unisat

Power Budget

Reference: ECSS-E-ST-20C (Space Engineering - Electrical and Electronic), CubeSat Design Specification Rev. 14

Profile scope: this file details the 3U CubeSat reference build. Per-class power envelopes (peak/average/battery/solar-capable) for every supported profile are in flight-software/core/form_factors.py.

1. Solar Generation

1.1 Panel Characteristics

Parameter	Value
Solar Constant (AM0)	1361 W/m²
Panel Type	GaAs Triple Junction (Spectrolab UTJ)
BOL Cell Efficiency	29.5%
EOL Cell Efficiency (2 yr)	27.1% (8% degradation)
Panel Area (per face)	60 cm² (0.006 m²)
Active Cell Area (per face)	52.8 cm² (88% packing factor)
Number of Body-Mounted Panels	6 (3 long faces + 2 end faces + 1 deployable)
Deployable Panel Area	300 cm² (1 wing, single-sided)
Total Active Area	616.8 cm²

1.2 Generation Calculation

The power generated by a solar panel is:

P_gen = S * A_cell * eta_cell * cos(theta) * F_packaging * F_degradation

Where:

S = 1361 W/m² (solar constant at 1 AU)
A_cell = active cell area per face
eta_cell = cell efficiency (BOL 29.5%, EOL 27.1%)
theta = angle of incidence (0 = normal)
F_packaging = 0.88 (packing factor, cell-to-panel area ratio)
F_degradation = 1.0 (BOL), 0.92 (EOL at 2 years)

For a tumbling 3U CubeSat, the average projected area is approximately:

A_avg = (2 * A_long + 2 * A_short) / pi  (for isotropic tumbling)
A_avg ~ 0.035 m² effective illuminated area

Condition	Generation (W)
BOL, sunlit, best orientation	8.2
BOL, sunlit, average orientation	5.6
EOL, sunlit, average orientation	5.1
BOL, orbit average (incl. eclipse)	3.6
EOL, orbit average (incl. eclipse)	3.3

1.3 Solar Panel Degradation Model

Degradation factors over 2-year mission (per ECSS-E-ST-20C):

Factor	Annual Loss	2-Year Cumulative
Radiation (trapped proton/electron)	2.5%	5.0%
UV degradation (coverglass darkening)	0.5%	1.0%
Thermal cycling fatigue	0.5%	1.0%
Micrometeorite damage	0.25%	0.5%
Contamination (outgassing)	0.25%	0.5%
Total degradation	4.0%	8.0%

2. Power Consumption by Mode

2.1 Operating Modes Definition

                    +---> SAFE MODE (autonomous, minimal power)
                    |
LAUNCH ──> DETUMBLE ──> COMMISSIONING ──> NOMINAL ──> SCIENCE
                                            |            |
                                            +--- COMM ---+
                                            |
                                            +---> ECLIPSE STANDBY

2.2 Detailed Power Consumption Table

Subsystem	Safe (W)	Eclipse Standby (W)	Nominal (W)	Comm Pass (W)	Science (W)	Peak (W)
OBC (STM32F4)	0.50	0.50	0.50	0.50	0.50	0.80
EPS Controller	0.15	0.15	0.15	0.15	0.15	0.15
COMM UHF RX	0.20	0.20	0.20	0.20	0.20	0.20
COMM UHF TX	OFF	OFF	OFF	1.50	OFF	1.50
COMM S-band TX	OFF	OFF	OFF	2.50	OFF	2.50
ADCS Sensors	0.10	0.20	0.30	0.30	0.30	0.30
ADCS Magnetorquers	0.20	OFF	0.50	0.50	0.50	0.80
ADCS Reaction Wheels	OFF	OFF	OFF	OFF	0.40	0.40
GNSS Receiver	OFF	OFF	0.30	0.30	0.30	0.40
Camera	OFF	OFF	OFF	OFF	3.00	3.00
Payload (Radiation)	OFF	OFF	0.50	OFF	0.80	0.80
Battery Heater	OFF	2.00	OFF	OFF	OFF	2.00
Harness Losses (5%)	0.06	0.15	0.12	0.30	0.31	0.64
Total	1.21	3.20	2.57	6.25	6.46	13.49

