GPS World, June 2009

INNOVATION GPS Modernization Hyperbolic sub reflector 4 m Sub reflector Å FIGURE 1 GNSS verification and analysis facility with 30 meter high gain antenna at Weilheim Germany 185 190 195 200 205 210 215 220 225 230 Spectral flux density dBW m 2 Hz 1 1155 1160 1165 1170 1175 1180 1185 1190 1195 Å FIGURE 3 L5 spectrum plot from data recorded on April 29 the GPS satellites LNA Antenna L band gain 50 dB Antenna beamwidth 05 Position accuracy 0001 each direction Max rotational speed 15 s 1 azimuth Online calibration system Hyperbolic reflector 30 m New L band feed Frequency MHz Although the Gore announcement proposed to introduce both of the new civil signals with the launch of the Block IIF satellites the addition of the L2C signal to the legacy signals was deemed a relatively straightforward task and the decision was made to modify the last eight Block IIR satellites for the provision of L2C The first modernized Block IIR satellite was launched on September 26 2005 and seven of these satellites are now in orbit The frequency selected for the L5 signal 117645 MHz is in a protected aeronautical radionavigation services ARNS band This frequency as with frequencies used by all satellite operators had to be coordinated with the International Telecommunication Union Radiocommunication Sector ITU R The ITU R registers frequencies essentially on a firstcome first served basis but a user must actually transmit signals on the assigned frequency from the designated satellite orbit type within seven years from the 10 s 1 elevation Vector signal analyzer Approx 108 samples s 1 PC for data storage and analysis Set of BP filters LNA 30 dB date of filing with ITUR This meant that L5 signals had to be transmitted before August 26 2009 to avoid the potential claim of the frequency by a different country A decision was made to modify an existing Block IIR M satellite to carry an L5 demonstration payload The L5 demo payload which was developed by Lockheed Martin and its subcontractors was 40 dB developed by DLR Measured Theoretical added to space vehicle number SVN 49 SVN49 was launched on March 24 2009 the seventh modernized Block IIR satellite to be placed in orbit Also known as PRN1 from the primary pseudorandom noise PRN codes assigned to the satellite the satellite began L5 transmissions on April 10 at 11 58 UTC and so satisfied the ITU R filing requirement with a few months to spare The L5 Signal Structure The structure of the future full L5 signal will differ significantly from the legacy L1 signal or even the modernized L2C signal It is fully described in the Navstar GPS L5 interface document IS GPS 705 We present just a brief overview of the signal here Two Component Signal The full L5 signal will offer two signal components one with and one without a superimposed navigation data message The two signal components in phase I and quadrature Q have equal power Both will 0 0 30 60 30 300 330 90 270 210 60 240 120 150 180 Å FIGURE 2 Skyplot of SVN49 pass at Weilheim Germany on April 29 2009 have a minimum received power of 157 dBW Each component is modulated with a different but synchronized L5 PRN code The in phase component the I or data channel is further modulated with a 100 symbol per second sps symbol stream carrying the navigation message data and the quadrature component the Q or data free channel also called the pilot channel is modulated only with a PRN code Different nearly orthogonal PRN codes referred to as I5 and Q5 are used in the two components to prevent tracking biases by making each component completely independent of the other except for the underlying carrier phase Another novel aspect of the L5 signal design is the use of Neuman Hoffman NH synchronization codes Code Structure As previously mentioned the I5 and Q5 channels are modulated with different PRN codes These codes differ significantly from the C A P and L2C codes used on L1 and L2 both in length and chipping rate The natural code chipping rate frequency of 1023 MHz as provided by the SV atomic frequency standards satisfies a number of requirements for a modernized signal within the bandwidth constraints increased bandwidth efficiency improved signal accuracy immunity to waveform distortion and improved rejection of narrowband interference The bandwidth constraints include rejection of out of band interference Accordingly a 1023 megachip per second Mcps chipping rate 10 times that of the C A and L2C codes was adopted for the L5 PRN codes Improved Cross Correlation There is a trade off between code period and the capability to do direct acquisition A GPS World June 2009 www gpsworld com 50