Next: 5.1.3 Level 0 Data Up: 5.1 Ground Telemetry Processing Previous: 5.1.1 Introduction
On board the ACIS experiment, a fundamental unit of data is the science run, which is a set of data obtained while ACIS has a fixed configuration. The design of the system for communicating science data from ACIS to the ground is organized around the science run concept. For example, changes in the configuration of ACIS are explicitly represented in the telemetry stream by the end of one science run and the beginning of another.
While transmitting science run data, the ACIS telemetry system must also transmit other sorts of data such as calibration data, command echoes, and software housekeeping information. This heterogeneous set of information is sent in an interleaved fashion using a packetized transport protocol similar to that used in computer networks. The boundaries between ACIS packets are represented by placing an ID word in the packet header at the beginning of each packet. Each packet can have a different length, which is also included in the packet header.
The packet stream produced by the ACIS computer is passed on to the satellite computer for transmission to the ground. When ACIS is in the focal plane, the ACIS packet stream is commutated into the science data fields in the AXAF major frame (see Section 2.4.4).
On the ground, the commutation and packetization steps must be inverted in order to recover each of the data streams produced by ACIS (science run data, calibration data, command logs, software housekeeping, etc.). Figure 5.1 shows a cartoon that summarizes the ACIS telemetry composition (on the satellite) and decomposition (on the ground) steps. In this simplified example, a Timed Exposure Faint Mode science run detects 9000 X-ray events. These data are formatted into three packets by the ACIS computer. These three packets are commutated into portions of two AXAF major frames by the AXAF computer. On the ground (at the ASC), a decommutation step recovers the ACIS packet stream and a de-packetization step recovers the actual science run data.