An absolute calibration site for radar altimeters in the continental domain: Lake Issykkul in Central Asia

Altimetry missions such as Topex/Poseidon, Jason-1, GFO and ENVISAT have been widely used in the continental domain over lakes, rivers and wetland although they were mostly dedicated to oceanic studies. Knowledge of the instrumental biases is a key issue. Numerous sites have been dedicated to calibration purposes, either in the oceanic domain (Harvest offshore platform in California, Corsica, Bass Strait in Australia) or over lakes (Lake Erie in United States). A new site (Lake Issykkul in Kirghizstan) is proposed for calibration in the continental domain. This lake is covered by past (T/P) and current radar altimetry satellites (Jason-1, T/P, GFO, and ENVISAT). Several in situ water levels and local meteorological variables are available at the site. Located in a mountainous area, it offers an opportunity for calibration far away from all other existing sites and very different environment contexts. Two GPS campaigns have been conducted on the lake in 2004 and in 2005. They consisted of cruises with stations installed onboard a boat following the satellite ground tracks, and onshore settings. This enabled estimating a bias for each altimeter and each tracking algorithm available. Biases obtained for Envisat, GFO, T/P and Jason-1 using the default ocean tracker (respectively, 48.1 ± 6.6, 7.5 ± 4.0, 0 ± 4.3 and 7.0 ± 5.5 cm) agree with biases published at the other calibration sites. For Jason-1, there is a significant disagreement with results obtained in the ocean field (7 cm instead of 13 cm) but is coherent with bias obtained on the Lake Erie site. Erroneous estimates of the sea state bias correction from non-oceanic-like waveforms is discussed as a possible explanation. Errors in the ionospheric, wet and dry tropospheric corrections for the continental domain are also highlighted and quantified.     

Integer ambiguity resolution in precise point positioning: method comparison

Integer ambiguity resolution at a single receiver can be implemented by applying improved satellite products where the fractional-cycle biases (FCBs) have been separated from the integer ambiguities in a network solution. One method to achieve these products is to estimate the FCBs by averaging the fractional parts of the float ambiguity estimates, and the other is to estimate the integer-recovery clocks by fixing the undifferenced ambiguities to integers in advance. In this paper, we theoretically prove the equivalence of the ambiguity-fixed position estimates derived from these two methods by assuming that the FCBs are hardware-dependent and only they are assimilated into the clocks and ambiguities. To verify this equivalence, we implement both methods in the Position and Navigation Data Analyst software to process 1 year of GPS data from a global network of about 350 stations. The mean biases between all daily position estimates derived from these two methods are only 0.2, 0.1 and 0.0 mm, whereas the standard deviations of all position differences are only 1.3, 0.8 and 2.0 mm for the East, North and Up components, respectively. Moreover, the differences of the position repeatabilities are below 0.2 mm on average for all three components. The RMS of the position estimates minus those from the International GNSS Service weekly solutions for the former method differs by below 0.1 mm on average for each component from that for the latter method. Therefore, considering the recognized millimeter-level precision of current GPS-derived daily positions, these statistics empirically demonstrate the theoretical equivalence of the ambiguity-fixed position estimates derived from these two methods. In practice, we note that the former method is compatible with current official clock-generation methods, whereas the latter method is not, but can potentially lead to slightly better positioning quality.     

