This list is very incomplete. All are encouraged to send in missing definitions.
Direct Digital Synthesizer.
The Radar Decoder correlates the received signal with the code used at transmission, generating the equivalent pulse-radar signal (digital). This process greatly reduces the output data rate over directly sampling with the Radar Interface, but requires that the ephemeris be very well known, so that the bins don't smear into one another. Decoding is an irreversible process, because phase information is lost, and, in order to actually reduce the data rate, you need to only keep part of the range information.
For radar purposes, Delay is the time between the transmission of a signal and the reception of the reflected signal. Since the signal transmission speed is known (though note atmospheric effects), this delay indicates the distance ("range") to the target. Because a round-trip path is required, the conversion is 998.009564 seconds/AU.
Frequency synthesizer that can generate the Doppler-shifted LO needed for radar reception of moving targets.
Electronics in the control building, or generally anywhere but Upstairs.
An Ephemeris is a position of an object at a particular time, or a list of such positions. Frequently, other time-dependent information is included, like the Apparent Visual Magnitude, and derivatives of the position. For radar astronomy, the position must include the range and the Doppler shift of the object, as well as the more usual astronomical Right Ascension and Declination, because the receivers must be tuned to the correct frequency to be able to detect the echo.
The IF/LO is really two units, one upstairs and one downstairs. The Upstairs IF/LO down-converts the signal to the intermediate frequency (generally 260 MHz) or, in some cases, just sends it down at RF on the optical fiber. The downstairs IF/LO converts the signal to the desired IF frequency, e.g., 260 MHz or 30 MHz. This unit is extremely flexible, with several variable-frequency oscillators and multiple channels.
Master Control Program
The Master Control Program allocates buffers for data acquisition and routes them to client devices, such as tape drives, array processors and monitor displays. Buffers can also be sent to the ethernet, and possibly directly to a Sun workstation at bus speed. The VME bus operates at 15 MB/s, but few of the devices can operate that fast in practice.
The Radar Interface samples an input input signal voltage (in quadrature), and provides it as a digital output to a VME bus for processing or recording. On this VME bus are also a control computer, an Array Processor, and a tape drive. The Radar Interface can sample the input signal at up to NNN bits, at up to 10 MHz sample rate (per polarization). There is currently no way to record data at that high rate, however. The maximum recording rate is 450-500 KB/s on one Exabyte, possibly going as high as 900 KB/s by striping two. In the future, faster tape drives and a VME <-> SBus interface may allow faster recording. The AP on the VME bus can handle 64K word transforms. The controller CPU can also direct data over ethernet to a Sun workstation. In principle, data can be both sent to tape and transformed in the AP and sent to the Sun, depending on the speed requirements.
The electronics on the observing platform, generally either inside the Gregorian enclosure or on the Carriage House.
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Last updated 1996 January 31