2.3 Power Mode Transition Criteria

Transition	Trigger	Battery SOC	Action
Any –> SAFE	V_bat < 12.4V (3.1V/cell)	< 15%	Disable all non-essential loads
SAFE –> NOMINAL	V_bat > 14.0V (3.5V/cell) AND sun detected	> 50%	Re-enable subsystems sequentially
NOMINAL –> COMM	GS in view AND schedule match	> 40%	Enable TX, prioritize downlink
NOMINAL –> SCIENCE	Imaging target in view	> 60%	Enable camera + RW pointing
NOMINAL –> ECLIPSE STANDBY	Eclipse entry detected	> 30%	Reduce ADCS, enable heater if T < 0C
ECLIPSE STANDBY –> NOMINAL	Eclipse exit + V_bat > 13.2V	> 25%	Restore nominal operations
SCIENCE –> NOMINAL	Imaging complete OR SOC < 40%	-	Disable camera + RW

3. Battery Specifications

Parameter	Value
Cell Type	Panasonic NCR18650B (Li-ion NCA)
Configuration	4S1P
Nominal Cell Voltage	3.6 V
Cell Capacity	3.4 Ah (12.24 Wh per cell)
Pack Nominal Voltage	14.4 V
Pack Capacity	48.96 Wh (3.4 Ah at 14.4V)
Usable Capacity (20-80% SOC)	29.4 Wh
Voltage Range	12.0 V (cutoff) – 16.8 V (full)
Charge Cutoff	4.2 V/cell (CC-CV charging)
Discharge Cutoff	3.0 V/cell (hardware cutoff)
Software Low Limit	3.1 V/cell (triggers SAFE mode)
Max Charge Rate	0.5C (1.7 A)
Max Discharge Rate	1C (3.4 A)
Operating Temp (charge)	0C to +45C
Operating Temp (discharge)	-10C to +45C
Storage Temp	-20C to +50C

3.1 Battery Cycling Analysis

At 550 km SSO, the satellite experiences ~15 orbits/day, yielding ~5,475 charge/discharge cycles per year.

Parameter	Value
Cycles per orbit	1
Cycles per year	5,475
Cycles over 2-year mission	10,950
Depth of Discharge (nominal)	10-15%
DoD (worst case, comm + eclipse)	25%
NCR18650B rated cycles at 15% DoD	> 15,000 cycles
NCR18650B rated cycles at 25% DoD	> 8,000 cycles
Capacity retention at EOL (2 yr)	~85% at 15% DoD avg

Capacity degradation model (empirical fit for NCR18650B):

C(n) = C_0 * (1 - 0.00002 * n - 5e-10 * n^2)

Where n = cycle count
At n = 10,950:  C(n)/C_0 = 0.852  (14.8% capacity loss)

EOL usable capacity: 29.4 Wh * 0.852 = 25.0 Wh

4. Orbit-by-Orbit Energy Balance

4.1 Orbit Parameters (550 km SSO)

Parameter	Value
Orbital Period	95.7 min (5742 s)
Sunlit Duration	62.2 min (65%)
Eclipse Duration	33.5 min (35%)
Maximum Eclipse (solstice)	36.1 min (37.7%)
Minimum Eclipse (near equinox)	0 min (full sun near summer solstice)

4.2 Energy Balance Per Orbit (Nominal Mode)

E_generated  = P_gen_avg * t_sun
             = 5.1 W * (62.2/60) h = 5.29 Wh  (EOL, avg orientation)

E_consumed   = P_nominal * t_sun + P_eclipse * t_eclipse
             = 2.57 W * (62.2/60) h + 3.20 W * (33.5/60) h
             = 2.66 Wh + 1.79 Wh = 4.45 Wh

E_net        = 5.29 - 4.45 = +0.84 Wh per orbit (POSITIVE)

4.3 Worst-Case Energy Balance (EOL, max eclipse, comm pass in eclipse)

E_generated  = P_gen_EOL * t_sun_min
             = 5.1 W * (59.6/60) h = 5.07 Wh

E_consumed   = P_nominal * t_sun + P_eclipse_comm * t_eclipse
             = 2.57 * (59.6/60) + 6.25 * (36.1/60)
             = 2.55 + 3.76 = 6.31 Wh

E_net        = 5.07 - 6.31 = -1.24 Wh per orbit (NEGATIVE)

This worst case requires battery supplementation. Recovery occurs in subsequent non-comm orbits.

