Network Telemetry: PracticalExperiences and Unique Features
Item Type text; Proceedings
Authors D’Amico, William P.; Stadter, Patrick A.; Lauss, Mark H.; Hooper,Andrew
Publisher International Foundation for Telemetering
Journal International Telemetering Conference Proceedings
Rights Copyright © International Foundation for Telemetering
Download date 11/05/2018 10:50:56
Link to Item http://hdl.handle.net/10150/606321
Network Telemetry: Practical Experiences and Unique Features
William P. D’Amico, PhD, Patrick A. Stadter, PhDJohns Hopkins University Applied Physics Laboratory
Laurel, MD
Mark H. Lauss, Andrew HooperMateriel Test Center
Yuma Proving Ground, AZ
ABSTRACT The US Army’s Yuma Proving Ground (YPG) uses a wireless local area network (WLAN) to gathertest data. It is desired to extend this WLAN to support tests of gun-launched munitions whereminiature and rugged data acquisition hardware will be required. The Two Way Robust Acquisition ofData (2-RAD) program has been initiated under the Central Test and Evaluation Investment Program(CTEIP) to develop a process to expand the use of WLAN technology, which is now primarily used atYPG for internal ballistic test data acquisition.
KEY WORDSWireless Local Area Network, IEEE802.11, Telemetry, Encryption
INTRODUCTIONIn order to evaluate the use of WLAN technologies for the acquisition of military test data, the potentialbenefits must be examined. Some potential benefits are higher data rates, shared processing of data,time sharing of network bandwidth, low ground station cost, natural 2-way links at the same frequency,and the support of network centric weapons and platforms. Developments from the 2-RAD project canmaterially augment the use of serial stream telemetry infrastructures with internet-like devices usingWLAN technologies to support a wider variety of test scenarios. The 2-RAD program goals are to (1)provide network and physical layer models to predict and extend the existing WLAN performance, (2)extend these models to other potential network telemetry applications (missile defense or the Army’sFuture Combat Systems), (3) understand the impact of network security, (4) define a path for miniature,high-g products for airborne munitions tests, and (5) conduct a flight test with a 70mm ballistic rocketas a fast mover airborne node in aWLAN trial.
YPG has used commercial-off-the-shelf (COTS) devices to implement an 802.11b WLAN. Theexisting YPG WLAN is shown in Figure 1. This system currently is configured using standardEthernet Bridging and Wireless Ethernet Access Point/Client operations with 6 units providing astationary backbone. Only one of the wireless backbone devices is tied to the fiber optical networkbackbone that services all of YPG’s computer systems. It is via this network connection that all datacollected from wireless sources are processed and archived. Similar wireless devices are used for datacollection at fixes sites (metrological data) and for slow moving ground and air vehicles. There are threetypes of sites: 6 fixed infrastructural repeaters, multiple fixed position data transponding nodes, andmultiple mobile test supporting nodes. The approximate East-West extent of YPG is 55 miles. Atypical infrastructural site is shown (Figures 2a, b, c, and d) in a field configuration with a set of solarpanels, automotive battery, and omni-directional whip antenna. Also shown are interior device views,showing network and data acquisition boards with a radio/antenna combination.
Figure 1. WLAN for test data acquisition at YPG.
Figure 1. YPG WLAN site locations.
Figures 2a. and b. A YPG Infrastructural Site.
Figures 2c. and d. Internal Views.
Under the 2-RAD program, The Johns Hopkins University Applied Physics Laboratory (JHU/APL)has produced network scaling and physical layer models that agree with the experiences at YPG. Thekey technical issues to be addressed are linked to the Doppler/Doppler rate and hand over problems ofa node that has high accelerations or velocity, a so-called fast mover. This paper will not discuss thoseaspects. Instead, it will focus on longer-term issues of frequency selection, encryption, and network-supported data processing.
