|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Sun Synchronous Orbit
On Monday, June 3, 1996 12:30:00 PM UTC+5:30, Frank Crary wrote:
In article , Pat wrote: ...the reason i heard sun synchonicity was desired was not measuring building heights (which can be measured at any time of day given the known sun angle, but rather to minimize shadow changes in the photo so as to maximize photo-analysts chances of spotting new construction or new building features. I disagree. You can not do equally good height measurements at any time of day. What's the length of a shadow at noon, on the equator, on June 21? You'd need very good resolution to measure the length of a nonexistent shadow... To get an accurate measurement of height, you need shadows that are much longer than the resolution of the image. How much longer depends on the desired accuracy, but longer shadows certainly help and that is a motive for sun-synchronous orbits with early morning passes. As Henry pointed out, it isn't the only reason as far as the Earth is concerned. I guess I'm too used to thinking of observations of other planets... Frank Crary CU Boulder hye, I was reading about sun synchronous orbits and i had a lot of confusion reg arding that topics so i was looking into various group pages which discusse d this topic.I was wondering if a satellite is in sun synchronous orbit and if it completes 14 orbits per day then will the satellite be passing over one particular location on the earth twice a day.ie 7 am in morning and 7 p m at evening or will it pass over one location on the earth only once. The inclination of the orbit can be taken as around 98 degrees and time per iod as 101 minutes.Altitude will be around 800 kms. Please help if possible |
#2
|
|||
|
|||
Sun Synchronous Orbit
On 04/17/2013 5:13 AM, Smit Kamal wrote:
On Monday, June 3, 1996 12:30:00 PM UTC+5:30, Frank Crary wrote: In article , Pat wrote: ...the reason i heard sun synchonicity was desired was not measuring building heights (which can be measured at any time of day given the known sun angle, but rather to minimize shadow changes in the photo so as to maximize photo-analysts chances of spotting new construction or new building features. I disagree. You can not do equally good height measurements at any time of day. What's the length of a shadow at noon, on the equator, on June 21? You'd need very good resolution to measure the length of a nonexistent shadow... To get an accurate measurement of height, you need shadows that are much longer than the resolution of the image. How much longer depends on the desired accuracy, but longer shadows certainly help and that is a motive for sun-synchronous orbits with early morning passes. As Henry pointed out, it isn't the only reason as far as the Earth is concerned. I guess I'm too used to thinking of observations of other planets... Frank Crary CU Boulder hye, I was reading about sun synchronous orbits and i had a lot of confusion reg arding that topics so i was looking into various group pages which discusse d this topic.I was wondering if a satellite is in sun synchronous orbit and if it completes 14 orbits per day then will the satellite be passing over one particular location on the earth twice a day.ie 7 am in morning and 7 p m at evening or will it pass over one location on the earth only once. The inclination of the orbit can be taken as around 98 degrees and time per iod as 101 minutes.Altitude will be around 800 kms. Please help if possible It is amusing to see a reply to a post 17 years later. The answer is basically only once. You can separate the orbit in two halves. On the first half, let's say the satellite is heading northwards, and on the second half it is heading southwards. On the northward half orbit the satellite will always pass at a given latitude at the same local time. So if the satellite passes over your head at 7 am while the satellite is heading north, then on its northern bound half orbit it will pass over your head every day at 7 am and at no other time. What about, on its southern bound half orbit 12 hours later? Exactly twelve hours later, the satellite, which you said completes 14 orbits per day will have completed exactly 7 orbits and will be over the same latitude on its northern bound half orbit, not on its south bound half orbit. If instead of 14 orbits per day, you have an odd number of orbits per day. Let's say 15 orbits per day. Then the satellite can pass over your head in the south bound half orbit about 7.5 orbits after passing over your head in the north bound orbit. But that would happen only at one specific latitude. Why only at one specific latitude? Well