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D i* ; . TECHNICAL ADVISEMENT MEMORANDUM NO. 106- 1 1 b t GEOS A TELEMETRY SUBSYSTEM RELIABILITY ASSESSMENT PRC D-1116 8 October 1965 Prepared by Winifred C. Graham Charles E. Bloomquist Under Contract No. NASW-11 Q n Approved by * / H I l i P 0 PRICE E CFSTI PRICE(S) $ Hard copy (HC) Robert J . Mulvihill ' d m Microfiche (MF) ff 653 July 65 P L A N N I N G R E S E A R C H C O R P O R A T I O N L O S A N G E L E S , C A L I F . N 6 6 1 3 1 5 4 I ii (ACCESSION NUMBER) I P>- (PAGES)$1 - J B L (NASA CR O R TMX OR AD NUMBER) W A S H I N G T O N , D. C. (THRU) L I C O D ~ IGATLOORY)

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TECHNICAL ADVISEMENT MEMORANDUM NO. 106- 11 To: P r o g r a m Manager, Geodetic Satellite Physics and Astronomy Programs, Office of Space Science and Applications, NASA Headquarters From: PRC GEOS Reliability Assessment Team Subject: GEOS A Telemetry Subsystem Reliability Assessment 1. Introduction PRC has completed an analysis of the GEOS A telemetry subsystem. Details of this analysis, a s well a s the results and conclusions, a r e discussed below. Briefly, two tasks were accomplished: (1) an engineering analysis was performed, including a failure mode and (2) the subsystem was modeled, using inforand effect analysis, mation generated in the first task, and effectiveness figure-of-merit numerics we r e calculated. 2. Functional Desc ription Since the experiments do not use telemetry for transmitting experiment results, the telemetry data a r e composed solely of operational information on the spacecraft equipment. These data consist of commutated PAM data, memory readout, and the time marker. The commutated data and related hardware will be discussed first. There a r e two commutators, but they a r e not redundant, since The commutation rate i s 0.63 secthey do not handle the same data. onds p e r channel, o r approximately 24 seconds per frame. A common group of circuits provides timing and control to both commutators ( s e e Exhibit 1). The timing and control signals a r e derived f r o m a 1,628-cps tuning fork oscillator. The commutator switches, a s discussed below, a r e set up to provide 35 data channels, but 38 channels per commutator a r e actually transmitted. This is accomplished by flholdingffthe channel following

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channel 35 f o r 4 channel times. These 4 channels a r e designated numb e r s 3 6 , 37, 3 8 , and 1, and their t0.25 volt input is read continuously during this period on both commutators. Essentially, a two-level gating scheme is employed in each commutator (see Exhibit 2). The A1 through A7 t e r m s commutate seven channels simultaneously in each of the five "switch boxes." The outputs of the switch boxes a r e further commutated by the B1 through B5 t e r m s to provide the output. Associated with the commutators a r e subcommutators and telltale (TT) registers. There a r e two subcommutators, each consisting of eight subcommutated channels transmitted as channel 18 of each of the main commutators. A common group of circuits provides timing and control to both subcommutators. Both of these subcommutators contain channels assigned to the optical beacon and its power supply. Channel 8 on each subcommutator (optical system current and optical battery voltage) i s transmitted when a flash sequence is not occurring. When a sequence occurs, signals from memory activate the timing and control circuitry, causing the other optical system information a s signed to the subcommutators to be telemetered. The telltale registers a r e 15-bit words, with each bit representing the "off/on" status of a command o r some other binary function. Commutator 1 has two telltale registers read in as channels 17 and 29. Commutator 2 has one telltale register read in as channel 17. A common group of circuits provides timing and control for all three telltale registers. These circuits a r e , in turn, under the control of other encoder timing and control circuits. Either main commutator can be stopped by ground command S O that any channel can be continuously read out. In addition, each of the commutators modulates a subcarrier oscillator (SCO), and each SCO phase modulates the transmitter. The other two outputs of the telemetry system (memory readout and time marker) will now be discussed. A signal f r o m memory transf e r s a relay which causes data from cornmutator 2 t o be replaced by 2

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----- Timing Control r I i I I i Countdown and Circuits Control Cor Memory Spacecraft Data 0 I Command Status Register 1 Information ( 1 5 B i t s ) Optical Channel 18 System mutator 1 Data ( 8 Channels) ---. Information --- --- -- I Spacecraft Data 0 I Command Status Register 3 Information ( 1 5 B i t s ) I Optical Channel 18 System mutator 2 Data (8 Channels) EXHIBIT 1 - FUNCTIONA 3 I

