Sun Synchronous Orbit
lbru...@erols.com
I am seeking a clear understanding of the sun sync orbit. So far I have
it that it involves giving the orbit a nodal regression equal to 360
degrees per year so that the orbit plane maintains a constant orientation
with respect to the sun as the earth moves in its orbit around the sun.
I have it that the reason for wanting a sun sync orbit is so that the
spacecraft will continuously view the surface of the earth at the same
local time at any given latitude. If I pick the latitude of the equator
I would expect that everytime the sub-satellite point crossed the equator
(ascending -exclusive or- descending) you could note the local time at
the sub-sat point and find that it was always the same. Am I
understanding this correctly?
Also, I have heard of a 6am or 9am, etc. sun sync orbit. Does this mean
that that local time (described above with respect to the equator) is
always 6am or 9am, etc?
Rick Cooper
lbru...@erols.com wrote:
>I am seeking a clear understanding of the sun sync orbit. So far I have
>it that it involves giving the orbit a nodal regression equal to 360
>degrees per year so that the orbit plane maintains a constant orientation
>with respect to the sun as the earth moves in its orbit around the sun.
>I have it that the reason for wanting a sun sync orbit is so that the
>spacecraft will continuously view the surface of the earth at the same
>local time at any given latitude. If I pick the latitude of the equator
>I would expect that everytime the sub-satellite point crossed the equator
>(ascending -exclusive or- descending) you could note the local time at
>the sub-sat point and find that it was always the same. Am I
>understanding this correctly?
Yes. Because the Earth is not a perfect sphere it tugs on a satellite
differently at the equator than at the pole. This causes the line of nodes
(where the orbit passes through the equator) to move with respect to an
inertial reference. (This is impossible with a perfect sphere or point mass)
The rate at which the nodes move is dependant on the semi-major axis (the
average of the closest and farthest distances from the earth) and the
inclination of the orbit.
A sun synchronous orbit rotates the line of nodes at a rate of 360 degrees/
year which makes the relationship between the line of nodes and the the line
between the sun and earth constant.
The period of the orbit (dependant on the semi-major axis) may vary as long
as the correct inclination is selected to keep the regression of the nodes
at the sun synchronous rate. You are correct in that you will pass over
the earth at the same local time, but it is not necessary to pass over that
spot of the earth on every orbit if the period of the orbit and the rotation of
the earth are not the same. (This is in contrast to the geo-synchronous orbit
where the period of the orbit must the the rotation rate of the earth)
>Also, I have heard of a 6am or 9am, etc. sun sync orbit. Does this mean
>that that local time (described above with respect to the equator) is
>always 6am or 9am, etc?
The local time is a reference to the angle between the orbit plane and the
earth-sun vector which dictates what the local time is below the satellite,
and there is no restrictions on that from a pure orbit mechanics point of
view.
--
ss8...@den.mmc.com - Rick Cooper, Lockheed Martin
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
"If you don't want to go around in circles, you have to
be somewhat eccentric." - Equations of Orbital Motion
Frank Crary
In article <4nfcl9$f...@news7.erols.com>, <lbru...@erols.com> wrote:
>I am seeking a clear understanding of the sun sync orbit. So far I have
>it that it involves giving the orbit a nodal regression equal to 360
>degrees per year so that the orbit plane maintains a constant orientation
>with respect to the sun as the earth moves in its orbit around the sun.
>I have it that the reason for wanting a sun sync orbit is so that the
>spacecraft will continuously view the surface of the earth at the same
>local time at any given latitude. If I pick the latitude of the equator
>I would expect that everytime the sub-satellite point crossed the equator
>(ascending -exclusive or- descending) you could note the local time at
>the sub-sat point and find that it was always the same. Am I
>understanding this correctly?
Pretty much. It isn't quite perfect: You need some fuel for station
keeping, since the orbit is only synchronous if you neglect perturbations
from the Moon, etc. And even so, the local time of day at the
equator crossing will vary by plus of minus 15 minutes or
so, over the course of a year, since the Earth's orbit isn't
perfectly circular. (The difference, by the way, is the
same as the "equation of time" used to correct sun dials
for the non-circularity of the Earth's orbit.)
>Also, I have heard of a 6am or 9am, etc. sun sync orbit. Does this mean
>that that local time (described above with respect to the equator) is
>always 6am or 9am, etc?
That's exactly what it would mean. 6 AM sounds a bit early, but
early morning (and late afternoon) are good sun angles for
some observations. Usually, the height of an object on the
surface is estimated from the length of the shadows it
casts. That means seeing a place at a constant time of
day is convenient (hence the use of a sun synchronous
orbit) and the accuracy of the measurement depends on
the length of the shadows. Ideally, you want a time of
day when there is enough light to see things, but when
the sun angle is low enough that they cast long shadows.
That's early morning and late afternoon.
Frank Crary
CU Boulder
Eric Kouba
> I have it that the reason for wanting a sun sync orbit is so that the
> spacecraft will continuously view the surface of the earth at the same
> local time at any given latitude.
**********
You can also set up sun synchronous orbits that will have no eclipses...
makes things much easier for satellite batteries (load shed when you lose
the sun, emergency cycles only, high depth of discharge) and a bit easier
for thermal cycling.
Satellite batteries are heavy...
Eric The Kouba
Joachim Fuchs
Dear lbrunson,
The one reason for a SSO (Sun Synchronous Orbit) is - as you say - to
observe the Earth surface at the same local time. However, there is
another argument as well, which is to facilitate the design of the
spacecraft. because its orientation towards the sun is (almost) the same
during the entire mission.
