Wednesday, March 15, 2017

Satellite dishes and satellite phones for radio astronomy and passive radar detection.

Copyright 2017 Robert Clark

Asteroid Detection.
 In the blog post "Combined amateur telescopes for asteroid detection", I suggested using multiple small amateur telescopes in concert to act as a giant astronomical instrument to make dim observations in the optical range. Could we do the same with multiple satellite dishes or satellite phones to make dim radio observations? 

 There is a technique called "passive radar" that uses reflected radio waves from aircraft that originate from surrounding radio transmissions such as from television and radio stations:

Passive Radar.
3. Typical illuminators
Passive radar systems have been developed that exploit the following sources of illumination:
Analog television signals
FM radio signals
Cellular phone base stations
Digital audio broadcasting
Digital video broadcasting
Terrestrial High-definition television transmitters in North America
GPS satellites (GPS reflectometry).
Satellite signals have generally been found to be inadequate for passive radar use: either because the powers are too low, or because the orbits of the satellites are such that illumination is too infrequent. The possible exception to this is the exploitation of satellite-based radar and satellite radio systems. In 2011, researchers Barott and Butka from Embry-Riddle Aeronautical University announced results claiming success using XM Radio to detect aircraft with a low-cost ground station.

 The difficulty in using satellite transmissions for the detections previously is that they just use a single ground station for the reception of the reflected signals. Instead of this, suppose we used millions of satellite dishes or radios or satellite phones to make the detections?

 As with the case of multiple amateur telescopes, you couldn't form a coherent signal from this method. But like in the optical case you could make correlations from which you could make a probabilistic estimate of the likelihood of an actual detection.

 There is an additional difficulty however. We are envisioning using satellites at geosynchronous orbit, about 35,000 kilometers out in space. We would detect asteroids closer than this distance by their blocking the satellite signals from being detected by satellite dishes or phones.

 However, the asteroids would tend to direct the reflected signals back out to space rather than towards the Earth, except for the case where the asteroid is along a line from the satellite towards the limb of the Earth, and with the dishes/phones along the limb. But this would be relatively few asteroids and dishes/phones so precisely placed in the right position.

 So in actuality for this method to work we would be looking for holes, deletions, in the signal. Such deletions in the satellite signal would be small for each dish or phone. But by correlating the signals of millions of them we can determine statistically that it represents a real detection.

 This would only be for detecting asteroids rather close in, since they would be inside the distance of geosynchronous orbit. This would still be useful since from multiple observations we could determine their orbits. And such asteroids that came so close in would have a higher probability of presenting an impact hazed on a future orbital pass.

 But could we also detect asteroids further out? Some proportion of the signal from the GEO satellites likely escapes past the sides of the Earth to proceed to the other side. And this proportion of the signal likely is increased by the signals bouncing off the ionosphere. Then these signals could proceed further outwards to be reflected back to Earth by more distance asteroids.

 The strength of the signal leaking past Earth would be reduced so the reflected signals would also be reduced. But in this case you are making actual positive detections rather than looking for holes in the signal so all in all the results could be just as effective as in the close in asteroid case.

Aircraft detection.
 A problem with detecting aircraft on intercontinental flights is that when they fly over the oceans they fly too far from the radar stations on land to be detected. Then perhaps the method of satellite signal detections by multiple dishes/phones can be used to track such aircraft as well. This may give a us a method to finally locate the missing airliner Malaysian Airlines Flight 370. The flight was lost three years ago but there may have been some satellite TV, radio or phone customers who saved programs or phone conversations at that time for which the recorded digital data can be reviewed to reconstruct a detection of the aircraft.

  Bob Clark

Tuesday, March 14, 2017

A smaller, faster version of the SpaceX Interplanetary Transport System to Mars, Page 2: triple cores for larger payloads.

 Copyright 2017 Robert Clark

 In the blog post "A smaller, faster version of the SpaceX Interplanetary Transport System to Mars", I suggested using just the upper stage of the ITS to get a booster for a Mars rocket, using an existing Ariane 5 core as an upper stage. This would be much cheaper and faster than the 7,000 metric ton, 42 engine booster that SpaceX was planning.

 Elon Musk says SpaceX plans to have the smaller upper stage built by 2020. So we could possibly have a Mars transport system by then since the Ariane 5 as an upper stage already exists. However, by using triple cores of the ITS upper stage we could also get a system of the larger size SpaceX is proposing.

 We'll input the data into Dr. John Schillings payload estimation program. In the calculator, select "No" for the "Restartable Upper Stage" option, rather than the default "Yes", otherwise the payload will be reduced. Select Cape Canaveral as the launch site, and input 28.5 degrees for the launch inclination to match the latitude of Cape Canaveral, otherwise the payload will be reduced.

 We'll also use the 382 s Isp of the vacuum version of the Raptor. Altitude compensation allows even engines used on first stage boosters to have the same vacuum Isp as upper stages engines.

 We'll use also crossfeed fueling. As I have argued before this is a well-known technique having been used for decades on jet airliners. To emulate crossfeed fueling with the Schilling calculator, enter in 2/3rds the actual propellant load in the field for the sideboosters, and enter in (1 + 2/3) times the actual propellant load in for the first stage propellant load.

 So in the side boosters propellant field enter in (2/3) * 2,500,000 kg = 1,667,000 kg. And in the first stage propellant field enter in (1 + 2/3) * 2,500,000 = 4,167,000 kg.

 For the thrust fields, enter in the vacuum thrust for 9 vacuum Raptors, since the calculator always takes as input the vacuum values, even for first stages and side boosters. The vacuum thrust for the 382 Isp vacuum Raptor is 3.5 meganewtons, 3,500 kN. So 9 would be 31,500 kN. Enter in also the vacuum Isp 382 s.

 For the second stage, we'll increase the vacuum thrust of the Vulcain engine on the Ariane 5 to 1,450 kN in accordance with an increased vacuum Isp of 465 s, since we can get this higher vacuum Isp by just using a nozzle extension. For the dry mass input 12,000 kg and propellant 158,000 kg. Inputting these specs in the calculator results in:

Mission Performance:
Launch Vehicle:  User-Defined Launch Vehicle
Launch Site:  Cape Canaveral / KSC
Destination Orbit:  185 x 185 km, 28 deg
Estimated Payload:  504575 kg
95% Confidence Interval:  426107 - 597674 kg

"Payload" refers to complete payload system weight, including any necessary payload attachment fittings or multiple payload adapters

This is an estimate based on the best publicly-available engineering and performance data, and shou
ld not be used for detailed mission planning. Operational constraints may reduce performance or preclude this mission.

 This is comparable to the payload mass of the expendable version of SpaceX's ITS. This would save greatly on development costs when not having to develop the larger booster. The launch cost would also be greatly reduced since judging by the Falcon Heavy, using triple cores only increased the price 50% over that of the single core rocket.

    Bob Clark

Could ‘Oumuamua be a spent rocket stage?

Copyright 2023 Robert Clark  I was interested to read of a supposed asteroid passing near Earth turning out to be a Centaur upper stage laun...