Telemetry data captured during operation of the ground station must often
be archived for post acquisition analysis and to satisfy legal requirements
that the vehicle under test was properly certified. Typically, data is stored immediately in the ground system, as close
to the received signal as possible (expensive instrumentation recorders
store the PCM signal immediately after bit synchronization). Data is also
archived to disk after decommutation and/or processing and is then backed
up to inexpensive cartridge tape (e.g., DAT, 8mm, DLT). Selected measurands
and processed parameters or the entire PCM frame can also be archived.Current single disk technology permits storing an entire continuous 40
Mbps PCM stream for only 3 hours, and less if measurand and time tags
are required. Archiving time may be extended by storing only time segments
or reducing the number of measurands, i.e, data compression (see real-time
processing). Conversely, if archival of EU-converted and processed
data is required, storage requirements increase and archiving bandwidth
may not be adequate. Bandwidth and volume size may be increased by recording
to multiple disks in parallel (RAID subsystems). Single UltraSCSI disks
achieve 10 MB/sec continuously over their entire 72 GB capacity, while
an UltraSCSI RAID can archive rates approaching 40 MB/sec and 80 MB/sec
for Fibre Channel. Archiving directly to commodity tape drives is limited
to a few MB/sec and up to 15 MB/sec for expensive proprietary architectures.
The system design using real-time tape must incorporate large buffers
to accommodate the time required for the drive to reach operating speed.
Archiving the time of acquisition with data consume a large segment of
both storage space and bandwidth. One extreme tags each measurement with
either a minor time (least significant portion), only placing the entire
time record periodically (once per telemetry frame). A more economic solution
for synchronously acquired data is to insert time periodically; for example,
at the end of each frame or block. The time associated with the acquisition
of a particular measurement can be interpolated from its position in the
telemetry frame. Storing aperiodic data requires time-tagging. Data playback
from disk offers a challenge since measurements must be continuously metered
to recreate continuous or real-time displays on workstations or strip
charts.
Archival
Media and Device Summary |
Media/Device
Type |
Uncompressed
Capacity |
Continuous
Rate |
(GB) |
(MB/s) |
| Random
Access |
Floppy |
0.001 |
0.125 |
Optical |
0.65 |
1 |
Disk drive |
72 |
10 |
RAID |
576 |
37 |
| Sequential |
1/4" cartridge |
0.15 |
0.09 |
1/2" reel |
0.18 |
0.75 |
Beta |
7 |
1 |
IBM |
0.8 |
3 |
DAT (DSS-4) |
20 |
3 |
VHS |
10 |
4 |
DLT |
40 |
5 |
Instrumentation |
0.46 |
12 |
DTF |
42 |
12 |
DCRsi |
47 |
30 |
IDI |
95 |
32 |
|
