Microwave Space Equipment Design for Production: The Globalstar2 Challenge
Suriani, A.; Barletta, F.; Basile, G.; Belperio, F.; Mannocchi, G.; Ranieri, P
Thales Alenia Space Italia S.p.A

The second generation of the Globalstar constellation foresees the launch starting in 2009 of 48 low earth orbit satellites providing mobile satellite voice and data services complementing and gradually replacing the present constellation in orbit since 1998 . All of the industrial entities of Thales Alenia Space and partners involved on the project are committed to the unprecedented challenge of production rates peaking up to one satellite per week while maintaining a quality level compatible with a guaranteed 15-year in-orbit operation, i.e. twice the requirement of its predecessors .

The contribution of Thales Alenia Space Italia to the microwave on board equipment comprises 1) the L band LNA and filter assemblies of the active antenna for the return (user to gateway) L to C transponder; 2) the C band LNAs of the forward (gateway to user) C to S transponder; 3) the C band frequency generator units for both forward and return transponders; and 4) the C band telemetry transmitters. This paper describes and motivates the technical solutions adopted to face such a challenge.

The L band LNA assemblies foresee the highest production volumes, with 52 radiating elements per antenna per satellite, thus 2444 deliverable LNAs: key performance are noise figure < 1.6 dB at 1.6 GHz, filter rejection of the 2.5 GHz transmit band > 65 dB, and a gain and phase tracking within 0.2 dB and 3 degrees for a lot of 52 LNA+filter assemblies. The tradeoffs in designing for cost and producibility led to a solution where the LNA is a macrohybrid composed of a single ceramic Al2O3 substrate in a kovar hermetic housing, with blindmate RF and DC interfaces. The microwave lineup consists of a balanced amplifier employing 0.18 um E/D HEMT MMICs, which has the advantage of avoiding costly isolators; a distributed passive network on Al2O3 realizing a select-on-test attenuator and phase shifter where a computer-controlled microwire bonder optimizes the connection to match the lot average; an output amplifier MMIC and a positive thermal coefficient thin film attenuator to achieve the gain stability. Simplicity was the key for this development that has proven to be well producible in the lot already delivered for the EQM antenna.

The development of the Frequency Generation Unit is not less challenging in that each unit comprises 32 microwave oscillators to be locked to a common crystal reference, thus 1536 deliverable PLLOs: key requirements are phase noise < 1.2 degrms at 7.59-7.74 GHz and 5.29-5.44 GHz, and frequency stability given by a GPS PPS-locking loop. The most important design tradeoffs regarded the microwave active devices which were selected as InGaP/InGaAs HBT MMICs, the only mature technology capable of complying with the requirement with an on-chip resonator, thus avoiding use of dielectric resonators which posed packaging concerns and would have forced to a lengthy assembly process. The selected HBT VCOs were successfully subjected to severe life-testing in representative hybrids withstanding beyond 4000 hours operation at 175°C case temperature. In terms of packaging technology, each PLLO stands on a single Al2O3 substrate which integrates MMIC VCO, CMOS-SOS prescaler and integer-N PLL synthesizer, plus all of the loop circuitry, the regulating and biasing circuitry, and the HF reference buffer. Four PLLOs are collectively housed in an aluminum hermetic housing along with redundancy combiners. Eight of these quad-oscillators plus the dual redunded reference oscillator module compose the FGU. Special care was devoted in speeding up the assembly process by eliminating all manual wire soldering, via use of multisection rigid-flex PCBs for the reference oscillator module and for all module interconnects: this approach results in a very compact unit weighing < 5 kg .

The telemetry transmitter belongs to a new generation of this class of products, organized as complex macrohybrids which integrate VCO, fractional-N PLL, direct modulator and power amplifier in a single cavity: use of the fractional-N synthesizer enables in the specific application to externally hardwire the selected downlink frequency, so that production can be independent of the specific spacecraft where the transmitter will be mounted. The required volumes (96 to be delivered) are comparable to the entire worldwide business of telemetry transmitters for communication satellites in the period, and as such an industrialization effort was necessarily spent to reduce parts count, manual interconnects and tuning, by extensive use of MMICs.

Finally, the C band LNAs operating at 5050-5250 MHz were also developed as single-cavity macrohybrids: the criticality of a fairly high (>60 dB) specified gain drove the packaging design to a solution with internal field-confining walls and partial covers. The moderate bandwidth allowed to achieve input matching without a costly isolator while yielding a 1.6 dB noise figure performance, via use of a 0,18 um E/D HEMT MMIC amplifier. The volumes required are moderate (3 per spacecraft, i.e. 144 chains) but still significantly higher than the standard C band market.