The Global Positioning System: A Shared National Asset (1995)
Chapter: Findings and Recommendations
Findings and Recommendations
The NRC committee determined that the addition of a new, L-band signal, L4, offers civilian users much improved precision in many reception environments as well as preserving selective denial options for the military. The NRC committee anticipates that domestic suppliers of commercial GPS receivers, who also are the suppliers of dual-frequency military receivers, would enjoy some advantage over foreign competitors in providing dual-frequency civilian receivers.
The NRC committee believes that the L4 signal could be added to several Block IIR spacecraft using the existing volume and power on the Block IIR spacecraft. If it is assumed that the L4 signal transmits at a radiated power similar to the L1 or L2 signals, then approximately 180 watts of DC power is required.25 The exact amount of power however, will depend on the specific frequency selected for L4. Since the current Block IIR L-band (L1, L2, and L3) navigation payloads and harnesses weigh around 160 kilograms (353 lbs), the L4 signal generation system is expected to weigh approximately one-fourth to one-fifth that amount.26 Based on information provided to the NRC committee through various presentations, it is believed that the sufficient power for an additional frequency can be made available on the Block IIR spacecraft by utilizing the currently unused Reserve Auxiliary Payload power margin, and by re-definition and re-allocation of other existing margins.
In order to add a new signal, several Block IIR hardware modifications are required, including the addition of a frequency synthesizer, modulator/intermediate power amplifier, a high-power amplifier, and a payload processor.27 The NRC committee believes that adequate space for this additional hardware currently exists on the Block IIR spacecraft. Based on cost information for the current Block IIR L-band navigation package, the committee believes that the addition of another, unencrypted L-band signal would cost approximately $1.3 million per Block IIR satellite.28
Immediate steps should be taken to obtain authorization to use an L-band frequency for an additional GPS signal, and the new signal should be added to GPS Block IIR satellites at the earliest opportunity.