PRACTICAL EXPERIENCESThe COTS WLAN devices were not originally intended for long distance transmissions, but clearly thesuccess of the YPG implementation has produced a more quantitative examination of the approach andfuture use. A short summary of the 802.11 standards is shown in Table 1. The “b” standard has beenthe first commercial implementation and is known as “WiFi.” The “a” standard is just nowexperiencing the release of commercial products. The “g” standard implements the “b” standard witha modulation scheme that can support the higher data rates. The number of non-overlapping channelsis for 802.11a in the United States is three. Using a 2 bit/Hz rule of rule of thumb, then prudentpractices will restrict the other standards to fewer channels when guard bands are employed. Guardbands are regions in a spectral map that provide unused buffer zones between channels. Givenfavorably low radiated power levels and large distances, a fourth channel has been used at YPG, but thisis not a standard operational mode.
IEEEStandard
Frequency Band(GHz)
Max Data Rate(Mbps/channel)
Max NumberChannels
802.11a 5.725 to 5.850 54 1-2
802/11b 2.400 to 2.4835 11 3
802.11g 2.400 to 2.4835 54 1
Table 1. A Short Description of the 802.11 Standards.
It is interesting to examine the number of tests and the associated data flows that are typically supportedon a daily basis at YPG. Clearly the management of the test data by a network with 11 Mbps/channel
maximum through put shows the inherent capability and flexibility of this approach. A typical array ofnodes and data rates is shown in Table 2. Essentially, most of the YPG WLAN is comprised of“bridge” radios that can transmit data using different channels. A judicious use of buffering can alsobe applied, so that time can become a new and independent variable in the data acquisition strategy. Thedata traffic shown in Table 2 would not be a surprising feat for a ground-based network, but this is aWLAN. A simple test was recently run where a specific data set was moved through the network whilethe usual background support was in progress. The “receive rate” for 120 packets per second thatcontained 272,142 Bytes translated into approximately 2.2 Mbps.
Data Source/Network Node Data Flows (Mbytes) Directories/Files
Stationary MET Sites 1.3 multi-reads/not stored
Video (4 sites/low resolution) 13 multi-reads/not stored
Indirect Fire Weapon Data (1test/day)
272 79 /2299
Direct Fire Weapon Data (2tests/day)
254 43 /3672
Mortar Weapon Data (2 tests/day) 70 51/404
Table 2. Typical Traffic Summary for the WLAN on a Test Day at YPG.
The practical considerations of operating a WLAN in the 2.4 or 5.7 GHz bands, which are notcontrolled like the usual L or S-Band frequencies, represents a challenge for the future. The test andevaluation community has shown interest in non-standard frequency bands (4.4-4.9 GHz, 5.4-5.9 GHz,and 7.125-8.175 GHz). Performance trades for higher frequencies /higher data rates/multiple channelswith lower link margins (shorter distances) must also be examined. These frequencies do not coincidewith the 802.11 standards, but new physical layer chips could provide new frequencies for DoD rangeuse. The tools developed under the 2-RAD effort could be used to assess the utility of these higherfrequencies. Of course antenna systems must be developed to support these new frequencies and thatcould be problematic simply from a development and cost standpoint. For example, it has been asimple matter to identify inexpensive COTS antennas for implementations of the mobile units, asshown in Figure 3. The antenna issue will need more attention in the demonstration of a munition(such as a projectile or missile) as a node in the network. It is typical that conformal antennas formuntions have nulls, and these nulls could produce time varying changes in link margin and impact theperformance of the network. The use of inexpensive additional ground nodes can mitigate lower linkmargins that will occur due to higher transmission frequencies and antenna nulls. Any of theseantennas must also have sufficient isolation to properly function with a GPS antenna. Moving tohigher radio frequencies could mitigate GPS interference problems.
Figure 3. COTS Stub Antenna for Mobile User WLAN sites.
UNIQUE FEATURESOnce a network is in place, however, the question remains – what to do next? The top list can include:extremely high data rates, network security, and miniaturization. The issue of high data rates isseemingly very attractive since the 802.11a maximum data rate for single channel use is 54Mbps. Ifvery high frequencies are used, then very high data rates could be achieved. One must be very careful tounderstand that similar data rates would be possible if normal telemetry methods were employed athigher frequencies, but a WLAN could perhaps more easily manage and efficiently use these widerbandwidths. It is also true that guard bands for any wireless method must be employed. The uniquefeature of a network is the use of time phasing and buffering to control the data flow coupled withautomatic repeater and 2-way capabilities. These techniques could be employed in a standard telemetrysystem at great expense, but they are part of the normal nature of a network.
