During the past six years of combat in Afghanistan and Iraq, all doubts about the utility of Unmanned Aerial Vehicles (UAVs) in providing mission critical ISR (Intelligence, Surveillance, Reconnaissance) for war fighters have been erased. In 2002, UAV systems (a ground station supporting a number of aircraft) deployed totaled 16. In 2008 the number of systems in theatre exceeded 1,000, with nearly 6,000 aircraft fielded. In these few years the UAV industry has moved from a state of infancy to a condition that might be described as adolescence, since the technology has advanced sufficiently that its full potential can be envisioned, but there is a lot more growing up to do.
Although they are small and have very low cost compared to manned ISR platforms, good ISR UAVs are an impressive aggregation of technologies the airframe, engine, autopilot, and navigation system, sensor package, and communication link each must excel for a UAV to achieve its mission goals. (Photo: Predator UAV)
As the principle purpose of these aircraft is to obtain quality, high bandwidth ISR data and move that information into the hands of its consumers as quickly as possible, the communication link is a key element of the UAV system. The requirements for these links vary as much as the types of missions and different UAV classes.
The general perception of UAVs tends to be driven by news articles featuring larger aircraft such as the Predator that are flown by pilots in climate-controlled rooms half-way around the world, and whose ISR output travels over Ku-band Common Data Link (CDL) satcom links. Those links allow viewing of data in real time by CONUS mission commands including Creech Air Force Base and the Pentagon. The reality is the role of the lions share of UAVs is to provide ISR data to soldiers within 50 miles of the aircraft using line-of-sight air-to-ground links
Today these links are predominantly analog FM links for standard definition video, but a change to sophisticated digital links is underway, driven by a range of factors. Tomorrows needs include security, improved range, efficient use of spectrum, and support for a variety of complex sensors such as high definition video, laser designators, synthetic aperture radar (SAR), ground moving target indicators, and multi-spectral imagers.
This article will offer some insight into how the variety of mission needs has resulted in a wide range of different UAVs, each filling a particular mission niche. The focus then moves on to how these needs drive the communication system design on the UAV, focusing especially on the need for a broad consideration of spectrum management, since this is the next major hurdle in the UAV maturation process.
UAVs for Intelligence, Surveillance and Reconnaissance
As there are many types of missions, there are also many types of UAVs. Each has a broad range of characteristics to allow it to satisfy the needs of a specific mission. Size (measured by payload capacity) and endurance (maximum flight time) are the biggest differentiators, but mode of flight also matters. Observers sometimes divide the UAV industry into three classes, called Tiers. Although the armed services differ on how to define the tiers, the Army Tier concept is shown here:
- Tier I UAVs are highly portable and can be hand launched (many can be carried in a soldier backpack). They are intended to allow small troop units to find out whats happening behind the next building or on the other side of a nearby hill. Tier I UAV payload capacity is typically less than three to four pounds and their endurance is an hour or less. They rarely carry more than a single standard definition EO camera. Examples of Tier I UAVs are the AeroVironment Raven and the Lockheed Martin Desert Hawk.
- Tier II UAVs can often be lifted by two men and might carry five to 30 pounds of payload, which could include several different types of sensors including EO, IR or SAR radar. These aircraft support larger troop formations with more wide-ranging missions and can operate out to the line-of-sight horizon, with endurance of as much as 12 hours or more. Examples of Tier II UAVs are the Insitu Scan Eagle, AAI Shadow, or DRS Sentry.
- Tier III UAVs rival small passenger aircraft in size and payload. They carry a wide range of sensors, including sophisticated on-board image and sensor data processing payloads. In recent years it is becoming increasingly commonplace that Tier III UAVs carry weapons so that they can fill a hunter/killer role, working in concert with ground warfighters to find, fix and finish a target. Examples of Tier III UAVs are the fixed wing General Atomics Predator and Northrop Grumman Global Hawk, or the rotary wing Boeing FireScout.
Tier I and Tier II UAVs almost never use satellite links because they cant support the size and weight of high-gain tracking antennas, and they often change attitude too rapidly for a tracking antenna to stay locked on the satellite. Line-of-sight, air-to-ground links are the norm for both Tier I and Tier II. Tier III UAVs commonly have satellite links to allow for the rapid dissemination of their ISR content across the globe.
Within the Tiers, there are many additional differentiating factors. For example, some Tier II UAVs focus on maximizing endurance. Endurance requires minimum aircraft weight and maximum flight efficiency, which can often best be achieved at the expense of greater complexity in the required ground equipment. For example, a Tier II UAV might use an RF transmitter with very low output power to minimize its power consumption (and thus conserve fuel). The range penalty resulting from the low power transmitter can be offset by use of a one to two meter parabolic reflector antenna at the ground site.
Larger ground-based launchers enable a lower peak-thrust engine design to save weight. Using ground-based in-flight retrieval devices, aircraft can be designed without landing gear. But such a large and complex ground segment can be an inviting target for an enemy, requiring a greater degree of security than might be needed with a smaller ground footprint.
