X-33 TELEMETRY BEST SOURCE SELECTION, PROCESSING, DISPLAY, AND SIMULATION MODEL COMPARISON
Item Type text; Proceedings Authors Burkes, Darryl A.
Publisher International Foundation for Telemetering
Journal International Telemetering Conference Proceedings Rights Copyright © International Foundation for Telemetering Download date 26/12/2021 22:06:34
Link to Item http://hdl.handle.net/10150/609673
X-33 TELEMETRY BEST SOURCE SELECTION, PROCESSING, DISPLAY, AND SIMULATION MODEL COMPARISON
Darryl A. Burkes
Air Force Flight Test Center
ABSTRACT
The X-33 program requires the use of multiple telemetry ground stations to provide continuous coverage of the launch, ascent, re-entry and approach phases for flights from Edwards AFB, California, to landings at Dugway Proving Grounds, Utah, and Malmstrom AFB, Montana. This paper will discuss the X-33 telemetry requirements and design,
including information on the fixed and mobile telemetry systems, automated best source selection system, processing/display support for range safety officers (RSO) and range engineers, and comparison of real-time data with simulated data using the Dynamic Ground Station Analysis model. Due to the use of multiple ground stations and short duration flights, the goal throughout the X-33 missions is to automatically provide the best telemetry source for critical vehicle performance monitoring. The X-33 program was initiated by National Aeronautics and Space Administration (NASA) Cooperative Agreement No. NCC8-115 with Lockheed Martin Skunk Works (LMSW).
KEY WORDS
X-33, Telemetry, Best Source Selection, Telemetry Data Processing and Display, Dynamic Ground Station Analysis
INTRODUCTION
Multiple telemetry ground stations are required to provide continuous coverage of the flight test of the autonomous single-stage-to-orbit X-33, a scaled version of next
generation reusable launch vehicle (RLV). A combination of fixed and mobile telemetry systems will be used to track the X-33 vehicle from launch at Edwards AFB (EAFB), California, to wheels stop for landings at Dugway Proving Grounds (DPG), Utah, and Malmstrom AFB (MAFB), Montana. See Figure 1 for the overall telemetry data flow for the MAFB missions. An innovative best source selection system will be utilized to
automatically determine the best source based on the frame synchronization status of the incoming telemetry streams from the multiple ground stations. These systems will be used
to select the best source at the landing sites and at NASA Dryden Flight Research Center (DFRC) at EAFB to determine the overall best source between the launch site,
intermediate sites, and landing site sources. The best source at the landing sites will be decommutated to display critical flight safety parameters for the RSOs. The overall best source will be sent to the LMSW’s Operational Control Center (OCC) at EAFB for performance monitoring by X-33 program personnel and for monitoring of critical flight safety parameters by the primary RSO. The real-time telemetry data (received signal strength) from each of the primary ground stations will also be compared during each mission with simulation data generated using the dynamic ground station analysis (DGSA) software model. An overall assessment of the accuracy of the model will occur after each mission, and will provide the means to assess any re-entry plasma attenuation.
FIXED AND MOBILE TELEMETRY SYSTEMS
A combination of fixed and mobile telemetry systems are required to support the continuous coverage of the X-33 flights. The systems selected have to be capable of receiving a 1.44 mega bits per second (Mbps) pulse code modulation (PCM)/frequency modulation (FM) signal in the S-band frequency range while maintaining a bit error rate (BER) of 1x10-6 with a 6 dB margin. This margin is above the 13.1 dB signal-to-noise ratio required to meet the BER. The encoding format of the data is randomized non return to zero-level (RNRZ-L), and the polarization of the X-33 transmitting antenna is right hand circularly polarized (RHCP). All the systems selected have the capability of
simultaneously receiving left hand circularly polarized (LHCP) and RHCP signals and will use polarization diversity combining to match the polarization of the incoming signal which can change relative to the ground stations as the X-33 vehicle maneuvers. Polarization diversity combining can also help combat the effects of fading caused by plasma and multipath. The only exception is the Air Force Space and Missile Center’s (AFSMC) Detachment 2 (DET2) 23-foot telemetry tracker, which only receives RHCP signals. This will not present a problem due to the high gain of this system compared to the other systems.
