Following significant interaction with the NSF in the 1980's, a proposal to the NSF for the construction of LIGO was successful in 1989. The US LIGO Observatory sites, in Livingston, Louisiana and Hanford, Washington were first identified by the LIGO Project through a search for technically and programmatically suitable sites with adequate separation. Sets of suitable site pairs were considered by NSF and the sites named. These criteria were later adjusted and used for the identification of a suitable LIGO-India site.  The Hanford and Livingston observatories were constructed during the period 1994--1999, and were inaugurated in November 1999. After three years of commissioning, the Initial LIGO detectors commenced scientific operations with the first science run in August 2002.

The configuration of two sites, separated by a continental baseline, was dictated by the need to demonstrate an initial detection by instruments which were plausibly independent from common environmental effects. It was evident as well that the additional timing information available from the two sites would yield valuable position information. An additional advantage has become clear in the sequence of commissioning and operation of two sites with very similar detectors under one management organization: the two site teams can address challenges in parallel, and then communicate solutions to the other site. Staff circulate between the sites, keeping perspectives fresh, and teams highly motivated. 

The facilities were designed anticipating a regular sequence of upgrades performed as technological advances allowed for more sensitive detectors. The LIGO facilities' performance limits~\cite{LIGO-SRD} are set at high frequencies ($f \gtrsim 50~ \mathrm{Hz}$) by path-length fluctuations due to the residual gas~\cite{Zucker:1994ix} at a strain noise level of a a few parts in $10^{-25} \big/\sqrt{\mathrm{Hz}}$, and at low frequencies ($f \lesssim 50~ \mathrm{Hz}$) by Newtonian gravity gradient noise. 
%In principle, both of these can be further  reduced; the former by introducing additional pumping along the interferometer arms, and the latter by using distributed arrays of seismic sensors arranged around the test masses to sense local ground motion and thereby estimate the gravity gradient noise in real-time~\cite{Coughlin:2016tmu}.

The initial LIGO era culminated in several upgrades that demonstrated technologies that would be utilized by Advanced LIGO (direct `DC' readout of the GW strain signal, active seismic isolation, frequency-independent squeezing). The Advanced LIGO era began in September 2015, when almost at the start of the first observing run (O1), the two LIGO detectors observed a spectacular first detection of gravitational waves emanating from the merger of a pair of massive black holes \cite{firstdiscovery}. The 2017 Nobel Prize in Physics\cite{nobelprize2017} recognized the three principals who conceived of and brought LIGO into existence as a novel facility. Since then Advanced LIGO has seen a first upgrade, A+~\cite{McCuller:2020yhw}, which is largely implemented for O4 and will be completed for O5 (refer to Fig. \ref{schedule}, above). There is also a planned further future upgrade, \Asharp{} (\cite{T2200287}, discussed above).

LIGO has been jointly operated by Caltech and MIT since its inception under cooperative agreements (CA) between Caltech and the NSF. LIGO is part of the NSF's Major Research Infrastructure and as such the CAs for maintenance and operation (M\&O) of the observatories are typically limited to a 5-year periods subject to renewal or recompetition. The time horizon under consideration in this white paper will span three or four NSF CA cycles.





