Memorandum Considerations for new IVS-Radiotelescopes ========================================== Version: May 27, 2001 Hayo Hase Background ---------- The International VLBI Service (IVS) was founded in 1999 ( http://ivscc.gsfc.nasa.gov ). The IVS is responsible for the data acquisition with radiotelescopes with the Very Long Baseline Interferometry (VLBI) technique around the world. The product oriented service was adopted by the International Astronomical Union (IAU) and the International Association of Geodesy (IAG). The indispensible products of the IVS are used in the International Earth Rotation Service (IERS) for the derivation of the IERS Celestial Reference Frame (ICRF) and the IERS Terrestrial Reference Frame (ITRF) which represent the most accurate reference systems on Earth and in space. Within the IVS a global network of about 30 radiotelescopes is scheduled regularly and supervised by the IVS network coordinator. The data quality assurance is done by the IVS analysis coordinator. Therefore the performance of network stations is continously monitored. The majority of the IVS network stations operates radiotelescopes which had not been designed primarily for the geodetic use of VLBI. Many had been constructed either for astronomical or for telecommunication purposes. Only very few radiotelescopes had been designed with respect to its geodetic needs. Nowadays some of the existing IVS network stations will need to replace or renovate their radiotelescopes due to aging. New institutions had been able to raise funds for completely new radiotelescope constructions in order to become an IVS member after completion of the facilities. The intension of this memorandum is to summarize the considerations for the construction of a radiotelescope which will support the needs of the IVS and its observation programs in an optimized manner. With well designed instruments the quality of IVS products can be improved, if less modelling of possible radiotelescope behaviour is necessary. 1. Needs for Geodesy -------------------- The geodetic VLBI allows the determination of baseline lengths between 1.000 to 10.000 km with a precision of about 1 mm. This level of precision requires, that - the thermal expansion and gravitational deformation of the radiotelescope must be minimized, - the invariant point as reference point does not change its position under any pointing/tracking conditions, - the invariant point must be accessible with surveying instruments during annual control measurements and/or panel adjustments. The geodetic VLBI is different from radioastronomy VLBI in the manner of observation. The geodetic VLBI requires, that - as many observations as possible can be executed per time unit and hence slewing times should be minimized, - the observations are made in S-band (13cm) and X-band (3.6cm) simultaneously (for ionospheric corrections). 2. Construction Elements ------------------------ The turning-head radiotelescope has the advantage over the wheel-track radiotelescope, that the - bottom part can be realized with concrete rather than steel (better thermal coefficient), - bottom part can be better isolated with heat shield, - moving masses are smaller, - axes can intersect, - invariant point is accessible and can be materialized. The Cassegrain/Gregory construction has the advantage over the offset construction, that the - reflector panels can be manufactured as multiple elements of the same type (cheap) compared with the (expensive) individual panels of the offset construction. The secondary focus has the advantage over the primary focus, that the - spillover of the feed system sees the cold sky (3K) rather than ground noise (300K), - the accessibility of receiver (maintenance) in secondary focus is much easier and more safe. The backstructure of the reflector can be realized by - aluminium, which optimizes stiffness, - carbon fibre, which optimal therma coefficient. A combination of both properties is ideal. A system of hollow axes has the advantage over a system with closed axes, that the - the invariant point can be monitored from the outside and inside of the radiotelescope with appropiated geodetic instrumentes (theodolite, precision plumblining, installation of vertical monitor system), - future compatible by using optical fibres along the axes for data transmission without cable wrap. A hollow axes construction involves the installation of hollow encoders. The reflector is deformed by gravitational forces, drive forces, wind, water/rain/ice/snow. The deformation can be partly compensated with the construction by optimizing it for a constant ray path length. The constant ray path length is a condition for VLBI! This will normally increase the pointing error due to deformation, but the pointing error can be corrected by a deformation model in the control circuitry of the antenna control unit. The reflector size determines the signal-noise-ratio (SNR). The larger, the better. But the deformation behaviour is also proportional to the dish size. An appropiate compromise between high SNR and sufficient stiffness is a diameter of about 20m. In geodetic VLBI observation programs many sources per time unit must be tracked. The slowest antenna determines the idle time in the network. Therefore the velocity requirements are >3deg/s for azimuth and >1.5deg/s for elevation (good values are 6deg/s and 3deg/s). However the acceleration must not be of that order. For a long life of the motor and gearings a maximum acceleration of 1deg/s/s is beneficial. The ranges in azimuth should be 0..540deg (1.5turns) and 0..180deg in elevation. The possibility to point at 90..180deg in elevation shortens the slewing time between opposite azimuth directions which is beneficial for geodetic VLBI. Modern servo control systems are digital. This allows a deep insight into the controller with appropiate diagnosis software (to be specified!). This has advantages in maintenance cases and trouble-shooting over former more analog systems. 3. Software Interface Elements ------------------------------ The IVS network stations are using the NASA PC Field System based on Linux operating system. The communication between the remote computer (Field System PC) and the Antenna Control Unit (ACU) can be realized in various ways. The ethernet connection has the advantage over a serial line (RS232, RS422) or GPIB that the data transmission rate is higher. Modern servo control systems are digital, which allows access to many sensors in the control circuitry. Its monitoring is of advantage in maintenance cases or trouble shooting. In addition the use of GPS-clocks like the CNS-clock ( http://www.cnssys.com/cnsclock/CnsClock.html ) allows the installation of a NTP-Server (Network Time Protocol). The ethernet connection between ACU and the NTP-Server enables the synchronization of ACU-time via a standard Unix service likewise the remote computer in the most simple way. This guarantees the same time in the remote computer which controls the VLBI equipment and in the ACU which controls the radiotelescope pointing. The ACU should support the following modes: - STOW, specific survival position, - IDLE, brakes in, - STANDBY, position hold by controller, brakes released, - RATE, move antenna at continous speed (for maintenance), - PRESET, pointing to a specific position, - PROGRAM TRACK, tracking according to azimuth/elevation values from remote computer, which sends periodically future values to replace past values, - AUTOMATIC TRACK, tracking according to right ascension, declination, catalog epoch, meteorological data (for refraction correction), - BURST MODE, tracking according to preloaded stack of curve elements (used for satellite tracking) - temporary OFFSET for azimuth, elevation, time, to be applied during trackings for radiometric measurements, - permanent OFFSET for azimuth, elevation, time to compensate offsets or delay. Any communication between remote computer and ACU should be acknowledge. Any control command (settings) should be allowed to be monitored with a separate command. For safety reasons the ACU should know whether the remote controller is still active or not. Usually this is recognized by a periodic request of the remote computer for the general status of the ACU, which closes the emergency chain.