A major issue with the wireless data is always security. As usual, some form of data encryption will berequired. In addition, a network will probably also need intrusion protection. Fortunately there is anear term solution to the encryption aspect based upon the SecNet 11TM product from the HarrisCorporation. SecNet 11TM is a Type 1 Secure Wireless LAN (SWLAN) available in a PC card(PCMCIA) format. It is based upon the Baton encryption algorithm and secures the RF link. Thissolution supports the data rates (to include the 5.5, 2, and 1 Mbps auto-fallbacks) used with the IEEE802.11b standard. At the top level, the implementation is based upon an application specific integratedcircuit (ASIC), two programmable logic chips, and a key fill device to augment the PRIISM II chip setfrom Intersil. The SecNet 11TM product will also feature a separate power amplifier and RF switch toprovide transmission frequencies in the upper S-band or in the 2.4GHz band. Release of the SecNet11TM products is expected by the summer of 2002. There is also the potential of conversion to otherfrequencies in the C-, L-, and X-band spectra. Pricing quotes for the Harris products are roughly in the$3000 range.
Any modern telemetry implementation would be driven by cost, power, and size. The SecNet 11TM
product addresses both of those issues, but the unique needs of the DoD test community must also beconsidered – especially if WLAN technology is to be used with airborne munitions. The HardenedSubminiature Telemetry and Sensor System (HSTSS) program, which is also funded by the CentralTest and Evaluation Investment Program (CTEIP), has made great strides in the use of commercialpractices and government developments to provide a new generation of highly packaged telemetryproducts. The modular approach of the HSTSS program assumes that a single hardwiredconfiguration telemetry device is not suitable for most testing applications. As such the HTSSSprogram has produced and/or demonstrated a series of modular products to include batteries, sensors,programmable logic devices, crystal oscillators, antennae, and telemeters. HSTSS has been
instrumental in the use of commercial-off-the-shelf MEMS sensors (accelerometers and angular ratesensors). In addition, the electronic devices are available in die form and have been assembled usingmixed packaging techniques to provide highly miniature system solutions. The HSTSS program hassupported a highly integrated solution for an artillery fuze with the development lead by the ArmyResearch Laboratory’s Weapons and Materials Directorate at Aberdeen Proving Ground, MD. Figure4 shows the board level configuration (diameter of 1.125”) that is inserted into a NATO standardartillery fuze configuration (Ref 1,2,3). These components are integrated with a circular disc antennathat radiates at approximately 2.2 GHz.
Figure 4. HSTSS Boards for a NATO Standard Fuze: ((a) Sensor Board (courtesy D. HepnerARL,(b) Encoder Board (courtesy P. Muller), (c) Assembly with M/A-COM transmitter (courtesy P.
Muller), CAD Rendition of Assembly (courtesy P. Muller, ARL), and (e) NATO Standard Fuze.)
The HSTSS program has not pursued miniature encryption technologies, however. A goal would be tofollow this integration and miniaturization path replacing the M/A-COM transmitter board with aWLAN and to provide a miniature encryption capability if needed. Of course swapping the M/A-COMtransmitter for a WLAN with encryption requires the use of more devices and would require morevolume or substantially higher electronic integration. The layout of the Harris PCMCIA 802.11 cardwith encryption is shown in Figure 5 (courtesy Harris Corporation).
(a)
(b))
(c)
(d)
(e)
Figure 5. An 802.11b PCMCIA Card from Harris.
The devices that are shown are certainly more numerous than the major components in the HSTSStransmitter, which are the high-g crystal oscillator, the voltage controlled oscillator, the phase-lockedloop, and the power amplifier. Under the 2-RAD program, inquiries as into die level componentavailability are underway to support higher packaging densities. However, if HSTSS-like mixedpackaging techniques were used, the rectangular PCMCIA layout could be transformed into a set ofcircular boards. The battery that is shown is needed for the encryption device, but the HSTSS programis very knowledgeable in these technologies for use with their products.
