chapter9

Slide1: 

9-1 Lesson objective - to discuss UAV Communications including … RF Basics Communications Issues Sizing Expectations - You will understand the basic issues associated with UAV communications and know how to define (size) a system to meet overall communication requirements

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9-2 Importance Communications are a key element of the overall UAV system A UAV system cannot operate without secure and reliable communications - unless it operates totally autonomously A good definition (and understanding) of communications requirements is one of the most important products of the UAV concept design phase

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9-3 RF basics Data link types Frequency bands Antennae Equations Communications issues Architecture Function Coverage Etc. Sizing (air and ground) Range Weight Volume Power Example problem Discussion subjects

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9-4 Data link types Simplex - One way point-to-point Half duplex - Two way, sequential Tx/Rx Full duplex - Two way, continuous Tx/Rx Modem - Device that sends data sent over analog link Omni directional - Theoretically a transmission in all directions (4 steradian or antenna gain  0) but generally means 360 degree azimuth coverage Directional - Transmitted energy focused in one direction (receive antennae usually also directional) - The more focused the antennae, the higher the gain Up links - used to control the UAV and sensors Down links - carry information from the UAV (location, status, etc) and the on-board sensors

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9-5 Frequency bands

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9-6 UAV frequencies Military and civilian UAVs communicate over a range of frequencies An informal survey of over 40 UAVs (mostly military, a few civilian) from Janes UAVs and Targets shows: Up links Band % using VHF (RC) 13% UHF 32% D 6% E/F 11% G/H 21% J 15% Ku 2% Down links Band % using VHF 0% UHF 17% D 19% E/F 13% G/H 23% J 17% Ku 9% Higher frequency down links provide more bandwidth

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9-7 More basics Carrier frequency - The center frequency around which a message is sent - The actual communication or message is represented by a modulation (e.g. FM) about the carrier Bandwidth - The amount (bandwidth) of frequency (nominally centered on a carrier frequency) used to transmit a message - Not all of it is used to communicate - Some amount is needed for interference protection - Sometimes expressed in bauds or bits per second but this is really the data rate

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9-8 Data rate Many people use band width and data rate as synonymous terms. Even though not rigorously correct, we will do likewise

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9-9 Polarity The physical orientation of an RF signal - Typically determined by the design of the antenna - But influenced by ground reflection Two types of polarization, linear and circular - Linear polarity is further characterized as horizontal (“h-pole”) or vertical (“v-pole”) - A simple vertical antenna will transmit a vertically polarized signal. The receiving antenna should also be vertical - V-pole tends to be absorbed by the earth and has poor ground reflection (tracking radars are V-pole). - H-pole has good ground reflection which extends the effective range ( used for acquisition radars) - Circular polarity typically comes from a spiral antenna - EHF SatCom transmissions are usually circular - Polarization can be either right or left hand circular

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9-10 And more Antenna gain - a measure of antenna performance - Typically defined in dBi = 10*log10(P/Pi) - where P/Pi = ability of an antenna to focus power vs. theoretical isotropic (4 steradian) radiation - Example - an antenna that focuses 1 watt into a 3deg x 3 deg beam (aka “beam width”) has a gain of 10*Log10(1/3^2/1/360^2) = 41.6 dB - For many reasons (e.g., bit error rates) high gain antennae (>20dBi) are required for high bandwidth data Example - 10.5 Kbps Inmarsat Arero-H Antenna - For small size and simplicity, low gain antenna (< 4 dBi) are used………... for low bandwidth data Example - 600 bps Inmarsat Aero-L Antenna

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9-11 Examples Inmarsat I (4.8 Kbps) Weight = 18 lb, 6 dB Data and pictures from http://www.tecom-ind.com/satcom.htm, weights = antenna + electronics Inmarsat H (≈9.6 Kbps) Weight = 102 lb, 12 dB Inmarsat L (600 bps) Weight = 8 lb, ? dB