Access to the EGNOS signal in space over mobile-IP

The EGNOS service will provide better positioning availability and accuracy than that from the standalone GPS solutions. However, in order to access the EGNOS service, the end user needs to access the corresponding GEO satellites that broadcast the augmentation information for the region. This is not a problem normally for aviation and maritime applications because an open sky is always available for such applications. However, an open sky is not always available for land applications because of the obstacles in the vicinity of the end users, for example, in the city canyons. The situation gets worse for the regions at high latitudes because the elevation angles to the GEO satellites are rather low (e.g. 4–22° in Finland). This article describes briefly the SISNeT technology, designed and developed by the European Space Agency, which allows accessing the EGNOS SIS via the Internet. It will describe in detail the handheld SISNeT receiver, designed and developed by the Finnish Geodetic Institute under ESA contract. The SISNeT data server is an IP-based server that acquires the EGNOS messages from an EGNOS receiver, and broadcasts them over the Internet in real-time. The handheld receiver consists of a GPS PC-card receiver, a GPRS (or GSM) card phone, and a pocket PC as the host platform. The receiver software is a Windows CE-based package with a multi-process and multi-thread architecture. It simultaneously receives: (1) the EGNOS SIS over a GPRS wireless connection and the Internet and (2) the NMEA messages from a serial connection to a GPS receiver. It decompresses and decodes the EGNOS messages, and utilizes the information in the messages to estimate the EGNOS-corrected coordinates, which are finally delivered to the end user via a virtual COM port. The virtual COM port has been implemented as a stream interface driver in the Pocket PC. It can be accessed in the same way as the physical COM port in a GPS receiver is accessed. Therefore, it is easy to interface to any third-party applications. The test results show that the handheld SISNeT receiver can provide a positioning accuracy of about 1–2 m for the horizontal components, and 2–3 m for the vertical component in real time. Due to the poor performance of the wireless connection, 10–30% of the EGNOS messages can be lost depending on the services provided by the wireless network operators. The impact of the messages lost on the positioning accuracy is about 0.5 m in both the horizontal and vertical components. Electronic Publication     

Determining cooling rates from mica 40Ar/39Ar thermochronology data: Effect of cooling path shape

Tectonic models are commonly underpinned by metamorphic cooling rates derived from diffusive‐loss thermochronology data. Such cooling ages are usually linked to temperature via Dodson's closure temperature (T_C) formulation, which specifies a 1/time‐shaped cooling path (Contributions to Mineralogy and Petrology, 1973, 40, 259). Geologists, however, commonly discuss cooling rates as a linear temperature/time shape. We present the results of a series of simple finite‐difference diffusion models for Ar diffusion in muscovite and biotite that show that the difference in recorded age between 1/t and linear cooling paths increases significantly with hotter starting temperatures, slower cooling rates and smaller grain sizes. Our results show that it is essential to constrain the cooling path shape in order to make meaningful interpretations of the measured data.     

The Arctic Coastal Dynamics Database: A New Classification Scheme and Statistics on Arctic Permafrost Coastlines

Arctic permafrost coasts are sensitive to changing climate. The lengthening open water season and the increasing open water area are likely to induce greater erosion and threaten community and industry infrastructure as well as dramatically change nutrient pathways in the near-shore zone. The shallow, mediterranean Arctic Ocean is likely to be strongly affected by changes in currently poorly observed arctic coastal dynamics. We present a geomorphological classification scheme for the arctic coast, with 101,447?km of coastline in 1,315 segments. The average rate of erosion for the arctic coast is 0.5?m? year^?1 with high local and regional variability. Highest rates are observed in the Laptev, East Siberian, and Beaufort Seas. Strong spatial variability in associated database bluff height, ground carbon and ice content, and coastline movement highlights the need to estimate the relative importance of shifting coastal fluxes to the Arctic Ocean at multiple spatial scales.     

Kimberlites distribution in Angola and prospective areas for new discoveries

Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.

     

Capital and Empire: Geographies of Imperial London

The notion of empire has often been regarded in Europe as a matter of diffusion and expansion; something which happened over there rather than close to home. Yet the form, use and representation of modern European cities have been shaped by the global history of imperialism in ways that continue to matter even in an apparently post-imperial age. The signs of empire were prominently displayed within the built environments of all the major cities of late-nineteenth century Europe, as they came (in different ways) to play the role of regional, national and imperial capitals. In what was evidently a pan-European discourse on the imperial city between the mid-nineteenth century and the mid-twentieth, national models were defined in relation to other national models, in a spirit of competition as much as emulation. This paper examines the case of London. British architects and planners frequently complained that London lagged behind its rivals in the struggle for imperial primacy, given the absence of state-sponsored projects to parallel Haussmann's rebuilding of Paris or Leopold's grand plans for Brussels. At the intra-urban scale, the imperial city had a geography which mattered: in the case of London, different parts of the city were associated with different aspects of empire. More generally, it is clear that national debates over imperial urbanism were conditioned not simply by understandings of the global reach of European empires, but also by attitudes towards social, cultural and political change within Europe itself.