The Global Positioning System for the Geosciences Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications (1997) / Chapter Skim
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9 Dynamic Positioning and Navigation
9 Dynamic Positioning and Navigation
Pages 164-200
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From page 164... ...
nyn~mIc roskIoning and Neon
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From page 165... ...
Differential GPS techniques for modern aerogeophysical studies include both realtime navigation of the aircraft and post-mission recovery of the precise positions for data reduction. Major science problems which have been addressed recently with aerogeophysics include deciphering the dynamics of the world's major ice sheets, imaging surface displacements due to earthquakes and decoding the structure of the continental lithosphere.
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From page 166... ...
An example of a global change issue addressed with aerogeophysics is ice sheet mass balance where real time differential techniques are necessary to accurately locate the experiment and precise positioning in three dimensions The Global Positioning System for the Geosciences is critical to the analysis of the ice surface. Erosion processes and landform evolution can also be studied with an accurately navigated aerogeophysical platform used to trace region changes in topography.
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From page 167... ...
This positioning is recovered by post-mission reduction of the carrier phase data acquired from a fixed base station and a GPS system mounted aboard the aircraft. This data must be recorded at 1 Hz data rate at both the dual frequency receivers.
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From page 168... ...
(Figure modified from Doll et al., 19931. DGPS REQUIREMENTS The aerogeophysical requirements for DGPS include use of dual frequency receivers recording carrier phase data at a high data rate (1 Hz)
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Kinematic Positioning with DGPS: Expanding Frontiers in Aerogeophysics National Research Council ~C) , 1995, Airborne Geophysics and Precise Positioning.
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From page 170... ...
Precise elevation measurements can also be made from an aircraft borne laser altimeter when the aircraft is positioned by differential GPS, with overall elevation accuracies in the 10-20 cm range. Such measurements have been made by NASA, starting in 1991, the first year in which the GPS constellation was adequate to support the 6-8 hour missions necessary to gather elevation measurements over ~ 1000 km profiles.
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From page 171... ...
-- ~ en, ~_ _ ~ ~ `.,- ~. , ~ ~# ~ ~ .~ ~ .~ FIGURE 1 Flight lines flown by NASA over Greenland during 1993-1995.
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172 The Global Positioning System for the Geosciences
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From page 173... ...
In assessing the accuracy of the elevation profiles (or swaths) obtained, the major error sources can be divided into the following categories: The aircraft trajectory as determined from differential GPS; Aircraft pitch and roll as provided by the aircraft INS; Laser bias, either constant or slowly varying; Laser measurement noise.
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From page 174... ...
(cm) ~, 950519 -8.57 5.90 950520 -3.46 6.18 950520 11.21 5.51 950522 -4.73 7.35 950522 7.70 6.69 950523 -5.95 6.13 950523 -2.95 5.79 950524 2.28 6.08 950524 1.17 5.59 950526 -3.79 6.79 950527 3.67 6.36 950527 -10.40 7.00 950530 -3 28 5.60 950530 -3.99 6.46 The Global Positioning System for the Geosciences The primary error source which should not be significant in the differences shown is GPS positioning error which should have negligible contributions from satellite ephemeris and tropospheric propagation errors, due to the fact that the reference station is only a few kilometers from the runway.
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From page 175... ...
. 930624 E-W -2.3 930628 W-E 1.1 930701 W-E 0.3 930701 E-W 0.9 930708 W-E 5.6 930709 E-W -5.4 940523 E-W 4.5 940526 W-E 2.1 940526 E-W 2.0 940528 E-W ~.4 940531 W-E 6.3 940531 E-W 1.8 940601 E-W 3.5 940606 W-E -3.5 940606 E-W -1.1 940607 W-E 4.3 940607 E-W 7.5 940608 W-E -8.0 940608 E-W 0.7 940609 E-W -2.7 940614 W-E -10.5 175 We now would like to show several of year to year comparisons of elevation profiles which demonstrate the capability of aircraft laser altimetry with differential GPS positioning to measure changes in ice surface elevation.
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We infer that elevation changes along this flight line are quite stable. We do not yet have sufficient confidence in the GPS trajectories and elevation measurements to state that the differences shown are geophysical changes.
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330 FIGURE 5 Differences between elevation measurements along EGIG flight line between 1993 and 1994.
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From page 178... ...
This quantity must be monitored in order to use the altimeter data to measure accurately any change in global sea level and to provide a basis for relating altimeter measurements from one mission to the next. The GPS receiver provides the data necessary to relate the altimeter range measurement to the range measurement inferred from the tide gauge and orbit determination data.
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From page 179... ...
MSS95, Yi, 1995 values in a five Km circle about the Harvest platform The differences may be explained by oceanographic features at the time of the experiment or by error sources in the either the GPS derived sea surface height or the OSUMSS95. This experiment showed that detailed mapping of ocean topography is feasible and could be used to obtain usefill information on the geoid and circulation features in areas of limited geographic extent.
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From page 180... ...
As stated before, the major problem with using GPS TEC data for oceanographic applications is the lack of stations in ocean areas and in the southern hemisphere. Consequently, any augmentation of the global GPS network with dual frequency receivers on islands or in coastal areas would be of benefit to oceanographic applications of satellite altimetry.
