Retrieving Precipitable Water Vapor From Shipborne Multi‐GNSS Observations

©2019. American Geophysical UnionPrecipitable water vapor (PWV) is an important parameter for climate research and a crucial factor to achieve high accuracy in satellite geodesy and satellite altimetry. Currently Global Navigation Satellite System (GNSS) PWV retrieval using static Precise Point Positioning is limited to ground stations. We demonstrated the PWV retrieval using kinematic Precise Point Positioning method with shipborne GNSS observations during a 20‐day experiment in 2016 in Fram Strait, the region of the Arctic Ocean between Greenland and Svalbard. The shipborne GNSS PWV shows an agreement of ~1.1 mm with numerical weather model data and radiosonde observations, and a root‐mean‐square of ~1.7 mm compared to Satellite with ARgos and ALtiKa PWV. An improvement of 10% is demonstrated with the multi‐GNSS compared to the Global Positioning System solution. The PWV retrieval was conducted under different sea state from calm water up to gale. Such shipborne GNSS PWV has the promising potential to improve numerical weather forecasts and satellite altimetry

Evaluation of earth rotation parameters from modernized GNSS navigation messages

Modernized navigation messages of global navigation satellite systems like GPS CNAV include earth rotation parameters (ERPs), namely the pole coordinates and UT1-UTC (∆UT1) as well as their rates. Broadcast ERPs are primarily needed for space-borne GNSS applications that require transformations between earth-fixed and inertial reference frames like navigation in earth orbit as well as to the moon. Based on a global tracking network of 23 stations, broadcast ERP values are obtained for the global systems GPS and BeiDou as well as the regional QZSS and IRNSS. Subsequent data sets at daily intervals show polar motion discontinuities of 0.4 to 0.7 mas for GPS, QZSS, and IRNSS, whereas BDS is worse by a factor of about two. Discontinuities in ∆UT1 range from 0.17 to 0.45 ms. External comparison with the C04 series of the International Earth Rotation and Reference Systems Service results in polar motion RMS differences of 0.3 to 1.0 mas and ∆UT1 differences of about 0.13 ms for GPS, QZSS, and IRNSS. Due to less frequent update intervals, BDS performs worse by a factor of 2 – 4. In view of the current GNSS-based positioning errors at geostationary or even lunar distances, the accuracy of GPS, QZSS, and IRNSS broadcast ERPs is sufficient to support autonomous spacecraft navigation without the need for external data

On the Response of Polarimetric GNSS-Reflectometry to Sea Surface Roughness

Reflectometry of Global Navigation Satellite Systems (GNSS) signals from the ocean surface has provided a new source of observations to study the ocean-atmosphere interaction. We investigate the sensitivity and performance of GNSS-Reflectometry (GNSS-R) data to retrieve sea surface roughness (SSR) as an indicator of sea state. A data set of one-year observations in 2016 is acquired from a coastal GNSS-R experiment in Onsala, Sweden. The experiment exploits two sea-looking antennas with right- and left-hand circular polarizations (RHCP and LHCP). The interference of the direct and reflected signals captured by the antennas is used by a GNSS-R receiver to generate complex interferometric fringes. We process the interferometric observations to estimate the contributions of direct signals and reflections to the total power. The power estimates are inverted to the SSR using the state-of-the-art model. The roughness measurements from the RHCP and LHCP links are evaluated against match-up wind measurements obtained from the nearest meteorological station. The results report on successful roughness retrieval with overall correlations of 0.76 for both links. However, the roughness effect in LHCP observations is more pronounced. The influence of surrounding complex coastlines and the wind direction dependence are discussed. The analysis reveals that the winds blowing from land have minimal impact on the roughness due to limited fetch. A clear improvement of roughness estimates with an overall correlation of 0.82 is observed for combined polarimetric observations from the RHCP and LHCP links. The combined observations can also improve the sensitivity of GNSS-R measurements to the change of sea state

