Inter-satellite links for satellite autonomous integrity monitoring

A new integrity monitoring mechanisms to be implemented on-board on a GNSS taking advantage of inter-satellite links has been introduced. This is based on accurate range and Doppler measurements not affected neither by atmospheric delays nor ground local degradation (multipath and interference). By a linear combination of the Inter-Satellite Links Observables, appropriate observables for both satellite orbits and clock monitoring are obtained and by the proposed algorithms it is possible to reduce the time-to-alarm and the probability of undetected satellite anomalies.     

A search for counterparts of selected COS-B galactic gamma-ray sources: The X-ray survey and optical identifications

The Einstein X-ray Imaging Instruments have been used to explore, down to an unprecedented sensitivity, the X-ray behavior of 7 high-energy γ-ray sources discovered by the COS-B satellite. 32 low latitude (¦b¦ < 5°) IPC fields, mosaic-arranged to cover the few-square-degrees COS-B error circles, yielded 30 soft X-ray sources, the fluxes of which range from ～ 1/100 to few UFU, and no diffuse features. While the density of ～ 1 source/IPC field is consistent with the value found at higher latitudes, the percentage of ‘stellar’ identifications among these low-latitude sources is significantly higher than in non-galactic-biased samples. Unfortunately, the positional accuracy achieved with the IPC does not allow astronomical identification in the absence of obvious counterpart(s). However, after the exploratory coverage, the IPC data were used, when possible, to point out potentially interesting targets for the HRI instrument capable of an accuracy of ～ 3 arc sec. Due to the misfortunes which occurred to the Einstein satellite, this time-consuming process was feasible only in two cases: within the error circle of 2CG135 + 01, the radio variable star LSI61.303 was pinpointed by the HRI, while the HRI exposure of the brightest X-ray source discovered in 2CG 195 + 04 (Geminga) positioned a source in an empty POSS field. The latter case will be presented and the nature of the X- and γ(?)-ray source briefly discussed.     

Is the Vela pulsar associated with the Vela SNR?

Optical data taken in January 1987 from La Silla, when compared with the discovery plate of 1975.2 show no proper motion for PSR 0833-45, while a very significant one was expected if the pulsar originated in the center of the Vela SNR, which has so far been associated with it. If such an association is to be retained, either an extreme asymmetry of the SNR is required, or both objects are much older than has so far been thought. Both alternatives have their difficulties, thus possibly casting doubts on the reality of this classic SNR/pulsar association.Based on observations collected at the European Southern Observatory, La Silla, Chile.     

GNSS geodetic techniques for time and frequency transfer applications

We performed an initial analysis of the pseudorange data of the GIOVE-B satellite, one of the two experimental Galileo satellites currently in operation, for time transfer.¹ For this specific aim, software was developed to process the GIOVE-B raw pseudoranges and broadcast navigation messages collected by the Galileo Experimental Sensor Stations (GESS) tracking network, yielding station clock phase errors with respect to the Experimental Galileo System Time (EGST). The software also allows processing the Global Positioning System (GPS) P1 and P2 pseudorange data with broadcast navigation message collected at the same stations to obtain the station clock phase errors with respect to the GPS system time (GPST). Differencing these solutions between stations provides two independent means of GNSS time transfer. We compared these time transfer results with Precise Point Positioning (PPP) method applied to GPS data in combined carrier-phase and pseudorange mode as well as in pseudorange-only mode to show their relative merits. The PPP solutions in combined carrier-phase and pseudorange mode showed the least instability of the methods tested herein at all scales, at few parts in 10¹⁵ at 1 day for the stations processed, following a tau^−½ interval dependency. Conversely, the PPP solutions in pseudorange-only mode are an order of magnitude worst (few parts in 10¹⁴ at 1 day for the stations processed) following a tau⁻¹ power-law, but slightly better than the single-satellite raw GPS time transfer solutions obtained using the developed software, since the PPP least-squares solution effectively averages the pseudorange noise. The pseudorange noise levels estimated from PPP pseudorange residuals and from clock solution comparisons are largely consistent, providing a validation of our software operation. The raw GIOVE-B time transfer, as implemented in this work, proves to be slightly better than single-satellite raw GPS satellite time transfer, at least in the medium term. However, one of the processed stations shows a combined GPS P1 and P2 pseudorange noise level at 2 m, a factor 2 worst than usually seen for geodetic receivers, so the GPS time transfer results may not be at their best for the cases processed. Over the short term, the GPS single-satellite time transfer instability outperforms the GIOVE-B by an order of magnitude at 1 s interval, which would be due to the different characteristics of the tracking loop filters for GPS P1 and P2 on one hand and the GIOVE-B signals on the other. Even at this preliminary stage and using an experimental satellite system, results show that the GIOVE-B (and hence Galileo) signals offer interesting perspectives for high precision time transfer between metrological laboratories.