Sample noise level (SNOISE)
Use SNOISE to estimate the current noise level from the receiver, so that the noise can be subtracted from subsequent measurements.
Data are sampled for 256 pulses at 256 bins, beginning at a selectable range and spaced by the range resolution at that pulse width. The internal trigger generator is temporarily set to a special noise rate (usually much lower than the operating rate) during the process. It is the user's responsibility to make sure that no returned power is present within the approximately 32 km sampling interval. In some cases, it may be necessary to raise the antenna during the noise measurement to avoid thermal noise pickup from the ground, or from weather targets.
SNOISE has the option of setting up a new sampling range and trigger generator rate each time it is called. Two bits in the command word determine which, if any, of the new values overrides the current values stored in RVP10. The power-up sampling range is 250 km (input value of 250), and the power-up trigger rate is 200 Hz (input value of 30000). These initial values persist until such time as they are altered here. Both input words must always be supplied after the command, even if the command calls for ignoring one or both of them. The range is supplied directly in kilometers up to a maximum of 992 km. The trigger rate, resulting from a given input, is 6 MHz divided by the input value, that is, the input value is the trigger period in 0.1667 μsec increments. Note that the given rate is bounded against the minimum PRT allowed for the current radar pulse width.
The SNOISE command bounds the requested starting range of the noise sampling interval. This is to make sure the noise samples fit within the specified PRT, and within the range mask hardware RAM. RVP10 sets an error bit when an improper range is requested.
Reissue the SNOISE command periodically to compensate for drift in the RF and A/D systems.
The noise levels must be measured for the RVP10 to properly process data. This can be done by issuing the SNOISE at least once after power-up, or by setting the correct values for the power-up noise levels with the Mt setup command (see Mt<n> — Transmit sequence #n). RVP10 does not automatically take a noise sample as part of its initialization procedure.
The measured offsets are stored internally for all subsequent uses in RVP10. The offset values may be inspected through the GPARM command, as may the current range and rate values themselves. When the range or rate are changed, the user must make sure that the new trigger rate allows at least 32 km following the new noise range. If this requirement is not met, or if other failures are detected during the noise measurement, appropriate bits are set in theGPARM latched status word. This word should generally be checked after SNOISE to make sure that everything worked properly.
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| | Action |Rat|Rng| | 0 0 1 0 1 | Command
|--------------|-----------|---|---|--------|-------------------|- Rng
- If 1, the range in input word 1 is taken as the starting noise range for this and all subsequent SNOISE calls.
- Rat
- If 1, the trigger rate in input word 2 is taken as the noise rate for this and all subsequent SNOISE calls.
- Action
- Specifies what action is carried out by the command.
- 0
- Compute a new noise sample based on the present IFD input signals.
- 1
-
Do not compute a noise sample, but rather, read new noise values from the host computer and use them for subsequent processing. Four additional input words supply the noise information, and GPARM words
6,9, and44..50are changed to reflect the new noise settings.If the current transmit pulse is a hybrid pulse, and XARGS arguments are supplied, then the additional arguments set the noise level for the second pulse.
- 2
- Do not compute a noise sample, but rather, restore the powerup noise defaults.
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| Starting Range in km (Max 992km) of 32km Sampling Interval | Input 1
|---------------------------------------------------------------| 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
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| Internal Trigger Rate (6Mhz/N) to use During Noise Sampling | Input 2
|---------------------------------------------------------------|The following input words are optional, only if Action=1.
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|0 0 | (MSB) Log of Measured Noise Level(LSB) | Input 3
|------|--------------------------------------------------------|The is the same number as GPARM output 6. See Initiate processing (PROC) and Get processor parameters (GPARM).
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| Noise Level Standard Deviation (in 1/100 of a dB) | Input 4
|---------------------------------------------------------------|This is the same number as the GPARM output 49.
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| Ratio of Horizontal/Vertical Noise Power in Hundredths of dB | Input 5
|---------------------------------------------------------------|This is the same number as the GPARM output 50.
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| <Spare> |Err|Ttf|Ntg| Input 6
|---------------------------------------------------|---|---|---|The following XARGS input words are optional, only if Action=1 and hybrid pulse.
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|0 0 | (MSB) Log of Measured Noise Level, puse 2 (LSB) | XARG 1
|------|--------------------------------------------------------| 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
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| NOISE Level Standard Deviation, pulse 2 (in 1/100 of a dB) | XARG 2
|------|--------------------------------------------------------| 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
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| Ratio of Horizontal/Vertical Noise, pulse 2 (in 1/100 of a dB)| XARG 3
|------|--------------------------------------------------------|These fault bits are outputted in the latched
status GPARM word 9.
- Ntg
- No Trigger during noise measurement.
- Ttf
- Trigger too fast during noise measurement, that is, some of the noise sample bins were positioned past the trigger range.
- Err
- Error detected during the SNOISE command.