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1. A computer-implemented method comprising: receiving pressure telemetry data and pressurant temperature telemetry data for a propellant tank of a satellite over a time interval, the propellant tank having a known geometry and containing an amount of propellant and a volume of a gas-phase pressurant, a rate of propellant consumption in support of satellite operations over the interval being sufficiently small to create a quasi-isochoric condition for the pressurant over the time interval such that an actual pressure in the propellant tank is proportiona...
1. A computer-implemented method comprising: receiving pressure telemetry data and pressurant temperature telemetry data for a propellant tank of a satellite over a time interval, the propellant tank having a known geometry and containing an amount of propellant and a volume of a gas-phase pressurant, a rate of propellant consumption in support of satellite operations over the interval being sufficiently small to create a quasi-isochoric condition for the pressurant over the time interval such that an actual pressure in the propellant tank is proportional to an actual pressurant temperature in the propellant tank over the time interval;calculating a metric relating the volume of the pressurant, the pressure telemetry data and the pressurant temperature telemetry data, the metric calculation comprising reconstructing, based on the received pressurant temperature telemetry data, a predicted set of pressure data for the time interval and comparing the predicted set of pressure data to at least part of the pressure telemetry data, the reconstructing comprising assuming a direct proportionality between the predicted set of pressure data and the pressurant temperature telemetry data;solving for a most likely estimate of the volume of the pressurant to optimize the metric; anddetermining, based on the most likely estimate of the volume of the pressurant and the known geometry of the propellant tank, the most likely amount of propellant remaining in the propellant tank;wherein the receiving, calculating, solving and determining are performed by at least one system comprising computer hardware. 2. A computer-implemented method as in claim 1, further comprising: commanding that the satellite operate a valve isolation system to isolate the propellant tank to disallow use of propellant from the propellant tank during the time interval. 3. A computer-implemented method as in claim 2, further comprising: detecting, over the time interval, that the most likely estimate of the amount of propellant remaining in the propellant tank is not constant; anddetermining, for the time interval, a drift rate associated with a pressure transducer on the satellite that generates at least part of the pressure telemetry data, the drift rate being determined to best account for the detected non-constant amount of propellant remaining in the propellant tank. 4. A computer-implemented method as in claim 3, further comprising: performing the calculating, the solving, and the determining of the most likely amount of propellant remaining in the propellant tank over an additional iteration, the additional iteration comprising inclusion of the drift rate in the determining of the most likely amount of propellant remaining in the propellant tank. 5. A computer-implemented method as in claim 1, wherein the propellant tank is not isolated during the time interval. 6. A computer-implemented method as in claim 5, further comprising: detecting, over the time interval, at least one deviation between the most likely estimate of the amount of propellant remaining in the propellant tank and an estimate of the rate of propellant consumption in support of known satellite operations; anddetermining, for the time interval, a drift rate of a pressure transducer associated with a pressure transducer on the satellite that generates at least part of the pressure telemetry data, the drift rate being determined to best account for the detected at least one deviation. 7. A computer-implemented method as in claim 6, further comprising: performing the calculating, the solving, and the determining of the most likely amount of propellant remaining in the propellant tank over an additional iteration, the additional iteration comprising inclusion of the drift rate in the determining of the most likely amount of propellant remaining in the propellant tank. 8. A computer-implemented method as in claim 1, further comprising: determining an amount of inaccessible propellant in the propellant tank, the determining of the amount of inaccessible propellant comprising estimating a location of a spin axis of the satellite based on an estimation of the moment of inertia of the satellite using on-orbit telemetry data, and estimating a distance from the spin axis to a center of the propellant tank. 9. A computer-implemented method as in claim 8, wherein the estimating of the location of the spin axis of the satellite comprises observing a change in the moment of inertia of the satellite in response to a known amount of consumed propellant. 10. A computer-implemented method as in claim 1, wherein the calculating further comprises correlating the pressure telemetry data and the pressurant temperature telemetry data in at least part of the time interval. 