VLBI2010: Networks and Observing Strategies

1. VLBI2010: Networks and Observing Strategies Bill Petrachenko, chairman, Natural Resources Canada Brian Corey, MIT Haystack Observatory Ed Himwich, NVI, Inc/GSFC Chopo Ma, NASA Goddard Space Flight Center Zinovy Malkin, Institute for applied Astronomy Arthur Niell, MIT Haystack Observatory David Shaffer, Radiometrics, Inc. Nancy Vandenberg, NVI, Inc./GSFC

2. Topics for This Sub-group • Antenna-network structure and observing strategies • Frequency bands, RFI • Source strength/structure/distribution • Field system and scheduling

3. Science-based VLBI Accuracy Targets from NGO proposal ParameterVLBI nowFuture Science Requirement Scale (ppb) 0.2 0.1 (important) Coords (NEU,mm) 2, 2, 5 1, 1, 1 Nutation (mas) 0.2, 3/wk 0.05, daily (unique) from NASA’s SESWG report “…accuracy of global geodetic networks advances by about a factor of 10 per decade, with submillimeter-scale reference-frame accuracy likely in the near future. Continued improvements in accuracy are critical to a number of the recommendations of this report…” Primary VLBI Targets for 2010 and Beyond - Achieve significantly better long-term accuracy for scale and orientation of reference frames. - Do it in a manner such that daily operations can be sustained indefinitely. - Reduce latency between the taking of data and delivery of products.

4. Current Network Structure and Observing Strategies • IVS uses a total of about 30 stations, each with a limited number of operating days per year. • In any given observing session only 9 of theses can be used which is limited by the number of inputs to the correlator. • IVS delivers scale and orientation by contributing to the ITRF, ICRF and EOPs through a series of observation types, each optimized for its own specific purpose, e.g. • EOP: R1/R4/INT • TRF: T2, EURO, APSG, OHIG, SYW • CRF: CRF, CRFS, CRDS • This somewhat complex strategy does a good job of making the best use of available resources. • However, it requires compromises which limit its performance.

5. Factors Determining VLBI Performance • Number of stations per observing session • Since scale and orientation are aggregate parameters, more stations will help. • This provides a better connection of EOPs to the ITRF • Number of observations per year. • Since scale is determined over time, more observations per year will help. • Also, the science case requires EOPs to be determined daily anyway. • Distribution of stations. • A more uniform global distribution provides more geometric strength. • Quality of schedule. • Hi/Lo elevation sources, rapid source switching and sub-netting have all been shown to be powerful elements for improving performance. • Precision of the delay observable. • Whether or not schedules are identical from day to day • Whether or not the design is identical from station to station. • From RDV we know that VLBA stations perform significantly better.

6. Factors Determining VLBI Performance (contd) Factors not under our control • Performance of stations, e.g. receiver temp, lost channels, lost scans, maser instability, etc • Environmental conditions, e.g. ionosphere, troposphere, RFI, diurnal temperature range, source strength variation, etc • The extent to which systematic effects are not properly modeled or calibrated, e.g. gravity and thermal deformation of the antenna, instrumental and cable delay variations, stability of the antenna pier and local geology, source structure, etc. • Both the number of stations per observing session and the number of observing sessions per year will help. • First, by averaging out the systematic effects. • Second, by allowing correlations wrt either time or geometry to point to possible causal relations with the systematic effects.

7. Our Vision for Tomorrow’s Network Structure and Observing Strategies Network Structure • The network we envision will be a large global constellation of uniformly distributed, identical, robust, high-performance geodetic VBLI stations, specifically designed and situated to minimize bias. Observing Strategy • All stations will participate in observations on a daily basis using the same highly optimized schedule each day.

8. What Limits Us From Achieving Our Vision? • Number of stations which can observe simultaneously • Limited by the size of current correlators • Achieving Daily observations • Limited by the availability and high operating cost of stations • Total number and distribution of stations • Limited by the capital cost of stations, and in some cases by the availability of suitable host nations. • System Biases • Limited by antiquated electronics designed 3 decades ago • Site Biases • Limited by the fact that many stations were acquired on an “as-is where-is” basis.

9. A Possible Solution • Develop a VLBI station that is robust, high-performance and easy-to-operate but much lower in cost than current stations. • Since VLBI sensitivity depends on the product of antenna diameters, i.e. D1*D2, consider building small new antennas to operate with existing large telescopes. [SNR=20 can be achieved with D1=6m, D2=24m, Data Rate=1Gbps, Integration time=60sec, Tsys=50K, Efficiency=55% and flux=400mJ.] • As an example, look at the Allen telecsope Array (ATA) antenna system: 6.1m antenna, positioner, pedestal, broad-band feed (0.5-11 GHz) and cryogenic amplifier - all for $30k. A 12m antenna is also being proposed for a DSN array, estimated cost $150k • In addition, use new low cost VLBI electronics, e.g. Mk5 @ $17k and European digital DAS @ $20k. • Implement a frequency scheme to achieve phase delay at SNR=20. • Work hard to minimize systematic effects. • Design the system for robust unattended operation. • Conclusion: The potential may exist to produce a high quality VLBI station for an order or magnitude less cost so that for the cost of 1 old station we could afford 5-10 identical new stations.

