Space Environment
The requirements for a data acquisition system for space testing (launch vehicles, orbiters etc.) can seem very similar to those intended for lower altitudes. In practice however, there are important environmental and operational differences that present challenges for both users and vendors of instrumentation. Environmental challenges include the severe vibration and shock experienced on take-off, followed by a very sharp thermal shock, culminating (for orbital vehicles) in a low temperature, low pressure, high radiation operating environment. Operational challenges include the need to dynamically adapt to changing configurations (for example when an instrumented stage is released) and the need to telemeter data during the initial launch (the initial launch stage may not be recoverable and thus a recorder is not an option). Addressing these challenges requires simple, rugged and flexible solutions.
Environmental challenges encountered in space applications include
- Acoustic pressures range - 30Hz to 10kHz
- Engine induced vibration -10Hz to 2kHz typically with peak power spectral densities reaching 1.6g2/Hz or more.
- Separation transient shocks – 2,000g or more
- Very rapid temperature changes affects sensor measurements; condensation can damage electronics
- Very high energy particles (>10 MeV) cause single event effects possibly leading to damage
The first step to selecting flight test instrumentation for a space vehicle is a realistic assessment of the environmental stresses that the equipment will actually experience. When it comes to assessing system performance given the shock and vibration envelope there is no substitute for testing the unit. For example, the KAM-500 flight test data acquisition unit from ACRA CONTROL was put through a series of extended environmental tests that established it would operate as expected on a Delta II launch vehicle application. The effects of radiation can be mitigated by choosing radiation tolerant technologies such as Actel FPGAs. Systems based around large microprocessors with large amounts of vulnerable data and programs stored in memory devices should be avoided.
Operational issues encountered in space applications include
- FTI networks change during separation
- Electromagnetic noise and interference; limited available spectrum at launch
- Limited windows of opportunity to telemeter the data back to earth for orbital vehicles
- Long cable runs (up to 200ft) between units
Recoverable DAUs should continue to record data while the ‘Master’ System should keep operating without the discarded DAUs. For PCM systems, cabling needs to be designed so that the ‘personality’ of affected DAUs changes as cables are severed. For Ethernet systems it is simply a case of a node/s leaving the network.
To transmit valuable data collected at launch, data should be stored during the take-off stage and transmitted after a predetermined time (e.g. 30s) once stable flight has been achieved. This must be automatic. Craft in orbit can utilize remote controlled recorders capable of ‘read while write’ functionality that sends data and performs other operations as required when dealing with limited transmission windows.
Overcoming limited data rates on long cable runs, without using repeaters (which add wiring complexity, weight and increases the bit error rate) can be achieved using state-of-the-art transceivers. These can transmit PCM over distances greater than 200ft at 20Mbps. Ethernet systems can also be used and don’t require additional hardware as they natively transmit at up to 100Mbps over distances greater than 300ft.