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9-12 More basics - losses Free space loss - The loss in signal strength due to range (R) = (/4R)^2 - Example : 10 GHz (=0.03m) at 250 Km = 160.4 dBi Atmospheric absorption - Diatomic oxygen and water vapor absorb RF emissions - Example : 0.01 radian path angle at 250 Km = 2.6 dB Precipitation absorption - Rain and snow absorb RF emissions - Example : 80 Km light rain cell at 250 Km = 6.5 dB Examples from “Data Link Basics: The Link Budget”, L3 Communications Systems West

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9-13 Architecture Military Commercial “Common” Function Up link (control) Launch and recovery Enroute On station Payload control Down link (data) Sensor System status Communications issues Coverage Local area Line of sight Over the horizon Other issues Time delay Survivability Reliability Redundancy Probability of intercept Logistics

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9-14 Military vs. civil Military communications systems historically were quite different from their civilian counterparts With the exception of fixed base (home country infrastructure) installations, military communications systems are designed for operations in remote locations under extreme environmental conditions They are designed for transportability and modularity - Most are palletized and come with environmental shelters Civilian communications systems were (and generally still are) designed for operation from fixed bases Users are expected to provide an environmentally controlled building (temperature and humidity) Now, however, the situation has changed

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9-15 Communication types Military operators now depend on a mix of civilian and military communications services - Cell phones and SatCom have joined the military Global Hawk example

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9-16 Military communications Military communications systems generally fall into one of two categories Integrated - multiple users, part of the communications infrastructure Dedicated - unique to a system Dedicated

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9-17 UAV architectures UAV communication systems are generally dedicated The systems may have other applications (e.g. used by manned and unmanned reconnaissance) but each UAV generally has its own communications system US military UAVs have an objective of common data link systems across all military UAVs (e.g.TCDL) Multiple UAV types could be controlled Frequencies or geographic areas are allocated to specific UAVs to prevent interference or “fratricide” UAV communications equipment is generally integrated with the control station This is particularly true for small UAVs and control stations Larger UAVs can have separate communications pallets

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9-18 US common data links Excerpts from - Survey of Current Air Force Tactical Data Links and Policy, Mark Minges, Information Directorate, ARFL. 13 June 2001 A program which defines a set of common and interoperable waveform characteristics A full duplex, jam resistant, point-to-point digital, wireless RF communication architecture Used with intelligence, surveillance and reconnaissance (ISR) collection systems Classes & tech base examples Class IV (SatCom) - DCGS (Distributed Common Ground System) Class III (Multiple Access) - RIDEX (AFRL proposed) Class II (Protected) - ABIT (Airborne Information transfer) Class I (High Rate) - MIST (Meteorological info. std. terminal) Class I (Low Rate) - TCDL (Tactical CDL)

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9-19 Global Hawk GDT GDT = Ground “data terminal”

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9-20 Global Hawk ADT ADT = Air “data terminal”

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9-21 TCDL ADT & GDT Range goal - 200 Km at 15Kft See ASE261.RF LOS.xls LOS@alpha, Col C ...but until you read 9-47

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9-22 Architecture Military Commercial “Common” Function Up link (control) Launch and recovery Enroute On station Payload control Down link (data) Sensor System status Next subject Coverage Local area Line of sight Over the horizon Other issues Time delay Survivability Reliability Redundancy Probability of intercept Logistics

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9-23 Control functions

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9-24 Launch and recovery Located at the operating base Control the UAV from engine start through initial climb and departure….and approach through engine shut down Communications must be tied in with other base operations - Usually 2-way UHF/VHF (voice) and land line Also linked to Mission Control (may be 100s of miles away) Global Hawk Launch Recovery Element

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9-25 Enroute Launch and recovery or mission control responsibility Control the UAV through air traffic control (ATC) airspace - Usually 2-way UHF/VHF (voice) Primary responsibility is separation from other traffic - particularly manned aircraft (military and civil) - UAV control by line of sight, relay and/or SatCom data link Global Hawk Mission Control Element

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9-26 On station Primary mission control responsibility Control the UAV air vehicle in the target area using line of sight, relay and/or SatCom data link - Bandwidth requirements typically 10s-100s Kpbs Control sometimes handed off to other users - Mission control monitors the operation http://www.fas.org/irp/program/collect/predator.htm http://www.fas.org/irp/program/collect/predator.htm

