PDR slides for Tomo Sugano

Formation Flying - T.Sugano

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


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)


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


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


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


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


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


Orbit Lifetime Equation

• Circular Orbit Lifetime Equation (Approximation)

a0 = initial altitude

S = projected area of the space probe m = space probe mass


Exponential Atmospheric Model

• Scale height, H, obtained from tabulated data


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 = 6400 + 300 km (typical altitude for STS or ISS)

Δ = 150 – 300 = - 150 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)


Computation Result

• Based on the assumptions we made- T = tau_0 * 189.565- 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.


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(?)


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


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 = 0.385 m2 (nominal), CD = 3.0 (worst case), m = 50 kg


Computations (cont’d)

• Orbiter speed (assuming circular orbit)

• Definition of Delta-V (or specific impulse)

• Mass expenditure of propellant (i.e. GN2 cold gas)


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.

Altitude [km] Density [kg/m3] Dsat-Dsts [m/s2] Thrust Req [N] m_dot [kg/s] mass in 24hr [kg] Delta V for 24hr [m/s]300 2.4180E-11 1.5780E-05 7.8901E-04 1.2386E-06 0.1070 1.363350 9.1580E-12 5.9322E-06 2.9661E-04 4.6564E-07 0.0402 0.513400 3.7250E-12 2.3951E-06 1.1976E-04 1.8800E-07 0.0162 0.207


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


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.


Arcom VIPER 400 MHz embedded controller


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


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


Scheme of FCS/COMM Integration


Detailed Scheme of integration

PDR slides for Tomo Sugano

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