Descriptions of Payload
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Quick Click Index |
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| Optics | ||
| Explanation of Radiation Handling | ||
| Precautions against Scattering | ||
| Spacecraft Monitors | ||
| The Photometer | ||
| Bilevel Monitors | ||
| Telemetry | ||
| Launch Specs | ||
| Physical Description |
Order and wavelength selection is carried out by thin-film metallic filters. Figure 3 describes the filters used, the positions they occupy on the filter holder, and the spectral bands through which the filters are transparent. All the filters discriminate against the hydrogen Lyman alpha radiation that dominates the day and night sky by several orders of magnitude over all other FUV and EUV sources.
To shield against charged particle background, two magnets are attached to the collimator baffle. These provide a field of about 600 Gauss inside the collimator. This assembly is capable of rejecting electrons with energies up to tens of kilovolts.
Each electron pulse produced by the MCP's is proximity-focused onto the collecting resistive anode. The deposited charge flowed off the ends of the anode in inverse proportion to the resistance between the point of incidence and the ends. Both ends of the anode are connected to low noise -10^13 V/coulomb amplifiers. Output of the amps is a bipolar pulse whose amplitude is proportional to the input charge.
Two Pulse Position Analyzers (PPA) compute the ratio of pulse amplitudes. The PPA's could accept inputs from either of the long or short wavelength RANICON's. The ratio is computed by a successive approximation analog-to-digital circuit. The result, between 0 and 1, gives the 1-dimensional location of the incident photon across the width of the channel plate. Discrimination logic rejects runt pulses, overload pulses, and pulse pile-up.
At the end of the PPA computation cycle, the 128-word x 12-bit Add-one memory is strobed. The memory latches and uses the input number as an address to RAM. The contents of that memory location are incremented, thus recording the arrival of another photon at that position of the RANICON.
When the spacecraft requests a data word, the data is loaded into a parallel/serial shift register and is shifted out by the bit clock. Output addressing is done by incrementing a counter at each data word request. In low resolution (10 Angstroms) mode, 64 words are allocated to each data channel. In high resolution (5 Angstroms) mode, all 128 words are allocated to a single data channel.
The spectrometer also includes a photometer with a 2140 Angstrom filter (28 degrees FWHM) for detecting NO, and a stepping motor for positioning the grating +/- 0.5 degrees. Figure 4 is a block diagram of the EUV spectrometer electronics.
The GRATING monitor indicates the position of the reflection grating. The grating can be slewed +/-1.5 degrees by a 36 step stepping motor. The monitor comes from the arm of a potentiometer attached directly to the axle of the motor. A calibration curve of grating position versus voltage is given in Figure 5.
PH1 and PH2 are amplifier pulse height monitors associated with data channels 1 and 2. A sample-and-hold circuit samples the peak voltage of the summed bipolar pulses produced by the amplifiers. Each analyzable event causes the S/H circuit to acquire a new peak voltage. That voltage is held until another photon event occurs. The S/H output, however, is only sampled by the spacecraft 30 times per minute. The true peak voltage is 2.1 times larger than the telemetered PH1/PH2 value.
HVMON monitors the output voltages of the two high voltage detector supplies. HV1 is an EMR 638K, HV2 is a SpaCom 383. In order to get two sources of information into one monitor, HVMON is multiplexed between HV1 and HV2. The Master Frame Pulse clocks the multiplexor, thus the monitor alternates data every 2.048 seconds. The multiplexor is not synchronized with any other signals, so it is not possible to know whether a particular voltage is HV1 or HV2. HVMON is redlined such that if the monitor is at a voltage other than 0 V or 3 to 6 V, then an appropriate action is initiated. The typical action is to turn off both high voltage supplies. The true HV voltage is 1000 times the HVMON value.
Table 3 summarizes the commands and their corresponding bilevels. Both the mnemonic and the corresponding BBRC numerical designation are listed. The letter R designates a restricted command, one which can only be sent after the status of ECOM-721 has been fully determined.
Table 4 DSS 1 A/B and DSS 2 A/B commands are always sent as command pairs. This is to ensure that the operator is always cognizant of the state of both DSS commands, since they are also used as flags to the data processing software to indicate the state of the instrument as being either MEMORY DATA mode or SERIAL DATA mode. Table 4 shows the only allowed pairings.