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9-27 Payload Primary mission control responsibility Control the sensors in the target area using line of sight, relay and/or SatCom data links - Sensor control modes include search and spot - High bandwidth required (sensor control feedback) Sensor control sometimes handed off to other users EO/IR sensor control SAR radar control

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9-28 Down links Down links carry the most valuable product of a UAV mission UAV sensor and position information that is transmitted back for analysis and dissemination - Exception, autonomous UAV with on board storage Or UCAV targeting information that is transmitted back for operator confirmation Real time search mode requirements typically define down link performance required Non-real time “Images” can be sent back over time and reduce bandwidth requirements Line of sight down link requirements cover a range from a few Kbps to 100s of Mbps, SatCom down link requirements are substantially lower

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9-29 Radar “imagery” High resolution “imagery” (whether real or synthetic) establishes the down link bandwidth requirement Example - Global Hawk has 138,000 sqkm/day area search area at 1m resolution. Assuming 8 bits per pixel and 4:1 compression, the required data rate would be 3.2 Mbps to meet the SAR search requirements alone* - In addition to this, the data link has to support 1900, 0.3 m resolution 2 Km x 2 Km SAP spot images per day, an equivalent data rate of 2.0 Mbps - Finally there is a ground moving target indicator (GMTI) search rate of 15,000 sq. Km/min at 10 m resolution, an implied data rate of about 5Mbps Total SAR data rate requirement is about 10 Mbps *See the payload lesson for how these requirements are calculated

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EO/IR data EO/IR requirements are for comparable areas and resolution. After compression, Global Hawk EO/IR bandwidth requirements estimated at 42 Mbps* This is why Global Hawk has a high bandwidth data link * Flight International, 30 January 2002 9-30

Slide31: 

9-31 System status data Air vehicle system status requirements are small in comparison to sensors - Fuel and electrical data can be reported with a few bits of data at relatively low rates (as long as nothing goes wrong - then higher rates required) - Position, speed and attitude data files are also small, albeit higher rate - Subsystem (propulsion, electrical, flight control, etc) and and avionics status reporting is probably the stressing requirement, particularly in emergencies Although important, system status bandwidth requirements will not be design drivers - A few Kbps should suffice Once again, the sensors, not system status, will drive the overall data link requirement

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Coverage Local area Line of sight Over the horizon Other issues Time delay Survivability Reliability Redundancy Probability of intercept Logistics 9-32 Next subject Architecture Military Commercial “Common” Function Up link (control) Launch and recovery Enroute On station Payload control Down link (data) Sensor System status

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9-33 Local area communications Close range operations (e.g., launch and recovery) typically use omni-directional data links - All azimuth, line of sight - Air vehicle and ground station impact minimal Communications must be tied in with other base operations - Usually 2-way UHF/VHF (voice) and land line Omni-directional antennae

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Typically require directional data links - RF focused on control station and/or air vehicle - Impact on small air vehicles significant - Impact on larger air vehicles less significant - Significant control station impact Communications requirements include air traffic control - Usually 2-way UHF/VHF (voice) 9-34 Long range comms (LOS) Hunter http://www.fas.org/irp/program/collect/pioneer.htm

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Relay aircraft - existing line of sight equipment Minimal air vehicle design impact Major operational impact 9-35 Over the horizon options Low bandwidth - minimal design impact, major operational High bandwidth - major impact (design and operational) SatCom

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9-36 Global Hawk SatCom

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Coverage Local area Line of sight Over the horizon Other issues Time delay Survivability Reliability Redundancy Probability of intercept Logistics 9-37 Architecture Military Commercial UAV Function Up link (control) Launch and recovery Enroute On station Payload control Down link (data) Sensor System status

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9-38 The time required to transmit, execute and feed back a command (at the speed of light) - A SatCom problem Example: - 200 Km LOS @ c = 3x10^5 Km/sec - Two way transmission time = 1.33 msec - Geo stationary Satcom at 35,900 Km - Two way transmission time = 240 msec Other issues - time delay Raw data from, Automated Information Systems Design Guidance - Commercial Satellite Transmission, U.S. Army Information Systems Engineering Command (http://www.fas.org/spp/military/docops/army/index.html) Inmarsat M (500 msec?)

