How much data can a PocketQube download per day?

Contact time per day#

A spacecraft in a 525 km circular Sun-synchronous orbit ($i\approx 97.5^{\circ }$, period $95.1\text{min}$) completes $15.14$ revolutions per day[1]. Because the Earth rotates underneath the orbital plane while $J_2$ nodal precession advances the ascending node by $0.9856^{\circ }/\text{day}$, a mid-latitude ground station sees between four and six passes per day above a $10^{\circ }$ 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° is the latitude of the ZBEÜ station that tracked Grizu-263A; 78.2° that of Svalbard.

At $40^{\circ }$ latitude and a $10^{\circ }$ elevation mask, total daily visibility is roughly $25\text{min}$, split across four or five passes with a mean duration near $5.5\text{min}$. Raising the mask to $20^{\circ }$—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:

$$V_{\text{net}}=R_b\cdot T_{\text{contact}}\cdot {\eta }_{\text{frame}}\cdot (1-\text{FER})\cdot d_{\text{TX}}$$

• Preamble and synchronization: $8\text{-}16\text{B}$ of alternating bits and a $4\text{B}$ sync word precede each packet so the receiver clock and frequency loop can lock.
• Header and FEC parity: Standard AX.25 UI frames add $16\text{-}20\text{B}$ of callsigns and control fields ($et{a}_{\text{frame}}\approx 0.78$ on $128\text{B}$ payloads), whereas CubeSat Space Protocol (CSP) with Reed–Solomon $(255,223)$ parity reaches ${\eta }_{\text{frame}}\approx 0.85$.
• Half-duplex turnaround ($d_{\text{TX}}$): Because a single UHF antenna is shared between uplink and downlink, command acknowledgements and beacon intervals typically limit continuous payload transmission to $70\text{-}85\%$ 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 $4800\text{bit/s}$ with ${\eta }_{\text{frame}}=0.82$ and $80\%$ transmit duty during $22\text{min}$ of daily contact above $10^{\circ }$, a single mid-latitude station receives about $520\text{kB/day}$. Once housekeeping beacons ($\approx 40\text{kB/day}$) are subtracted, a $1\text{P}$ mission can budget $350\text{-}450\text{kB/day}$ for compressed imagery or sensor logs from one primary station, while open receiver networks such as SatNOGS capture beacon frames across all latitudes[4].