GPS World, May 2009

GPS Modernization SYSTEM DESIGN TEST cating a satellite to the L5 BIU mission was technically feasible the development cost the very tight schedule and the cost of a dedicated launch would have been prohibitive The search for an alternate host for GPS L5 led to the final two GPS IIR M spacecraft still at the Lockheed Martin factory in Valley Forge Pennsylvania SV09 and SV10 featured an auxiliary payload capability that had been designed into all IIR and IIR M spacecraft Fortunately for GPS spectrum managers SV09 and SV10 were not scheduled to carry auxiliary payloads After briefly considering modifying both spacecraft SV09 was selected for the L5 modification However SV09 which would become GPS Mission IIR 20 was a critical element of the GPS constellation sustainment strategy GPS is currently in the process of replacing the Block IIA spacecraft many of which are operating at twice their design life with Block IIRM Å L5 DEMONSTRATION PAYLOAD hardware satellites Every IIR M and subsequent GPS spacecraft will be crucial to this effort to maintain global services to GPS civil and military users Initiating the L5 modifications introduced a risk that the modifications could delay the availability of SV09 for constellation sustainment and a risk that the L5 modifications might interfere with the primary and secondary payload missions The programmatic and technical challenges were quickly addressed but the highest hurdle to clear was the challenge of moving forward with implementation to meet the need date while maintaining launch readiness The need to move out on this effort required that we get started but we also needed to balance the potential needs of the constellation and have the capability to fall back at certain points in the effort Being able to move smartly and expeditiously while also protecting the SV as a launch asset was key stated Col David Madden GPS Wing commander A look ahead to the projected Air Force launch schedule showed that the SV modifications and retest activities would have to be completed by April 2008 In the end once cost schedule and constellation sustainment factors were considered the risk of GPS failing to BIU its L5 filing was deemed higher than the risks posed by the SV09 modification and Lockheed Martin was authorized to proceed with detailed design of the L5 Demo payload in March 2007 L5 Demo Requirements The simple mission objective for the L5 Demo payload to transmit a GPS signal at the L5 frequency by August 26 2009 allowed the project to accept minimal requirements However recognizing the primary responsibility of sustaining the GPS constellation the GPS Wing mandated one overarching requirement ensure that any modifi cations needed to accommodate the L5 payload would not affect the primary GPS mission or SV capabilities The design also featured the ability to return the SV to the basic IIRM design if constellation needs mandated a launch before the L5 modifi cations were completed The first aspect of the Statement of Work was to define the signal characteristics necessary to satisfy the ITU R BIU requirements ITU R filings are mainly concerned with center frequency bandwidth power and satellite orbit Since the L5 Demo signal was not intended for navigation it could be an L5 carrier modulated by a simple spreading code without a navigation data message Like other modernized GPS signals L5 consists of a dataless pilot channel plus a data channel that carries the navigation message Because the data channel is not needed only one dataless Q5 spreading code was chosen from the L5 Interface Specification IS GPS 705 PRN 63 was chosen for the Demo L5 signal Because this code is from ISGPS 705 its autocorrelation and cross correlation properties have been verified to be compatible with the other 62 L5 PRN sequences Use of a hardwired PRN sequence was necessary because it was not known at the beginning of the L5 Demo project which of the satellites in the current constellation with PRNs from 1 to 32 the GPS IIR 20 satellite would replace As the highest PRN number available in IS GPS 705 the chances of another GPS IIF or GPS III satellite ever transmitting L5 PRN 63 essentially are zero since on these satellites the PRNs for all L1 L2 and L5 signals will be the same Hardware Design Once the L5 Demo signal parameters were set the next step was to design an antenna to transmit the signal Initial feasibility studies examined adding single or dual string payload components adding a single L5 dedicated antenna element or utilizing the existing antenna farm Parts availability electrical power thermal control and structural considerations were examined with the ultimate goal of not affecting existing IIR M capabilities Fortunately for the L5 Demo project ITT engineers quickly determined that an existing IIR M qualification transmitter unit originally designed for the L2 frequency would meet the L5 transmitter needs with minimal rework During the feasibility assessment engineers examined whether it would be better www gpsworld com May 2009 GPS World 29