Payloads

Definition

The payload is the reason a spacecraft flies; everything else is the bus that keeps it alive. In small satellites the line between the two is often blurred: on an amateur-radio PocketQube the radio is both the communication subsystem and the service the mission provides, and on a technology demonstrator the "payload" may be a new attitude sensor or a new board stack.

What PocketQubes have carried

Technology demonstration dominates the PocketQube population, often combined with amateur-radio services or education[1]. Within that envelope the class has flown a surprising range of payloads[2]:

Open payload designs

Open hardware has lowered the entry barrier. The IEEE Geoscience and Remote Sensing Society Open PocketQube Kit, developed at the UPC NanoSat Lab, provides a complete 1P design with power, computer, communications and ADCS boards, together with example payloads such as a VGA camera and L- and K-band radio-frequency-interference monitors[3].

Volume

A 1P board has about 42 × 42 mm of usable area once the integration margin to the 50 mm walls is subtracted, roughly a fifth of a PC/104 CubeSat board. Power, computer and radio usually take most of the stack, so payloads are either a single board or functions that share boards with the bus. Multi-unit formats buy volume: the 3P Unicorn-2 carries a camera and reaction wheels that no 1P could hold[4].

Power

The reference 1P power budget of nadir lab. gives its payload 120 mW at a 10% duty cycle, 12 mW on average, against an orbit-average generation of about 550 mW:

Load	Power when on	Duty cycle	Mean
On-board computer (mostly low-power mode)	15 mW	100%	15.0 mW
UHF receiver (continuous listening)	30 mW	100%	30.0 mW
UHF transmitter (0.5 W RF, 40% PA efficiency)	1,250 mW	4%	50.0 mW
Magnetometer and IMU	10 mW	100%	10.0 mW
Magnetorquer coils	60 mW	15%	9.0 mW
Payload (spectrum measurement)	120 mW	10%	12.0 mW
EPS self-consumption (MPPT, telemetry)	8 mW	100%	8.0 mW
Mean load			134.0 mW
Energy per orbit
Sunlight / eclipse	59.5 / 35.7 min
Available from the cells (mean 0.86 W in sunlight)	850 mWh
Needed for the loads and the eclipse recharge	221 mWh
Energy margin	284%
Drawn in eclipse / depth of discharge	80 mWh / 4.3%

Battery: 3.7 V, 500 mAh (1.85 Wh) lithium-polymer cell, 90% charge–discharge round-trip efficiency. Generation from Eq. 1 without the eclipse factor.

A payload that needs watts must run in short bursts on the battery, or the spacecraft must grow.

Downlink

Data volume is set by the energy per bit the radio can afford. For the reference 1P link at 9600 bit/s with at least 3 dB of margin:

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.

A station at mid-latitude sees a 525 km Sun-synchronous orbit only a few times a day above a 10° mask:

Station latitude (°)	Passes per day	Contact per day (min)	Mean pass (min)	Longest gap (h)
41.45	3.5	21	6.0	13.0

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.

A single uncompressed VGA image of 640 × 480 pixels at 8 bits is about 300 kB, roughly the raw capacity of one good pass. Imaging PocketQubes therefore compress hard, downlink thumbnails first, or move to S band.

Pointing

Most PocketQube payloads are designed to tolerate a tumbling or field-aligned spacecraft: spectrum monitors, radio services and reflectors need no pointing. Cameras and optical links do, which is why imaging appeared only with the three-axis control of Unicorn-2, specified for about 5° worst case[1].

Thermal

A payload on a 50 mm board shares the fast eclipse cycling of the bus, and its own dissipation creates hot spots. Active cooling of an instrument is very difficult at this scale[5]; detectors that need stable temperatures must be designed for drift.

Contamination

Optical payloads and the reflectors of laser-ranging missions depend on clean surfaces. In a shared deployer the outgassing of neighbouring spacecraft matters too: an AlbaPod v2 carries up to six PocketQube units, and Delfi-PQ flew a laser-reflector payload whose performance depends on clean optics[6].

Hosted payloads

POQUITO shows a model that suits the smallest classes: the PocketQube provides power, radio and attitude, while the hosted payload is itself a free-flying ChipSat a few grams heavy. The University of Luxembourg team printed the magnetorquers into the inner layers of the boards to make room[7].