Comparison of Atmospheric Gradients Estimated From Ground-Based GNSS Observations and Microwave Radiometry

Observations over four years from two nearby groundbasedGlobal Navigation Satellite System (GNSS) stationsand one microwave radiometer have been used toestimate linear horizontal gradients in the atmosphere.We find that gradients estimated by the radiometer havelarger amplitudes than those estimated using data fromthe Global Positioning System (GPS). One reason for thisis that they are estimated, every 15 min, independentlyof previous estimates, whereas the gradients from GPSare estimated every 5 min using constraints on their variability.We also find that the elevation cutoff angle has asignificant impact on the estimated GPS gradients. Decreasingthe cutoff angle results in smaller gradient amplitudes.The estimated gradients are not homogeneouslydistributed in all directions. When studying the largestgradients they all occur during the warmer period of theyear, beginning in April and ending in October. Specifically,for the 25 events with the largest gradient amplitudesfrom the GPS data, we find that the vast majority ofthem are associated with the passage of weather fronts

The 22 GHz radio-aeronomy receiver at Onsala Space Observatory

We present a radiometer system for regular long-term measurements of water vapour in the middle atmosphere. To be able to do continuous and long-term measurements a simple, robust, reliable and automatic system is needed. Our system therefore is based on a stable, uncooled, HEMT amplifier frontend and on a digital spectrometer backend. In order to minimise reflections in the frontend transmission line, which distort the signal due to standing waves, we have designed a corrugated receiver horn, which combines good characteristics (low return loss and sidelobes) and narrow beamwidth to simplify the receiver optics. In order to make the radiometer system as simple as possible, we use the sky as the calibration cold load. This is possible since we use the observed brightness temperatures of an already existing broadband dual-channel 21.0/31.4-GHz radiometer, at the observation site, to estimate the brightness temperature of the sky at . However, we have developed a calibration method, which makes it possible to estimate the sky brightness temperature even if we cannot use the dual-channel radiometer. Despite new measurements, which became available in recent years, the determination of middle atmospheric water vapour distribution still remains a challenge due to the fact that there is a large dispersion among the different measurement methods and data sets, which are obtained on a sparse and sporadic basis. This is the reason why several instruments similar to ours currently are developed in Europe

Water vapour radiometry in geodetic very long baseline interferometry telescopes: assessed through simulations

The accuracy of geodetic Very Long Baseline Interferometry (VLBI) is affected by water vapour in the atmosphere in terms of variations in the signal propagation delay at the different stations. This “wet” delay may be estimated directly from the VLBI data, as well as from independent instruments, such as collocated microwave radiometers. Rather than having stand-alone microwave radiometers we have, through simulations, evaluated the possibility to use radiometric data from the VLBI receiver in the VGOS telescopes at the Onsala Space Observatory. The advantage is that the emission from water vapour, as sensed by the radiometer, originates from the same atmospheric volume that delays the VLBI signal from the extra-galactic object. We use simulations of the sky brightness temperature and the wet delay together with an assumption of a root-mean-square (rms) noise of the receiver of 1\ua0K, and observations evenly spread between elevation angles of 10∘–90∘. This results in an rms error of the estimated equivalent zenith wet delay of the order of 3\ua0mm for a one frequency algorithm, used under cloud free conditions, and 4\ua0mm for a two frequency algorithm, used during conditions with liquid water clouds. The results exclude rainy conditions when the method does not work. These errors are reduced by a factor of 3 if the receiver error is 0.1\ua0K meaning that the receivers’ measurements of the sky brightness temperature is the main error source. We study the impact of ground-noise pickup by using a model of an existing wideband feed. Taking the algorithm uncertainty and the ground noise pickup into account we conclude that the method presented will be useful as an independent estimate of the wet delay to assess the quality of the wet delays and linear horizontal gradients estimated from the VLBI data themselves

Accuracy assessment of the two WVRs, Astrid and Konrad, at the Onsala Space Observatory