4.4 Daily Energy Budget Summary

Scenario	Orbits	E_gen (Wh)	E_cons (Wh)	E_net (Wh)	SOC Trend
Nominal (BOL)	15	84.4	66.8	+17.6	Charging
Nominal (EOL)	15	79.4	66.8	+12.6	Charging
With 4 comm passes/day (BOL)	15	84.4	75.3	+9.1	Charging
With 4 comm passes/day (EOL)	15	79.4	75.3	+4.1	Marginal
Worst case (EOL + science)	15	79.4	82.6	-3.2	Discharging

4.5 Power Margin Analysis

Per ECSS-E-ST-20C, minimum power margin shall be > 10% for nominal operations.

Power Margin = (P_generated - P_consumed) / P_consumed * 100%

Mode	P_gen (W)	P_cons (W)	Margin	Requirement	Status
Nominal (BOL)	5.6	2.57	+118%	> 10%	PASS
Nominal (EOL)	5.1	2.57	+98%	> 10%	PASS
Comm (BOL, sunlit)	5.6	6.25	-10.4%	> 10%	Battery assisted
Comm (EOL, eclipse)	0.0	6.25	-100%	N/A	Battery only
Science (BOL)	5.6	6.46	-13.3%	> 10%	Battery assisted
Safe Mode	5.1	1.21	+321%	> 10%	PASS

Note: Comm and Science modes are inherently battery-supplemented (short duration < 15 min). The relevant metric is per-orbit energy balance, which is positive for all nominal scenarios.

5. Eclipse Cycle Analysis

5.1 Eclipse Duration vs. Beta Angle

The beta angle (angle between orbital plane and sun vector) determines eclipse fraction:

Eclipse fraction = (1/pi) * arccos( sqrt(h^2 + 2*R_E*h) / ((R_E + h) * cos(beta)) )

Where: R_E = 6371 km, h = 550 km

Beta Angle	Eclipse Duration (min)	Eclipse Fraction	Annual Occurrence
0 deg	35.7	37.3%	Equinoxes
20 deg	34.1	35.6%	Common
40 deg	30.2	31.5%	Common
60 deg	22.3	23.3%	Moderate
71.6 deg	0 (full sun)	0%	Near summer solstice

5.2 Battery SOC Profile During Eclipse Season

SOC (%)
100 |
 90 |  ____        ____        ____
 80 | /    \      /    \      /    \
 70 |/      \    /      \    /      \
 60 |        \  /        \  /        \
 50 |         \/          \/          \
 40 |
 30 |.........SAFE mode trigger (15%)...........
    +----+----+----+----+----+----+----+----> Time
     Orbit 1   Orbit 2   Orbit 3   Orbit 4

Legend: /\ = sunlit period (charging), \/ = eclipse (discharging)

6. Power Distribution Architecture

                          +------------------+
  Solar Panels ──────────>| MPPT Controller  |
  (6 faces + deployable)  | (buck converter) |
                          +--------+---------+
                                   |
                          +--------v---------+
                          |   Power Bus       |
                          |   (14.4V nom.)    |
                          +--+--+--+--+--+---+
                             |  |  |  |  |
                   +---------+  |  |  |  +--------+
                   |            |  |  |            |
              +----v----+  +---v--v--v---+   +----v----+
              | Battery  |  | Load Switch |   | Heater  |
              | Charger  |  | Array (8ch) |   | Control |
              | (CC-CV)  |  | w/ current  |   | (PWM)   |
              +----+-----+  | limiters    |   +---------+
                   |         +--+--+--+---+
              +----v----+       |  |  |
              | 4S1P    |    3.3V 5V 12V
              | Battery |   (buck converters)
              | Pack    |       |  |  |
              +---------+    OBC GNSS ADCS...

7. Regulated Voltage Rails

Rail	Voltage	Max Current	Subsystems
VBAT	12.0-16.8V	3.4 A	Heater, S-band PA
5V0	5.0V +/- 2%	1.5 A	Camera, Payload
3V3	3.3V +/- 2%	2.0 A	OBC, ADCS, GNSS, UHF
1V8	1.8V +/- 3%	0.5 A	OBC core (if needed)

8. References

ECSS-E-ST-20C: Space Engineering - Electrical and Electronic (2008)
CubeSat Design Specification Rev. 14, Cal Poly SLO
Panasonic NCR18650B Datasheet, Rev. 2019
Spectrolab UTJ Solar Cell Datasheet
Wertz, J. R., “Space Mission Engineering: The New SMAD”, Chapter 11

This site is open source. Improve this page.