The International Space Station orbital period dictates the exact time between successive observations
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Retrieved literature shows that instruments like the Gas Slit Camera on the ISS complete scans corresponding to the 92-minute orbital period, linking the orbital period directly to successive observations.
The Gas Slit Camera (GSC) is an X-ray instrument on the MAXI (Monitor of All-sky X-ray Image) mission on the International Space Station. It is designed to scan the entire sky every 92-minute orbital period in the 2--30 keV band and to achieve the highest sensitivity among the X-ray all-sky monitors ever flown so far. The GSC employs large-area position-sensitive proportional counters with the total detector area of 5350 cm$^2$. The on-board data processor has functions to format telemetry data as well as to control the high voltage of the proportional counters to protect them from the particle irradiation. The paper describes the instruments, on-board data processing, telemetry data formats, and performance specifications expected from the ground calibration tests.
# ISS: time between two successive observations and orbital period
Tags: iss, orbit
- Score: 10
- Views: 3631
- Answers: 2
- Answered: yes
- Asked by: Hugh Wylam (101 rep)
- Asked: 2014-12-20
- Edited: 2014-12-20
- Site: space
## Question
Just had 2 evenings when the ISS was visible twice on successive passes.
Times for appearance were 94 minutes apart.
If I use the quoted value of g at 400 km above the earth of 0.89m/s/s I can calculate orbit time of 91 minutes which is pretty close BUT it makes no allowance for the earth's rotation in the 90 minutes which is not insignificant - about 22deg. For the ISS to 'catch up' with the earth's rotation surely it should need another 6 minutes or so.
Where is the flaw in my thinking?
## Answers
### Answer by David Hammen (score: 14)
Where is the flaw in my thinking?
This is a good question, but there are a few flaws in your thinking.
If I use the quoted value of g at 400 km above the earth of 0.89m/s/s I can calculate orbit time of 91 minutes.
Your calculation of the ISS' orbital period is a bit off. The ISS is currently (Dec. 20) in a 404 x 415 km orbit, or a mean altitude of 409.4 km. That corresponds to an orbital period of 92
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