11. A computer program product comprising a machine-readable medium storing instructions that, when executed by at least one processor, cause the at least one processor to perform operations comprising: receiving pressure telemetry data and pressurant temperature telemetry data for a propellant tank of a satellite over a time interval, the propellant tank having a known geometry and containing an amount of propellant and a volume of a gas-phase pressurant, a rate of propellant consumption in support of satellite operations over the interval being sufficiently small to create a quasi-isochoric condition for the pressurant over the time interval such that an actual pressure in the propellant tank is proportional to an actual pressurant temperature in the propellant tank over the time interval;calculating a metric relating the volume of the pressurant, the pressure telemetry data and the pressurant temperature telemetry data, the metric calculation comprising reconstructing, based on the received pressurant temperature telemetry data, a predicted set of pressure data for the time interval and comparing the predicted set of pressure data to at least part of the pressure telemetry data, the reconstructing comprising assuming a direct proportionality between the predicted set of pressure data and the pressurant temperature telemetry data;solving for a most likely estimate of the volume of the pressurant to optimize the metric; anddetermining, based on the most likely estimate of the volume of the pressurant and the known geometry of the propellant tank, the most likely amount of propellant remaining in the propellant tank. 12. A computer program product as in claim 11, wherein the operations further comprise: commanding that the satellite operate a valve isolation system to isolate the propellant tank to disallow use of propellant from the propellant tank during the time interval. 13. A computer program product as in claim 12, wherein the operations further comprise: detecting, over the time interval, that the most likely estimate of the amount of propellant remaining in the propellant tank is not constant; anddetermining, for the time interval, a drift rate associated with a pressure transducer on the satellite that generates at least part of the pressure telemetry data, the drift rate being determined to best account for the detected non-constant amount of propellant remaining in the propellant tank. 14. A computer program product as in claim 13, wherein the operations further comprise: performing the calculating, the solving, and the determining of the most likely amount of propellant remaining in the propellant tank over an additional iteration, the additional iteration comprising inclusion of the drift rate in the determining of the most likely amount of propellant remaining in the propellant tank. 15. A computer program product as in claim 11, wherein the propellant tank is not isolated during the time interval. 16. A computer program product as in claim 15, wherein the operations further comprise: detecting, over the time interval, at least one deviation between the most likely estimate of the amount of propellant remaining in the propellant tank and an estimate of the rate of propellant consumption in support of known satellite operations; anddetermining, for the time interval, a drift rate of a pressure transducer associated with a pressure transducer on the satellite that generates at least part of the pressure telemetry data, the drift rate being determined to best account for the detected at least one deviation. 17. A computer program product as in claim 16, wherein the operations further comprise: performing the calculating, the solving, and the determining of the most likely amount of propellant remaining in the propellant tank over an additional iteration, the additional iteration comprising inclusion of the drift rate in the determining of the most likely amount of propellant remaining in the propellant tank. 18. A computer program product as in claim 11, wherein the operations further comprise: determining an amount of inaccessible propellant in the propellant tank, the determining of the amount of inaccessible propellant comprising estimating a location of a spin axis of the satellite based on an estimation of the moment of inertia of the satellite using on-orbit telemetry data, and estimating a distance from the spin axis to a center of the propellant tank. 19. A computer program product as in claim 18, wherein the estimating of the location of the spin axis of the satellite comprises observing a change in the moment of inertia of the satellite in response to a known amount of consumed propellant. 20. A computer program product as in claim 11, wherein the calculating further comprises correlating the pressure telemetry data and the pressurant temperature telemetry data in at least part of the time interval. 21. A system comprising computer hardware configured to perform operations comprising: receiving pressure telemetry data and pressurant temperature telemetry data for a propellant tank of a satellite over a time interval, the propellant tank having a known geometry and containing an amount of propellant and a volume of a gas-phase pressurant, a rate of propellant consumption in support of satellite operations over the interval being sufficiently small to create a quasi-isochoric condition for the pressurant over the time interval such that an actual pressure in the propellant tank is proportional to an actual pressurant temperature in the propellant tank over the time interval;calculating a metric relating the volume of the pressurant, the pressure telemetry data and the pressurant temperature telemetry data, the metric calculation comprising reconstructing, based on the received pressurant temperature telemetry data, a predicted set of pressure data for the time interval and comparing the predicted set of pressure data to at least part of the pressure telemetry data, the reconstructing comprising assuming a direct proportionality between the predicted set of pressure data and the pressurant temperature telemetry data;solving for a most likely estimate of the volume of the pressurant to optimize the metric; anddetermining, based on the most likely estimate of the volume of the pressurant and the known geometry of the propellant tank, the most likely amount of propellant remaining in the propellant tank.
