NASA Technical Reports Server (NTRS) 20060014041: Applications of Clocks to Space Navigation & "Planetary GPS"

6.7 MB·English

by  NASA Technical Reports Server (NTRS)

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Applications of Clocks tH GPS determines user position via“triangulation” to three GPS satellites The user’s GPS receiver compareseach satellite’s unique pseudorandomcode to models stored in the receiver tomeasure the time delay, and hencedistance, to each GPS satellite –Requires that all GPS satellites be“synchronized” to “same” time –User does not need a good clock: afourth GPS measurement determinesthe user time offset from “GPS time” Each GPS satellite continuouslybroadcasts its ephemeris and offsetfrom “GPS time,” which is definedprecisely relative to highly stableground clocks. With these data, a GPSuser receiver can in real-time uniquelydetermine its location (and time offsetfrom “GPS time”) by tracking four GPSsatellites
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Applications of Clocks to Spa How GPS perational Control Segment CS consists of one Master Con hriever AFB in Colorado Spri ations at the MCS, Hawaii, Kw d Ascension Island. The stati nging data from all GPS in vie timates and predicts each sat d clock parameters and perio each GPS for re-transmission essage. PS ephemeris/clock uploads ar veral hours. During that time p hemeris degrades only moder GPS are in relatively high-altitude, well-be GPS all carry precise (atomic) clocks Knowledge of GPS time is maintained throclocks at the ground tracking sites
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Applications of Clocks to Spa How GPS e Design Trade for GPS. Th orporated … Accurate (atomic) clocks at operatio and onboard the GPS satellites A 24/7 global tracking network to ac determine and update GPS orbits a In contrast: the GPS user typically c equipment and does not require a g e U.S. government elected to able GPS space and ground/ rastructure), thus enabling th llions of users) to carry relati eap user equipment ughly $12B to develop GPS (
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Applications of Clocks to Spa GPS Perfor one commercial receiver (handh eters real-time positioning. erformance is limited by GPS clock & orbi rcial receiver with differential ser ters real-time positioning. Requires lce subscription. erformance is limited by GPS clock & orbi cision global differential GPS (G m real-time positioning accuracy. Rece from commercial partner. etwork processing improves orbits and el l-time (minutes to days) geodeti r than 1-cm non-real-time positioninork data + special software. etwork processing improves orbits and eli
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Applications of Clocks to “In Situ” Trackin e MER mission dramaticallye high value of having navigecom “infrastructure” durinallenging surface operationher planetary bodies hat is the best approach for r the Moon? Light travel time to Mars is tens of monly about 1 sec to the Moon •Importance of autonomy Some Earth-orbiting GPS can be useracked at Moon
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& “Planetary GPS”Jet Propulsion Laboratory California Institute of Technology s Infrastructure Other Mars Orbiter or low Mars orbit vehicle can bewith 1-way Doppler mance depends on user’s clock ocks, 1 orbiter only: 3 km in 24 hrs ocks, 1 orbiter only: 300 m in 24 hr cking two orbiters simultaneously,dence on user’s clock is reducedminated nd angle-sensing may improvemance or robustness e challenging if target vehicle isuvering Lichten - 10
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