The type of coverage is dependent upon where the systems are located. Single telemetry coverage is required at the launch site, NASA DFRC, and intermediate sites. Redundant telemetry coverage is required for landings at DPG and MAFB. The primary systems are required to provide coverage out to 235 nautical miles. This slant range figure was
determined using a minimum 2-degree look angle with the vehicle at 100,000 feet. The redundant systems need to cover at least the vehicle terminal area energy management (TAEM) maneuver (approximately 25 nautical miles out). This is to ensure that critical telemetry parameters such as landing gear status and vehicle global positioning system (GPS)/inertial navigation system (INS) position information for landing are still obtained in the event of the failure of one of the systems.
The data from the telemetry systems will be routed to different locations. The telemetry data from the single coverage systems will interface with the communication network shown in Figure1 to send the data to DFRC’s Range Operations Center (ROC). The telemetry best source at the sites where there are redundant telemetry tracking sources will interface with the communication network to send the data to the X-33 Mobile Operational Control Center (MOCC) and the ROC. The telemetry data from the single coverage sites and the best source from the redundant tracking sites will be recorded at the ROC. The local best source will be recorded at the landing sites. The overall range best source will be recorded at the ROC, and it is critical that the real-time best source is relayed to the OCC. The OCC at EAFB is the primary monitoring location by X-33 program personnel for all phases of the X-33 missions. The overall range best source will also be sent to all the downrange sites to allow the RSOs to look at telemetry data before the X-33 gets within range of the systems at their particular location.
All the telemetry assets required to support the X-33 missions were not available at EAFB.
Additional assets were made available through the creation of the ExTended Test Range Alliance (EXTRA) described in a companion International Telemetering Conference (ITC) 98 paper titled, The X-33 Extended Flight Test Range (Reference 4). The telemetry
assets which will be used are listed below, and distribution of the assets to support X-33 landings at MAFB is shown in Figure 1.
TM DATA
RANGE TM BEST SOURCE
SHADOW MOUNTAIN
8 FT
LAUNCH SITE
MALMSTROM AFB
MALMSTROM TM BEST SOURCE RANGE TM
BEST SOURCE
TM DATA
WFF 18 Ft
MOF
RF TM DATA TRIPLEX
RANGE OPERATIONS CENTER (ROC) BLDG 4800
OPERATIONS CONTROL CENTER (OCC)@HAYSTACK BUTTE EAFB
UMBILICAL TM DATA
DPG MOUNTAIN HOME
DFRC-9 DET2 23 FT
DPG TM DATA
MOUNTAIN HOME TM DATA
RANGE TM BEST SOURCE
TM DATA
TM DATA
RANGE TM BEST SOURCE
NASA DFRC@EAFB
MALMSTROM TM BEST SOURCE
EDWARDS AFB (EAFB)
RANGE TM BEST SOURCE
REQUIREMENTS - Receive downlink telemetry data during vehicle test and flight operations (X-33 SRD 604D0007 [Reference 1])
- Receive TM data at 1.44 Mbps - BER of 1x10-6
- Link Margin 3 dB (6 dB goal) (X-33 Vehicle-Ground Station RF Interface IC604Y0001 [Reference 2]) -Provide real-time information on in-flight X-33 vehicle behavior (AFFTC RSRD [Reference 3]) Data
Comm Network
Figure 1 X-33 Telemetry Support for MAFB Flights
AFFTC Shadow Mountain 8-Foot System Launch Site Coverage
DFRC Triplex 23-Foot System Primary Coverage after Launch
DFRC-9 30-Foot System Primary Coverage for DPG Landings and Overflight Support for MAFB Missions DFRC Mobile Operations Facility (MOF) Redundant Coverage for DPG and MAFB
6-Foot System Landings
AF Space and Missile Center (AFSMC) DET2 Overflight Support at Mountain Home AFB, 23-Foot System from Kirtland AFB, New Mexico Idaho, for MAFB Missions
Wallops Flight Facility (WFF) 18-Foot System Primary Coverage for MAFB Landings from Wallops Island, Virginia
Table 1 is a summary of the systems that will be used to support the X-33 missions:
TABLE 1 TELEMETRY SYSTEMS PARAMETERS AFFTC
Shadow Mountain @
Launch Site
DFRC Triplex @
EAFB
DFRC-9*
@ DPG
MOF
@ DPG &
MAFB
WFF-18
@ MAFB
AFSMC DET2
@ Mountain Home AFB
Diameter (ft) 8 23 30 6 18 23
G/T (dB/K)** 3.3 19 20 4 14.6 21.5
Polarization RHCP/
LHCP
RHCP/
LHCP
RHCP/
LHCP
RHCP/
LHCP
RHCP/
LHCP
RHCP Fixed or
Mobile
Fixed Fixed Fixed Mobile Mobile Mobile
Link Margin Above
Required 6 dB Margin
N/A 61 NM max range at 6 dB
3.9 @ 235 NM
4.9 @ 235 NM
8.4 @ 25 NM
-0.5 @ 235 NM
6.4 @ 235 NM
* The DFRC-9 system will be located to DPG for the duration of the X-33 program.