On the other hand, if shorter endurance is acceptable, the aircraft can be designed to land without assistance from ground hardware. The engine can have sufficient power that the launcher can be very small (or be eliminated), and the aircraft transmitter power can be increased to the point that small ground antennas with relatively low gain work just fine. The resulting UAV and all its support equipment can conceivably be carried in the back of a HumVee while drawing minimal attention, but its endurance will not match the previous example.
Neither approach can be designated as better outside the context of the mission. For some missions, endurance is critical. For others, minimal ground equipment is vital. There are clear niches for both concepts. The UAV ecosystem has many other niches as well for example, for surveillance between buildings in urban canyons, a fixed wing design that must fly in circular orbits is inappropriate and the ability to hover is key. Rotary wing UAVs include the aforementioned Boeing FireScout and the DragonFly
Tailoring To Meet Mission Requirements
Communication systems often require some degree of tailoring for best performance on a given UAV platform. Descriptions of some of the design issues follow.
- Antenna Placement With Tier II UAVs, the number of potential antenna mounting points is limited. Shadowing or blocking of the antenna is routinely a problem, as the single most common mission is flying in circles while monitoring a fixed or slowly moving point below. For many antenna locations, there will be some part of the orbit in which the antenna is shadowed by the aircraft fuselage, resulting in a link dropout. Use of antenna space diversity techniques in the aircraft can greatly improve shadowing performance.
- Self Jamming Almost every UAV has receivers for GPS and as well as for command and control for the payloads and the aircraft navigation systems. Care is required to ensure that the downlink transmitter does not de-sensitize these receivers, which typically involves analysis of transmitter emissions and receiver selectivity as well as care in antenna placement.
- Asymmetric Transmissions For most UAVs the downlink data transmission bit rate is typically much greater than the uplink rate. Coupled with the fact that aircraft power supply constraints limit the amount of transmitter power, this means that the downlink is substantially disadvantaged compared to the uplink. Techniques such as downlink antenna space diversity provide gain in multipath channels, and are one way to recover some of this disadvantage.
- Relay Capability It is easier to acquire good imagery when operating at low altitudes, since there is less atmospheric haze and shorter focal-length lenses are lighter and have less stringent stabilization requirements. Operational altitudes from 2,000-5,000 feet are commonplace, but these altitudes cut line-of-sight range to about 50-100 miles when the terrain is smooth, or even less in hilly or mountainous terrain. Using a second high-flying UAV to relay the ISR data can extend the range substantially.
The relay configuration introduces additional complexity. If the data link uses the common technique of frequency division duplexing to separate the uplink and downlink, it is very difficult for the relay UAV to avoid self jamming unless it can reverse the frequency plan for the link back to the ground station. Moreover, antenna placement on the distant UAV must allow some visibility upward, which may require different antennas from those used when there is no relay.
An important conclusion to be reached from this discussion is that a one-size-fits-all solution to the communication challenge is unlikely. However, a well conceived system approach can provide an interoperable system that can be tailored to a range of specific scenarios.
Spectrum Efficiency and Frequency Management is Vital
All branches of the U.S. armed forces and special operations units plan substantial increases in the number of fielded UAVs in the near future. Analog FM links used on the majority of fielded UAVs consume 20-25 MHz of bandwidth, and channel spacing often has to be even greater than that. It is very difficult to have more than three or four UAVs operating in a given region when bandwidth is consumed in such a wanton fashion.
Realizing the DoD vision of large numbers of UAVs operating simultaneously will require a very different approach. Many UAVs are in the process of the necessary first step: transitioning to digital transmission. But without a holistic view of the spectrum management problem, the industry is likely to find itself confronted by another retrofit cycle in five years.
To date, most of the thought about UAV spectrum efficiency with digital transmission has simplistically focused on the transmitted spectrum, but this is only one facet of a multi-dimensional problem. The UAV community needs to draw from the experiences of other wireless industries with bandwidth constraints, examining all the tools available for spectral efficiency including source coding, efficient modulation, receiver selectivity, antenna directivity, and transmit power control.
The cellular industry has a similar problem to the UAV industry. It is very constrained in bandwidth and needs to space channels as tightly as possible. Cellular engineers have long recognized that the fundamental problem caused by transmissions in the adjacent channel is that some amount of the energy of the unwanted adjacent channel signal makes its way through the receiver filters. This energy creates an effect comparable to additional receiver front end noise, thus degrading the minimum level of the desired signal that can be received with acceptable quality. A spectrally efficient system design uses every means possible to limit the amount of adjacent channel energy that makes its way through the receiver filters. The solution requires much more than just reducing out-of-band emissions in the transmitter spectrum, including these techniques:
- Maximizing receiver selectivity that is, designing the receiver to reject energy outside of its intended passband to the greatest possible extent
- Directional Antennas when the ground station antenna has most of its gain in the direction of the desired transmitter, a potential interferer that is not in the main lobe of the antenna experiences substantial attenuation before it reaches the receiver and thus interferes less
- Power control adaptively reducing the transmitted signal level to no more than what is needed to close the link, with acceptable margin, reduces the amount of interference that a given transmitter can cause
The remaining tactic to minimize channel spacing is an obvious one: the system should send as few bits as possible. Fewer bits implies the use of the most effective compression techniques, but it also includes simple (but not always followed) concepts such as using a modem whose bit rate can be matched to the throughput demand of the mission, rather than running at only one or two rates chosen to grossly overbound the maximum rate required among a broad range of missions. With H.264 compression and a high performance turbo code for FEC, it is easy to transmit high quality 30 fps standard definition video with a 3 Mbps modem bit rate.
