Ground Systems

The TIMED Ground System has all the facilities usually associated with space missions, such as operations centers and ground stations. But TIMED was engineered as a total space-ground system to streamline mission activities, so this Ground System is best understood as a set of components that are tightly integrated with their counterparts on the spacecraft. Figure 1 illustrates just how close this integration is. The spacecraft components are shown on the left, and the ground components on the right, but the physical distance and communications complexity that separates them evaporates when one focuses on the strongly-coupled flows of control and data

Figure 1. End-to-End Data Flow in the TIMED System -- Click for enlarged view.

Focusing on the outside (green) loop in Figure 1, we can see on the right an investigator in a Payload Operations Center (POC) who is generating a command to operate his instrument. He sends this command on its way over the Internet in a packet addressed to his instrument. The packet stops off briefly at a computer in the Mission Operations Center (MOC) at APL. If the command is authentic and there is a current radio link with the spacecraft, it is immediately transmitted to the spacecraft's Command and Data Handling (C&DH) system. The C&DH system automatically routes the command to the instrument, where it is executed. The Instrument puts its collected science data into telemetry packets that are addressed to the Mission Data Center (MDC) at APL. When a downlink is available, the telemetry is transmitted to the MDC. Now, an investigator at the POC (or anywhere else) can receive his (or anyone's) packets over the Internet. He has two ways to receive his data: as packaged files or a continuous time-ordered stream. He can see the packets as soon as they arrive on the ground, but since all telemetry is archived and on-line in the MDC, he may wait as long as he likes and then ask for a historical data stream.

Notice that there is also an inside (red) loop, almost entirely decoupled from the instruments, that is dedicated to maintenance and operation of the spacecraft "bus", which really includes everything needed to provide a comfortable "ride" to the four instruments on board. In this loop, we see a mission specialist in the MOC who is generating command packets for the bus, perhaps to perform a diagnostic test. The status of the bus is conveyed back to the MOC for assessment.

To summarize the essential feature of this architecture, the bus and instrument data loops share common uplink and downlink services, but these services keep the operations almost completely independent. eliminating the need for coordination between bus and instrument operators for routine activities. This decoupling provides substantial savings and simplification in the on-orbit mission operations.

Now turning to Figure 2, we see a more conventional view of the Ground System facilities and their interconnections. The space-ground link is provided through a primary ground station at APL, or via one of a network of offsite backup ground stations. The equivalent path for pre-launch testing is through a special assembly of Ground Support Equipment (GSE), also shown on the diagram. The red and blue color-coded flows are the command and telemetry that support mission operations already described.

Figure 2. TIMED Ground System Architecture -- Click for enlarged view.

Outlined in Figure 2 is a special subdivision of the Ground System called the Science Data System (SDS), which encompasses all science data analysis activities in the TIMED program. We see here that each of the four instrument POCs is actually subdivided into two areas, one for mission operations (MO), and the other for data analysis (DA). The green and violet flows illustrate interchange of science data among the many facilities involved, again over the Internet. Although the science product archives are distributed among these facilities, the MDC is a one-stop shop on the Web for locating and retrieving any of the information gathered by the program.