** G/T (G over T) is a figure of merit for the antenna/front end that takes into account both the geometric gain of the antenna and the equivalent noise temperature for the front end measured in decibels per degrees Kelvin.
BEST SOURCE SELECTION
The best source selection system is a critical part of the X-33 telemetry support system.
The system will ensure that the best information possible on vehicle performance is provided to X-33 program personnel and RSOs. The innovative system will be used to automatically determine the best source based upon the frame synchronization status of the incoming data between the landing site sources for the DPG and MAFB missions. Due to the short duration of the missions, it is required that the best source be determined automatically for quick response and minimal user intervention. It is required that a best source selector system be used at the landing sites to determine the best source between
the primary and redundant systems and to mitigate data latency concerns. The time delay to send the data from the two landing site telemetry systems to the ROC for best source selection, and to send the best source back to the landing site over the Data Comm network was unacceptable. It is more efficient and cost effective to use an on-site best source selector. The overall best source determination will be made by a system that will be located in the ROC at DFRC. The ROC system will need to select from three inputs for the DPG missions and from five inputs for the MAFB missions shown in Figure 2.
The best source determined at the ROC will also be sent downrange to allow the RSOs to monitor the X-33 performance before it gets within the coverage area of the telemetry ground stations at their location.
The configuration of the Avtec Systems, Inc. Programmable Telemetry Processor (PTP) (Reference 5) that will be used as the best source selector system is as follows:
o PC-based rackmount Pentium Pro 200 MHz CPU o Windows NT Operating System
o Peripheral Component Interface (PCI) and Industry Standard Architecture (ISA) Input/Output (I/O) Busses
o Receive up to six inputs using PCI Monarch Frame Synchronizer cards o Transistor-Transistor-Logic (TTL) or RS-422 I/O
o ISA Synchronized Time Code Generator card o Redundant power supply
o Serial Input, Serial Output, Best Source Select (BSS) Software Modules
Comm Interface to OCC
LHCP AGC Triplex TM Data
Shadow Mtn TM Data
Comm Interface to Downrange Sites VDA
Data Clock Data
Clock Data
Clock Triplex
RHCP AGC To TRAPS
VDA Dugway
AGCs RHCP AGC
LHCP AGC
Best Source
PTP Best Source Selector VDA
Bit Sync
VDA VDA
Data Clock Bit Sync
Data
Clock Bit Sync
Data
Clock Triplex
LHCP AGC
VDA
RHCP AGC
LHCP AGC To TRAPS
To TRAPS
Bit Sync
Bit Sync Mtn Home
TM Data
VDA
Data
Clock Clock Mtn Home
AGC RHCP AGC
To TRAPS Malmstrom TM
Best Source
VDA Malmstrom
AGCs RHCP AGC
To TRAPS Dugway TM Data
To TRAPS (Record)
To TRAPS (Record)
To TRAPS (Record)
Data
To TRAPS (Record)
To TRAPS (Record) Data
Comm Interface
TTL Data
& Clock
Video Distribution Amplifier (VDA) Telemetry and Radar Acquisition Processing System (TRAPS) Programmable Telemetry Processor (PTP)
Right Hand Circularly Polarized (RHCP) Left Hand Circularly Polarized (LHCP) Automatic Gain Control (AGC)
Figure 2 Telemetry Data Flow in the ROC for MAFB Flights
The PTP acquires telemetry data streams using serial data and clock signals from a bit synchronizer, and a frame synchronizer card. The bit synchronizer will also be used to derandomize the telemetry data before it is input into the PTP. The Monarch card
performs frame synchronization using an adaptive strategy, serial-to-parallel conversion, and time tagging. The frames will be time tagged with Inter-Range Instrumentation Group (IRIG)-B timing via the synchronized time code generator card. The frame synchronizer card outputs frame data to the PCI bus. Synchronous TTL I/O will be used for the X-33 program. The TTL data and clock signals out of the PTP will be used to interface with a PC-based decommutation system, Data Comm Interface, and digital recorder.