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I ?- Commutator 1 (38 Channels) Isolation Amplifier and SCO ~ (2.3 kc) Calibration Regulator L Amplifier and SCO I Commutator 2 Relay (38 Channels) T ransmiseion Driver Enable P h a s e Advance T e l e m e t r y RetardPhase4 - 1 -0- BLOCK DIAGRAM . Transmitter (-136 m c ) Data Control

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• • Data , A 1 Data 1 A2 Data A3 Data A4 Data A5 Data A6 Data A7 Seven Data 0 output Input. Seven Data Input8 Seven Data Input. Seven Data Input. EXHIBIT 2 - LOGICAL REPRESENTATION O F COMMUTATOR 5

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memory readout data at the input of the SCO. When readout is complete, the relay transfers back. The time m a r k e r circuits cause the phase advance and phase retard signals f r o m the spacecraft clock to modulate the transmitter directly. The n o r m a l operational plan for the telemetry subsystem calls for operation with only the transmitter and time m a r k e r on. The remainder of the unit (commutated data and memory readout) will be turned on only when the spacecraft is over a ground station assigned to receive these data. 3 . Engineering Analysis The appendix to this TAM contains the failure mode and effect analysis. The analysis revealed 20 failure states, ranging in s e - verity f r o m loss of the entire subsystem (due to loss of the transmitter) to loss of one T T bit. These failure states (none of which were unexpected in a subsystem of this type) a r e summarized in Exhibit 3 . An important aspect of failures in a telemetry system is that c e r - tain types of failure states may occur and not be recognized f r o m the telemetry data. Attention was given to this potential problem, and a discussion of it appears in Reference 1. Of the 20 possible failure states in the GEOS telemetry subsystem, it was f e l t that very few w e r e of this type. State 15 (the loss of one TT bit) would be the most difficult to identify via diagnosis of other spacecraft data and/or the r e - sponse of the spacecraft to commands. 4. Telemetrv Model Making use of the preceding sections, the telemetry subsystem m a y be thought of as providing 128 discrete ltoutputs" which are either present o r absent, depending on the state of the subsystem. The 128 outputs consist of the memory readout, the time m a r k e r , 45 telltale bits (utilizing 3 main commutated channels), 16 subcommutated channels (utilizing 2 main commutated channels), and, finally, 6 5 "other" main commutated channels. The state of the subsystem is a function of its failed and unfailed components. 6

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EXHIBIT 3 - FAILURE STATES Failure Loss State 1 All telemetry 2 Both commutators 3 60 channels (30 from each commutator) 4 56 channels (28 from each cornmutator) 5 Commutator 2 and memory readout 6 One commutator 7 Both subcommutator s 8 One subcommutator 9 All telltale registers 10 One telltale register 11 Seven comrnutated channels 12 One commutated channel 13 One subcommutated channel 14 Five telltale bits 15 One telltale bit 16 Flash intensity information 17 Memory readout 18 Time marker 19 Sync 20 Ternpe ratur e indications 7

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c Assume that the presence of all 128 outputs yields a value of unity for the subsystem. Further, assume that each output contributes a proportion, Vi , of this value independent of any other output and that the value of any particular subsystem state is the s u m of the values associated with each output. Under these conditions, i f Pi is the probability of output i being present, considered independently, it can be shown that the telemetry subsystem figure of m e r i t is given by 128 1 P i V i . i=1 Exhibit 4 explicitly defines all 128 outputs, as well as tabulating the probability and relative value of each. Derivation of the probability and value terms of Exhibit 4 is undertaken in the following two subsections. a. Derivation of Output Probabilities, Pi The probability of any particular output being present, at time t , is assumed to be given by an expression of the form Pi = exp (-hit) (2) where i = particular output Xi = total failure rate of -all parts required to achieve output i t = operating time (uniformly assumed to be 8,7 60 hours) The Xi a r e derived by consulting Exhibit 5, which indicates generally the portions of the telemetry subsystem required for each output and which i s cross-referenced to the appendix (via the item numbers). The appendix gives a comprehensive tabulation of the telemetry component 8

