* Posts by jimlux

2 publicly visible posts • joined 4 Oct 2021

NASA advised to study up on what open source, free software, and permissive licenses actually mean


Software is tricky

Software developed entirely by government employees is usually not copyrighted. Software developed by a NASA contractor might well be covered by copyright. Typically, what happens is that NASA gets a "limited non-exclusive government use" license. That is, NASA can use it, other government entities can use it, but the general public cannot. This comes about because NASA is saving money - they're paying to solve a NASA problem, not the general public's problem, and companies are free to charge more for a "free distribution no license" scenario.

It's also complicated by things like the Bayh-Dole Act, which says that educational institutions doing work for the government retain the intellectual property rights.

In general, though, one can get non-commercial use licenses fairly easily.

That all said, the whole area is one where there's a lot of inconsistency - and it gets muddled when you're talking about software that is the combination of multiple sources, each with different licenses. The "easy way" is to not redistribute such software - no redistribution, no worries about whether GPL applies, etc.

It's about time! NASA's orbital atomic clock a boon for deep space navigation – if they can get it working for long enough


Re: Why is that accuracy necessary?

No computers involved in the round trip measurement. Deep Space spacecraft fly what's known as a coherent transponder. It takes the input signal (e.g. at 7.15 GHz) and generates an output signal at precisely 880/749 * input frequency (usually around 8.45 GHz). Or, at X band up (7.15) and Ka-band down (at ~32 GHz) or other combinations. On the ground, the phase of the received signal is compared with a reference from a hydrogen maser which was used to generate the transmitted signal. Typically, over a path to, say, Jupiter, the time delay (and hence the range) can be resolved on the order of cm, and the range rate (Doppler) to mm/s. That is, on a billion kilometer path (1e12 meters), phase changes corresponding to a few cm can be measured - call it 1 part in 1E14.

The time delay through the transponder and the rest of the radio system is measured on the ground, before launch, over temperature, so that factor can be taken into account.