GNSS Observations for Remote Sensing in the Arctic

In a series of expeditions to the Arctic, high-rate observation data of Global Navigation Satellite Systems (GNSS) have been acquired. These expeditions include the cruises of research vessels Lance, Polarstern and Kronprins Haakon in a period between 2016 and 2021. The observations are designated for remote sensing and allow application for ionospheric monitoring (scintillation indices) and sea-ice characterization (signal reflection power). The objective of our ongoing studies is to develop new pathways for these applications with ships operating in the Arctic. The global coverage of GNSS observations, including the Arctic, give us the opportunity to extend existing observing systems for a better understanding of the exceptional Arctic environment in terms of space weather and sea-ice evolution. Current investigations aim to validate GNSS-based sea ice monitoring for different seasonal conditions, and to examine the space weather impact on GNSS reflection power estimates for long-term monitoring in the Arctic. Different GNSS receiver types, designated for reflectometry, scintillation detection and atmosphere sounding, were used. The data comprise cruises to Fram Strait (in 2016 and 2017), to the Central Arctic in 2019/20 during the one-year MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate) and seasonal cruises to Barents Sea in 2021 within the Norwegian Nansen Legacy project

Temperature-Induced Bias Variations of Multi-Frequency Receivers

Precise GNSS applications, like PPP or RTK, make use of pseudorange and carrier-phase data. Traditionally, the ionospheric delays are removed to first order by using dual-frequency data. More recently, three or more frequencies are available from modernized and newly emerged systems. Most observation models assume the inter-signal biases for pseudoranges and carrierphases to be time-invariant. A violation of this assumption may be uncritical for dual-frequency processing in many cases, but with three frequencies the variations are not absorbed any more by nuisance parameters and may affect actual estimation parameters of interest. Coping with variable receiver biases is essential for many GNSS applications particularly for high-precision positioning where fast ambiguity resolution relies on the use of triple-frequency observations. This study demonstrates the effects of temperature-dependent receiver bias variations for dual-frequency and triple-frequency data. Real tracking data from geodetic receivers is used together with simulated observations. The bias variations are studied using measurement residuals from positioning solutions as well as triple-frequency signal combinations. Possible means of mitigating the variations through adaptions of the estimation parameters as well as changes in receiver operation are shown

Coherent GNSS Reflections over the Sea Surface: A Classification for Reflectometry

The exploitation of GNSS signals for reflectometry opens several fields of application over the ocean, land and in the cryosphere. Coherence of the reflection allows precise measurements of the carrier phase and signal amplitude for accurate sea surface altimetry and sea ice characterisation. A coherence condition can be set by a threshold of the signal-to-noise power ratio (SNR). Previous simulations suggest that an SNR > 30 dB will ensure a coherent processing of the signal. This paper presents reflectometry measurements that provide signal coherence information. The measurements have been conducted on two research vessels: R/V Lance and R/V Polarstern. The objective is to reveal the required conditions for coherent reflectometry depending on sea state and sea ice occurrence. Three data sets from expeditions of the two research vessels to Fram Strait, the Northern Atlantic and the Arctic Ocean are analysed. On both ships a GORS (GNSS Occultation Reflectometry Scatterometry) receiver with three antenna links has been installed. A common up-looking link is dedicated to direct signal observations. Two additional side-looking links allow sampling the reflected signal with right- and left-handed polarization (RHCP and LHCP). The respective setups have suitable positions to observe grazing sea surface reflections (< 30 deg elevation angle). The antennas are mounted on Lance and Polarstern about 24 m and 22 m above sea level, respectively. Reflection events are recorded continuously covering more than 70 days. Each event comprises a track of the satellite signal in the grazing angle elevation range. On average 2-3 reflection events were recorded in parallel. The results of the analysis show that in coastal waters (German Bight and Svalbard fjords) up to 44%, 37% (RHCP, LHCP) of the measurements meet the coherence condition. On the high sea it is rarely met, only <0.5% of RHCP and LHCP records fulfill the coherence condition there. The rate of coherent observations increases up to 14%, 13% (RHCP, LHCP) in case of sea ice occurrence. It can be concluded that the sea state plays an important role for coherent reflectometry. Applications of coherent reflectometry over the ocean may concentrate on the retrieval of sea ice properties and altimetry in coastal waters. For the early data set, recorded in Fram Strait 2016, the estimation of sea concentration has been demonstrated. At present the Polarstern setup continues reflectometry measurements in the MOSAiC expedition with unique opportunities for sea ice observations in the central Arctic. The limits of coherent reflectometry at high sea became clear. However, it is worth noting that the direct signal link meets the SNR condition also at high sea with an average rate of 55%. This result motivates further investigations to exploit the direct link of shipborne GNSS for atmospheric and ionospheric soundings on the sparsely covered ocean using coherent phase delay measurements

