Geodesy in the Year 2000 (1990) / Chapter Skim
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GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000
GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000
Pages 124-177
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From page 124... ...
Given the current performance of GPS, particularly on regional baselines, one can say that GPS has arrived. Its attributes of high accuracy and relatively low cost make it the geodetic technique of choice in the l990s for most of the regional deformation studies that require high temporal and spatial resolutions.
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From page 125... ...
The total zenith delay of the dry troposphere is about 2 m and is readily determined barometrically to a precision of about 1 mm using standard atmosphere models. Departures from hydrostatic equilibrium of the dry component of the troposphere leads to zenith delay mix-modeling that can amount to a few millimeters in certain dynamical conditions.
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From page 126... ...
Also, for equatorial and high latitude observations, particularly those made during very high solar activity, ionospheric scintillations can be troublesome. The rapid phase variations and concomitant signal amplitude in the L-band carriers induces tracking errors in-the tracking loops of some receivers and even loss of lock; on the other hand, some receivers of more recent design are capable of tracking signal frequency accelerations of up to several g's, even in adverse signal conditions, without significant error at the millimeter level.
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From page 127... ...
For these regions, an equally successful approach is to stochastically model the tropospheric zenith delay as a piecewise constant random walk or as a first order Gauss-Markov process. The values of the constants in this time series are adjusted in a batchsequential least squares process in which both the transition from one value to the next and the length of time over which the vertical delay is held fixed are a priori constrained by the stochastic model.
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From page 128... ...
While it may be theoretically possible for GPS-based geodetic systems to achieve millimeter horizontal baseline accuracies with major capital outlays, as a practical matter we may be mostly limited to accuracies attainable with the stochastic modeling approach. Stochastic modeling will reach an impasse at some accuracy level.
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From page 129... ...
Figure 2 gives an example of multipath in carrier phase measurements. Here, the plots are the post-fit smoothed residuals of double-differenced carrier phase measurements of two GPS satellites from two ground receivers.
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From page 130... ...
Pseudorange measurements are made by matching local models of the ranging code generated at the receiver with codes being received from the GPS satellite. In general, the received signal is a combination of direct and reflected signals with different delays, phases and amplitudes.
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From page 131... ...
131 using a Rogue receiver which has a Dorne & Margolin antenna coupled with a choke ring backplane designed at JPL (see insert on Figure 4~. These results are from a single receiver tracking a single GPS satellite whose elevation with time is also shown.
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From page 132... ...
1. Tracking GPS satellites with dual band carrier phase and 5 cm pseudorange would yield highly accurate global ephemerides obtained with far less tracking per pass than is required today.
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From page 133... ...
It will also be used for high accuracy orbit determination of future Earth satellite missions such as TOPEX/Poseidon which will carry a GPS flight-rated receiver (Yunck et al., 1985~. This approach obviates the need for extensive dynamical models to generate the motion of the user platform.
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From page 134... ...
Without P and P2 the uncertainty in the differential ionospheric delay limits the reliability of this fixing to baselines or a maximum length that depends on solar activity, time of day, elevation angles of the GPS satellites, and geographical location.
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From page 135... ...
The limiting error source in this case probably will be the mix-modeling of the phase center variation of the GPS satellite antenna array. This varies with Foresight angle at the satellite and the latter ranges from zero up to a maximum of 14.3 degrees.
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From page 136... ...
in baseline solution comparisons made over successive experiments, even though intra-experiment repeatability might be substantially better. Moreover, even with antennas with identical phase center variability with azimuth and elevation, one is likely to incur a baseline length dependent error owing to the differing direction of a GPS satellite as viewed at the ends of the baseline.
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From page 137... ...
By processing concurrent tracking data from the fiducial and the geodetic sites and incorporating the a priori site coordinates, one effectively aligns the GPS orbits with a reference frame defined by these fiducial points and thereby transfers the fiducial information via the GPS ephemerides to the geodetic sites within the network. There are variations on this approach, mainly concerning how the ephemerides are generated and controlled, and how their information is transferred to the geodetic sites; but strong a priori fiducial information from VLBI and SLR is the hallmark of the fiducial approach.
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From page 138... ...
These currently cause errors in horizontal baseline estimates that are probably at the level of a few millimeters; they will need to be refined to achieve millimeter accuracy. GPS ephemeris errors for well-modeled data analysis systems appear to be somewhat data noise-limited in regional fiducial applications, particularly for GPS satellites with poor viewing geometry.
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From page 139... ...
Instead, it relies on the dynamical consistence of the GPS constellation for the reference frame as embodied in the GPS ephemerides, in the set of derived station coordinates of a globally distributed GPS tracking system, in the Earth orientation parameters obtained from a global monitoring system, and in the transformations between the conventional inertial system and the conventional terrestrial system. Why would the Global Framework be preferred by some over the fiducial approach?
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From page 140... ...
Figure 7 shows that the geocentric offset is determined to better than 5 cm accuracy after only 1 day of tracking (Wu and Malla, 1988~. In short, this system appears capable of providing on a continuous basis a reference frame whose accuracy may well approach 1 part in 109 by the mid-199Os.
