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Why Mark 5 at Arecibo?
- A Brief description of the Mark 5 system, what is it and what does it do to enhance our science? An Overview (from A. Whitney's documents on the subject).
- A simple block diagram of how Mark 5 interfaces with AO.
- May need major changes to the AO system (IF frequency/Bandwidth etc)?
1. A Brief description of the Mark 5 system, what is it and what does it do to enhance our science? An Overview (from A. Whitney's documents on the subject).
The Mark 5 system is being developed by Haystack Observatory as a Gbps VLBI data system based on magnetic disk technology. Incorporating primarily low-cost PC-based components, it can record data at a rate of up to 1024 Mbps on to an array of inexpensive, removable IDE/ATA disks.
At the heart of it lies the so called StreamStor disk interface card, specially designed (by Conduant Corp.) for high-speed real-time data collection and playback. It supports the following three physical interfaces in a triangle of connectivity as shown in the following schematic diagram:
- Front Panel Data Port (FPDP): This is a two-way 32-bit card-top bus through which realtime data may enter or leave the system. All 32 bits of the bus are always active.
- This port supports up to 16 IDE disks in 8 master/slave pairs.
- This is the standard connection to the host PC operating under a Linux OS.
The path exercised for regular VLBI is between 1 & 2. Path 2 - 3 is used to read the data off of the disk, and 1 - 3 can be used in eVLBI.
The I/O panel (located at the rear of the unit Fig1), contains the same set of connections as a Mark 4 or a VLBA tape drive, so that the Mark 5A is very easily substituted for a tape drive. Up to 64 (or 32 for VLBA formatter) tracks of formatted data can be directly recorded. The StreamStor card accumulates 32-bit 'words' (corresponding to the 32-bit wide FPDP bus) into 64-kB blocks and writes them sequentially in a round-robin fashion to the disk array. This alleviates the needs for "barrel-rolling" and "data-modulation" by the formatter.
For the near future, VLBA will still be operating using Tape recorders. Hence, if Arecibo were to get a MARK 5A unit within the next few months, the most useful configuration for a Mark 5A unit would be the so-called "pig-wire" mode whereby both the Tape recorder and the Mark 5 disk-recorder can be kept wired to the formatter in parallel, providing the opportunity to record up to 256 Mbps on tape or disk at any time. In this configuration, Arecibo would be able to participate in both VLBA (tape-based) and EVN's disk-based runs at short notice. For higher data-density runs, the choices would be to either, reconnect all 64 tracks from the formatter to the Tape recorder or to the Mark-5 disk recorder, providing 512 and 1024 Mbps data-rates respectively.
In its "straight-through" mode, Mark 5 will be able to record any 32-bit digitized signal, a feature that can be utilized by other AO backends as a fast recorder.
This system will have a re-designed I/O board facilitating internal data formatting. It will be fully VLBI Standard Interface (VSI) compatible. With Mark 5B, it will be possible to record data at the rate of 1024 Mbps from both Mark-4 and VLBA systems. Among other improvements, it will also have much simpler a '80-pin' Input/Output interface. The expected date for its full implementation is early 2004.
Upgrading from Mark-5A to B would involve changing the I/O board, and (most probably) the I/O ports. Cost of this is as yet undefined.
Mark 5 Software:
Much of the software for VLBI uses is already in place. The Mark 5 PC has its stand-alone Linux-based software, and Mark 5 units operations are also included within the 'Field System' (developed and maintained by NVI Inc.) that runs Arecibo's current VLBA4 recorder.
Functional and scientific enhancement:
Tape recorder-based VLBI systems require heavy maintenance being rather delicate systems. Headstack alignment and proper calibration is also very important for the success of the observations. For playing back, the correlators need a separate playback unit. Tape-based recording is also more error-prone when compared to any disk-based system. All these factors make the Mark 5 a much easier and reliable system to use.
In addition to the operational ease, a move to Mark 5 systems will enable Arecibo to provide 256-MHz bandwidth with 2-bit sampling, a factor of 2 improvement over its current capability. The data acquisition rack for Arecibo's VLBA4 system has 8 double-sided baseband converters, each of which can be set to a maximum bandwidth of 16 MHz. This configuration implies a maximum possible data output rate (for 2-bit sampling at the rate of 32 Mega-samples per sec, i.e. 32 x 2 x 16=) of 1024 Mbps. However, to date we have been limited to record only at maximum rates of 256 Mbps with the VLBA and 512 Mbps with the EVN stations even with our dual-headstack tape recorder. A move to a Mark 5 system would enhance the maximum sensitivity possible for baselines involving Arecibo by a factor of 1.4 without requiring any other changes in the DAS.
Although such a factor may not at first seem impressive, this enhancement will open up possibilities of conducting wider-band VLBI with the world's biggest antennas where Arecibo's presence is essential for both detecting the weakest sources and to form better synthesized beams for high dynamic range maps of weak features.
Another advantage of Mark 5 systems is that the data can be read out using the same unit (equivalent to "play-back units" for tape-based systems), or sent to a correlator directly via the 1-3 pathway mentioned above. This opens up the possibilities of e-VLBI for Arecibo.
A few research areas that will benefit immensely are:
- Host galaxies of low-luminosity AGNs can be studied by detailed mapping of their ISM and nuclear regions via both their continuum emission and atomic/molecular absorption effects.
- Investigations of weak-cored sources, the low-luminosity Seyfert and LINER galaxies, the most distant radio galaxies, and the search for gravitational micro-lensing. Observations of such extragalactic phenomena provide fundamental input both for testing the unification schemes of AGNs, and for estimating cosmological parameters.
- Polarization VLBI has already yielded information of the highest resolution on the magnetic fields within AGNs. As polarized signals are typically only about few percent of the total intensity, Arecibo with its enhanced sensitivity baselines is needed to make many observations even possible.
- Afterglows of Gamma Ray Bursts (GRBs) are usually very weak at centimeter wavelengths. VLBI imaging efforts for these enigmatic objects, where researchers have already traced out jet-like emission would benefit immensely from the highest possible sensitivity from Arecibo.
- Imaging of weak SNRs in other galaxies and especially in nearby starburst (ULIRG) galaxies.
- Within our Galaxy, VLBI is crucial for the study of flare stars, radio-emitting X-ray binaries, and other classes of radio stars, to say nothing of the galactic superluminal sources. This is also the case for the imaging of galactic maser sources of OH and CH3OH.
- High-sensitivity VLBI permits the astrometry of pulsars, and the
investigation of the interstellar scattering of their radiation during
propagation which, together with observations of low-latitude AGNs and
galactic masers, can provide unique information on turbulence in the
interstellar medium and the solar wind.