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I have a dream. A GPS/SBAS/QZSS dream ! In honour of MLK day.
I have a dream. All IIA satellites retired. All IIF satellites in
orbit. L2C signal set fully healthy with 19 operational satellites (12 IIF + 7 IIRM). L5 signal set healthy even though only 12 IIF satellites are broadcasting it. But their ranging signals are as perfect as predicted, not enough for stand alone L5 ranging, but who will use L5 without L1 C/A ??? Nobody in their right mind. That would be a dream for Jan 2016 actually. Two IIF launches per year starting this year. Back to reality, with the current status quo, the last IIF launch won't happen before 2020. I'm willing to bet on this. L5 FOC, not before 2030. Even if the doomsday mass IIA failure hypothesis from the GAO happens, that would only mean accelerated IIF launches, but FOC L5 requires another 12 GPS III launches, that will only happen as IIR satellites fail on mass, not likely at all. Back to the dream, a steady IIF launch schedule would first retire all undesirable IIA satellites without a working Rb clock. That would improve average GPS clock performance substantially. Then replace the rest of the IIA, even those with usable Rb clocks and enough buses and reaction wheels working, those would become residuals. A new informal 36 slot orbital scheme would be made. 30-32 active satellites with 4+ residuals. No more pairs due to fear of failing satellites, 6x6 orbital arrangement (today there's 3 primary slots for each orbit, an additional 3 secondary slots would be created half way between each consecutive pair of slots there is today). With 32 healthy satellites optimally spaced out and 4 residuals in the remaining 4 slots, any satellite failure/maintenance is a non-event. 95% of the possible dual satellite failures would also be a non event, with residuals brought online within 24 hrs of the failure event. No need to delta V satellites around, when a satellite fails, just schedule to replace that satellite and leave that slot open until launch. Remember there are today 3 WAAS, 4 EGNOS and 2 MSAS L1/L5 ranging sources, with 3 QZSS on the way. If those sources are considered part of the civil GPS constellation, even with 12 regular GPS L5 signals, an IOC can be declared. Since I'm pretty sure no one will listen... Why not float another crazy idea, err, DREAM. The american aviation authorities poll resouces and licence QZSS and launch a 4 (or perhaps 5) satellite american QZSS, using a flat wide eight figure orbit, and use those satellites to broadcast a replacement WAAS signal, replacing the current GEOs, with an additional 20 reference stations in south america, Caribean and a few more in central america (since Mexico is already covered right now). My understanding is that QZSS is broadcasting an L1 SBAS signal and since each satellite will have a Rb clock, their ranging should be on par with a IIR ranging, except that they broadcast L2C, L5 and L1C from the get go. With their wide orbit, Alaska and northern Canada integrity signals would be improved to perfect, as well as coverage all the way down to Antartica (wide figure 8 orbits are much higher than GPS orbits = wider signal coverage). Users outside the current WAAS coverage are will be required to have L1/L5 GBAS receivers, so they can generate their own IONO corrections = no longer need a station within 250nm. With stations spread all over south america and data exchange with EGNOS and MSAS all healthy GPS satellites should stay at a 3 meter UDRE all the time, with the extra QZSS ranging also at 3 meter UDRE, that would allow for LPV 200 coverage from -90 to +90 longitude in the American side of the globe. Remember that the current 3 GEO WAAS layout (assuming CRW back online on it's original location) uses satellites so close to each other that even with the 3 GEOs at their best ranging accuracy, they're just about as useful as a single GPS satellite for ranging purposes. And since they don't have an internal atomic clock, their ranging is limited to 7.5 meter UDRE instead of the usual 3 meter UDRE of regular GPS satellites. With an onboard Rb clock, hourly clock updates, QZSS ranging could reach better quality than regular GPS satellites due to triple frequency and on board laser reflector, sub inch ephemeris accuracy possible all the time. There, I said it, I have a dream. Suitable for MLK day. Cheers, Marcelo Pacheco |
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