The frame synchronization method consists of four states: SEARCH, CHECK, LOCK, and FLYWHEEL. The frame sync card will look for a valid 32-bit frame sync pattern during the SEARCH state. The board will enter the CHECK state if N programmable number (up to 7) of check frames are programmed. The LOCK state is entered when N consecutive valid check frames are received. If no check frames are programmed, the board will enter the LOCK state when a valid frame sync pattern is received. The board will return to the SEARCH state if an invalid frame sync pattern is received during the CHECK state. The LOCK state is maintained if consecutive valid frame sync patterns are received. The board will advance from the LOCK state to the FLYWHEEL or SEARCH state based upon the number of flywheel frames that are programmed. The number of M flywheel frames are programmable from 0 to 7 frames. If no flywheel frames are
programmed, the board will go from the LOCK state to SEARCH state when an invalid frame sync pattern is received. The board will go from the LOCK state to the
FLYWHEEL state when an invalid frame sync pattern is received if flywheel frames are programmed. If M consecutive invalid frame sync patterns are received during the
FLYWHEEL state, the board returns to the SEARCH state. The board advances back to the LOCK state from the FLYWHEEL state when a valid frame sync pattern is received.
This will ensure that the LOCK state is maintained even in the presence of random bit errors.
The Monarch supports a frame sync bit slip window from 0 to + 3 bit periods wide. If a bit slip occurs and a valid sync pattern is received within the programmed window, it will adjust the bit count to accommodate the bit slip. The X-33 frame sync pattern will be the 32-bit pattern shown in Appendix C of the IRIG 106-96 Telemetry Standards document (Reference 6).
The BSS module in the ROC PTP for the X-33 application will accept five data stream inputs shown in Figure 2 for the MAFB missions and output the ‘best’ stream. The
module maintains a count of the number of good frames (valid frame sync code) received on each input port and uses the frame counts to select the best stream according to the
input that passes the highest number of good frames to the module. The module will only receive good frames. The frame counts are zeroed each time a best source is selected.
The following settings are available for setup of the BSS module:
Error Threshold - Defines frame count value for switching from one stream to another. For example: If the error
threshold is set for 10, then the best source will switch to another stream if the current stream exceeds 10 frame sync errors.
Sample Millisecs – Defines sample period in millisecs (1,000 to 10,000) to count number of frames received from each stream.
Manual Override – Allows user to manually select output stream.
Allow Switch During Sample – As soon as error threshold is reached, the BSS select module will switch streams instead of at end of sample period.
Fixed Priority – Starts checking from stream 1 or lowest numbered stream available to check for ‘good’ streams. For example: If the current best source is stream 3, and the error threshold is reached, the system will go back to stream 1 or lowest available to check for
‘good’ streams.
Rotational Priority - If Fixed Priority is not selected, the system starts checking from the current stream to check for
‘good’ streams. For example: If the current stream 3 exceeds error threshold, the system will start checking at stream 4.
