22
votes
Accepted
Radar altimeter in a space shuttle
The radar altimeter was intended only for landing, to provide a direct, precise measurement of the altitude of the vehicle above the runway. It locked on around 5000 feet altitude but was primarily ...
- 189k
15
votes
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Detailed radar imaging of Tiangong-1; how do they do that?
It was done with one single dish,
...in Ku-band (16.7 GHz) and ... currently equipped with a high target resolution.
The Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR)'s ...
- 1,805
14
votes
Accepted
Why are Titan's lakes "black" in radar images rather than transparent?
This article suggests that the radar can penetrate the lakes and reports them to be hundreds of meters deep. The space.com article referenced seems to be sourced from a Geophysical Research Letters ...
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13
votes
Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit?
The explanation has to do with the operation of the radar transmitters and the round trip light travel time.
It takes about 3 seconds for a radar pulse to travel from the Earth to the Moon and back. ...
- 131
10
votes
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How does the SpaceBEEs' experimental passive radar reflector work?
The original 4 satellites were 0.25U cubesats (as shown in the picture in the question) while the newer ones are 1U (cubical). The larger size of the newer sats provides space for Van Atta reflectors ...
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9
votes
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Why do radar maps of the surface of Venus have missing slices?
These maps were produced with doppler-delay imaging, which uses a combination of range and motion to identify the radar return from each specific point on the surface and build a map.
Longitude is ...
- 2,267
9
votes
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Is there really a frozen lake near the equator on Mars?
This paper reports SHARAD observations of this site, and also references previous publications based on an earlier radar instrument MARSIS. It's complicated and the data seems to be ambiguous, but the ...
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9
votes
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How could a hydrazine and N2O4 cloud (or it's reactants) show up in weather radar?
Hydrazine has a lot of spectral structure in the microwave regions (see paper), as does dinitrogen tetroxide (sorry, only paper I have is paper). This means that they’re pretty good at absorbing and ...
- 12.9k
8
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What are the pros and cons of Doppler radar versus Doppler lidar?
NOTE: This discusses autonomous car sensors but the same physics will apply to any longwave vs. shortwave sensor comparison (cost being the wildcard)
An interesting parallel is Tesla Inc.'s decision ...
- 622
8
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A black cube 1 mile per side appears in geosynchronous orbit around the earth. Who would spot it and how would they do it? How easy would it be?
The initial detection would almost certainly be a ground based telescope that could no longer see a particular star (because there is a giant cube in the way, cf. occultation). From that point, once a ...
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7
votes
What steel ball was thrown out of the ISS to help test how well ground stations can track orbital debris?
The ODERACS (Orbital DEbris RAdar Calibration Spheres):
Five-centimeter balls were detected by the Don-2N radar with an experimental processing program with incoherent accumulation of about a dozen ...
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7
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How (the heck) do they know some lakes on Titan are 100 meters deep?
The paper itself (I have access) says and shows that they are detecting the bottom of some lakes with their radar with a maximum depth of 105+/-6 meters. The radar reflection off the bottom is much ...
- 286
7
votes
How (the heck) do they know some lakes on Titan are 100 meters deep?
From Cassini observations the methane-ethane mixture (with methane by far the largest component, maybe with some dissolved nitrogen) appears so pure that its absorptivity at the RADAR instrument's Ka-...
- 18.4k
7
votes
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Did the Apollo LM's cockpit altitude displays account for the pointing angle of the radar altimeter?
Yes and No. The altitude data presented on the altitude indicator (tapemeter) depends on the mode select switch setting.
MODEL SEL switch
LGD RADAR: Radar altitude, altitude rate... are displayed.
...
- 2,001
7
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A black cube 1 mile per side appears in geosynchronous orbit around the earth. Who would spot it and how would they do it? How easy would it be?
Remember, the moon is black, or at least almost as dark as fresh asphalt (the moon reflects about 7% of incoming light, compared to 5% for asphalt). It's just brightly lit against a truly dark ...
- 11.6k
6
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How could a hydrazine and N2O4 cloud (or it's reactants) show up in weather radar?
