decrease in service revenue was primarily due to decreases in MUAS, small UAS and HAPS service revenue, partially offset by increases in revenue from customer-funded research and development efforts primarily in our All Other and TMS segments. We expect the lower levels of MUAS service revenues to continue into fiscal 2024 related to the completion of certain MUAS site locations. Due to the higher backlog, we expect the Small UAS product revenues to be significantly higher in the fourth quarter of fiscal 2023 and during the first half of fiscal 2024 as compared to the first half of fiscal 2023.
Cost of Sales. Cost of sales for the nine months ended January 28, 2023 was $249.4 million, as compared to $220.5 million for the nine months ended January 29, 2022, representing an increase of $28.9 million, or 13%. The increase in cost of sales was a result of an increase in product cost of sales of $26.4 million and an increase in service costs of sales of $2.5 million. The increase in product costs of sales was primarily due to an increase in product revenue. The increase in service cost of sales was primarily due to accelerated depreciation charges of certain deployed fixed assets related to the anticipated completion of certain MUAS site locations of $8.1 million, partially offset by a decrease in service revenue and a decrease in intangible amortization and other related non-cash purchase accounting expenses. Cost of sales for the nine months ended January 28, 2023 included $10.4 million of intangible amortization and other related non-cash purchase accounting expenses as compared to $14.6 million for the nine months ended January 29, 2022. As a percentage of revenue, cost of sales remained consistent at 70%.
Gross Margin. Gross margin for the nine months ended January 28, 2023 was $105.1 million, as compared to $92.6 million for the nine months ended January 29, 2022, representing an increase of $12.5 million, or 13%. The increase in gross margin was due to an increase in product margin of $18.4 million, partially offset by a decrease in service margin of $5.9 million. The increase in product margin was primarily due to the increase in product sales combined with a favorable product mix. The decrease in service margin was primarily due to accelerated depreciation charges of certain deployed fixed assets related to the anticipated completion of certain MUAS site locations of $8.1 million and a decrease in service revenue. As a percentage of revenue, gross margin remained consistent at 30%. Additionally, we expect inflationary and supply chain constraint trends to continue throughout our fiscal year 2023, which are currently and will continue to negatively impact our gross margin across all our segments.
Selling, General and Administrative. SG&A expense for the nine months ended January 28, 2023 was $70.3 million, or 20% of revenue, as compared to SG&A expense of $74.5 million, or 24% of revenue, for the nine months ended January 29, 2022. The decrease in SG&A expense was primarily due to a decrease in acquisition-related expenses of $3.9 million and a decrease in intangible amortization and other related non-cash purchase accounting expenses of $2.7 million, partially offset by an increase in employee related costs.
Research and Development. R&D expense for the nine months ended January 28, 2023 was $47.8 million, or 13% of revenue, as compared to R&D expense of $41.0 million, or 13% of revenue, for the nine months ended January 29, 2022, primarily due to an increase in development activities regarding enhanced capabilities for our products, development of new product lines and to support our acquired businesses.
Interest Expense, net. Interest expense, net for the nine months ended January 28, 2023 was $6.7 million compared to interest expense, net of $4.2 million for the nine months ended January 29, 2022. The increase in interest expense, net was primarily due to an increase in interest expense resulting from higher interest rates on our debt facility, partially offset by lower average outstanding balances.
Other Expense, net. Other expense, net, for the nine months ended January 28, 2023 was $2.2 million compared to other expense, net of $10.4 million for the nine months ended January 29, 2022. The decrease in other expense, net is primarily due to a legal accrual of $10.0 million for the settlement of all claims made by the buyers of our former EES business recorded during the nine months ended January 29, 2022. Other expense, net for the nine months ended January 28, 2023 includes unrealized losses associated with decreases in fair market value for equity security investments.
Benefit from Income Taxes. Our effective income tax rate was 38.3% for the nine months ended January 28, 2023, as compared to 69.1% for the nine months ended January 29, 2022. Historically, we calculate the provision for income taxes during interim reporting periods by applying an estimate of our annual effective tax rate (“AETR”) for the full fiscal year to the pretax income or loss for the interim reporting period. For the nine months ended January 28, 2023, we