An apparatus, comprising: a jarring tool operable for coupling within and generating an impact imparted to a downhole tool string, wherein the jarring tool comprises: an upper housing comprising a first bore extending longitudinally therethrough; a lower housing (265, 502) comprising a second bore extending longitudinally therethrough; a shaft coupled with the lower housing and extending into the first bore of the upper housing, wherein the upper housing moves axially relative to the lower housing along the shaft; and characterized in that the jarring tool further comprises a sensor system disposed within the second bore of the lower housing and comprising: a chassis slidably insertable into the second bore of the lower housing, wherein the chassis comprises a curved outer surface contacting a cooperatively curved inner surface of the lower housing, wherein the chassis is retained within the second bore via at least one fastener (532, 572) engaging the chassis and the lower housing, and wherein the chassis does not support weight of a lower portion of the downhole tool string coupled below the jarring tool; and an accelerometer connected to the chassis and operable to generate an output signal indicative of acceleration caused by an impact generated by movement of the upper housing away from the lower housing until corresponding impact features (490, 495) of the upper and lower housings collide. an upper housing comprising a first bore extending longitudinally therethrough; a lower housing (265, 502) comprising a second bore extending longitudinally therethrough; a shaft coupled with the lower housing and extending into the first bore of the upper housing, wherein the upper housing moves axially relative to the lower housing along the shaft; and characterized in that the jarring tool further comprises a sensor system disposed within the second bore of the lower housing and comprising: a chassis slidably insertable into the second bore of the lower housing, wherein the chassis comprises a curved outer surface contacting a cooperatively curved inner surface of the lower housing, wherein the chassis is retained within the second bore via at least one fastener (532, 572) engaging the chassis and the lower housing, and wherein the chassis does not support weight of a lower portion of the downhole tool string coupled below the jarring tool; and an accelerometer connected to the chassis and operable to generate an output signal indicative of acceleration caused by an impact generated by movement of the upper housing away from the lower housing until corresponding impact features (490, 495) of the upper and lower housings collide. a chassis slidably insertable into the second bore of the lower housing, wherein the chassis comprises a curved outer surface contacting a cooperatively curved inner surface of the lower housing, wherein the chassis is retained within the second bore via at least one fastener (532, 572) engaging the chassis and the lower housing, and wherein the chassis does not support weight of a lower portion of the downhole tool string coupled below the jarring tool; and an accelerometer connected to the chassis and operable to generate an output signal indicative of acceleration caused by an impact generated by movement of the upper housing away from the lower housing until corresponding impact features (490, 495) of the upper and lower housings collide.