Trade accounts receivable, net (excluding notes receivable) are reflected in the following table (in thousands):

	AS OF SEPTEMBER 30,
	2021			2020
Trade accounts receivable	$	12,635		$	14,090
Allowance for doubtful accounts		(428	)		(496	)
	$	12,207		$	13,594

The allowance for doubtful accounts represents the Company’s best estimate of probable credit losses. The Company determines the allowance based upon historical experience and a current review of its accounts receivable balances. Accounts receivable balances are charged off against the allowance whenever it is probable that the receivable balance will not be recoverable.

	AS OF SEPTEMBER 30,
	2021			2020
Promissory notes	$	5,432		$	184
Sales-type lease		2,464			—
Total financing receivables		7,896			184
Unearned income:
Sales-type lease		(6	)		—
Total unearned income		(6	)		—
Total financing receivables, net of unearned income		7,890			184
Less current portion		(4,952	)		(184	)
Non-current notes receivable	$	2,938		$	—

Promissory notes receivable are generally collateralized by the products sold, and bear interest at rates ranging up to 8% per year. The promissory notes receivable mature at various times through January 2023. The Company has, on occasion, extended or renewed notes receivable as they mature, but there is no obligation to do so.

During the second quarter of fiscal year 2020, the Company partially financed a $12.5 million product sale by entering into a $10.0 million promissory note with the customer. The note has a three-year term with monthly principal and interest payments of $0.3 million. Due to the financial condition of the customer, the Company had concerns over the probable collectability of the promissory note. As a result, the promissory note was not reflected on the Company’s consolidated balance sheet through its first quarter of fiscal year 2021. During the second quarter of fiscal year 2021, as a result of a new information received from the customer, management determined that it is probable that the customer will satisfy its remaining payment obligations to the Company and recognized the promissory note on its consolidated balance sheet as of March 31, 2021. See Note 2 for more information on this matter.

During the third quarter of fiscal year 2021, the Company entered into a sales-type lease with a customer on land-based wireless seismic equipment from its rental fleet. The lease has a term of six months. Scheduled minimum payments due and outstanding under the lease at September 30, 2021 were $1.5 million. Minimum leases payments of $1.0 million are due in the first quarter of fiscal year 2022. Interest income of $37,000 was recognized during the fiscal year ended September 30, 2021. The ownership of the equipment will transfer to the customer at the end of the lease term.