| The largest remaining uncertainty in GNSS (GPS, Galileo, etc) position measurements is the ionosphere. For the most common type of receiver it is typically in the order of a few meters. Since LOFAR is very good at measuring the ionosphere, it seems logical to investigate whether LOFAR calibration data might be used to improve GNSS positioning. This is exactly what we are doing together with the Dutch Aerospace Laboratory (NLR), in a contract issued by the European Space Agency (ESA).
For this purpose, we have recently installed an advanced dual-frequency GNSS receiver (the blue thingy) in the concentrator hut near the LOFAR superterp. The image shows our own Menno Norden with Hein Zelle(*) and Arnoud van Kleef(**) of NLR.
The idea is to observe a sequence of bright radio sources with LOFAR, along the path of one or more GNSS satellites as they move across the sky. Thanks to the selfcal technique, the LOFAR data represent very accurate relative measurements of the ionosphere in the direction of the source (and thus approximately the satellite), as seen from the various LOFAR stations. These can be compared to the absolute measurements of the ionosphere by the GNSS receiver, which has an accurately known position.
Of course there are some complications caused by the troposphere, multipath and receiver noise, and by the difficulty to measure the absolute ionosphere with LOFAR(***). Nevertheless, in view of the obvious importance of accurate GNSS positioning, it is worth exploring all available avenues.
(*) It is a small world. Hein happens to be the partner of our own Agnes Mika.
(**) Arnoud also happens to have grown up in the same village (Middelstum) as Menno.
(***) LOFAR only needs the absolute ionosphere for polarization observations. For imaging, relative measurements are sufficient.
|Madroon Community Consultants (MCC)