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TUTORIAL: Rotary Joints Mobilize Tethered Aerostat Mobile Platforms

Aerostats have long been considered more spectacle than practical machines. Their low speed and large size have limited them to the role of advertising (banners) and events telecasting (cameras). The advent in fiber optic communication, however, has leased a new life to the gigantic flying machines that are slow and yet stable, large and yet mobile, and high-flying and yet low cost to operate.

Equipped with modern fiber optic based equipments, aerostats are excellent mobile platforms for surveillance, communication, and early warning systems. They can be either sea or land based mobiles systems flying as high as 5 km, much greater reach in altitude than alternatives, such as terrestrial towers. The aerostat system can accommodate active or passive monitoring devices that have unique application to such difficult problems as control and protection of coastal areas and forests. As applied to internet coverage, a system configured to the requirements of a specific area such as a city can be fulfilled quickly with the flexibility of expansion as demands growth. A full range of direct broadcast-to-user services (WIFI, cell phone, TV, AM/FM, mobile radio, etc.) can be provided from the platform, blanketing an area in excess of 130,000 sq km (400 km diameter area).

Aerostat platform’s operation and on-station time is not encumbered by energy-supporting considerations. Operating from this platform, modern electronic and fiber optic equipment gains a substantial advantage in cost-effective area coverage, installation speed, and flexibility for a wide variety of applications. The aerostats are stable in winds below 65 knots (120 km/h). Airborne time is generally limited only by the weather and insignificant routine maintenance downtime.

A typical aerostat mobile system consists of the aerostat, the mooring system, the tether, the payload, and the ground control unit. In order to maintain continuous signal and power passage during accent or descent an electrical and fiber optical rotary joint (FORJ) is used inside the cable drum on the ground level. The cable reel is sitting on top of a turn-table that tracks the spin of the vehicle and follows accordingly (Fig. 1) such that the structure rotates on one axis to permit weather vaning of a deployed aerostat. The movement of the aerostat in azimuth maintains alignment with the turn-table, thus providing ease of operation regardless of the dynamic movement of the aerostat.

In another design, a second rotary joint housed in a mechanical swivel with high tensile load capacity is placed near the bottom of the aerostat to allow the vehicle to spin freely without twisting the cable below (Fig. 2). This approach eliminates the need for the heavy turn-table and the complex tracking system in the ground vehicle. It greatly simplifies the mechanical and control system. The drawback is reduced aerostat payload (~25 kg) due to the presence of the rotary joint and the swivel. However, with the possible payload of 1,000 kg, it seems a small price to pay.

The mobile system typically requires 1-4 fibers, single or multimode, and 4 to 8 electrical circuits with high voltage and high current capacity (1-2 kV and 5-10 A). In reality, almost all telemetry bandwidth requirement can be handled with just one fiber, typically singlemode (SM) by employing WDM (wavelength division multiplexing-2 channels), CWDM (course wavelength division multiplexing-up to 16 channels) (Fig. 3), and circulators (allow bidirectional transmission in each singlemode channel) (Fig. 4). Single-fiber rotary joint is also much smaller and more rugged. It requires smaller slipring and swivel and therefore promises much lighter unit.

WDM, CWDM, and circulators are all standard passive fiber components. Their presence requires no electrical power and poses no adverse effect on the system performance. WDM employs two of three wavelengths, 850/1310 nm for multimode fiber and 1310/1550 nm for singlemode fiber. CWDM requires the optical window of 1310 to 1610 nm depending upon the number of channels used. Circulators are also wavelength specific. They can be made over all CWDM wavelengths. The most common ones are made for 1310 or 1550 nm. Both CWDM and circulator are singlemode devices.

The typical insertion loss of a single-channel and multiple-channel FORJ is 0.1-2 dB and 1-5 dB respectively. WDM, CWDM, and circulator have max insertion loss of 0.7 dB. As one can see, the combination of single-channel FORJ and WDMs or CWDMs will have a total loss figure of 3.4-dB max still lower than that of multiple-channel FORJ. Adding two more circulators in the discussed channel for bi-direction passage brings the total maximum loss to 4.8 dB. The cable weight reduction from 1 or 2 core to multiple cores should not be overlooked either since most aerostat is this application fly 1 km or higher.

WDM and circulators are both cylinder shaped with 3-6 mm diameter and 30-50 mm length. CWDM is typically palm sized (10-13 mm thick) for an 8-channel device and doubled width for the 16-channel device. Their weight is relatively insignificant.

A typical single strand of optical fiber consists of the silica core doped with rare earth element, the silica cladding, and a layer of protective jacket, typically acrylic. The core diameter and its index of refraction relative to the cladding determine the mode structure of the beam. The standard singlemode telecom fiber core diameter is around 8.2 microns. The two most popular multimode fiber diameters are 50 and 62.5 microns.

The selection of fiber primarily depends on the system's current and, more importantly, its future need for bandwidth. Multimode fiber has limited bit rate, up to 100 Mbps for lengths up to 40 km; shorter lengths support higher bit rates (Table 1). Standard singlemode fiber supports up to tera bits per second over 100km without amplification.

If one can be confident that the need for bandwidth is not going to exceed a few hundred megabits-per-second over a short distance in the lifetime of the system, then multimode can be the choice for lower cost. However, if any branching device (such as a coupler) was necessary in the system, singlemode fiber is recommend for its much better stability in branching devices. Keep in mind that singlemode systems have been growing much faster than multimode systems due to their much higher level of upgradeability (more channels of signals in the existing fiber).

Given the outdoor nature of the mobile systems, all rotary joints need to be sealed for weather. The airborne rotary joint and swivel must be compact and light weight. Aircraft aluminum is often selected for structural construction of the mechanical swivel. In one design a 25 kg unit has the pulling capacity of 9 tons.

Fusion splicing or fiber connectors are the two main ways to connect the rotary joint to the tether. Fiber connectors are convenient for system maintenance, repair, and parts replacement while fusion splicing offers much lower insertion loss (<0.05 dB) and refection (<-60 dB). ST connectors are the connector of choice for most outdoor applications (Fig. 5). The typical ST connector insertion loss and refection are 0.1 dB and -40 dB under ideal conditions. Overtime though, their performance will degrade gradually. A tender loving care of the connectors can yield tens or even hundreds of mating cycles without serious performance degradation.

Given the increasing availability of CWDM based E/O media converters; single-channel (SM) fiber based mobile system will become more and more practical. They cost less. The cable’s weight density is low allowing higher altitude flying or increased payload. They are more reliable and long lasting. And finally, they are easier to maintain and replace in the field.

To conclude, rotary joints provide aerostat based mobile systems the freedom of accenting and dissenting without losing connection. The airborne rotary joint adds the second degree of freedom, spin, so the system can be completely free moving in the air without active control. The tethered aerostat is a unique system whose platform capability significantly enhances the use of modern electronic and fiber optic equipment to provide a broad range of cost-effective services with fast turn around time for full system utilization.


Reference

Tethered telecommunications, broadcast, and monitoring systems, Hirl, J. P., Lighter-Than-Air Systems Technology Conference, Palo Alto, Calif., July 11-13, 1979, Technical Papers. (A79-42378 18-01) New York, American Institute of Aeronautics and Astronautics, Inc., 1979, p. 173-180.

Tethered Aerostat Radar System, wikipedia, http://en.wikipedia.org/wiki/Tethered_Aerostat_Radar_System

Tethered aerostat radar system, http://www.acc.af.mil/library/factsheets/factsheet.asp?id=2359, January 2003


Terminology

TARS: Tethered aerostat radar system
STARS: Small Tethered Aerostat Relocatable Systems

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