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## 10. Feasibility of Long-term Project Goals
**ALPACA long-term project goals**
1. Deliver a wide-field-of-view 40 beam L-band cryogenic phased array feed (PAF) instrument to GBO at the conclusion of this project. The design and construction will be robust, will provide sensitivity competitive with the existing GBT L-band receiver but with 40 times the on-sky field of view, and will be capable of expansion for future performance enhancement projects.
1. Provide the community of L-band HI, pulsar, FRB, and SETI scientists with a US instrument which will be transformative in performing faster surveys and mapping studies. The ALPACA instrument will be upgradable with future funding to support technosignature science.
1. In collaboration with GBO, complete the steps necessary to qualify ALPACA as a facility user instrument, available to the broader scientific community through the Open Skies GBT observing time proposal process.
1. Complete Goals 1 - 3 within the ALPACA instrument budget and schedule.
**Feasibility of these Goals**
We believe that the ALPACA project is on track to complete each of these goals, with the possible exception of goal 4 with respect to budget requirements. The following list addresses the feasibility of each goal in sequence.
1. Key ALPACA technologies which enable achieving Goal 1 include:
a) The 40 beam Cryogenic L-band phased array feed is a first-of-its-kind system to bring cryo-cooling to an astronomical PAF of this large size at L-band. The CSIRO Cryo-PAF project for the Parkes telescope has similarities, but ALPACA will be commissioned first. The full cryo-cooling of both array antennas and LNAs will yield outstanding sensitivity and low $`T_{\rm sys}`$. Though this is groundbreaking technology, the ALPACA team has been working together on related projects for over a decade, including successfully proving this design approach with the AO19 prototype 19-element PAF array. BYU led the FLAG project at GBO to develop that 19 element PAF array with cryo-cooled LNAs and its real-time digital beamformer back end. BYU has been the leading US research group for phased array feeds for two decades. Refer to Sections [1.3](/1-intro-alpaca/1.3) and [3.2](/3-front-end-design/3.2) for further detail.
b) The goal of robust design is addressed by many aspects of the ALPACA system. For example, the innovation of easily removable antenna/LNA modules in the array cryostat makes maintenance and repair of so many LNA modules feasible even though they are enclosed by the cryostat window (see [Section 3.3](/3-front-end-design/3.3)). Ample spare units will be provided with the ALPACA delivered instrument for all critical subsystems, including antenna/LNA modules, custom RF over fiber transmitter and receiver boards, RFSoC boards, and full HPC servers with all associated GPUs, memory, and peripheral I/O boards. Further, the digital back end hardware systems (RFSoCs, server HPCs, and GPUs) are configured in large enough numbers and sized with excess processing capacity to provide significant headroom in the real-time computational requirements (see [Section 5.1](/5-dbe/5.1)).
The direct sampling 14-bit ADCs and tight double-stage custom bandpass filtering for the ALPACA observation band provide extraordinary signal fidelity, absence of mix-down image artifacts, wide dynamic range for signals, and protection from RFI saturation of the signal chain (see Sections [4.2](/4-signal-transport/4.2),[4.4](/4-signal-transport/4.4) and [5.2](/5-dbe/5.2)).
c) Highest priority to design details which enable high sensitivity and low $`T_{\rm sys}`$ was given at each stage of ALPACA development. Many man-months and computing resources were spent in numerical optimization of both individual antenna element geometry and the array grid pattern to maximize receiver sensitivity (see Section [3.1](/3-front-end-design/3.1)). As mentioned above, the challenging approach of cryo-cooling both the LNAs and antennas is unique for such a large PAF in this band, but was undertaken to improve sensitivity (See Section [3.2](/3-front-end-design/3.2)). High-performance custom LNAs and packages were developed by Arizona State University for this same purpose (See Section [3.5](/3-front-end-design/3.5)). The RF-over-fiber downlinks were designed with the requirements that they contribute no more than one degree Kelvin to $`T_{\rm sys}`$ while still providing a high dynamic range link (see Section [4.4](/4-signal-transport/4.4)). And finally our efforts towards low system noise are backed up with our past success on the 7 beam 19 dual-pol antenna FLAG PAF at GBO were we measured a $`T_{\rm sys}`$ of 26K. We can thus reasonably expect the formed beam ALPACA sensitivity to be competitive with the existing GBT L-band horn feed.
d) ALPACA was designed with future expansion for new capabilities in mind. The first figure in [Section 5.1](/5-dbe/5.1) shows that 23 spare 100 GbE network ports are available on the 60-port switch. These are intended to pass high data rate signals to future secondary back ends, such as a pulsar DM search and detection engine, or a BTL SETI technosignature processor. The system includes 25 dual CPU data processing servers, with 50 high-end graphical processor units. Our software design distributes the processing load in a way that keeps all of these units well below full resource utilization. Thus we expect that this hardware will be able to support additional functional capabilities with the addition of new software applications in the future.
1. An important feasibility indicator for Goal 2 is how the instrument is accepted by potential users. We have received universal enthusiasm from the pertinent science communities as they consider the capabilities ALPACA will bring to the GBT. [Section 2](/2) presents an extended discussion of the new science which will be enabled. We note that though SETI science will require some future work to support real-time beamformed sample voltage outputs, this application was anticipated in the design so all needed hardware is present and software upgrades will be modest.
1. Regarding Goal 3, the ALPACA project is being developed entirely for the purpose of providing a general access permanent user instrument to the observatory (initially for AO, but now for GBO). It was never conceived as a PI instrument. Indeed, the PI and ALPACA development team are primarily engineers and program administrators who have been engaged in radio astronomical instrument development for decades. Even so, throughout the project, ALPACA developers have collaborated closely and have relied heavily on technical guidance and performance requirement specifications from a large team of scientists who are potential users. We have done this to insure that the end product will satisfy needs of the broader scientific community. Co-PI Jim Cordes, Cornell Professor and pulsar and transient/FRB scientist, played a major role in architecting the ALPACA receiver and its specifications. Additionally, an unfunded 10 member team of advisors who are committed collaborators and interested parties has provided substantial guidance to shape the design and performance specifications of the ALPACA receiver.
A list of members of this advisory team and their qualifications is found [here.](uploads/4ab585a92b8b4fc9ef9da54a0bc13390/ALPACA_Advisor_team.pdf)
Additionally, working closely with GBO software engineers, we will develop a fully integrated Monitor and Control interface system which will be compatible with all GBO requirements for a user instrument. This is described in [Sections 7.3](/7-interfaces/7.3) and [7.4](/7-interfaces/7.4). Also, the engineering and commissioning tests shown in the [project schedule](/9-project-management/9.2) as WBS task number 5.8 will be used to validate and demonstrate on-sky performance and procedures as part of the process for qualifying as a GBO user instrument. These tests take place in the final two quarters of the project.
1. The [project schedule](/9-project-management/9.2) includes a 9-month extension as mitigation for delays incurred by the COVID-19 pandemic and collapse of the Arecibo Telescope. We believe this will be adequate if the joint collaborative working agreement with GBO can be finalized shortly following completing this design review.
[Section 9.3](/9-project-management/9.3) addresses causes and mitigation for a budget shortfall due to COVID-19 delays, collapse of the Arecibo Telescope, and costs of moving ALPACA to the GBT. We are pursuing a supplementary funding request for $1,485,645, which will fully fund project completion through installation, integration, and commissioning as a GBO user instrument. Without this or other supplemental sources, the instrument itself can be completed, but installation, integration, and commissioning as a GBO user instrument would be unfunded.
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