VLBI - DORIS compatibility tests at the Geodetic Observatory Wettzell

Thomas Kluegel (BKG, Geodetic Observatory Wettzell, Germany)

Event: 2016 IDS Workshop

Session: Network and constellation: status and evolution

Presentation type: Type Oral

The co-location of DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) with other geodetic space techniques is of special interest for the realisation and maintenance of global reference frames like the ITRF (International Terrestrial Reference Frame). While GNSS (Global Navigation Satellite System) receivers are often operated together with DORIS beacons, co-locations with Very Long Baseline Interferometry (VLBI) or Satellite Laser Ranging (SLR) are less frequent. The Geodetic Observatory Wettzell in Germany, which includes 3 VLBI radio telescopes, 2 SLR, several GNSS receivers and a dense local tie network, is therefore an interesting co-location site for the IDS.

However, the common operation of VLBI and DORIS at one site generates Electromagnetic Compatibility (EMC) problems due to the DORIS transmitting power of 40 dBm at 2036 MHz. The power difference can be as high as 15 orders of magnitude between the power of astronomical VLBI sources and the emitted power of the DORIS beacon. While the recorded frequency band itself (S band 2.1-2.4 GHz) is not expected to be influenced directly, the high power at the Low Noise Amplifier (LNA) input could generate spurious signals and overload or even damage the LNA.

In order to find a solution for a common operation, Radio Frequency (RF) interferences at the LNA inputs of the 20 m radio telescope (RTW) and the classical S-/X-/Ka-band TWIN telescope (TTW-1) were investigated varying the telescope azimuths and elevations and testing different locations and RF blocking structures. It has been shown that obstacles like buildings, earth mounds or absorber plates attenuate the signal up to 20 dB. However, the power received at the LNA input is still at a critical level when the radio telescopes point towards the DORIS beacon. In order to minimize the impact of the DORIS emission, an operation mode where the beacon is kept in stand-by mode when no satellite is visible was implemented.

A four weeks lasting test in operation-on-demand mode was performed at the preferred location behind an earth mound. First results indicate that the perfomance is satisfying and the quality of the correlated VLBI signals is not affected by the operation of the DORIS beacon.

 

Oral presentation show times:

RoomStart DateEnd Date
Casoar Mon, Oct 31 2016,11:00 Mon, Oct 31 2016,11:15
Thomas Kluegel
BKG, Geodetic Observatory Wettzell
Germany
kluegel@fs.wettzell.de