NOAA KLM User's Guide

Section 3.5

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3.5 Space Environment Monitor (SEM-2)

3.5.1 Instrument Operation

The SEM-2 Space Environment Monitor is a multichannel charged-particle spectrometer which senses the flux of charged particles at the satellite altitude, and thus contributes to knowledge of the solar-terrestrial environment. SEM-1 units have been in orbit on the TIROS-N series since 1978. SEM-2 is a new design produced by Panametrics, Inc. of Waltham, Massachusetts to specifications written by the Space Environment Center of NOAA.

Solar particle emissions travel to Earth in the form of the solar wind, which consists of streams of charged particles moving at hundreds of kilometers per second. It is a source of particles for the Earth's trapped radiation belts. In addition to the solar wind, there occur times of intense fluxes of energetic electrons, protons and alpha particles (energies up to 1,000 MeV). As a consequence of both the solar wind particle stream and the occurrences of very energetic particle emissions from the Sun, there can be large changes to the energy input to the magnetosphere, ionosphere and high altitude atmosphere. Some of this energy is deposited into the ionosphere to produce the aurora visible around both magnetic poles.

Large, abrupt changes occur in the solar wind. These changes produce changes in the magnetosphere and the ionosphere. This can result in hazards among which are:

1. Radiation hazard to people in space and the potential of increased radiation exposure to people in high flying aircraft;

2. Disruption of radio navigation;

3. Absorption, even blackout, of radio waves so that radio communication is disrupted;

4. Induced voltages and currents in electric power circuits leading to circuit breaker trip, damage to equipment, and failure of transformers;

5. Induced currents in buried pipe lines causing accelerated corrosion;

6. Change in density of the high altitude atmosphere causing change in drag on satellites;

7. Change in magnetic torque on satellites; and

8. Damage by electrons and protons to satellite circuits and solar panels.

For these reasons, SEM-2 is specified to sense particles over a broad range of energies. SEM-2 consists of two detectors: the total energy detector (TED) and the medium energy proton and electron detector (MEPED) along with a data processing unit (DPU).

3.5.1.1 Total Energy Detector

The Total Energy Detector (TED) measures electron and proton energy fluxes in the 0.05 to 20 keV energy range.

Two independent measurements of the particle energy flux are made at zero and 30 degrees from the local vertical. The total energy measurement is divided into two ranges: 0.05 to 1 keV and 1 to 20 keV and each measurement is made independently for electrons and protons. The TED also measures the maximum differential energy flux density and the energy at which it occurs for each direction and particle type (electron and proton).

TED operation is verified by a commandable In-Flight Calibration system to track changes in the TED electronics.

3.5.1.2 Medium Energy Proton Electron Detector

The Medium Energy Proton Electron Detector (MEPED) provides both directional and omni-directional measurements. The directional sensors, called telescopes, make independent measurements of the particle types in the energy intervals shown in Table 3.5.1.2-1. Directional measurements are made near the local vertical and near 90 degrees to the local vertical. The particle channels are designated by direction of view (0 or 9), particle type (E or P), and sequential energy range (1-6 for protons, 1-3 for electrons). Table 3.5.1.2-1 contains the particle types and energy intervals measured by the MEPED directional sensors.

Table 3.5.1.2-1. Particle types and energy intervals measured by the MEPED directional sensors.
Particle type Channel Designations Energy Interval (keV)
Protons 0P1 and 9P1 30-80
0P2 and 9P2 80-250
0P3 and 9P3 250-800
0P4 and 9P4 800-2500
0P5 and 9P5 2500-6900
0P6 and 9P6 >6900 integral
Electrons 0E1 and 9E1 >30 integral
0E2 and 9E2 >100 integral
0E3 and 9E3 >300 integral

The omni-directional sensors measure proton energy in the following ranges: >16 MeV, >35 MeV, >70 MeV and >140 MeV. Each sensor consists of a dome of moderating material which absorbs energy from the particle (and so sets the detection energy threshold), a silicon solid state detector (SSD), a preamplifier, and a level comparator which responds to particles with enough energy to go through the moderator and produce a pulse from the detector large enough to exceed the level in the comparator.

The MEPED has an In-Flight Calibration (IFC) circuit started by command from the ground. Using pulses of increasing size, the calibration provides data for calculating the energy of each threshold and the noise (FWHM) of each directional channel.

3.5.1.3 Data Processing Unit

The Data Processing Unit (DPU) is the interface between the sensors and the spacecraft. It converts spacecraft power to the voltages required by the SEM-2. It counts pulses, scales them, combines some, and formats a data stream for Digital A telemetry. It digitizes analog monitors of TED, MEPED, and DPU circuits. Independent of whether SEM-2 power is on or off, some temperatures are monitored by the spacecraft.

The DPU provides bi-level monitors, via Digital B telemetry, of SEM-2 status. The DPU receives and processes commands from the ground through the spacecraft to operate SEM-2.

3.5.2 System Description

3.5.2.1 Total Energy Detector

The Total Energy Detector (TED) consists of eight Electrostatic Analyzers (ESA), pulse height iscriminators (PHD), an In-Flight Calibrator (IFC), two high voltage (HV) supplies, a sweep voltage supply and housekeeping circuits.

The ESA selects particles according to their charge and energy using curved plates which apply an electric field to the particles as they pass through the ESA. Only particles of the selected charge and energy can pass entirely through the ESA. At the outlet of the ESA is a continuous dynode electron multiplier (CDEM). The CDEM produces a pulse for each particle passed by the ESA.