Frequency Band Choice
A related problem is the question of what frequency band or bands to use. While the Office of the Secretary of Defense has mandated the use of Ku-band CDL links for Tier II and Tier III UAVs, to date the Tier II UAVs have not made this transition, mainly because of the technical challenge of using Ku-band on an aircraft that demands minimal size, weight and power consumption.
While the OSD goal of providing data link interoperability has undeniable merit, a link budget problem results because steerable antennas are impractical in a small UAV and also because of the losses due to atmospheric effects. Range is extremely compromised at Ku-band compared to what is possible at lower RF frequencies. At L-band (1700-1850 MHz), for example, it is easy to close a 5-10 Mbps link at ranges up to 75 miles with a simple, manually pointed ground antenna. Even C-band (4-6 GHz) provides a far more favorable link budget than Ku-band. Thus many Tier II UAVs use links in L- or C-band as well as S-band. Although there is less bandwidth available at the lower frequencies, if the spectrum is used efficiently, a large number of UAVs can share it.
Data Security Also Driving UAV Data Link Design
FM analog links are easily intercepted by low cost off-the-shelf receivers, so they dont provide even rudimentary data privacy. Even without encryption, digital links provide a substantial step forward in privacy if they are not based on common standards. Commercial grade AES encryption is simple to add to a digital link, and maximum security is afforded by Type 1 military encryption.
The security issue also creates some conundrums that must be addressed. One of the most valuable ISR assets for the warfighter has been the remote video terminal (for example, the ROVER terminal), which is a single multi-band FM video receiver that allows soldiers on the ground to view video from a range of UAVs. RVTs turn the lack of security in the video into an advantage, since multiple users can exploit the ISR without the need to manage the distribution of encryption keys and passwords.
On the other hand, Type 1 encryption creates its own set of issues, since it carries with it a requirement to manage keying material with a level of care that often precludes its use by foreign nationals or contract employees who may be part of the mission team. Provisions for destruction of keying material in the event of compromise make equipment bigger, more power hungry, and more expensive all of which are serious impediments to Type 1 use in Tier I and Tier II UAVs.
The use of commercial grade encryption with password-based keys seems to be a good compromise, but in many cases there are concerns about how secure such a system really is. Some of these concerns may be more cultural than real, but perception can be important in security. And any encryption system will reduce the ability of the RVT user to exploit the video. Ultimately, it seems likely that providing a range of security options will allow tailoring of security to meet the needs of the mission, but there is more work ahead to reach an industry consensus in this regard.
Converging Issues = Solutions
In the last six years, UAVs have demonstrated the ability to provide critical real-time ISR data to warfighters at a fraction of the cost of manned platforms, in spite of the relative immaturity of the systems. Timely and accurate ISR increases the effectiveness of our armed forces and also helps to greatly reduce casualties through improved situational awareness and shortened sensor to shooter chains.
The universal recognition of these benefits has brought the need for improvement in next generation UAV systems into sharp focus. An understanding of the different platforms supporting the range of DoD missions will drive the deployment of many varieties of aircraft, and it will be necessary for these aircraft to be able to cooperate spatially and spectrally while disseminating the ISR from platforms in standardized waveforms and formats that are spectrally very efficient and designed to coexist and interoperate. The user community is recognizing the need for interoperability, but recognition that existing systems may not provide the required spectrum efficiency is still to come. The next few years promise to be interesting as these issues converge to a solution.
About the author
Steve Gardner became General Manager of Enerdyne in December 2007 after serving the company as Chief Technical Officer since May of 2003. Under his technical leadership, Enerdyne is a mainstream provider of digital data links for manned and unmanned applications in ISR. Steve was the architect of the EnerLinksII family full-duplex data link, which has been deployed on a variety of manned and unmanned aircraft and surface vessel platforms. The EnerLinksII family includes the EnerLinksII DVA (Digital Video over Analog FM Links) system that converts legacy analog data links to digital systems using the existing sensors and RF equipment in both the aircraft and ground system. Drawing on nearly 30 years of experience in the field of communication system design, he restructured Enerdynes engineering department to add best-in-class talent in RF hardware, modem design, signal processing and networking design.