Have a look at this:
Chase PaymentTech: Orbital Gateway .NET (2.0) SDK[
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It states:
The .NET SDK is made up of two components:
• ptDotNetSDK.dll – The primary dll containing the Transaction and Response classes.
• PTDotNetService.exe – A service containing the core engine within a separate process as a Windows Service. This service is used when the SDK is operated in Service mode.
Thus, it should be the part of the SDK that you are about to use. Further, document states you need to make sure that the DLL is present in a specific folder. If you still face issue after going through the document then I would suggest you to contact them regarding the same.