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9-39 Also known as data “latency” or “lag” - Limited by speed of light and “clock speed” All systems have latency - Human eye flicker detection - 30 Hz (33 msec delay) - Computer screen refresh rate - 75 Hz (13 msec) - Computer keyboard buffer latency - 10 to 20 msec - LOS communications - 2 msec - LEO SatCom - 10 msec - MEO Satcom - 100 msec - GEO Satcom - 200 to 300 msec - Typical human reaction - 150-250 msec Acceptable overall system lag varies by task < 40 msec for PIO susceptible flight tasks (low L/D) < 100 msec for “up and away” flight tasks (high L/D) When OTH control latency > 40 msec, direct control of a UAV is high risk (except through an autopilot) Time delays and UAVs

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9-40 The preferred reliability solution Separate back up data link(s) Most modern UAVs have redundant data links Global Hawk has 4 (two per function) - UHF (LOS command and control) - UHF (SatCom command and control) - CDL (J-band LOS down link) - SHF (SatCom Ku band down link) Dark Star also had four (4) Predator, Shadow 200 have two (2) Most UAVs also have pre-programmed lost link procedures - If contact lost for TBD time period (or other criteria) return to pre-determined point (near recovery base) - Loiter until contact re-established (or fuel reaches minimum levels then initiate self destruct) Other issues - redundancy

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9-41 Probability of intercept Probability that an adversary will be able to detect and intercept a data link and be able to 1. Establish track on the UAV position 2. Interfere with (or spoof) commands Purely a military UAV issue No known civil equivalent Some well known techniques - Spread spectrum - Random frequency hopping - Burst transmissions - Difficult to detect and track - Power management - No more power than required to receive - Narrow beam widths - Difficult intercept geometry

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9-42 More issues Power and cooling Communications equipment (especially transmitters) require significant power and cooling to meet steady state and peak requirements - At low altitudes, meeting these power and cooling requirements typically is not an issue - At high altitude, both are a problem since power and cooling required ≈ constant and …. - Power available approximately proportional  - Cooling air required(cfm) approximately proportional 1/; one reason why high-altitude aircraft use fuel for cooling (also keeps the fuel from freezing!)

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9-43 A significant part of transport requirements are associated with communications equipment C-141B transport configuration Other issues - logistics

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9-44 Next subject RF basics Data link types Frequency bands Antennae Equations Communications issues Architecture Function Coverage Etc. Sizing (air and ground) Range Weight Volume Power Example problem

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- Given 2 platforms at distance (D1+D2) apart at altitudes h1 and h2 above the surface of the earth: D1+D2  Re*{ArcCos[(Re+hmin)/(Re+h2)]+ ArcCos[(Re+hmin)/(Re+h1)]} (9.1) Re ≈ 6378 km (3444 nm) hmin = intermediate terrain or weather avoidance altitude (≈ 20kft)* ArcCos[ ] is measured in radians *not applicable if h1 and/or h2 lower than hmin - From geometry where and 9-45 Geometric line of sight (LOS)

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9-46 RF line of sight Due to earth curvature and atmospheric index of refraction, RF transmissions bend slightly and the RF line of sight (LOS) is > the geometric LOS by a factor ≈ √4/3 (Skolnik, Radar Handbook, page 24-6) Another equation for communication LOS can be found using a simple radar horizon equation from Skolnik (page 24-8) where: - LOS(statute miles) ≈ √2*h(ft) (9.2) or - LOS(nm) ≈ 0.869√2*h(ft) (9.3) Note that the ratio of Eqs 9.1 and 9.3 for h1 = hmin = 0 and h2 = h is √4/3 ; e.g. LOS (Eq 9.1) = 184 nm @ h2 = 30Kft while LOS (Eq 9.3) = 213 nm - We will assume that the √4/3 factor will correct any geometric LOS calculation including 9.4 when h1 and h2min ≠ 0

Slide47: 

Ignore the small differences between LOS and LOS’ The equation predicts published Global Hawk comm ranges at   0.75 9-47 Grazing angle effects Given a platform at altitude h at grazing angle  above the horizon: Re h Local horizon  LOS’ LOS Use this method ( = 0.75) to size the air-ground comm. segment for your projects You can use SpreadSheet ASE261.RF LOS.xls to eliminate hand calculations

