9. Explain the two philosophies of accuracy in collecting GPS data presented by Kennedy.
• Good enough • The best that is reasonable Good enough can be used if you know what you need for a particular application such as bringing a ship into harbor The best that is reasonable can be used when you don't really know what the application is or what the application calls for or future applications might need a higher degree of accuracy. In this case, collecting data with the best accuracy that your budget and technology can allow could be a consideration.
32. How do you usually get base station files?
• Over a computer communication network such as the Internet • Download the file to your computer through anonymous FTP (file transfer protocol) • Or you could request a base station file be sent to you through the mail on a CD-ROM if the base station does not have a Web site or anonymous FTP (which does not occur that often) • Sometimes base station files are available as zip files or self extracting . exe files Base stations usually record a data fix every few seconds from all the satellites in view, using an elevation mask of 10 degrees
10. List the major factors that contribute to GPS error.
• Satellite clock errors • Ephemeris errors (satellite position errors) • Receiver errors • Ionosphere errors (upper atmosphere errors) • Troposphere errors (lower atmosphere errors) • Multipath errors (errors from bounced signals) In the long run, you can do a lot about these errors by a method called "differential correction" (to be discussed later?)
26. What conditions must be satisfied for differential correction to work?
• The base and the rover have to be taking data at the same time • The base has to be taking data frequently • If the base station is to serve the rover, wherever it may be (within the 500 km limit) regardless of when data are taken, the base station must take data from all satellites the rover might see