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EXHIBIT 4 - TELEMETRY MODEL TERMS Output, Probability, Value, p; v: 1 0.855 31 2 0.825 2 3-32 0.246 119 33-47 0.244 119 /4 Subcommutated channels from 48-52 0.323 1 /4 Subcommutated channels from 53-55 0.340 1 56-63 0.336 1/4 58/65 Commutated channels from 64-71 0.377 58/65 Other commutated channels 72-95 0.408 OutDut Definition Memory readout Time m a r k e r Telltale bits f r o m commutator 1 (30) Telltale bits from commutator 2 (15) commutator 1 with flash intensity information (5) commutator 1 without flash intensity information (3) Subcommutated channels from commutator 2 (8) commutator 1 with temperature information (8) f r o m commutator 1 (24) 96-100 0.373 58/65 Commutated channels from commutator 2 with temperat u r e information (5) 101-128 0.404 58/65 Other commutated channels 128 from commutator 2 (28) 2 P i V i = 0.54 i = l 9

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EXHIBIT 5 - TELEMETRY EQUIPMENT REQUIRED FOR AVAILABILITY O F VARIOUS OUTPUTS Output/Telemetry Equipment Required A. Memory Readout (Output 1) 1. Transmitter 2. Isolation amplifier Item Number (11 60 6 3 Subcarrier oscillator (SCO) 6 5 3, 4. Calibration and SCO regulator 61 5. Relay module B. Time Marker (Output 2) 1. Transmitter 2. Time m a r k e r generator C. One Telltale Bit (Outputs 3-47) 1. Timing and control 2. Commutator 3. Telltale register 4. Isolation amplifier 66, 67 60 68 1- 19 1/35 of 20; 1/5 of 21 and 23; 22, 2 4 46, 47; 1/15 of 48, 1/3 of 49 6 2 Subcarrier oscillator (SCO) 64 5. Calibration and SCO regulator 61 6. 7. Transmitter 8. Relay module(2) 9. Trigger circuits 60 66, 67 58, 59 (worst c a s e ) D. One Subcommutated t h a n n e l (Outputs 48-63) 1. Timing and control 2. Commutator 3. Subcommutator 4. Isolation amplifier 10 1-3, 5-19, 33, 34 1/35 of 20; 1/5 of 2 1 and 23; 22, 24 37, 1/8 of 38 6 2

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EXHIBIT 5 (Continued) Output y Equipment Requir ed Item Number (1) / Telemetr 5. Subcarrier oscillator (SCO) 64 6. Calibration and SCO regulator 6 1 7. Transmitter 8. Relay module(3) 9. Flash intensity circuits( 4 ) 10. Attenuators 60 66, 67 39, 43 Assume two R's per channel E. One Commutated Channel (Outputs 64-128) 1. Timing and control 2. Commutator 3. Isolation amplifier 1-38 5-19 1/35 of 20; 1/5 of 2 1 and 23; 22, 24 62 4. Subcarrier oscillator (SCO) 64 5. Calibration and SCO regulator 61 6. Transmitter 7. Relay module(5) 60 66, 67 8. Thermistor regulator(6) 69 9. Attenuators Notes: (1) See appendix. (2) Outputs 33-47 only. (3) Outputs 56-63 only. (4) Outputs 48-52 only6 (5) Outputs 96-128 only. Assume two R's per channel (6) Outputs 96-100 and 64-71 only. 1 1

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parts. Component part failure rates a r e as given previously in Reference 2. The numerical results for the probability of each output being present a r e tabulated in Exhibit 4. b. Derivation of Output Relative Values, V i In order to derive a rational assignment of relative values to each state, assume f i r s t that each main commutated channel has a value of unity. Then, since the 16 subcommutated channels utilize 2 main commutated channels, assign a value of 1 / 8 to each sub- The 45 telltale bits utilize 3 main commutated commutated channel. each bit may be assigned a value of 1/15. The channels, and, hence, memory readout essentially replaces one commutator when it i s in service; thus, assign it a value of 35. There is no direct relationship between the time m a r k e r and a main commutated channel, but a value of 2 i s judged to be reasonable in the present context. If these values a r e normalized to total unity, the Vi shown in Exhibit 4 a r e the result. If the operation of Equation (1) is performed using the data of Exhibit 4, the telemetry subsystem figure of m e r i t is found to be approximately 0.54. A cursory examination of Exhibit 4 indicates that the memory readout alone contributes nearly half of the subsystem figure of merit. Since the probability associated with this output i s relatively high, reducing the relative value of the memory readout will reduce the telemetry subsystem figure of m e r i t , and vice versa. 5. Results The design and implementation of the telemetry subsystem a r e considered by P R C to be good. Since telemetry i s not used to transmit experiment results on GEOS, it i s felt that the nonredundant design i s adequate. The telemetry subsystem figure of m e r i t of 0.54 (indicating roughly that the subsystem is expected to be 54-percent available at the end of one year) i s judged to be not unreasonable in view of the generally pessimistic assumptions used throughout the GEOS subsystem assessments. The figure of m e r i t i s highly dependent on the relative values a s signed to the individual outputs. 12

