All of the Company's leases are classified as operating leases. The Company's rental income is comprised of both fixed and variable income. Fixed and in-substance fixed lease income includes stated amounts per the lease contract, which are primarily related to base rent. The Company’s leases also provide for reimbursement from recoverable costs. A portion of our operating cost reimbursement revenue is estimated each period and is recognized as rental income in the period the recoverable costs are incurred and accrued. Income for these amounts is recognized on a straight-line basis. Variable lease income includes the tenants' contractual obligations to reimburse the Company for their portion of recoverable costs incurred. The following table provides a disaggregation of lease income recognized as either fixed or variable lease income three months ended March 31, 2023 and 2022:


	2023		2022
Rental income
Fixed and in-substance fixed lease income	$	1,203,924	$	1,085,741
Variable lease income		79,821		71,253
Other related lease income, net:
Amortization of below-market leases		26,114		23,841
Straight line rent revenue		16,848		1,100
Total rental income	$	1,326,707	$	1,181,935

For the three months ended March 31, 2023 and 2022, we had four tenants that each account for more than 10% of our annual rental revenue as indicated below:


	2023			2022
General Services Administration - Norfolk, VA & Manteo, NC		23	%		22	%
PRA Holdings, Inc. - Norfolk, VA		16	%		16	%
Pratt & Whitney Automation, Inc. - Huntsville, AL		15	%		15	%
Kohl's Corporation - Tucson, AZ		17	%	Less than 10%

The following table presents future minimum rental cash payments due to the Company over the next five calendar years and thereafter as of December 31:


	As of March 31,
	2023
2023 (9 months remaining)	$	3,543,139
2024		4,785,452
2025		4,635,711
2026		4,513,724
2027		3,919,118
Thereafter		5,210,921
	$	26,608,065

The Company acquired one property on March 9, 2022 that is subject to a non-cancelable, long-term ground lease where a third party owns the underlying land and has leased the land to the Company. Accordingly, the Company owns only a long-term leasehold in this property. This ground lease expires in 2084 including those options the Company deems probable of exercising. The ground lease expense is recognized on a straight-line basis over the term of the lease, including management's estimate of expected option renewal periods. Operating lease expense was approximately $93,762 and $16,000 for the three months ended March 31, 2023 and 2022, respectively. There are no variable lease expenses required to be paid by the Company as lessee per the lease terms. Cash paid for amounts included in the measurement of the lease liability, net was $58,175 and $14,387 for the three months ended March 31, 2023 and 2022, respectively.

The following table summarizes the undiscounted future cash flows for subsequent years ending December 31 attributable to the lease liability as of March 31, 2023 and provides a reconciliation to the lease liability included in the accompanying Consolidated Balance Sheets as of March 31, 2023.


	As of March 31,
	2023
2023 (9 months remaining)	$	174,525
2024		244,077
2025		245,111
2026		245,111
2027		245,111
Thereafter		21,820,644
Total undiscounted liability	$	22,974,579
Present value discount		(16,603,853	)
Lease liability	$	6,370,726
Discount rate		4.58	%
Term Remaining	61 years