The voltage applied to the ESA curved plates steps up through a series of exponential values to allow particles of increasing energy to pass through. Timing by the DPU determines the energy counter in which the CDEM pulses are accumulated.

There are commandable HV supplies for the electron CDEMs (up to 3,700 V) and for the proton CDEMs (up to 2,700 V) that compensate for gain degradation of the CDEMs with use.

3.5.2.2 Medium Energy Proton Electron Detector (MEPED)

The MEPED consists of two proton telescopes, (each containing two solid state detectors (SSDs)), two electron telescopes (each containing a single solid state detector) and four Omni-directional sensors (each containing a single solid state detector), Charge Sensitive Preamplifiers, Analog Signal Processors, Proton and Electron Coincidence Logic, In-Flight Calibrator, Low Voltage Regulators, SSD Bias Supply and Analog Housekeeping.

The two proton telescope sensors are located at 0 and near 90 degrees to the local vertical. Each sensor has logic circuits which select six energy ranges from 30 keV to greater than 6900 keV. The energy intervals detected are shown in Table 3.5.2.2-1.

Table 3.5.2.2-1. Proton telescope detection energy intervals.
Energy Interval Designation Ep Range (keV)
0P1 and 9P1 30-80
0P2 and 9P2 80-250
0P3 and 9P3 250-800
0P4 and 9P4 800-2500
0P5 and 9P5 2500-6900
0P6 and 9P6 >6900

The two electron telescopes sensors are located at 0 and near 90 degrees to the local vertical. Each sensor has logic circuits which select three energy thresholds from 30 keV to 300 keV. Each energy channel has a veto for particle energy deposits of >2500 keV, which reduces the contamination from high energy proton fluxes. The energy intervals are shown in Table 3.5.2.2-2.

Table 3.5.2.2-2. Electron telescope detection energy intervals.
Energy Interval Designation Ee Range (keV)
0E1 and 9E1 >30
0E2 and 9E2 >100
0E3 and 9E3 >300

The four Omni-directional Proton Sensors, D4, D5, D6 and D7 have energy thresholds at 16, 35, 70 and 140 MeV, respectively.

A charged particle enters an SSD where it releases charges from the crystal until its energy is gone or it exits the crystal. The amount of charge released is proportional to the energy deposited in the crystal. The charge is collected by a charge sensitive preamplifier which converts the charge to a voltage pulse. After passing through an analog signal processor circuit, which shapes the pulse by pole-zero cancellation, single differentiation, double integration and baseline restoration, coincidence logic separates the pulses into the various channels for counting by the DPU.

Regulated voltage for the SSDs is provided by an SSD bias supply. An in-flight calibrator (IFC) puts pulses into the charge sensitive preamplifier inputs to check the complete analog signal processor and coincidence logic circuitry. The IFC data are telemetered the same as the normal particle data. Thresholds and noise values are calculated on the ground.

3.5.2.3 Data Processing Unit

The Data Processing Unit (DPU) is the sole electrical interface between the detectors and the spacecraft except for the TED and MEPED heaters. It converts spacecraft power to the voltages required by the SEM. It counts pulses, scales the counts, combines some, and formats a data stream for Digital A. It digitizes analog monitors of TED, MEPED and DPU voltages and temperatures. Independent of whether SEM-2 power is on or off, some temperatures are monitored by the spacecraft. The DPU provides bi-level monitors via Digital B of SEM-2 status. The DPU receives and processes commands from the ground through the spacecraft to control SEM-2.

The DPU interfaces with, and controls, the TED and MEPED. The DPU performs the following tasks:

The DPU contains four "active" Printed Circuit Boards (daughter boards) and one "passive" PCB (mother board). The DC/DC converter is contained in a Faraday cage to reduce radio frequency interference (RFI) which might be caused by "high power" switching. SEM-2 operation is controlled by two redundant microprocessors, one of which is selected by ground command to be active.

3.5.2.3.1 Telemetry

Within the DPU three types of telemetry are generated: Digital A, Digital B and Analog.

3.5.2.3.2 Digital A

Digital A contains most of the telemetered data and consists of two 8-bit serial words made available to the spacecraft every 0.1 second, i.e., every minor frame.

3.5.2.3.3 Digital B

Digital B (DIGB) consists of 9 bits always available to the spacecraft. DIGB is updated by the DPU each 2.0 seconds. The spacecraft samples these each 3.2 seconds.

3.5.2.3.4 Spacecraft Analog Telemetry

A third type of data, analog telemetry, goes to the spacecraft. In this type, thirteen analog monitors are provided. Nine of these monitors verify command execution and indicate general state of health. These nine are also digitized and included in Digital A. The remaining four are temperature monitors powered by the +28 V Analog Telemetry Bus, which allows the DPU, MEPED and TED housing temperatures, as well as one MEPED proton telescope temperature, to be monitored regardless of whether the instrument is on or off. These temperature monitors are not digitized by the DPU, but are processed directly by the TIP.

3.5.2.3.5 Timing

All timing is controlled by the microprocessor and is synchronous with the spacecraft 1.248 MHZ clock. Various data acquisition timing signals are spawned from a single timing chain to ensure proper signal phase. DPU timing has the following parts:

3.5.3 Calibration Requirements

The SEM-2 provides an In-Flight Calibration (IFC) for the MEPED and the TED.


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