Marching To C-Band
August 17, 2017 | BY: Paul Cook
By Paul Cook in Aerospace Testing International
The auction of airwaves for use in mobile broadband held in November 2014 was done to support the demand for internet access by smartphones and tablets. Frequencies that were sold included two blocks in the 1,695-1,710MHz band, and four paired sets of frequencies at 1,755-1,780MHz and 2,155-2,180MHz that were traditionally used for telemetry applications both in aircraft flight and missile tests, and on launch platforms. New C-band frequencies were established in the 4,400-4,940MHz, 5,090-5,150MHz and the 5,925- 6,700MHz for telemetry usage.
The Sell-off Effects
With the governmental auctions and sell-off of the telemetry frequency bands, the everyday operation of the flight test community is drastically affected, with reduced operational availability due to increased frequency crowding. The US government responded with new standards for bandwidth-efficient modulation schemes to ‘pack’ more data bandwidth in the remaining telemetry workspace, allowing the private sector to develop hardware in the new C-band allocations for telemetry. The current transition status to C-band is that a dual-band operation is needed for telemetry that must still operate for a short time while the systems are upgraded to C-band.
The telemetry supplier community responded first with spectrum-efficient hardware for both air and ground applications, while the requirements for C-band applications evolved from initial experiments to products actually meeting testing requirements. The landscape for the transition to C-band has changed recently with the increased government spending as the result of the sell-off. Range operations now have the funding for full-range upgrades. The upgrades will support the transition to C-band over the next five years at least.
C-BAND Hardware Discoveries
Testing of the initial C-band hardware demonstrated the impact on link margin caused primarily by cable and connector losses. Increasing the RF power to compensate for the loss also increases the current draw by 20% on average comparing S-band to a C-band 5W transmitter. The aircraft platforms are implementing 20W C-band transmitters in instances that previously flew 10W variants. Tracking accuracies and water vapor effects on the link margins are still being evaluated for C-band, but are viewed as problem areas requiring additional Radio Frequency (RF) power or enhanced performance to obtain good data download reception.
Technology Road Maps
The transition to C-band requires not only multiband operation, but higher RF power as well on the new frequencies. Once the grace period is over, all telemetry will reside in C-band in the continental USA. Higher gain coding schemes have been introduced to the telemetry market to obtain additional link margin. These coding schemes have been used for many years in the launch and space industry segments and may have a new home in flight test.
Commercial RF devices are leading the RF efficiency challenge as the cell industry has increased its frequency dependency. The desire for broadband access fuels the spectrum sell-off both in funding and in the development of RF devices that operate in higher frequency bands with greater efficiency, allowing entry into the telemetry market with increased RF power efficiency and better performance in C-band. The trend of the past 20 years continues as the commercial market develops better devices that are applicable to the telemetry application. The lower cost, higher gain and smaller size eases the transition to C-band.
Suppliers such as TTC are supporting the transition with singleband and multiband to allow the dual-band usage as long as the grace period continues. This allows the end user to gracefully enter into the C-band, and to evaluate the system performance in both bands. TTC also offers the TTS-9689-2 Airborne Dual Band Multimode Transmitter that is programmable in lower L-band (1,435- 1,534MHz) upper L-band (1,700- 1,850MHz), lower C-band (4,400- 4,940MHz) and middle C-band (5,090-5,150MHz). The TTS-9600 provides an RS-232 programming port and a parallel interface port to select the frequency of interest along with modulation type and RF output level. TTC also offers singleband C-band transmitters when the dual-band operation is no longer necessary. A full range of ground receiver equipment is available from lab use to full-range operation. Receivers in both PCI and rack-mounted form factors support all aspects for system testing and support.
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Curtiss-Wright acquired TTC in 2017 - read more here about the acquisition and Curtiss-Wright's expansion in Data Acquisition & FTI Markets