Data budget
Spacecraft Data Budget
Can the ground link keep up? Balance how fast a satellite makes data against how fast it can get it down — and watch onboard storage fill in between.
// payload + housekeeping generation vs downlink capacity over ground contacts. daily data balance, storage saturation time, break-even rate / contacts. orbit-mean trade study. flight design needs a full link budget + contact-schedule simulation.
How this model works & what it omits
Every Earth-observation or science satellite faces the same arithmetic. The payload generates data continuously — an imager, a radar, an RF survey receiver — while the spacecraft can only get that data to the ground during the few minutes per pass that a ground station is in view. A data budget (Space Mission Analysis and Design (SMAD), Ch. 13 — communications architecture; Ch. 9 — mission operations) checks whether the second number can keep up with the first, and sizes the onboard mass-memory that buffers the gap.
Data generation is the sum of two streams: the payload data rate (the science or imaging stream) and the housekeeping rate (bus telemetry — voltages, temperatures, attitude, fault counters). Generation runs around the clock, so data generated per day is simply (payload + housekeeping) × 86 400 s. For a 5 Mbit/s imager that is roughly 432 Gbit every day.
Downlink capacity is bounded by contact opportunities. A single ground station in low Earth orbit (LEO) sees a satellite for only a handful of passes per day, each lasting a few usable minutes. The day's downlinked data is contacts/day × usable-minutes × 60 × downlink-rate × efficiency. The efficiency factor (~0.75 nominal) discounts the channel time lost to pass acquisition, ranging, framing, and forward-error-correction overhead before useful payload bits flow. Contacts can be entered directly per day, or per orbit and scaled by the orbits-per-day the period implies.
The daily balance is generation minus downlink. A non-positive balance means the link keeps up — onboard storage never saturates. A positive balance means a backlog accumulates: onboard storage fills at the balance rate until it hits capacity, after which new data is lost or overwrites the buffer. The storage-vs-time chart plots exactly that fill curve against the capacity ceiling. Two break-even figures close the trade: the minimum downlink rate, and the minimum contacts per day, that each drive the balance to zero with the other inputs held fixed.
What this tool does not capture: per-pass elevation-angle and slant-range variation, weather outages, station scheduling conflicts, variable-rate adaptive coding, payload duty-cycling (most imagers do not run continuously), data compression, and the difference between a single station and a ground-station network. It is an orbit-mean trade study to size storage and sanity-check a contact plan early — flight design needs a full link budget and a contact-schedule simulation.
// pick a mission profile, then dial data rates / contacts / storage.
Orbit
// circular orbit; period derived from altitude or supplied directly.
period 94.6 min · 15.2 orbits/day
Data generation
// payload + housekeeping streams, summed.
Downlink channel
// ground link rate × usable fraction after overhead.
Ground contacts
// contacts supplied per day or per orbit.
Onboard storage
// mass-memory capacity available for buffering.
Data budget
// 6.0 contacts/day · 48.0 min total contact
// link verdict
Downlink falls behind — storage saturates in 1.1 days
The spacecraft generates more data each day than the contact schedule can clear. The backlog grows until onboard mass-memory fills, after which new data overwrites or is lost.
440.6 Gbit
Data generated / day
216.0 Gbit
Data downlinked / day
+224.6 Gbit
Daily balance
1.1 days
Storage fill time
204.0 Mbit/s
Break-even downlink rate
12
Break-even contacts / day
29.0 Gbit
Data per orbit
5.10 Mbit/s
Generation rate
// onboard storage fill vs time
// shareable URL encodes every input. no backend.
// ai-generated breakdown of what these numbers mean — with diagrams.
References
- // Wertz, J. R., Everett, D. F., Puschell, J. J. (eds.) (2011). Space Mission Engineering: The New SMAD, ch. 13 — Communications Architecture.
- // Wertz, J. R., Larson, W. J. (eds.) (1999). Space Mission Analysis and Design, 3rd ed., ch. 9 — Mission Operations (data-rate & ground-contact analysis).
- // CCSDS 130.0-G — Overview of Space Communications Protocols.
- // Vallado, D. A. (2013). Fundamentals of Astrodynamics and Applications, 4th ed., ch. 2 — circular-orbit period law.