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6 . Summary a. The design a n d implementation of the GEOS telemetry subsystem are good. b. The expected proportion of telemetry subsystem capability available at the end of one year is approximately 5 4 percent. 1 3

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. I REFERENCES 1 . Operational Reliability Assessment of the GEOS A Spacecraft, Technical Advisement Memorandum No. 106- 10 (PRCD- 10561, 29 October 1965. 2. Component Part Failure Rate Assignments for Reliability Assessment of the GEOS Satellite, Technical Advisement Memorandum No. 106-6 (PRC D-1027), 8 June 1965. 1 5

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APPENDIX FAILURE MODE AND EFFECT ANALYSIS

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Item Drawing Number Number C i r c u i t Q D D Z I 1 72 11-482 1 Tuning f o r k oscillator 3 3 1 2 721 1-4821 T h r e e - stage ripple counter 3 2 3 721 1-4821 F i v e - s t a g e ripple counter a n d amplifier 4 7211-4821 t 3, O R gate, T T r e s e t amplifier 5 721 1-4941 Sync one -shot 2 4 2 5 6 721 1-4941 Synchronizable m u l t i v i b r a t o r 2 8 7 721 1-4941 F l i p - flop 2 2 I 8 721 1-4941 Duty cycle c i r c u i t s 4 4 9 721 1-4941 R e set gener a t or 1-112 4-112 10 721 1-4941 R e s e t g e n e r a t o r 1-112 4-112 f 11 7211-4941 F r a m e m a r k e r g e n e r a t o r 5 20 12 7210-491 1 S t e p one- shot 2 3 1 13 7210-491 1 B counter input c i r c u i t 2 2 1 14 7210-491 1 B counter 15 7210-4911 B counter 5 11-112 112 F a l s e 5 11-112 112 A counter input c i r c u i t Any 2 2 1 16 7210-4821 17 7210-4821 A counter 18 7210-4821 A counter T r u e 8 17 112 False 8 17 Dead time c i r c u i t Any 2 19 7210-4821 19

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Parts Count Failure R1 C2 Other State Comments c1 9 1 6 1 tuning fork 2 6 3 1 2 IC 2 5 3 5 IC 2 5 3 IC '9 6 2 ~2 11 5 ' 2 I 8 4 2 10 1 2 5- 11 2 1 2 A s s u m e s failure holds timing c i r c u i t s r e s e t 5-112 1 19 Indeterminate 26 10 2 W o r s t - c a s e assumption 5 2 2 6 1 2 10-112 2 - 1 1 2 2 If counter f a i l s so that e i t h e r no outputs a r e t r u e o r m o r e than one output i s t r u e , all encoded data a r e lost; i f counter fails s o that only one output is t r u e , then i t s data groups w i l l be continuous ly t e l e m e t e r ed 10-112 2 - 112 4 (See above comment) 6 1 2 16 4 2 If counter fails so that e i t h e r no outputs a r e t r u e o r m o r e than one output is t r u e , all encoded data a r e lost; if counter f a i l s so that only one output is t r u e , then one channel f r o m e a c h data g r o u p will be continuously t e l e m e t e r e d 16 4 3 (See above comment) 2 1 2