Use Triggered Sample Period – When enabled, it synchronizes Sample Period timer to first detected error.
Extensive testing will be conducted to determine the optimum settings/configuration for the frame synchronization boards and BSS module. The number of check frames and flywheel frames to be used with the frame sync boards needs to be determined. The impacts on the best source selection of different time delays between the remote sites and the ROC system need to be analyzed. The entire X-33 Extended Flight Test Range will be simulated to help model the conditions under which the systems will be utilized. For the X-33 application, the system will most likely be setup to select the best source on a rotational basis because the best source will move to the downrange systems as the X-33 vehicle progresses along its flight path. If the inputs for the MAFB flights are numbered from 1 to 5 starting with the launch site source and ending with the landing site source, it
would not be acceptable to go back and check number 1 if the current source is number 4. It is preferable to check number 5 first if number 4 exceeds the error threshold rather than going back to check at number 1. An input can also be disabled so that it is not checked when looking for the best source. For example: The input corresponding to the Shadow Mountain tracker can be disabled after the X-33 is out of its tracking range, and the Triplex tracker has a solid lock on the telemetry signal. The system can be used in the manual override mode to select the best source if the occasion arises.
TELEMETRY DATA PROCESSING AND DISPLAY
The telemetry best source needs to be processed to display parameters for RSOs and X- 33 range engineers. The requirements for monitoring by LMSW personnel are not covered in this paper. The X-33 Range is only responsible for providing the raw telemetry best source to the OCC. The processing & display systems are the following: Veda Series 3000 PC-based system running Windows NT for RSO support; AFFTC developed Test Evaluation Command and Control System (TECCS) Instantaneous Impact Prediction (IIP) system for RSO support; and a ROC Telemetry and Radar Processing System (TRAPS) for X-33 Range engineering support. The processing and display requirements for each of the systems are described below.
The X-33 RSOs located at the OCC (primary RSO location), intermediate, and landing sites (backup RSO location) are required to monitor the performance of the X-33 vehicle and subsystems to ensure that public safety is maintained. The telemetry data at the various sites will be decommutated by the Veda system to display critical vehicle system parameters for the RSOs such as vehicle Mach number, vehicle altitude, vehicle attitude, vehicle angle of attack, voltage and current monitoring of each flight termination system (FTS) battery, FTS battery temperature, etc. The RSOs at the landing sites will have the ability to look at the best source sent from the ROC or the local best source via an A/B data switch as shown in Figure 3.
The vehicle GPS/INS data that is decommutated will also be output over Ethernet at a 10- hertz rate to the TECCS network shown in figure 4. The display of telemetry data on the TECCS systems is a unique development. The present TECCS situational
awareness/mission control systems only display position data of airborne systems from Federal Aviation Administration (FAA) and instrumentation radars. The GPS/INS decommutated data will be repackaged into the TECCS message format to output over Ethernet. Tied to the network are two (primary and backup) TECCS IIP systems that will take in the GPS/INS data and also instrumentation and FAA radar data via the TECCS Data Receive Unit (DRU)
for redundancy so that the RSOs can monitor the position of the vehicle throughout the missions. The GPS/INS data could also be used for slaving data for the other X-33 ground systems. The IIP systems are required to do calculations to display the following:
o Display nonlifting (post-FTS) IIP for intact vehicle o Display footprint for post break-up debris field
o Display background data including destruct limits, population centers, airspace boundaries,
glideslope, and runway alignment
o Use GPS/INS and instrumentation radar data for vehicle position information
T M B e s t S o u r c e F r o m R O C
R a n g e T M D a t a C o m m Interface
P T P
V D A
V D A
D a t a S w i t c h Local Best
S o u r c e T o R O C
Local B e s t S o u r c e
R e c o r d i n g S y s t e m
Local or ROC Best Source