10. A Possible Solution (cont’d) • Correlator: • Investigate cost-effective architectures, strategies and technologies for significantly increasing correlator size. • Surprisingly, the 3 current geodetic correlators could be upgraded to 16 stations each simply by adding 7-8 extra Mk5B systems at each correlator, at a cost of about $20k for each Mk5B. If current trends persist, correlator efficiencies of about 1 might be achievable. • Conclusion • The ingredients may be available to make a significant leap toward achieving our rather ambitious vision for 2010 and beyond.

11. Multi-beam VLBI • Motivation: • Nothnagel et al at IVS2002 showed that strong correlation exists between the station Up-component and the DC term of the clock. • This is confirmed by R1 simulations in which the station Up-component improved by a factor of >3 when the DC term was removed from the simulation. • Question: Could removal of the DC term of the clock be a missing link in the solution to the Up-component precision problem? • Possible approaches: • Approach #1: Develop a 2-way satellite link to compare clocks. Unfortunately, this is unexpectedly difficult because many other effects are lumped into the DC term such as cable and electronic delays. • Approach #2: Use multiple VLBI antennas to observe more than 1 source at a time. Since the clock behaviour is identical for all sources, it disappears in the difference. • Using 2 antennas sounds expensive but the second antenna could be small (e.g. the 6.1m ATA antenna for $30k), if that antenna was always scheduled to observe a strong source.

12. Should we Move to Higher Frequency Bands? • Motivations: • Avoid the S-band interference currently afflicting many stations. • Extend ICRF to Ka-band to support DSN tracking at X/Ka band. • Sources have less structure at higher frequencies. • Since wider spanned bandwidths are available at higher frequencies, use this fact to get better delay precision. • Problems with higher frequencies: • Everthing is more demanding at higher frequencies, e.g. antenna surface accuracy, antenna support structures, antenna pointing systems, coherence, RF/IF electronics, etc • Most of our existing geodetic VLBI antenna’s are not up to spec at higher frequencies and many of them could not even be upgraded. • An alternate solution to the S-band interference problem: • Use other low-RFI windows below 15 GHz. • An alternate solution for improving delay precision: • Find frequency schemes which deliver phase delay at modest SNR’s.

13. Are there Major Frequency BandsFree of Commercial Downlinks and Broadcast? Frequencies (GHz)Bandwidth(GHz) 2.69 – 3.40 0.71 4.80 – 6.70 1.90 7.75 – 10.70 2.95 12.75 – 17.30 5.50 22.00 – 37.50 15.50 Clearly there is a lot of available frequency space above 7.75GHz but there is also still potential at lower frequencies, in particular we shouldn’t rule out the possibility of a modified S/X band capability.

14. Develop an RFI-resistant VLBI System • Motivation: • Growing RFI is a fact of life and no matter how we select our bands RFI will eventually cause us a problem. • RFI, if not treated properly, can render useless an entire sampled channel, and not just the part of the spectrum where the RFI occurs. • What does RFI-resistant mean? • Only the parts of the spectrum suffering from RFI are removed. • Requires that no isolated part of the spectrum is essential to good performance. • Here is a concept for an RFI-resistant station design: • Use broad-band channels instead of critically spaced narrow bands. • Build the RF/IF electronics to be linear for 20-30 dB above the receiver noise. • Use large multi-bit samplers to avoid saturation by the RFI. • Filter out the RFI with an adaptive digital filter • Requantize to 1 (or 2) bits to allow efficient data transmission to the correlator.

15. Summary of Challenges Arising from ‘Observing Strategies Sub-group’ • Consider cost-effective designs for a large correlator. • Develop strategies to reduce site operating costs. • Investigate the development of a robust, high-performance, easy-to-operate VLBI station that is also very low-cost. • Investigate designs for an RFI-resistant VLBI system. • Investigate the need for source structure corrections. • Investigate frequency schemes for achieving phase delay at modest SNR. • Produce a comprehensive list of site selection criteria. • Produce a comprehensive list of site design goals. • Investigate the performance benefits of multi-beam VLBI and consider cost-effective implementations.

16. Future Work for the Sub-group? • Start putting hard numbers beside system parameters, e.g. • Optimum number of stations in a network • Suggested locations for new stations • Minimum delay precision. • Target SNR’s • Correlated flux of weakest source to be used. • Minimum antenna diameter • Sustainable record rate forecast for 2010 and beyond. • Etc, etc • Get more of the IVS community involved • e.g. propose an IVS-sponsored workshop to study the major error sources currently limiting geodetic VLBI and to suggest solutions.