Weather radar is not specifically measuring clouds or water in the atmosphere. What is measured is the amount of microwaves in the 5 GHz range reflected by anything in the atmosphere. This can be ...
- 15.2k
6
votes
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How does Deep Space Network's DSS-14 transmit radar and receive it at (almost) the same time? (monostatic radar)
Evidently the DSS-14 does rely on separate periods of transmitting and receiving, but it is able to switch between the transmitting and receiving modes rapidly using a "quasi-optical switch"....
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6
votes
Accepted
Is it easier to detect a satellite than an aircraft with a land-based radar?
In addition to Steve’s comments, there are some other factors
Target Reflectivity is affected by size (most satellites are smaller than most planes), material (planes are completely covered in metal), ...
- 28k
5
votes
Space Travel Advanced Recon
Within the solar system you cannot really travel 'ahead' of a transfer orbit since it would have required you to have departed from a point in space ahead of the origin body (for example Earth) in it'...
- 25k
5
votes
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How many antennas does Kurs use in toto, and what are each of their functions?
I see different versions of Cyrillic transliterations in my sources; I have chosen consistency.
There are 6 Kurs antennae. For some reason NASA labeled two of them with the single number "3".
Refer ...
- 189k
5
votes
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Why are there so many waveguide feeds (?) near the focus of Cassini's dish antenna?
Some of those are the Ku-band antenna feeds for the Radar. The high-gain antenna also serves (served) as part of the Radar science instrument. The HGA was pointed at Titan many times to get synthetic-...
- 58.4k
5
votes
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What is "TFR" in the context of operating a marine radar on top of a "water tower" at a launch site?
It stands for Temporary Flight Restriction.
Source: https://en.wikipedia.org/wiki/Temporary_flight_restriction
Presumably the TFR is in place for whatever SpaceX is doing and the radar will be ...
- 189k
5
votes
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How (the heck) was coherent synthetic aperture radar (SAR) implemented using photographic emulsion aboard Apollo 17?
Synthetic Aperture Radar does not involve interferometry. As such, it is unrelated to aperture synthesis techniques in radio astronomy. (The question mentions interferometry and perhaps this should be ...
- 166
4
votes
Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit?
I know nothing of the activity you're asking about, but I do know something about radar.
All the radar systems I've worked with used a single antenna to both transmit and receive. The power of the ...
- 1,174
4
votes
Accepted
How does a three-way doppler shift measurement work?
Three-way Doppler tracking was developed for when the spacecraft (specifically Voyager) is so danged far away, that by the time the round trip signal gets back to Earth the original transmitting ...
- 58.4k
4
votes
Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit?
Since the deep space network can perform ranging on spacecraft much farther away (tens of thousands of times farther than the moon) by itself, why was it necessary to use a non-colocated, non-DSN dish ...
- 131k
4
votes
How did NORAD notice Kosmos 954 was making erratic manoeuvres?
NORAD uses radar to track the position of satellites. When you observe a satellite a few times, you can calculate its orbit and predict where the satellite will be in the future.
When a satellite ...
- 131k
4
votes
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What is Space Policy Directive 3 and how does it relate to the Space Fence?
Broadly speaking, SPD-3 establishes new roles and responsibilities for SSA (Space Situational Awareness) and STM (Space Traffic Management).
It shifts the responsibility of managing SSA data (which ...
- 596
4
votes
How does Deep Space Network's DSS-14 transmit radar and receive it at (almost) the same time? (monostatic radar)
Supplemental to @hobbs's answer I've found Soar System Radar Group 332F; Communications Architectures & Research Section 332
with these static image and this animated GIF (it runs about ten times ...
- 151k
4
votes
How (the heck) was coherent synthetic aperture radar (SAR) implemented using photographic emulsion aboard Apollo 17?
The CSAR (coherent synthetic aperture radar) did not use very high frequencies and short wavelengths like 3 GHz (0.1 m) or 30 GHz (0.01 m) allowing small narrow beam directional antennas.
Very low ...
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