Table 5 summarizes the 35 commands that are provided to us by the spacecraft. Table 6 summarizes the analog and bilevel monitors provided to us by the spacecraft. All monitors are transmitted in word 111 of the Main Frame. The Sub Frame column lists which subframe word (of 64 possible subframe words) within mainframe word 111 that the monitor actually appears. The bilevels are further divided among the 8 individual bits of subframe words 47 and 55.
1 Main frame = 128 words 32 milliseconds
1 Master frame = 64 main frames 2.048 seconds
1 ECOM science word/msec = 2048 science words/master frame
1 spectrum = 128 x 12 = 1536 bits or 192 8-bit words
or 6 main frames
or .192 seconds
Therefore we obtained 5.2 spectra/sec or 10.66 spectra/masterframe 0.183 revolutions/sec = 66 degrees/sec
For a fuller description of the spacecraft telemetry format, see (DIRECTIONAL GIVEN HERE).
ECOM-721 has three modes of outputting science data:
1. low resolution MEMORY data, both channels
2. high resolution MEMORY data, one channel (selectable)
3. SERIAL data, 8-bit data directly from a PPA (selectable)
In SERIAL mode, every 8-bit word that comes from the S/C telemetry represents one analyzed photon. The value of the number (0 to 255) corresponds to the position across the channel plate (from short to long wavelength, see Figure 3) at which the photon struck. Obviously the science data rate is the main frame ECOM word rate: 1000/sec. This is shown schematically in Figure 7(a). The PPA used as the SERIAL data source is selected by the HRES command.
In MEMORY mode, the 8-bit telemetry words are repacked into 12-bit words corresponding to the 128 12-bit words comprising the TRW add-one memory. The memory readout is synchronized to the Master Frame, the beginning of which is signalled by a Master Frame Pulse (MFP). The timing relationships between MFP, Data Strobe, Main Frame word, and the beginning of a new spectrum is (DIRECTIONAL GIVEN HERE).
Repacked 12-bit data in high resolution mode, Figure 7(b), is in final form and will not be further processed by either BBRC or the SCF. Each spectrum consisted of 128 words, 10.66 such spectra are telemetered before the next MFP occurs. The high resolution channel was selected by the HRES command.
But in low resolution MEMORY mode, a final reordering step was needed. As each memory word was read out, the data would alternate between data channels 1 and 2. Thus, for two separate spectra the data was segregated into the even-numbered words (Channel 1) and the odd-numbered words (Channel 2). This is shown in Figure 7 (c).
Before actually powering up the experiment, one has to know the state of various command relays. This was not easily done without energizing the ECOM power supplies (and not all at once in the package is integrated into the S/C), so it was recommended that before turn-on, the INITIALIZATION command sequence be given (Table 7). Once integrated the sequence was always sent before ECOM was turned on. And when the satellite was operational, the sequence could be modified based on the operating history and performance of the experiment.
As implemented, the discriminator and HV programming commands had the following affects within ECOM-721:
| HV | MSB | LSB | HV1 | HV2 |
| 1 | 1 | -2653 | -2420 VDC | |
| 1 | 0 | -2464 | -2257 | |
| 0 | 1 | -2323 | -2143 | |
| 0 | 0 | -2226 | -2033 |
ECOM-721 power consumption is 270 mA at 28 VDC, or 7.5 watts. ECOM weighs 31 pounds.
A pictorial representation of the spacecraft is given in Figure 8.
The space vehicle was injected into orbit with its spin axis lying in the orbit plane. An acquisition maneuver was then executed to bring the spin axis perpendicular to the orbit plane. The spin rate was 11 rotations per minute.
The I/F requirements resulting from the mission profile was applied across all spacecraft systems and included requirements imposed on the total P78-1 space vehicle. ECOM-721 requirements are in the Interface Control Document (ICD) and Drawings, BBRC documents 49318, 49319, 49320.
BBRC provided space in their Integration and Test Building for receiving and shipping the P78-1 Space Vehicle Payloads/Payload AGE. The pre-spacecraft installation tests as well as the four integrated system tests was conducted in the Space Vehicle assembly and test room. The payload was allocated approximately 100 square feet floor space in the Space Vehicle assembly and test room for test activities.
The P78-1 Space Vehicle was launched from the SLC-3E launch pad at Vandenberg Air Force Base. Space Vehicle launch preparation was conducted in the Vehicle Support Building, 766 and Spin Test Facility, Bldg. 1610.