let's imagine the orbit has zero eccentricity and passes exactly over the poles (which is impossible but it will help understand) and that the satellite has an odd number of orbits per day, let's say 2n+1 orbits per day. If it passes over you on the equator on its north bound half orbit, then half a day later, the satellite will have done n + 1/2 orbits, it will therefore be over the equator on its south bound half orbit, exactly over your head. But if you aren't on the equator, let's say you are in the norther hemisphere, then half a day later, the satellite will be in the southern hemisphere, not over your head. So you see, that in such an (impossible) sun synchronous 90 degree circular orbit, the satellite goes twice a day over the same point over the equator. It also goes 2n+1 times a day over the poles, but for other points under its path, only once a day. For a real sun synchronous orbit, the points where the satellite passes over head twice a day got to be placed in such a way that it takes exactly 12 hours to do n orbits plus the remaining "half orbit". But because of eccentricity, the said "half orbit" has to be a half orbit in time, but does not have to be a half orbit in distance, so the points where this works aren't necessarily over the equator. Alain Fournier |
#3
|
|||
|
|||
Sun Synchronous Orbit
In article ,
Smit Kamal writes: I was wondering if a satellite is in sun synchronous orbit and if it completes 14 orbits per day then will the satellite be passing over one particular location on the earth twice a day.ie 7 am in morning and 7 p m at evening or will it pass over one location on the earth only once. Twice a day, once going northbound and once southbound. Imagine yourself near the Sun looking back at the Earth with the satellite going round. From that perspective, the Earth rotates once in 24 hours, and a given spot on Earth (at least a spot not near either pole) passes under the satellite path twice. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#4
|
|||
|
|||
Sun Synchronous Orbit
On 4/17/2013 8:05 PM, Alain Fournier wrote:
On 04/17/2013 5:13 AM, Smit Kamal wrote: It is amusing to see a reply to a post 17 years later. [...snip...] Alain Fournier Smit, So what's life like around HR4550? Is your planet earth-like? And if so or even if not, how many Earth-like planets in your system? Hopefully I'm still around in the year 2030 to read your reply.... ;-) Dave |
#5
|
|||
|
|||
Sun Synchronous Orbit
HR4550 (aka. Groombridge 1830)...
Oh well I misread this map, I thought you had to determine distance by projecting down into the circled plane. No, apparently this is a 3D projection. I should have looked at the table at the end.... http://www.atlasoftheuniverse.com/50lys.html So I'm stuck with Sirius... Maybe Smit in on a space platform orbiting Sirius... :-) Dave |
#6
|
|||
|
|||
Sun Synchronous Orbit
On 04/17/2013 8:06 PM, Steve Willner wrote:
In article , Smit Kamal writes: I was wondering if a satellite is in sun synchronous orbit and if it completes 14 orbits per day then will the satellite be passing over one particular location on the earth twice a day.ie 7 am in morning and 7 p m at evening or will it pass over one location on the earth only once. Twice a day, once going northbound and once southbound. Imagine yourself near the Sun looking back at the Earth with the satellite going round. From that perspective, the Earth rotates once in 24 hours, and a given spot on Earth (at least a spot not near either pole) passes under the satellite path twice. As I said in an other post, if it passes exactly over your head while northbound, it is unlikely to past exactly over your head while southbound. Of course, because not all days are 24 hours according to apparent solar time, if the satellite passes exactly over your head, it is unlikely to pass again exactly over your head 24 hours later. So one can assume less precision and then yes, there will be one orbit where it will pass "not too far" from your location on the southbound leg, but in that case "not too far" could be about 13 degrees of longitude for a satellite doing 14 orbits per day. Alain Fournier |
#7
|
|||
|
|||
Sun Synchronous Orbit
On Tuesday, April 23, 2013 8:29:41 PM UTC+5:30, Alain Fournier wrote:
On 04/17/2013 8:06 PM, Steve Willner wrote: In article , Smit Kamal writes: I was wondering if a satellite is in sun synchronous orbit and if it completes 14 orbits per day then will the satellite be passing over one particular location on the earth twice a day.ie 7 am in morning and 7 p m at evening or will it pass over one location on the earth only once. Twice a day, once going northbound and once southbound. Imagine yourself near the Sun looking back at the Earth with the satellite going round. From that perspective, the Earth rotates once in 24 hours, and a given spot on Earth (at least a spot not near either pole) passes under the satellite path twice. As I said in an other post, if it passes exactly over your head while northbound, it is unlikely to past exactly over your head while southbound. Of course, because not all days are 24 hours according to apparent solar time, if the satellite passes exactly over your head, it is unlikely to pass again exactly over your head 24 hours later. So one can assume less precision and then yes, there will be one orbit where it will pass "not too far" from your location on the southbound leg, but in that case "not too far" could be about 13 degrees of longitude for a satellite doing 14 orbits per day. Alain Fournier could you please explain the eclipse patterns in the sunsynchronous orbits which are not experiencing van allen radiations and the altitude is around 800km. |
#8
|
|||
|
|||
Sun Synchronous Orbit
On Tuesday, April 23, 2013 3:46:27 PM UTC+5:30, David Spain wrote:
HR4550 (aka. Groombridge 1830)... Hye Dave Thankyou for your help. I was asking about satellites at an altitude of 800 kilometres which is in sun sychronous orbit. Oh well I misread this map, I thought you had to determine distance by projecting down into the circled plane. No, apparently this is a 3D projection. I should have looked at the table at the end.... http://www.atlasoftheuniverse.com/50lys.html So I'm stuck with Sirius... Maybe Smit in on a space platform orbiting Sirius... :-) Dave |
#9
|
|||
|
|||
Sun Synchronous Orbit
On Tuesday, April 23, 2013 8:29:41 PM UTC+5:30, Alain Fournier wrote:
On 04/17/2013 8:06 PM, Steve Willner wrote: In article , Smit Kamal writes: I was wondering if a satellite is in sun synchronous orbit and if it completes 14 orbits per day then will the satellite be passing over Thankyou Alain!!! Your reply was pretty helpful one particular location on the earth twice a day.ie 7 am in morning and 7 p m at evening or will it pass over one location on the earth only once. Twice a day, once going northbound and once southbound. Imagine yourself near the Sun looking back at the Earth with the satellite going round. From that perspective, the Earth rotates once in 24 hours, and a given spot on Earth (at least a spot not near either pole) passes under the satellite path twice. As I said in an other post, if it passes exactly over your head while northbound, it is unlikely to past exactly over your head while southbound. Of course, because not all days are 24 hours according to apparent solar time, if the satellite passes exactly over your head, it is unlikely to pass again exactly over your head 24 hours later. So one can assume less precision and then yes, there will be one orbit where it will pass "not too far" from your location on the southbound leg, but in that case "not too far" could be about 13 degrees of longitude for a satellite doing 14 orbits per day. Alain Fournier |
#10
|
|||
|
|||
Sun Synchronous Orbit
On 3/25/2014 2:21 PM, Smit Kamal wrote:
On Tuesday, April 23, 2013 3:46:27 PM UTC+5:30, David Spain wrote: HR4550 (aka. Groombridge 1830)... Hye Dave Thankyou for your help. I was asking about satellites at an altitude of 800 kilometres which is in sun sychronous orbit. Smit, Sorry I was not being very helpful. I was joking about your response time. Either you are very busy, have an extremely slow USENET service, or are very far away. However if the latter, at one month shy of 1 year for a response, puts you at slightly 0.5 light-year away, so you've moved significantly closer! You are no longer at Sirius but somewhere in the Oort Cloud! Just kidding (I think), if not, I'll see your response in February 2015.... :-) Dave |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
an efficient orbit to fly a space craft from the L1 lagrange point from earth to mars sun-synchronous orbit? | kelvin | Space Shuttle | 1 | November 11th 05 03:21 AM |
An efficient orbit to fly a space craft from the L1 lagrange point to mars sun-synchronous orbit? | kelvin | Technology | 2 | November 10th 05 08:01 PM |
reccomendations on an efficient orbit to fly a space craft from the L1 lagrange point from earth to mars sun-synchronous orbit? | kelvin | Space Station | 1 | November 8th 05 02:56 PM |
an efficient orbit to fly a space craft from the L1 lagrange point from earth to mars sun-synchronous orbit? | kelvin | Astronomy Misc | 0 | November 8th 05 02:48 PM |
What's a sun-synchronous orbit? | Ultimate Buu | Technology | 0 | July 24th 03 09:45 AM |