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F a i l u r e Item Drawing Number Number Circuit Mode 2 0 721 0 - 4 7 6 1 Commutator 1 (five data groups of seven channels each) 2 1 7 2 1 0 - 4 7 6 1 Commutator 1 (five data groups of seven channels each) 2 2 7 2 1 0 - 4 7 6 1 Commutator 1 (five data groups of seven channels each) 2 3 7 2 1 0 - 4 7 6 1 Commutator 1 (five data groups of seven channels each) 2 4 7 2 1 0 - 4 7 6 1 Commutator 1 (five data groups of seven channels each) 2 5 7 2 1 0 - 4 7 6 1 Commutator 2 26 7 21 0 - 4 7 6 1 Commutator 2 27 7 2 1 0 - 4 7 6 1 Commutator 2 2 8 7 2 1 0 - 4 7 6 1 Commutator 2 2 9 7 2 1 0 - 4 7 6 1 Commutator 2 3 0 7 2 1 0 - 4 7 4 1 Attenuators, commutator 31 7 2 1 0 - 4 7 1 1 Attenuators, commutator 32 7 2 1 0 - 4 7 3 1 Atte nuat or s , com m uta tor at or reg ulator Any 1 2 3 3 7 2 1 1 - 4 7 4 1 Subc oIII HIut 3 4 7 2 1 1 - 4 7 4 1 Subcommutator control Any 35 7 2 1 1 - 4 7 4 1 Attenuators, subcommutator 1 36 7 2 1 1 - 4 7 4 1 Attenuators, subcommutator 2 37 7 2 1 1 - 4 7 2 1 Subcommutator 1 3 8 7L11-4721 Subcommutator 1 39 721 1 - 4 7 6 1 Light intensity d e t e c t o r s Light intensity d e t e c t o r s 1 2 4 0 7 2 1 1-4761 Light intcnsity d e t e c t o r s 1 2 4 I 7 2 1 1 - 4 7 6 1 Light intensity detectors 1 L 4 2 7 2 I 1 - 4 7 6 I

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P a r t s Count i Failure R1 c 1 c2 Other State Comments 7- I 5 5 1 5 IC 2 I C L IC 1 1 12 Open input 11 Superimposed data 6a Supe r imp0sed data 1 1 2 Worst-case as sumption 1 2 Open input 1 1 Superimposed data 6b Superimposed data 1 1 2 W o r st - case a s sumption 1 2 Worst- case as sumption 1 2 Worst-case assumption 1 2 Worst-case assumption 7 7 1 3 a Worst-case assumption 13b Worst-case as sumption 8a 1 3 a 16a Lose flash intensity indication 1 and flash control pulse 16c Lose f l a s h intensity indication 2 16c Lose f l a s h intensity indication 3 16c L o s e flash intensity indication 4

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\ Item Drawing .1 I +.5+ F a i l u r e Number Number Circuit Mode Q D D Z 43 721 1-4761 Light intensity detectors 3 1 44 721 1-4721 Subcommutator 2 16 (SI 9 45 721 1-4721 Subcommutator 2 16 (0) 46 721 0-5221 Telltale register 1 Any 47 721 0-5221 Telltale register 1 T r u e 48 7210-5221 Telltale register 1 F a l s e I 49 721 0-5221 Telltale register 1 F a l s e 50 721 0- 522 1 Telltale register 2 Any 51 7210-522 1 Telltale register 2 T r u e 52 721 0- 522 1 Telltale register 2 F a l s e 53 7210-5221 Telltale register 2 F a l s e 54 7210-5221 Telltale register 3 Any 55 7210-5221 Telltale register 3 T r u e 56 72 10- 522 1 Telltale register 3 F a l s e 57 Telltale register 3 F a l s e 7210-5221 58 7 2 10-485 1 T r i g g e r circuits, r e g i s t e r Any 2 1 59 7210-4851 T r i g g e r circuits (5 circuits) Any 10 10 60 721 1-4501 Transmitter 61 f Calibration and SCO regulator Any 9 7 62 721 1-4641 Isolation amplifier 1 63 721 1-4641 Isolation amplifier 2 64 N/A Subcarrier oscillator 1 65 N / A Subcarrier oscillator 2 66 721 1-4621 Relay module 67 721 1-4621 Relay module 68 721 1-4871 T i m e m a r k e r generator 1 69 7207 -498 1 T h e r m i s t o r regulator 21

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2- .. Y P a r t s Coiint Failure .R l c 1 c2 Other State Co m m e nt s 11 3 I6b Lose a l l flash intensity 16 2 IC 2 IC 13b 3 11 IC 1Oa 2012 IC 1Oa 29 1512 IC 15a . 512 IC 14a 3 11 IC 10b 2012 IC 1Ob 29 1 5 / 2 IC 15b 512 IC 14b 3 11 IC i o c 20/2 IC 1oc 29 1512 IC 15c 512 IC 14c 3 19 indications Bb Inde te r m inate 25 15 19 Indeterminate 35 69 1 IC, 1 9 inductors, 1 c r y s t a l 10 2 7 5 7 5 8 2 2 1 inductor 8 2 2 1 inductor 1 112 r e l a y 1 1 / 2 r e l a y 17 15 1 2 1 inductor 18 1 1 A s s u m e d p a r t s count 6a 5 6a A s s u m e d p a r t s count 5 A s s u m e d p a r t s count 6b 3 2 4 20 W o r st- c a s e a ss urn pt ion