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9-48 Airborne relay A system level solution for an organic over the horizon (OTH) UAV communications capability Requires that relay UAV(s) stay airborne at all times - For extended range and/or redundancy Also requires separate communication relay payload - In addition to basic UAV communication payload But relay platform location is critical. Example: Four (4) WAS UAVs loiter at 27 Kft and one (1) ID UAV loiter at 10 Kft over a 200 nm x 200 nm combat area located 100 nm from base Two (2) WAS UAVs closest to base function as communications relays for the three other UAVs Typical terrain altitude over the area is 5 Kft How would a WAS relay have to operate to provide LOS communications to the ID UAV at max range?

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9-49 LOS defines max communication distance for relay - At  =0.75, LOS from base= 156.7 nm vs. 158.1 nm req’d At hmin = 5 kft, LOS from ID UAV at 10 Kft to WAS relay at 27 Kft = 269.2 nm vs. 212 nm req’d WAS altitude inadequate to meet base relay requirement Example problem relay 100 nm 200 nm x 200 nm 158.1 nm 10 Kft 27 Kft 156.7 nm 269.2 nm 212 nm Altitude increase to 27.4 Kft required See SpreadSheet ASE261.RF LOS.xls for details

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9-50 There is little public information available on UAV data links to use for initial sizing - Including both air and ground data “terminals” Short hand notation - ADT and GDT Three sources 1. Janes UAVs and Targets, Issue 14, June 2000 - Mostly military UAV data links 2. Unpublished notebook data on aircraft communications equipment - Both military and civil, not UAV unique 3. Wireless LAN data - Collected from the internet, not aircraft qualified - Indicative of what could be done with advanced COTS technology For actual projects, use manufacturer supplied data Next - sizing data

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9-51 ADT range and power Calculate LOS range Equations 9.1-9.4 Estimate RF output power required

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9-52 Initial sizing - ADT Satcom Parametric correlation basis Known correlation between band width or data rate and frequency - Bandwidth availability increases with frequency Parametric data source All Satcom data links Frequency range 0.24 - 15 GHz Bandwidth range 0.6 Kbps - 5.0 Mbs Select Bandwidth Select frequency

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9-53 ADT power required Parametric data source Military line of sight data links Frequency range 30 MHz - 15 GHz Bandwidth range 0.01-5.0 Mbs Estimate input power requirements - LOS - SatCom (GEO)

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9-54 ADT weight Parametric data source Janes and unpublished data Frequency range 30 MHz - 15 GHz Bandwidth range 0.01-5.0 Mbs Estimate weight - LOS - SatCom (GEO) Note - excludes antennae

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9-55 ADT volume Parametric data source All LOS data links & modems Frequency range 30 MHz - 15 GHz Bandwidth range 0.01-5.0 Mbs Estimate volume - LOS - SatCom (GEO)

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Parametric correlation basis Known correlation between bandwidth required and size Antenna characteristic “size” defined as following: - For EHF : square root of antenna area (when known) or cube root of installed volume - For UHF : antenna length (blade) or diameter (patch) 9-56 ADT Satcom antenna Parametric data source All Satcom data link antenna Frequency range 0.24 - 15 GHz Bandwidth range 0.6 Kbps - 5.0 Mbs Estimate antenna “size” Calculate area, volume or length as appropriate

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9-57 ADT satcom antenna Parametric data source All Satcom data link antenna Frequency range 0.24 - 15 GHz Bandwidth range 0.6 Kbps - 5.0 Mbs Estimate antenna weight

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9-58 More ADT LOS data Median = .025 Median = .045 Parametric data source All LOS data links & modems Frequency range 30 MHz - 15 GHz Bandwidth range 0.01-5.0 Mbs

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9-59 All systems on an air vehicle have an installation weight and volume penalty (more in Lesson 19) We will assume a typical installation at 130% of dry uninstalled weight We will make this assumption for all installed items (mechanical systems, avionics, engines, etc.) Installed volume is estimated by allowing space around periphery, assume 10% on each dimension Installed volume = 1.33 uninstalled volume For frequently removed items or those requiring air cooling, we will add 25% to each dimension Installed volume = 1.95 uninstalled volume Payloads and data links should be installed this way Installation considerations