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From page 141... ...
SA dithers GPS Pcode clock frequency in a pseudo random fashion but the clock epoch varies within certain bounds as prescribed by the 100 m 2 arms point position accuracy requirement. This means that the effects of SA clock frequency dithering manifest themselves as GPS satellite clock epoch errors and, potentially, as a mean error in the length scale when the carrier phase and pseudorange measurements are converted from units of light-see to centimeters.
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From page 142... ...
The fiducial sites are particularly suited for this; they frequently can provide H-maser timing to the GPS receiver; also, the fiducial baseline vectors are known a priori and thus, their solutions in the estimation process should be essentially invariant to any excursions in mean clock frequency. For example, one could treat the SA frequency dithering as a piecawise constant stochastic process and the clock epochs as a pseudorandom walk process.
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From page 143... ...
conventional standardization of procedures and of the configuration of data acquisition and information systems, and g. use of low Earth orbiters flying GPS receivers in conjunction with ground arrays to reduce the sensitivity of baseline estimates to tropospheric mix-modeling, to improve the vertical components, and to enhance the temporal resolution of baseline variability.
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From page 144... ...
of each line item to the horizontal baseline components, after all calibrations, modeling and estimation procedures have been applied using state-of-theart procedures. If present trends continue, I would expect the vertical component to be about three times worse, but the LEO satellites carrying GPS receivers might enable one who uses this tracking information to reduce it to only a factor of two.
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From page 145... ...
Major advances in VLSI and solid state technology made during the 1980s are pivotal in reducing the capital costs of GPS receivers in the 1990s. These advances will result in greatly reduced component and fabrication costs.
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From page 146... ...
Figure 11 diagrams the high level organization of this digital front end chip (DFE) showing its four major functions: a RF sampler and A/D conversion section, a reference clock synthesizer section, a control logic section, and a digital processing section.
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From page 147... ...
; its digital samples can be sent by a fiber optic link to the receiver, which may be kilometers away. A single receiver could service an array of antennas via multiple fiber optic links.
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From page 148... ...
Here are some of the operational features that I would expect smart GPS receivers of the 1990s to have: 1. Extensive on-board processing with special purpose hardware, 2.
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From page 149... ...
Probably the biggest factor in reducing data analysis costs is establishing standardization across many system elements: hardware and software configurations; data formats; procedures in site selection, monumentation, data acquisition and data processing; tracking network; reference system; scenarios; archiving; and so on. Nothing takes more time in data analysis than encountering and dealing with the unexpected.
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From page 150... ...
Networks for maintenance of terrestrial reference systems including GPS ephemerides and fiducial site coordinates plate motions and Earth orientation variability; 3. Tracking and navigation systems for satellite dynamics and for satellite and/or air-borne remote sensing applications; 4.
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From page 151... ...
Post-processing by the Center would be greatly streamlined over current off-line capabilities, which run at roughly one-tenth the real-time rate. Powerful parallel processing architecture will be commonplace; sophisticated post-filter editors will be used for combining separate network information, e.g., yesterday's solutions and information matrices with today's, or information from one array with that from another or from a reference or global tracking network, the Center will provide highly interactive capabilities for users and efficient and automated data management and archival systems.
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From page 152... ...
Future kinematic surveying will also be effected with a single roving GPS receiver operating in conjunction with a remote array, thereby enabling virtual baselines among sites occupied by the rover to be measured with carrier range accuracy using only the one receiver. GPS Global Tracking System.
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From page 153... ...
On the other hand, preliminary results from the global tracking network used in the CASA UNO experiment suggest that it facilitates cycle ambiguity resolution on the continental-sized baselines of that experiment. If this is true, it would lead to significant accuracy advances on these baselines.
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From page 154... ...
Ultimately, centimeter accuracy should be achieved using precision orbit determination strategies that synergistically combine carrier phase and pseudorange. A critical component in GPS-based satellite POD is the GPS Global Tracking System; the ground stations of this network must concurrently track the GPS satellites to achieve these orbit accuracies in differential positioning.
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From page 155... ...
By also flying a GPS receiver, which is now planned, it should recover valuable gravity information up to a wavenumber of about 60. This maximum wavenumber falls short of the 200-300 that could be achieved with the Gravity Research Mission using a micron accuracy microwave satellite-to-satellite tracking technique, or alternatively, with superconducting three-axis gravity "radiometer.
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From page 156... ...
7. An internationally sponsored and cooperative GPS global tracking network will be operating to maintain a terrestrial reference system approaching an accuracy of 0.001 ppm, to support Earth orientation monitoring, and to support ground programs and Earth satellite missions requiring high accuracy positioning.
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From page 157... ...
They are: Sassan Bassiri, Geoff Blewitt, Tim Dixon, Michael Janssen, Steve Lichten, Tom Meehan, Ruth Neilan, Benno Rayhrer, John Scheid, and Larry Young, all from JPL. This work was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract from the National Aeronautics and Space Administration.