Polarimetric GNSS-R Sea Level Monitoring using I/Q Interference Patterns at Different Antenna Configurations and Carrier Frequencies

Coastal sea level variation as an indicator of climate change is extremely important due to its large socioeconomic and environmental impacts. The ground-based global navigation satellite system (GNSS)-reflectometry (GNSS-R) is becoming a reliable alternative for sea surface altimetry. We investigate the impact of antenna polarization and orientation on GNSS-R altimetric performance at different carrier frequencies. A one-year dataset of ground-based observations at the Onsala Space Observatory using a dedicated reflectometry receiver is used. Interferometric patterns produced by the superposition of direct and reflected signals are analyzed using the least-squares harmonic estimation (LS-HE) method to retrieve sea surface height. The results suggest that the observations from global positioning system (GPS) L1 and L2 frequencies provide similar levels of accuracy. However, the overall performance of the height products from the GPS L1 shows slightly better performance due to more observations. The combination of L1 and L2 observations (L12) improves the accuracy up to 25% and 40% compared to the L1 and L2 heights. The impacts of antenna orientation and polarization are also evaluated. A sea-looking left-handed circular polarization (LHCP) antenna shows the best performance compared to both zenith- and sea-looking right-handed circular polarization (RHCP) antennas. The results are presented using different averaging windows ranging from 15 min to 6 h. Based on a 6-h window, the yearly root mean squared errors (RMSEs) between GNSS-R L12 sea surface heights with collocated tide gauge observations are 2.4, 3.1, and 4.1 cm with the correlation of 0.990, 0.982, and 0.969 for LHCP sea-looking, RHCP sea-looking, and RHCP up-looking antennas, respectively

On the Impact of Sea State on GNSS-R Polarimetric Observations

We investigate a long-term ground-based GNSS-R dataset to evaluate the effect of sea state on the polarization of the reflected signals. The dataset consists of one-year polarimetric observations recorded at Onsala space observatory in Sweden in 2016 using right- and left-handed circular polarization (RHCP and LHCP) antennas. One up-looking antenna to receive direct signal and two side-looking antennas to collect reflections are installed at about 3 meters above sea level. The data is collocated with the measurements from a nearby tide-gauge and meteorological station. We focus on precise power estimation using a polarimetric processor based on Lomb–Scargle periodogram at precisely observed sea levels. The processor converts 0.1 Hz coherent in-phase and quadrature correlation sums provided by a reflectometry receiver to power estimates of the direct and reflected signals. The power estimates are reduced to three power ratios, i.e. cross-, co-, and cross to co-polarization. A model, describing the elevation dependent power loss due to sea surface roughness, is then utilized to invert the calculated power ratios to the standard deviation of sea surface height. Analysis of about 14000 events found in the dataset (about 40 continuous tracks per day) shows a fair agreement with the wind speeds as an indicator of the sea state. Although an increasing sensitivity to sea state is observed for all the power ratios at elevation angles above 10 degrees, the measurements from the co-polar link seem to be less affected by the surface roughness. The results reveal that the existing model cannot predict the effect of sea surface roughness in a comprehensive way. The different response of RHCP and LHCP observations to roughness is evident, however, the polarization dependence is not covered by the model. The deviations from the model are particularly clear at lowest elevations (<5 deg) where the roughness effect is expected to vanish. The results indicate that roughness also affect observations at lowest elevation angles. In this elevation range the expected dominance of the RHCP component above the LHCP component is not observed. A different approach is required to model the influence of sea state in GNSS-R. The increasing amount of reflectometry data may allow to retrieve an empirical relation between coherent reflection power and sea state in future investigation