R S O D e c o m / D i s p l a y S y s t e m E t h e r n e t
T o I I P S y s t e m s
R e c o r d i n g S y s t e m
B i t S y n c B i t S y n c
M O F D a t a
D a t a
D a t a C l o c k
C l o c k
B i t S y n c D a t a
C l o c k
W F F - 1 8 D a t a
V D A
B i t S y n c D a t a
C l o c k T o M O C C C o m m Interface
B i t S y n c D a t a
C l o c k
Video Distribution Amplifier (VDA)
F i g u r e 3 T e l e m e t r y D a t a F l o w a t M A F B
RSO TM Decom/
Display
TECCS Data Receive Unit (DRU)
TECCS IIP System
TECCS IIP System TM Best
Source
10 Mbps Ethernet TECCS Network FAA Radar Data
Tracking Radar Data
Positive knowledge of X-33 vehicle position throughout ascent and powered flight. Range Safety also needs information on the X-33 position during transition, descent, and approach phases. (AFFTC RSRD [Reference 3])
GPS/INS Data
Figure 4 Telemetry Data to TECCS Network
The X-33 range ground systems engineers will also monitor telemetry parameters in the ROC to help verify that the X-33 ground systems and some of the vehicle systems are operating correctly. A ROC TRAPS will be used to process and display the required information. Uplink, downlink, GPS, and FTS range engineers will be located in the ROC to monitor the following types of parameters:
o Automatic Gain Control (AGC) voltages for the FTS command receiver decoders o Uplink L-Band Receiver AGC voltages
o GPS receiver status (Have DGPS corrections been received?) o GPS/INS data for vehicle position
o Vehicle telemetry status including primary versus backup system activations
o Vehicle S-band telemetry transmitter forward power, temperature, current, reflected power
o Frame sync status of data from the various ground stations
DYNAMIC GROUND STATION ANALYSIS
The Dynamic Ground Station Analysis (DGSA) model developed by NASA Goddard Space Flight Center in Greenbelt, Maryland will be used to generate simulated data that will be compared with real-time data during the X-33 missions. Detailed information on the DGSA model is included in the ITC 98 paper titled, X-33 Integrated Test Facility,
Extended Range Simulation (Reference 7). The AGC voltages from the LHCP and RHCP primary telemetry receivers at the various locations will be sent over the X-33 Data Comm network to allow monitoring by a TRAPS in the ROC as shown in Figure 2. The AGCs are an indication of the received signal strength from the X-33 vehicle. The DGSA model will calculate predicted received signal strength values based upon parameters such as frequency, antenna gain, space loss, plasma effects, vehicle trajectory, etc. A display similar to Figure 5 will be generated in the ROC to show the predicted signal strength values versus the real-time AGC voltage values converted to dB scale. These values will be recorded for later analysis to define how the model compared with actual real-time data.
CONCLUSION
The X-33 range telemetry requirements, design, and implementation have been described in detail. The requirement for continuous coverage of the X-33 vehicle from launch to wheels stop at landing will be met by a combination of fixed and mobile systems from a variety of organizations. The best source selector system will be used to automatically determine the best source so that the best information is used for vehicle performance monitoring by RSO, LMSW, and X-33 Range personnel. Extensive testing on the best
source selector system will occur to determine the optimum settings for real-time support.
The X-33 telemetry stream will be decommutated to display parameters for the RSOs and range engineers, as well as provide information in the unique TECCS message format to calculate IIP plots to help ensure that public safety is maintained. Finally, real-time
telemetry data will be compared with data generated using the DGSA model to define how accurate the data can be predicted using a software model.
REFERENCES
1. X-33 SRD 604D0007 X-33 Systems Requirements Document 2. IC604Y0001 X-33 Vehicle-Ground Station RF Interface
3. AFFTC RSRD Range Safety Requirements Document for X-33 Flights from EAFB
4. Mackall, Dale, “The X-33 Extended Flight Test Range,” ITC Proceedings Paper, ITC/USA 98, San Diego, CA, 26-29 October, 1998
5. Avtec Systems, Inc. PTP_NT Programmable Telemetry Processor for Windows NT, User’s Manual Version 1.0, June 1997
6. IRIG 106-96 Telemetry Standards
7. Sharma, Ashley, “X-33 Integrated Test Facility, Extended Range Simulation,” ITC Presentation Paper, ITC/USA 98, San Diego, CA 26-29 October, 1998
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