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From page 158... ...
Dong, D., and B Bock, GPS Network AnalYsis with Phase Ambiguity Resolution Applied to Crustal Deformation Studies in California, J
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From page 159... ...
Krabill, Decimeter Level Aircraft Positioning with GPS Carrier-Phase Measurements, Bulletin Geodesique, in press, 1989. Melbourne, W., Series X Cycle Ambiguity Resolution Using Bandwidth Synthesis and "Angle SYnthesis " IOM #3300-82-119, Jet Propulsion Laboratory, 1982.
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From page 160... ...
19-23, 1988, Fort Lauderdale, FL, 1988. Sims, M, Phase Center Variation in the Geodetic TI4100 GPS Receiver SYstem's Conical Spiral Antenna, First International Symposium on Precise Positioning with the Global Positioning System, Positioning with GPS-1985, Vol.
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From page 161... ...
Bletzacker, GPS Satellite Multipath:_ An Experimental Investigation, First International Symposium on Precise Positioning with the Global Positioning System: Positioning with GPS-1985, Vol.
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From page 162... ...
a) and by Difference in Horizontal Components of Baselines Versus Baseline Length; Continental U.S.
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From page 163... ...
a) Double Differenced ionosphere Corrected, Post-Fit, and 2 Minute Gaussian Smoothed Carrier Phase Residuals Using a Pair of Tl-4100 Receivers with the Standard Tl Antenna Observing SV 9 and SV 11 from Mojave and OVRO.
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From page 164... ...
164 o o o CD o o oo LL z ~: LLJ CL o cn ~ Z U)
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From page 165... ...
P-code pseudorange multipath obtained by a Rogue receiver operating with a Dorne & Margolin antenna coupled with a quarter wavelenth choke-ring backplane designed by JPL; a) Ordinate is the linear combination of Equations 1 and 2 that eliminates non-dispersive and ionospheric delays and should be a constant in absence of multipath and SNR errors.
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From page 166... ...
166 z o 6 of c G UJ LL LL in J cn 6 m A O In LL a: ILL 1 I T Z C)
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From page 167... ...
F Z LL O Z Z , Y · C: ~ 6 z tar O ~ F A: ~ z m ~r O 1,LI J Z cn rNc ~~"'$ ~ ~.
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From page 168... ...
Predicted Accuracy Versus Tracking Time for Recovery of Geocentric Offset in Reference Frame Using Full 21-Satellite Block 11 Constellation and a 12-Station Global Tracking Network.
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From page 169... ...
169 BROAD COVERAGE IMPROVES SHORT-TERM GPS ORBIT DETERMINAT10N SERVES AS AN INTERMEDIATE OBSERVING POINT BETWEEN WIDELY SEPARATED RECEIVERS EXTENDS VERTICAL VIEWING GEOMETRY REDUCES SENSITIVITY TO TROPOSPHERE , LEO/ / ~ J / / Figure 8. Utility of LEO in enhancing accuracies of baselines and GPS ephemerides using a global tracking network.
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From page 170... ...
ICY . B.P Fiber B.P liter _ >' DIGITAL BASE BAND DATA L-BAND RF data _ DOWN- CONVERTER AC'C'=~.AOI~ 11 1~C~11~1 i ~: I'm 1 ~_ DSP BOARD === _ ~ ~ Cos 0 _ ~ ~ Sin0 ~ _ _ F Gen 1 1~l~ Satellite B Satellite A ~~ =~ ~ D , a, 1 11 ~ Hey.
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From page 171... ...
. The Rogue to Turbo-Rogue Transition utilizing the DFE chip for RF down conversion to baseband digital sampling and the turbo-chip for baseband signal processing.
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From page 172... ...
Functional Schematic of an Analog Front End for RF Down-Conversion to Baseband, A/D Conversion, and Digital Sampling. Digital Front End Eliminates Analog Components that are Expensive and Sources of Error.
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From page 173... ...
DIGITAL BASEBAND O 10 - 600 MEGA SAMPLES 4 TO 1 0 BIT QUADRATURE SAMPLES SELECTABLE COMPLETED DESIGN PRELIMINARY FA3RlCATION IN PROCESS - mom b) Figure 11.
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From page 174... ...
174 ~ -- ~.-~;~ Or, stat ~ ons `Th, ee Is ~' ~ e'-~`v' =~w,-`edi GAG - ~4 I ,- ~t ~ 0~ Add: AN OF ~ GLOE! ~L | ~.~tF=rr,at~ ones Cavern ng 1_ IFS Tract..nc~ System arid C:ompe' i In:: i ^1 Beat i u.ns -- ALE.
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From page 176... ...
Predicted Gravity Recovery from GP-B Using a TOPEX/Poseidon Class GPS Flight Receiver in Conjunction with a GPS Global Tracking System. Mission Duration is 6 Months.
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From page 177... ...
Predicted Gravity Recovery from GP-B Using a TOPEX/Poseidon Class GPS Flight Receiver in Conjunction with a GPS Global Tracking System. Mission Duration is 6 Months.
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