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991.
Assessment of the LandStar Real-Time DGPS Service under Several Operational Conditions 总被引:1,自引:0,他引:1
LandStar is a differential global positioning service (DGPS) that provides 24-h real-time positioning for various applications
on land, water, and air in North America, Australia, New Zealand, Europe, and Africa. Its focus is on real-time applications
requiring a submeter positioning capability such as agriculture, forestry, Geospatial Information Systems (GIS), survey/mapping,
and land/vehicular navigation. LandStar uses a Wide Area Network of reference stations to derive DGPS corrections to model
the variation of GPS error sources over a large area. These model parameters are used by the Virtual Reference Station processors
to calculate standard corrections that are available for all predefined locations in the network. The corrections are transmitted
to the user by L-band satellite communication in the standard RTCM SC104 DGPS correction format. This article investigates
the performance of the LandStar Mk III system under various operational conditions and assesses its performance in both static
and kinematic modes. Four field tests were conducted during 12 months that tested the sysem in clear static and kinematic
conditions as well as suboptimal environments associated with low and heavy foliage conditions. Both the accuracy and availability
of the system under these conditions is investigated, with an emphasis on whether the above variables are caused by the LandStar
system differential corrections, the GPS measurements, or a combination of both. ? 1999 John Wiley & Sons, Inc. 相似文献
992.
Three methods to correct for the atmospheric propagation delay in very-long-baseline interferometry (VLBI) measurements were
investigated. In the analysis, the NASA R&D experiments from January 1993 to June 1995 were used. The methods were compared
in correcting for the excess propagation delay due to water vapour, the “wet” delay, at one of the sites, the Onsala Space
Observatory on the west coast of Sweden. The three methods were: (1) estimating the wet delay using the VLBI data themselves;
(2) inferring the wet delay from water vapour radiometer (WVR) data, and (3) using independent estimates based on data from
the global positioning system (GPS). Optimum elevation cutoff angles were 22∘ and 26∘ when using WVR and GPS data, respectively. The results were found to be similar in terms of reproducibility of the estimated
baseline lengths. The shortest baselines tend to benefit from external measurements, whereas the lack of improvement in the
longer baselines may be partly due to the large amount of data thrown away when removing observations at low elevation angles.
Over a 2 week period of intensive measurements, the two methods using external data showed an overall improvement, for all
baseline lengths, compared to the first method. This indicates that there are long-term systematic errors in the wet delay
data estimated using WVR and GPS data.
Received: 27 October 1998 / Accepted: 20 May 1999 相似文献
993.
994.
V. K. Srivastava A. M. Rai R. K. Dixit M. P. Oza A. Narayana 《International Journal of Applied Earth Observation and Geoinformation》1999,1(3-4)
Sal (Shorea robusta) is an important forest tree species in north and north-eastern India. Large-scale plantations of this species have been raised there under taungya and coppice system of management. The conventional volume table prepared for high sal forest is referred to infer the volume of production of this species. Earlier workers have used aerial remote sensing data to develop volume tables of this species. In the present study a volume table for sal is developed based on remotely sensed satellite data using a regression technique. A two-step method was developed to estimate mean tree volume from satellite data. In step 1, mean crown diameter — an intermediate variable - was estimated from satellite data. In step 2, the estimated mean crown diameter was used to estimate the mean tree volume. Addition of age of the crop as an independent variable improved the predictive ability of the regression equation. 相似文献
995.
996.
R. S. Tiwari Manoj K. Arora T. Kailash 《Journal of the Indian Society of Remote Sensing》1999,27(4):225-234
With increasing resolution of the remotely sensed data the problems of images contaminated by mixed pixels arc frequent. Conventional
classification techniques often produce erroneous results when applied to images dominated by mixed pixels. This may load
to unrealistic representation of land cover, thereby, affecting efficient planning, management and monitoring of natural resources.
Consequently, soft classification techniques providing sub-pixel land cover information may have to be utilised.
From a range of soft classification techniques, the present study focuses on the utility of conventional maximum likelihood
classifier and linear mixture modelling for sub-pixel. land cover classifications. The accuracy of the soft classifications
has been assessed using distance measures and correlation co-efficient. The results show that linear mixture modelling has
produced accuracies comparable to maximum likelihood classifier. Besides this the correlations between actual land cover proportions
and proportions from linear mixture modelling, though not strong, arc statistically significant at 95% level of confidence.
