5 ms·
> I assume with one gyro the satellite still has pointing function because one gyro senses in 3 axis, Each of the Hubble's gyroscopes only measures rotation ar
by teraflop 8y ago
> I assume with one gyro the satellite still has pointing function because one gyro senses in 3 axis,
Each of the Hubble's gyroscopes only measures rotation around a single axis.
> But... it’s a telescope. Couldn’t it determine direction from images received from a spotting or wide angle lens? Does it not have one?
Hubble does have imaging sensors to allow it to track the stars and orient itself. But the orientation needs to be not only measured very accurately (to within a few millionths of a degree), but also kept stable over long periods of time, in order to avoid motion blur. So the attitude control system needs to have low latency.
In principle it would probably be possible to do this using only image sensors, but it would have been very challenging when that Hubble was designed in the 1980s. It has a number of CPUs, the fastest of which is an 80486 running at 25 MHz.
- therein 8y ago> It has a number of CPUs, the fastest of which is an 80486 running at 25 MHz. Not unexpected but reading that now... Absolutely remarkable.
- Lammy 8y agoSpecifically, a radiation-hardened 486. The type of equipment that can survive in space lags significantly behind what you would consider modern on a desktop computer. For example, the radiation-hardened PowerPC G3 is a popular choice. It will power the 2020 Mars rover too: https://en.m.wikipedia.org/wiki/RAD750 https://en.m.wikipedia.org/wiki/RAD750
- bluedino 8y agoThe 80486 was state of the art in 1990 when the Hubble launched, as it was released in 1989. Was it installed as a later update? The PowerPC G3 was released in Apple computers in 1997, just a few years later.
- Sharlin 8y agoYep, the original main computer was a 1.25 MHz (!) DF-224 [1]. It was augmented on Servicing Mission 1 in 1993 with a 15 Mhz 80386 co-processor [2]. The 80486 was installed in 1999 on Servicing Mission 3A (STS-103) [2]. [1] https://en.wikipedia.org/wiki/DF-224 https://en.wikipedia.org/wiki/DF-224 [2] https://en.wikipedia.org/wiki/STS-61 https://en.wikipedia.org/wiki/STS-61 [3] https://en.wikipedia.org/wiki/STS-103 https://en.wikipedia.org/wiki/STS-103
- ams6110 8y agoReading more about the DF-224... It was roughly 45 centimeters (1.48 ft) by 45 centimeters (1.48 ft) by 30 centimeters (0.98 ft), weighed 50 kilograms (110 lb) It had about 96KB of usable plated-wire memory (32K 24-bit words). That seems like a lot of weight. I wonder how much was radiation shielding?
- stephengillie 8y agoSo not a data center, but like an Arduino.
- uncletammy 8y ago> But the orientation needs to be not only measured very accurately (to within a few millionths of a degree) The fact that we can even achieve this level of precision from a piece of hardware blows my mind.
- ISL 8y agoOne need not have something as expensive as Hubble to have such angle sensitivity. This instrument can resolve an angle change of < 0.3 millionths of a degree in one second. https://arxiv.org/abs/1309.4828 https://arxiv.org/abs/1309.4828 Disclaimer -- am an author. Edit: and this instrument, using the autocollimator above, can do the same thing for inertial sensing. Same disclaimer applies. https://aip.scitation.org/doi/10.1063/1.4862816 https://aip.scitation.org/doi/10.1063/1.4862816