1. PDR slides for Tomo Sugano
Tasks done so far:
In-flight Delta V estimation of the mission
Atmospheric Drag Analysis
Orbital Decay Life
Communication/CPU selection
Formation Flying - T.Sugano

2. Presence of Atmospheric Drag in LEO orbit
Atmospheric density is largest at perigee
Largest drag is experienced at perigee
Atmospheric drag shall be considered if orbit perigee height is <1000 km
Atmospheric drag acceleration (D):
1/(ACD/m) is the ballistic coefficient, a measure of resistance to fluid
A (projected area normal to flight path)
m (mass of spacecraft)
f (latitude correction coefficient)
Formation Flying - T.Sugano

3. Effect of Atmospheric Drag to Orbit Profile
Atmospheric drag tends to circularise the probe’s orbit
Drag effect greatest at perigee
Apogee height consequently reduced
Overall altitude is lost unless orbit correction is done
Determinant of satellite decay time
Formation Flying - T.Sugano

4. Drag Coefficient of STS and other LEO probes
STS Orbiter (aka the Space Shuttle)
STS has a CD of 2.0 at typical mission altitudes in LEO
Above 200 km of orbit altitude, use 2.2 < CD < 3.0
Cylindrical probes have larger CD than those of spherical probes
Exact CD is hard to predict as LEO environment is not fully understood
Currently best determined by actual flight test
Formation Flying - T.Sugano

5. Consideration of Drag in Formation Flying
FF mission is required to last at least 24 hours
STS orbiter (primary) typically performs a trim burn once a day
Trim burns correct orbit altitude and ascending node
Drag differentials present between primary and satellite(s)
Possible consideration of LEO drag in our mission
Formation Flying - T.Sugano

6. Formation Flying - T.Sugano
Orbital Decay
Perturbation in LEO is mainly due to atmospheric drag
Orbital decay of space probes (e.g. Space Shuttle, ISS, satellites)
Altitude correction “trim burns” necessary to keep probes in orbit
Orbit will decay in the absence of trim burns
Formation Flying - T.Sugano

7. Orbit Lifetime Estimation
Estimation of the orbit lifetime of our satellite after mission
Consider atmospheric drag effect only
Mission orbit is assumed virtually circular for simplicity
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8. Orbit Lifetime Equation
Circular Orbit Lifetime Equation (Approximation)
a0 = initial altitude
S = projected area of the space probe
m = space probe mass
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9. Exponential Atmospheric Model
Scale height, H, obtained from tabulated data
Formation Flying - T.Sugano

10. Assumptions set forth for our lifetime computation
Assumptions: (Made for worst case or shortest decay)
m = 50 kg (maximum); S = 0.385m2 (spherical correction of max volume)
CD = 3.0 (upper bound value in LEO probes)
a0 = km (typical altitude for STS or ISS)
Δ = 150 – 300 = km (typical re-entry altitude, note the minus sign)
f = 1 (ignore latitude effect; not significant (<10%))
ρ0 = 2.418x10-11 kg/m3 (Table, 300 km base altitude)
Unavoidable uncertainty  Scale height, H
- Not constant between orbit and re-entry altitude
- Take H = 30 km, so β = 1 / (30 km)
Formation Flying - T.Sugano

11. Formation Flying - T.Sugano
Computation Result
Based on the assumptions we made
- T = tau_0 *
- T = (approx. 1.5 hr of initial orbit period)*(190) = 12 days
LEO Nanosat at 300 km of altitude will take 12 days to decay.
Formation Flying - T.Sugano

12. Formation Flying - T.Sugano
Conclusion
Our Nanosat does not decease for 12 days
Retroburn delta-V input to decelerate the Nanosat for faster decay will be costly without a compelling space debris concern(?)
Unless allowed to dispose of the Nanosat in space, retrieval is rather recommended(?)
Retrieval may be attained fairly easily by using robot arm of STS perhaps equipped with capture net(?)
Formation Flying - T.Sugano

13. Drag Differential Compensation
Different ballistic coefficients between the orbiter and the Nonosat
Consequent difference in drag forces exerted during mission
Ballistic Coeff. of STS >> Ballistic Coeff. of Nanosat
Nanosat must expend Delta-V to keep up with STS orbiter
Formation Flying - T.Sugano

14. Formation Flying - T.Sugano
Computations
Atmospheric drag acceleration (Da):
Drag (acceleration) difference between the two spacecraft:
STS: S = 64.1 m2, CD = 2.0, m = 104,000 kg (orbiter average)
sat: S = m2 (nominal), CD = 3.0 (worst case), m = 50 kg
Formation Flying - T.Sugano

15. Computations (cont’d)
Orbiter speed (assuming circular orbit)
Definition of Delta-V (or specific impulse)
Mass expenditure of propellant (i.e. GN2 cold gas)
Formation Flying - T.Sugano

16. Formation Flying - T.Sugano
Results
Using Isp = 65 sec; assume 50 kg for satellite weight
Conclusions
- At the typical 300 km LEO, Delta-V for 1 day mission is 1.36 m/s
- Satellite will need at least 107 grams of GN2 to compensate drag
- Besides this Delta-V requirement, we have orbit transfer Delta-V (currently estimated at 1.17 m/s) and ADCS Delta-V.
Formation Flying - T.Sugano

17. Formation Flying - T.Sugano
FCS and COMM
FCS – Flight Control System
COMM – Communications (camera is assumed to be part of COMM)
Satellite needs to handle both FCS and COMM systems
Use of COTS (Consumer Off-the-Shelf) computer(s) aimed
COMM utilizes a low-cost COTS transceiver radio
Formation Flying - T.Sugano

18. CPU selection for the Nanosat
Arcom VIPER 400 MHz CPU recommended
VIPER is suitable because of its
- Light weight, 96 grams
- Operable temperature range, -40 C to + 85 C
- Windows Embedded feature, easy to program
- Computation speed, 400 MHz
- Memory capacity, up to 64MB of SDRAM
- Embedded audio I/O, necessary for COMM with voice radio
Redundancy can be implemented.
Formation Flying - T.Sugano

19. Arcom VIPER 400 MHz embedded controller
Formation Flying - T.Sugano

20. Radio selection for the Nanosat
Kenwood Free Talk XL 2W transceiver recommended
Kenwood Free Talk XL is suitable because of its
- COTS nature, low cost
- 2W of transmission power, more than enough for non-obstructed space communication, but higher wattage than FRS 500 mW radio
- Ability to use both GMRS and FRS frequencies
- FRS frequencies recommended because by international treaty FRS (Family walkie talkie) is restricted to 500 mW
- 500 mW is too weak to penetrate into space
- MilSpec cetified
Formation Flying - T.Sugano

21. Kenwood Free Talk XL 2W FRS/GMRS Transceiver
15 UHF channels (7 FRS and 8 GMRS)
2W output for both categories
DC 7.2 V (600mAh)
Circuit board weighs only 60 grams
Speaker/Microphone/Encapsulation Removed
Formation Flying - T.Sugano

22. Scheme of FCS/COMM Integration
Formation Flying - T.Sugano

23. Detailed Scheme of integration
Formation Flying - T.Sugano