NOAA KLM User's Guide

Section 7.5.1

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7.5.1 TED Calibration

7.5.1.1 TED Particle Calibration

The TED ESAs are all calibrated with electron and ion beams to provide measured values for the geometric factors. This corrects for the variations in absolute CDEM detection efficiency, and requires that a unique set of calibration constants for each TED be used in processing the telemetered data. Each TED has a Calibration Report which provides all of the necessary calibration constants.

The primary TED measurements are the electron and proton (ion) energy fluxes measured at 0 and 30 degrees to the local vertical. The energy fluxes are measured for 0.05 to 1 keV and for 1 to 20 keV, so there are 8 primary energy flux values and calibration constants. Approximate values for the conversion of telemetered energy flux counts into energy fluxes are listed in Table 7.5.1.1-1. Note that the 8-bit telemetry count is a compressed count, and must first be converted into an uncompressed count. The actual calibrated constants for a given TED may differ by as much as 50% from the values listed in Table 7.5.1.1-1.

Table 7.5.1.1-1. Nominal Values for TED Energy Flux Calibration Factors.
Channel Designation Particle Type Direction Measured Energy Range
(keV)
Calibration Factor
(erg/(cm2-s-sr-count))
0EFL Electrons 0 0.05 - 1 2×10-6
0EFH Electrons 0 1 - 20 5×10-5
3EFL Electrons 30 0.05 - 1 2×10-6
3EFH Electrons 30 1 - 20 5×10-5
0PFL Protons (Ions) 0 0.05 - 1 1×10-6
0PFH Protons (Ions) 0 1 - 20 4×10-5
3PFL Protons (Ions) 30 0.05 - 1 1×10-6
3PFH Protons (Ions) 30 1 - 20 4×10-5

The TED also provides four point energy spectra and a measurement of the peak flux point in the spectrum for each particle type/measurement direction. The four point spectra require a total of 16 calibration constants to convert the telemetered counts, after decompression, into particle energy fluxes. The peak flux channel can be any of the 16 channels measured for a given particle type/measurement direction, and requires the full set of channel calibration factors, a total of 64 constants. These calibration constants are obtained from the Calibration Report for the TED, and may vary by 50% from the nominal values.

7.5.1.2 TED In-Flight Calibration

Each of the eight CDEMs has a threshold set to one of four values with each design value twice the previous one, as shown in Table 7.5.1.2-1.

Table 7.5.1.2-1. CDEM Threshold Levels.
Level Binary Level Design Value Voltage
0 00 0.2375 V
1 01 0.475 V
2 10 0.950 V
3 11 1.900 V

The threshold rejects low level noise which appears in the signal processing circuitry. Natural signal pulses from the CDEM are normally larger than the highest threshold so any threshold can be used. At the beginning of life, the lowest threshold is used. If noise appears, it can be rejected by choosing a higher threshold. In addition, CDEM gain may go down with age. This is expected to show up with the highest threshold where weaker pulses may not be counted. This effect can be corrected by using a higher CDEM operating voltage, selected by ground command.

The thresholds in use are commanded from the ground, separately for electrons and for protons. The threshold selected for electrons is used by all eight electron channels. Likewise all eight proton channels have one threshold. All thresholds in each of the eight channels are measured by the In-Flight Calibrator, a total of 32 values.

TED In-Flight Calibration (IFC) starts on command from the ground. Because the IFC changes settings, the settings in use before the IFC starts are remembered and are restored when the TED IFC terminates. Settings commanded from the ground during TED IFC are remembered and are the ones "restored" on termination. TED IFC has two phases, phase 0 and phase 1, both controlled by the DPU.

Phase 0 measures the thresholds using one major frame (32 seconds). This is done with increasing pulses put in at the CDEM with the Electrostatic Analyzers (ESAs) turned off so no natural pulses occur. In eight seconds, the calibration pulses rise in a linear series of steps to 2.50 V at the discriminator, exceeding the highest of the four thresholds. This is done four times, once for each of the four thresholds, taking a total of 32 seconds.

Phase 1, which starts at the end of Phase 0, measures CDEM detection efficiency. Phase 1 has no calibration pulses but the ESAs are turned back on so that operation is normal except that the thresholds are cycled through their four levels (0, 1, 2, 3, 0, ...). It runs for about one spacecraft orbit (189 x 32 second major frames = 100.8 minutes). This allows natural pulses to exercise all thresholds thereby testing the CDEMs as well as the signal processing circuitry. A full revolution is used to assure that there will be pulses, most of which appear in the auroral region. Data from this phase of the IFC is used to determine if any CDEMs have degraded, and if any CDEM operating voltage adjustments are required.


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