First day, Post-EDL |
First day, Post-EDL |
May 26 2008, 02:31 AM
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#1
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Member Group: Members Posts: 428 Joined: 21-August 06 From: Northern Virginia Member No.: 1062 |
So, I think we can say now that we are in post-EDL operations, now that the solar panels have been verified. Still on the list of things to come:
1. Full panoramic from Phoenix, I would expect it to be coming in pieces over the next day, every hour and a half or so. 2. Imagery from HiRISE and possible Mars Express, which might show Phoenix while landing. The odds of success are slim, but it sure would be cool! 3. Imagery of HiRISE on it's landing spot. I find this unlikely to come by this Wednesday release, for a few reasons. First of all, it's likely due to a late parachute opening that Phoenix overshot it's landing a bit. If it overshot it far enough, then it would miss a HiRISE window (Remember, the FOV on HiRISE isn't that high...) Secondly, it takes about 4 advanced notice to change an image location, so the next opportunity won't be for a while. Given also the length of a downlink, I put a HiRISE image for the 4th of June. Anyways, please post your topics on Sol 1 (Or is it 0) of Phoenix. I have a hunch we'll have to do a post for each of the first few days, but... By the way, for those trying to see the Phoenix Images, and want to download the JPGs, look at http://phoenix.lpl.arizona.edu/ , and then click view images now, then the image you want, then look at the right. You will see an area to download the image now, in JPG format. |
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May 26 2008, 12:06 PM
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#2
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Newbie Group: Members Posts: 9 Joined: 12-March 08 Member No.: 4065 |
Hey people,
I'm one of the hundreds of people that have been lurking here yesterday! For my first post I have a question: does anyone know what was the event that triggered the parachute deploy? I'm wondering if it was a specific altitude, speed, density, or something else. If it's something like that, then the Martian atmosphere models will have to be recalculated. The 7 seconds delay of the parachute deploy at that speed caused a significant deviation from the center of the landing ellipse. |
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May 26 2008, 01:36 PM
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#3
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Member Group: Members Posts: 611 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
For my first post I have a question: does anyone know what was the event that triggered the parachute deploy? I'm wondering if it was a specific altitude, speed, density, or something else. If it's something like that, then the Martian atmosphere models will have to be recalculated. The 7 seconds delay of the parachute deploy at that speed caused a significant deviation from the center of the landing ellipse. For lots of background you can read Ball et al, 'Planetary Landers and Entry Probes' Usually the specification on the parachute is for deployment in an allowed dynamic pressure range (i.e. not too high density*speed^2 or you shred the thing when it deploys) and a Mach number range for probe/parachute stability (typically Mach 1.4 - as you get to the transonic regime, the sphere-cone entry configuration can start to tumble, so you deploy before you slow down to this point) But, you cant measure Mach directly. So for a range of plausible models you calculate the entry deceleration history, and it turns out that there is typically a function of the form Right_Time_to_deploy = time_g_falls_below_X + the_time_between_g_rising_above_y_and_falling_below_X (with maybe an extra offset term or something) This can be implemented with simple logic and g-switches (literally microswitches with weights on springs) or more typically an accelerometer (both Huygens and Galileo used this approach - for Huygens the accels were prime, with g-switches as a backup). I think in Phoenix's case it was probably a full-up IMU (i.e. accels integrated to derive speed) Interestingly (see Harland and Lorenz 'Space System Failures') on Galileo the two switches were miswired, and the parachute was deployed late (meaning the descent data only began from lower down than desired). I think for Phoenix the hardware and software is rather more sophisticated, so a simple miswiring like this is unlikely to explain the late chute for Phoenix - it may be as you say an atmospheric profile issue, but it's too early to know. |
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May 26 2008, 02:06 PM
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#4
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Merciless Robot Group: Admin Posts: 8784 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
I think for Phoenix the hardware and software is rather more sophisticated, so a simple miswiring like this is unlikely to explain the late chute for Phoenix - it may be as you say an atmospheric profile issue, but it's too early to know. VERY interesting, Ralph; I need to get these books, too. Question: On Phoenix, was the descent radar turned on prior to parachute deployment? Everything happened so rapidly that I lost track. Reason I ask is that I can see a combination of IMU deceleration data & ground proximity to cue chute deployment. The only other thing I can think of if the radar was NOT on is that they were relying on IMU data alone, which would include a Martian "geoid" model for surface reference (assuming that ambient atmospheric pressure was not used as a reference, and can't see why it would be based on the highly variable density of the upper atmosphere). Or did they do it all based just on deceleration data, which seems quite risky? -------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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