[Gmat-users] Optimizing trajectories with GMAT: how to understand the Vary parameters, and how to know what values to use for them?
Hello all, I've been using GMAT in my free time to optimize trajectories, and have varied burn component values and spacecraft states, usually with success. The vary command in GMAT, with the Yukon optimizer that I am using, has the following parameters that can be changed: * Initial value: The initial guess. I know the gradient descent optimization method that GMAT uses is very sensitive to initial conditions and so this must be feasible or reasonably close. * Perturbation: The step size used to calculate the finite difference derivative. * Max step: The maximum allowed change in the control variable during a single iteration of the solver. * Additive scale factor: Number used to nondimensionalize the independent variable. This is done with the equation xn = m (xd + a), where xn is the non-dimensional parameter, xd is the dimensional parameter and this parameter is a. * Multiplicative scale factor: Same as above, but it's the variable d in the equation. For the initial value, I can usually see when my chosen value is feasible by observing the solver window or a graphical display of the orbit in different iterations. The max step is the most intuitive of these parameters for me, and by trial and error, observation of the solver window and how sensitive my target variables are to changes in the control variables I can usually get it right and get convergence. It is still partially trial and error though. However, I do not understand the effect of the other parameters on the optimization. I read a bit about finite difference and nondimensionalization/rescaling, and I think I understand them conceptually, but I still don't understand what values they have to be to get an optimal optimization process. This is especially a problem now because I have started to vary epochs (TAIModJulian usually) or time intervals (e.g. "travel for x days" and find optimal x, or to find optimal launch windows), and I cannot get the optimizer to vary them properly, even when I use a large step size. The optimizer usually stays close to the initial values, and eventually leads to a non-convergence message. I have noticed that using large values for the two scale factors sometimes gives me larger step sizes and occasionally what I want, but it's still trial and error. As far as perturbation goes, I do not understand its influence on how the optimization works. Sometimes for extremely small values I get array dimension errors, sometimes for very large values I get similar results to if I'm using too large a max step size, and that's about it. I usually use 1e-5 to 1e-7 and it seems to work most of the time. So I guess my question is two-fold: how to understand the optimization parameters of GMAT and what they should be in different situations, and what should they be when I want GMAT to consider a wide array of possible trajectories with different values of control variables, especially when those control variables are epochs or time intervals? Is there a procedure or automatic method that takes into account the scale of the optimization problem and its sensitivity, and gives an estimate of what the optimization parameters should be? Best regards, Panagiotis Karakatsanis ___ GMAT-users mailing list GMAT-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/gmat-users
[Gmat-users] How do I prevent infinite propagation into the future, which causes the "requested epoch 95008.513478505 is not on the DE file" error?
Hello all, I have a spacecraft I am trying to get from Earth to Mars. I use an Earth propagator until the SC is out of the Earth's sphere of influence, then a deep space propagator until Mars' sphere of influence, and then I switch to a Mars propagator. The only stopping conditions the propagators have are the distances (i.e. propagate until 924,000 km from Earth with Earth propagator, propagate until from 578,000 km from Mars with deep space propagator, and so on). I want GMAT to decide how much time the spacecraft should spend in each propagator since I don't how many days it would take the SC to go from, e.g., LEO to the end of Earth's SOI, or the edge of Mars' SOI to my desired Mars altitude or periapsis, so the only conditions I give are the desired arrival date to Mars, and the distances in the trip in which it should switch propagators. However, whenever there are no time-dependent stopping conditions in the propagators, GMAT propagates the spacecraft with what I assume is the aim of reducing delta-V as much as possible, leading to the trip lasting hundreds of years, which causes the optimizer to reach the end of GMAT's ephemeris data and the "PlanetaryEphem (sub)class exception: Requested epoch 95008.513478505 is not on the DE file" error. I sometimes get this error when I put impossible stopping conditions in the propagate commands, but this shouldn't be the case here. I have tried adding a second stopping condition to the last propagator in the sequence, being the desired arrival date, or adding a non-linear constraint on the epoch of the SC upon arrival, but the optimizer still tries to consider unrealistic dates. I also tried minimizing total travel time in the cost function, same problem. The mission sequences from my attempts can be seen in this link: https://imgur.com/a/myLEmM6 Finally, I created three variables: earth_prop_time, deepspace_prop_time and mars_prop_time, being the amount of days spent by the spacecraft in each propagator. I added an "ElapsedDays" stopping condition in each propagator, equaling those variables, and then a constraint on their sum being my desired total travel time. I added some plausible initial values to the Vary commands for them, but it does not vary them properly, disregarding my initial values in one iteration and leading to a non-converge message: https://imgur.com/iPKM0M7 So, is there a way to constrain the dates that the optimizer analyzes without specifying time conditions in the propagators? I essentially want the same thing that happens when you specify a time-dependent stopping condition, like ElapsedDays, but with different propagators for each part of the trip. If that isn't possible, and I must always specify how long each part of the trip should be, how can I get those intervals? PS: the commands labelled as "propagate to Mars periapsis" in the pictures are actually "propagate to Mars altitude", where the altitude equals the number I have on the constraint (500 km). Best regards, Panagiotis Karakatsanis ___ GMAT-users mailing list GMAT-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/gmat-users
[Gmat-users] Installing SNOPT
And my last question: I managed to get to the full version of the SNOPT optimizer due to being a student. How do I install it on GMAT? ___ GMAT-users mailing list GMAT-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/gmat-users