It has also been observed that the normalised likelihoods of maximum likelihood classifier also show strong correlations with
the actual land cover proportions on ground and therefore has the potential to be used as a soft classification technique. 相似文献
997.
Anjana Pant S. G. Chavan P. S. Roy K. K. Das 《Journal of the Indian Society of Remote Sensing》1999,27(3):133-139
Habitat analysis for sambar in terms of food, cover, water, space and extent of edge in Corbett National Park using remote sensing and GIS has been attempted. Other physical parameters include climate, topography, fire history, disturbance regimes, weeds etc. IRS-IB LISS II data (FCC, hardcopy) on 1:50.000 scale was interpreted to generate vegetation cover and density map. Other maps showing drainage, water bodies, roads, human habitations and contours were prepared using Survey of India topographical maps. During evaluation of sambar habitat information regarding habitat parameters and their tolerance was collected from existing literature as well as during field observations. Twenty-two transects of one km. length were laid down in all the strata randomly to collect information regarding the structure and composition of the forest and also habitat use (direct and indirect evidences) by sambar. This was then integrated using condition-based equations in the GIS domain to generate suitability maps. Actual sightings on the ground to a large extent supported the results. 相似文献
998.
G P Singh 《Journal of the Indian Society of Remote Sensing》1999,27(1):1-8
An attempt is made to mark regional geology of part of Lalitpur district. The area is characterized by flat topography gently sloping east ward with isolated hillocks. The well contrasted lithology has been marked on the basis of variation in tone, drainage pattern and texture. The main lithounits are foliated granites/gneiss, banded iron formation, granites, ultra basic, quartz reef and basic dykes. The structural elements of lineaments are visible on aerial photos and imagery which range in length from 1 to 10 km. The lineaments at places intersect with each other. The major trends are NNW- SSE. NE- SW. NNH- SSW. These NNW- SSE and NE-SW lineaments are retlected by well aligned body of quartz- sericite schist and quartz reef respectively. 相似文献
999.
M D Shamim Ansari R. K. Mahey S. S. Sidhu 《Journal of the Indian Society of Remote Sensing》1999,27(4):185-192
To predict the crop yield from spectral parameters, a field experiment was conducted on cotton crop during 1997-98 Kharif season on a sandy loam soil at the Punjab Agricultural Unjversity, Ludhiana. India. Spectral reflectance and agronomic measurements were made for cotton species (American and Desi cotton), sown on two dates (May 1 and May 29) under five nitrogen levels (0, 40, 80, 120 and 160 kg/ha). Regression analysis showed that growth variables had poor correlation with seed cotton yield for all three models, however, yield attributes were significantly and highly correlated for second degree model with seed cotton yield. The integrated Radiance Ratio (RR) and Normalized Difference Vegetation Index (NDVI) measured over time were significantly correlated quadratically with seed cotton yield on three time segment periods viz., 81–110, 111–140 and 141–200 DAS, but highest correlation values were obtained during 81–110 DAS, In American cotton, the highest correlation coefficient for RR and NDVI were 0.91 and 0.81, respectively; whereas for Desi cotton these values were 0.88 and 0.84, respectively. 相似文献
1000.
The least-squares ambiguity decorrelation adjustment: a method for fast GPS integer ambiguity estimation 总被引:71,自引:26,他引:71
P. J. G. Teunissen 《Journal of Geodesy》1995,70(1-2):65-82
The GPS double difference carrier phase measurements are ambiguous by an unknown integer number of cycles. High precision relative GPS positioning based on short observational timespan data, is possible, when reliable estimates of the integer double difference ambiguities can be determined in an efficient manner. In this contribution a new method is introduced that enables very fast integer least-squares estimation of the ambiguities. The method makes use of an ambiguity transformation that allows one to reformulate the original ambiguity estimation problem as a new problem that is much easier to solve. The transformation aims at decorrelating the least-squares ambiguities and is based on an integer approximation of the conditional least-squares transformation. This least-squares ambiguity decorrelation approach, flattens the typical discontinuity in the GPS-spectrum of ambiguity conditional variances and returns new ambiguities that show a dramatic improvement in correlation and precision. As a result, the search for the transformed integer least-squares ambiguities can be performed in a highly efficient manner. 相似文献