Two Water Vapour Radiometers (WVRs), Astrid and Konrad, have been operating at the Onsala Space Observatory during the time period 2013–2016. There are several data gaps due to different types of instrument failures and therefore we also use estimates of the equivalent zenith wet delay (ZWD) from the two GNSS reference stations: ONSA and ONS1. They providean almost continuous time series during the four years. ZWD root-mean-square differences are 0.38 cm between ONSA and ONS1, 0.92 cm between ONS1 and Astrid, and 0.75 cm between ONS1 and Konrad. For the horizontal linear gradients we see correlation coefficients of the order of 0.9 between ONSA and ONS1 and 0.5 between ONS1 and Konrad

Hvor mange kompromiser har økologien råd til? Ægproduktion som eksempel

Mens konventionelt landbrug blot forpligter sig til at producere, hvad efterspørgslen dikterer, påtager økologisk landbrug sig et etisk ansvar for at producere i overensstemmelse med standarder, der er afledt af økologiens egne værdier og prioriteringer. Dette forhold er formentlig forklaringen på den bredt udtrykte sympati, der hersker for økologien. Men det gør også økologien sårbar i de tilfælde, hvor produktionen ikke synes at leve op til økologiens egne værdier. Gennem en analyse af økologiens værdier identificeres en fundamental prioritering i økologien, nemlig at miljø, dyrevelfærd og menneskelig sundhed skal tjenes gennem et ’sundt’ økosystem, snarere end gennem isoleret behandling af symptomer. Det følger heraf, at hvis dyr, mennesker eller miljøet vedholdende fungerer dårligt i økosystemet, så skal løsningen søges i at få økosystemet til arbejde ’sundere’. Nogle af de velfærdsproblemer, som findes hos økologiske æglæggende høner, ville ifølge økologiens ideer løses ved små flokstørrelser, men denne løsning realiseres ikke på grund af økonomiske hensyn. For andre problemer mangler der ideer til løsning. Denne case er måske ikke i sig selv repræsentativ for al økologisk produktion, men den rejser alligevel et principielt spørgsmål om, i hvilket omfang økonomiske hensyn tillader økologien at gå på kompromis i forhold til egne værdier – et spørgsmål, som det er relevant for økologien at overveje i lyset af, at nye markedsandele ofte vil være betinget af, at merprisen i forhold til konventionelle produkter må være beskeden. Artiklen peger på to mulige strategier: den pragmatiske, hvor man indrømmer, at det kan være nødvendigt at gå på kompromis, og den idealistiske, hvor man afviser alvorlige kompromiser på bekostning af, at visse former for produktion ikke vil være realisable

Radiometry performance of the VGOS receivers of the Onsala twin telescopes

With the introduction of the VLBI Global Observing System (VGOS) the parallel use of the VGOS receiver asradiometer in order to estimate the wet propagation delay was recognised as a future possibility. That is whenobservations can be carried out at higher frequencies, closer to the water vapour emission line at 22.2 GHz.An advantage of having the radiometer in the VLBI telescope, compared to the use of a stand-alone WaterVapour Radiometer (WVR), is that the radiometer will observe the same atmospheric volume that is causing thesignal propagation delay.We have assessed this method using simulations and arrived at the following two important conclusions:(1) the receiver’s measurements of the sky brightness temperature is likely to be the main error source, rather thanthe algorithm error introduced when calculating the wet delay from the observed sky brightness temperatures;(2) the method requires an extension of the frequency range of the receiver well beyond 14 GHz in order toincrease the sensitivity for water vapour. The radiometric measurements shall be made within a couple of GHzfrom the emission line at 22.2 GHz.In spite of the fact that the present VGOS receivers observe at too low frequencies we find it meaningful toassess the radiometric stability of these receivers at the higher end of the frequency band. We have used one ofthe Onsala Twin Telescopes for this purpose, which is able to observe both polarizations in the frequency band15.36–15.58 GHz. The system temperature has been observed at different elevation angles in order to separatethe atmospheric sky brightness temperature and the receiver noise temperature. The observations are carried outduring different atmospheric conditions and the estimated sky temperatures are compared to the observationsdone with one of our stand-alone WVRs. By using one-frequency algorithms we may also, during cloud-freeconditions, compare the wet propagation delays using 20.7 GHz observations from the stand-alone WVR and15 GHz observations from the VGOS receiver