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From page 181... ...
Ionospheric path delay correction for single frequency altimeter missions requires additional ground sites equipped with dual frequency GPS receivers, especially in ocean areas and in the southern hemisphere. A possible alternative to more ground sites would be the use of ships of opportunity to carry dual-frequency GPS receivers across ocean basins.
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From page 182... ...
Since the altitude of both satellites is lower than T/P, the mission objectives cannot be achieved without a dual frequency GPS receiver because of greater ionosphere effects than experienced at the higher T/P altitude. Dual frequency GPS receivers which do not carry the AS-decryption keys have been developed and demonstrated in orbit.
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From page 183... ...
ANALYSIS AI? PROAClIES Centimeter-level orbit determination of a LEO satellite requires dual frequency carrier phase measurements, which have a precision of a few millimeters, as well as pseudorange measurements, which have a precision of a few decimeters.
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From page 184... ...
Such adjustments, however, have been shown by analysis of T/P data and simulations to be the preferred approach. The centimeter-level accuracy requires use of dual frequency GPS receivers for removal of the ionosphere, even for the LEO satellite.
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From page 185... ...
1996. GPS-based precision orbit determination for low altitude geodetic satellites, Ph.D.
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From page 186... ...
186 Over the past one to two decades considerably more effort in physical oceanography has been devoted to measuring absolute, as opposed to relative geostrophic, horizontal ocean currents. A major reason for this is that earlier assumptions (such as a "level of no motion")
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1~. ~"d - ~% /, RR ,1 pros FIGURE 1 Arrangement of Fast PEGASUS land reference GPS station and two "roving" GPS receivers which form the acoustic baseline for the observation site at sea..
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From page 188... ...
Therefore, initial tests were carried out using highly accurate Trimble SSE receivers at both buoys and et the land reference station (Leaman end Rocker, 1994; Leaman et al, 1995~. These are dual frequency p-code receivers capable of storing carrier phase information.as well; however, in addition to their high cost they are not particularly well suited to deployment from sea-going buoys, in part because of their size.
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From page 189... ...
We anticipate that this step will allow us to generate effectively "real-time" velocity profiles, in particular if a standard grid of land reference stations can provide DGPS data to the ship in a timely manner and at a sufficiently high data rate when needed. It should be noted that these reference sites do not have to be particularly close to the locations where we are making the profiles.
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From page 190... ...
Wilson, W.D., 1994. Deep ocean current profiling with a lowered broadband acoustic Doppler current profiler, OCEANS '94: Proceedings, Institute of Electrical and Electronics Engineers, New York 660-665.
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From page 191... ...
This paper describes the use of carrier differential techniques to determine the position, velocity, attitude and angular velocity of an autonomous aircraft in real time. The full state GPS estimates are used in linear quadratic (LQ)
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From page 192... ...
Each phase packet from the attitude TANS is processed to generate an attitude and angular velocity datum. Each phase packet from the position TANS must be combined with reference phase data with a matching time stamp from the reference station receiver to generate a position and velocity datum.
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From page 193... ...
Position and Velocity Position is calculated relative to the reference receiver antenna in a differential manner. One has two antennae, typically one on the ground (the reference station)
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From page 194... ...
Velocity and angular velocity solutions are calculated using phase-rate measurements stroked from the receiver's phase tracking loops at 10 Hz. The differentiation is achieved at the lowest signal processing level within the receiver by filtering the fundamental beatphase measurements against a steady state Kalman filter.
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From page 195... ...
Due to uplink latency of approximately 50ms, position and velocity solutions are calculated subsequent to the attitude/angular velocity calculation, as soon as timestamps from the local and reference position receivers can be matched. The aircraft position and velocity vectors, originally calculated in ENU, are subsequently expressed in runway relative and body relative bases respectively, using the known runway azimuth and the current attitude estimate.
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From page 196... ...
The yaw rate commensurate with the commanded bank angle of 25 degrees is approximately 10 deg/s. Heading hold is achieved by regulating the integral error from the commanded heading to zero while commanding a zero bank angle.
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During takeoff roll, bank angle is damped by ailerons and heading error is regulated by feedback to the nosewheel steering and rudder. Elevator is set to neutral and throttle to maximum.
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From page 198... ...
Alpha and beta measurements were not used in control laws directly, however a wind estimate derived from the alpha and beta vanes was used at a supervisory level to correct for the wind in predicting turn commencement for waypoint tracking. This is distinct from earlier flight tests on commercial aircraft, where GPS was used in auto-coupled approaches A
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From page 199... ...
CONCLUSIONS AND FUTURE RESEARCH We have demonstrated that GPS can be used to generate real time position, velocity, attitude and angular velocity signals that have utility for automatic control. We have used these signals to achieve autonomous takeoff to landing of a real airplane for the first time.
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From page 200... ...
This in turn leads to velocity and angular velocity solutions that are equivalently more noisy than the analogous position or attitude solution. The noise in all measurements may be reduced at the expense of tracking performance by reducing the bandwidth of the phase state estimator.
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