Contact time per day
A spacecraft in a 525 km circular Sun-synchronous orbit (, period ) completes revolutions per day[1]. Because the Earth rotates underneath the orbital plane while nodal precession advances the ascending node by , a mid-latitude ground station sees between four and six passes per day above a elevation mask[2].
| Station latitude (°) | Passes per day | Contact per day (min) | Mean pass (min) | Longest gap (h) |
|---|---|---|---|---|
| 0 | 2.4 | 15 | 6.1 | 11.8 |
| 20 | 2.9 | 17 | 5.8 | 11.9 |
| 41.45 | 3.5 | 21 | 6.0 | 13.0 |
| 60 | 5.5 | 32 | 5.9 | 11.3 |
| 78.2 | 11.0 | 71 | 6.5 | 9.5 |
Ten-day propagation with Earth rotation, two-body motion and a spherical Earth. 41.45° represents a mid-latitude reference ground station; 78.2° that of Svalbard.
At latitude and a elevation mask, total daily visibility is roughly , split across four or five passes with a mean duration near . Raising the mask to —necessary when local UHF noise or obstructions degrade low elevations—cuts the usable contact time by more than half.
Protocol framing and ARQ overhead
Not every transmitted bit carries payload data. On amateur UHF links[3], three overheads reduce the net throughput:
- Preamble and synchronization: of alternating bits and a sync word precede each packet so the receiver clock and frequency loop can lock.
- Header and FEC parity: Standard UI frames add of callsigns and control fields ( on payloads), whereas CubeSat Space Protocol (CSP) with Reed–Solomon parity reaches .
- Half-duplex turnaround (): Because a single UHF antenna is shared between uplink and downlink, command acknowledgements and beacon intervals typically limit continuous payload transmission to of the pass above mask.
| Maximum elevation | Above the horizon (min) | Margin ≥ 3 dB (min) | Raw data (kB) |
|---|---|---|---|
| 10° | 9.0 | 0.0 | 0 |
| 20° | 10.5 | 0.9 | 68 |
| 30° | 11.0 | 3.6 | 262 |
| 45° | 11.4 | 4.6 | 329 |
| 60° | 11.5 | 4.9 | 350 |
| 90° | 11.6 | 5.0 | 362 |
Channel bits before framing, coding and protocol overhead; Earth rotation neglected. Low passes, the most frequent kind, deliver little or nothing.
Sizing the daily payload allocation
At with and transmit duty during of daily contact above , a single mid-latitude station receives about . Once housekeeping beacons () are subtracted, a mission can budget for compressed imagery or sensor logs from one primary station, while open receiver networks such as capture beacon frames across all latitudes[4].