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9-60 GDT options There are a few GDT system descriptions in Janes and on the internet for UAV applications. - Little technical data is provided but in general they are large - The CL-289 GDT is integrated into a truck mounted ground control station and includes a 12 meter hydraulic antenna mast - The Elta EL/K-1861 has G and I-band dish antennae (6 ft and 7ft diameter, respectively) - The AAI GDT appears to be about a 2 meter cube excluding the 1.83 m C-band antenna - Smaller man portable systems are also described but little technical performance data is included The following parametrics are very approximate and should be used only until you get better information from manufacturers

Slide61: 

9-61 GDT parametrics

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9-62 Expectations You should understand Communications fundamentals UAV unique communications issues How to calculate communication line of sight How to define (size) a system to meet overall communication requirements

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9-63 Final subject RF basics Data link types Frequency bands Antennae Equations Communications issues Architecture Function Coverage Etc. Sizing (air and ground) Range Weight Volume Power Example problem

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9-64 Example problem Five medium UAVs, four provide wide area search, a fifth provides positive target identification WAS range required (95km) not a challenge Only one UAV responds to target ID requests No need to switch roles, simplifies ConOps No need for frequent climbs and descents Communications distances reasonable (158nm & 212 nm) Speed requirement = 280 kts Air vehicle operating altitude differences reasonable We will study other options as trades What is a reasonable communications architecture? How big are the parts?

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9-65 Parametric data is used to size (1) a basic UAV data link and (2) a communications relay payload We assume both are identical and that all UAVs carry both, allowing any UAV to function as a relay Provides communication system redundancy Parametric sizing as follows (for each system) Max range = 212 nm  RF power = 110 W (Chart 51)  Power consumption = 500 W (Chart 53)  Weight = 27 lbm (Chart 54)  Volume = 500 cuin (Chart 55) We have no non-Satcom antenna parametric data and simply assume a 12 inch diameter dish, weighing 25 lbm with volume required = 2 cuft If you have no data, make an educated guess, document it and move on We will always check the effect later We include communications in our payload definition ADT sizing

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9-66 We have little GDT parametric sizing date and simply assume an ADT consistent input power requirement (500W) and use the chart 61 parametrics to estimate weight and volume 250 lbm and 9.5 cuft Antenna size will be a function of frequency and bandwidth which we will select after assessing our payload down link requirements GDT sizing

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9-67 Requirements update System element GDT weight/volume/power excluding antenna (each) = 205 lbm/9.5 cuft/500 W GDT installations required = 2 Payload element Installed weight/volume/power = TBD WAS Range/FOR /resolution/speed = 95 km/45/10m/2mps Uninstalled weight/volume/power = TBD ID Type/range/resolution = TBD/TBD/0.5m Uninstalled weight/volume/power = TBD Communications Range/type = 212nm/air vehicle and payload C2I Uninstalled weight/volume/power  52 lbm/2.3 cuft/500 W Range/type = 158nm/communication relay Uninstalled weight/volume/power  52 lbm/2.3 cuft/500 W Air vehicle element Cruise/loiter altitudes = 10 – 27.4Kft

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9-68 Homework 1. Update your team architecture to meet new req’ments (a) Redundancy – one back-up for all flight and mission critical comm. functions (air vehicle and payload control up-link and down link and payload data down link) – note: payload down link can be back-up for air vehicle control down link and vise versa, payload back-up downlink can be at reduced bandwidth (b) Refined LOS methodology – Update your estimates Min. grazing angle ( ) = 0.75 for air-ground comms If you require airborne relay, recalculate req’ments for your operating altitudes @ hmin = 1000 ft (c) Refined (Chapter 9) ADT sizing methodology 2. Refine your team ADT req’ment (wt., vol. and power) 3. Resize your vehicle for updated comm. req’ments Note : # 1 and #2 - One input per team (do them in class?) # 3 - Individual submittals

Slide69: 

9-69 Intermission