CROSSLINK TRANSCEIVER TECHNOLOGIESThe use of a WLAN is typically for connectivity and ease of data transmittal. There is another highlynovel utilization that could in certain circumstances be of great value to the test community. Thisapplication is the Crosslink Transceiver (CLT) (Ref 4,5). A block diagram is shown in Figure 6 withan expanded view of the CLT hardware for space applications in Figure 7. The concept is relativelysimple – use the network to provide a near real time solution that is based on the shared data from thenodes. This presumes that the network has sufficient bandwidth to co-process data using a Kalmanfilter.
Figure 6. High Level CLT BlockDiagram
.
Figure 7. Crosslink Transceiver in an Expanded View
JHU/APL has pioneered the use of the CLT where communication, control, and navigation functionsare combined. The hardware is unusually large since all components are space qualified, but thecomponents could be significantly reduced in size for other applications. In the present CLT format,absolute GPS solutions can be filtered across the network to achieve more accurate relative positionsolutions for the nodes. In addition, ranging data from the transceiver waveforms can be used to reducethe RF front-end biases, thus providing additional position improvements. If sufficient processing ratesand bandwidth are available, then improved real time, relative position solutions can be achieved. CLTtechnology has been developed for the autonomous operation of groups of spacecraft, but thistechnology could be implemented for vector scoring scenarios common to the test community. TheCLT concept could be broadened to include other types of sensors where those data are fused in realtime as opposed to the usual post processing case.
CONCLUSIONSThis paper has described a WLAN at Yuma Proving Ground that is used to collect test data on a dailybasis. The success of using this WLAN has spawned a closer examination into the utility andextension of wireless network methods and the expanded use COTS-based products. The chip set fora WLAN appears ready for further miniaturization using mixed packaging techniques using die levelcomponents. In addition, the requirement to encrypt test data (up to the SECRET level) can apparentlybe met based upon a commercial product, which has a relatively small footprints and low power needs.With the potential of continued miniaturization and the release of NSA-approved encrypted devices,novel applications of WLAN technology should grow. Given the drive of the DoD to network centriccombat system, a WLAN offers a natural environment to support the test of these systems of systems.
ACKNOWLEDGEMENTSThe Central Test and Evaluation Investment Program (CTEIP) sponsors the Two-Way RobustAcquisition of Data (2-RAD) program at Yuma Proving Ground and at the Johns Hopkins UniversityApplied Physics Laboratory under the Test Technology and Development Demonstration phase. Theauthors are also indebted to Mr. Peter Muller and Mr. David Hepner of the Army ResearchLaboratory’s Weapons and Materials Directorate at Aberdeen Proving Ground, MD, for the use oftheir figures, which were presented at the HSTSS symposium in Denver, CO, in August of 2001.
REFERENCES
1. Hepner, David, et al, “An Aeroballistic Diagnostic Fuze,” 26th Joint Services Data Exchange forGuidance, Navigation, and Control Symposium, Oxnard, CA. 23-26 October 2000.
2. Muller, Peter and Hollis, Michael, “HSTSS CPLD-Based PCM Encoder,” Hardened SubminiatureTelemetry and Sensor System (HSTSS) and Joint Advanced Missile Instrumentation (JAMI)Symposium 2001, Denver, CO. August 21-23, 2001.
3. Osgood, Karina, et al “HSTSS Transmitter Chip Set,” Hardened Subminiature Telemetry andSensor System (HSTSS) and Joint Advanced Missile Instrumentation (JAMI) Symposium 2001,Denver, CO. August 21-23, 2001.
4. Stadter, Patrick, et al “Confluence of Navigation, Communication, and Control in DistributedSpacecraft Systems,” Aerospace Conference, 2001 IEEE Proceedings, 7 Volume Set, Institute ofElectrical and Electronics, Inc. (IEEE), New Brunswick, New Jersey, May 1, 2001.
5. Stadter, Patrick, et al “Enabling Distributed Spacecraft System Operations with the CrosslinkTransceiver,” Aerospace Conference, 2002 IEEE Proceedings, 7 Volume Set, Institute of Electrical andElectronics, Inc. (IEEE), New Brunswick, New Jersey, May 1, 2002.