Instead of talking of the local time on the ground track, another
possibility is to say that the angle between the orbital plane and the
axis Earth-Sun remnains the same. This facilitates the way to imagine it
in inertial space. This angle can then directly be translated in a local
time.
However, these issues are explained in any basic Mission Analysis and
Design handbook.
--joachim fuchs
////
(o o)
------------------------------oOO--(_)--OOo-------------------------
Joachim Fuchs European Space Agency
ESA/ESTEC-JPP Earth Observation Preparatory Programme
email: jfu...@eoppsun.estec.esa.nl
tel: +31-71-565-5296
snail: POBox 299, 2200 AG Noordwijk, The Netherlands
Henry Spencer
In article <4njh4b$g...@lace.colorado.edu> fcr...@rintintin.Colorado.EDU (Frank Crary) writes:
>...It isn't quite perfect: You need some fuel for station
>keeping, since the orbit is only synchronous if you neglect perturbations
And from the Sun! Solar perturbations are particularly troublesome because
the orbit, fairly obviously, tends to be in resonance with them. The major
effect is a slow drift of the inclination.
>...early morning (and late afternoon) are good sun angles for
>some observations. ... Ideally, you want a time of
>day when there is enough light to see things, but when
>the sun angle is low enough that they cast long shadows...
Actually, for observing the Earth's surface, a more fundamental issue
tends to be the desire to minimize cloud cover. Cloud cover generally
increases during the day, because solar heating makes the atmosphere more
active. On the other hand, the first little dose of heat early in the
morning also tends to "burn off" fog and low cloud. Overall, cloud cover
hits a minimum at about 10AM, which is why there's a tight little cluster
of remote-sensing-satellite orbits around 10AM.
--
Unix was a breakthrough. | Henry Spencer
Windows 95 is more like a smash-and-grab. | he...@zoo.toronto.edu
Henry Spencer
In article <ekouba-1905...@slip22110.rmii.com> eko...@rmii.com (Eric Kouba) writes:
>You can also set up sun synchronous orbits that will have no eclipses...
>makes things much easier for satellite batteries (load shed when you lose
>the sun, emergency cycles only, high depth of discharge) and a bit easier
>for thermal cycling.
Unfortunately, if you want to stay out of the Van Allen belts, you can't
get a sun-synchronous orbit that is permanently eclipse-free. The problem
is that the sun-synchronous orbits precess around the Earth's axis, which
is not in the plane of the ecliptic, so they don't hold a truly constant
angle to the Sun. Around the summer and winter solstices, when one of
Earth's poles is in permanent shadow, sun-synchronous satellites which
pass that pole low on the spaceward side will pass through Earth's shadow.
All practical sun-synchronous orbits pass low on the spaceward side of
one pole or the other.
By launching just after the relevant solstice, you can arrange for a
satellite to be in permanent sun for about ten months, but that's all.
To get a truly eclipse-free sun-synchronous orbit, you have to go to higher
altitudes... up in the Van Allen belts where the radiation fries your
electronics rather quickly.
Pat
In article <Drv2F9.73G%spen...@zoo.toronto.edu>,
now this is a new one for me, but 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.
pat
--
Overbreeding leads to weakness, it is a slow death -- Major Katsunagi -
L.C. Scholz
In article <Drv2F9.73G%spen...@zoo.toronto.edu> Henry Spencer
wrote:
>Date: Thu, 23 May 1996 13:49:57 GMT
>From: Henry Spencer <he...@zoo.toronto.edu>
>To: sci-spa...@agate.berkeley.edu
>Newsgroups: sci.space.tech
>Subject: Re: Sun Synchronous Orbit
>
<snipped>
>Actually, for observing the Earth's surface, a more fundamental
issue
>tends to be the desire to minimize cloud cover. Cloud cover
generally
>increases during the day, because solar heating makes the
atmosphere more
>active. On the other hand, the first little dose of heat early in
the
>morning also tends to "burn off" fog and low cloud. Overall,
cloud cover
>hits a minimum at about 10AM, which is why there's a tight little
cluster
>of remote-sensing-satellite orbits around 10AM.
>--
There may be a desire to actually study the cloud cover and then
afternoon orbits are used. NOAA generally flies two orbits, a
morning and an afternoon node crossing time, for the TIROS weather
satellites. The EOS projects in the Mission To Planet Earth
programs also include both morning and afternoon orbits. EOS-AM is
primarily concerned with Earth surface observations while EOS-PM is
looking at Earth radiation balance and other cloud cover effects.
Frank Crary
In article <4o9u05$9...@clarknet.clark.net>, Pat <p...@clark.net> 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
Smit Kamal
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
Alain Fournier
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
Steve Willner
> 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.
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 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
David Spain
> On 04/17/2013 5:13 AM, Smit Kamal wrote:
> It is amusing to see a reply to a post 17 years later.
>
> 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
David Spain
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
Alain Fournier
> In article <2a7c6d00-6bd9-4ddc...@googlegroups.com>,
> Smit Kamal <smit...@gmail.com> 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.
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
revathi...@gmail.com
Smit Kamal
> 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 Kamal
>
> > In article <2a7c6d00-6bd9-4ddc...@googlegroups.com>,
>
> > Smit Kamal <smit...@gmail.com> 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
Your reply was pretty helpful :)
David Spain
> 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.
>>
>>
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
Smit Kamal
lol..I am now getting close to planet earth :D...actually I did not get any mail about the replies in this group which caused my delays :P..apologies for the delay
Alain Fournier
about sun synchronous orbits with respect to going into Earth's shadow
except for the fact that if the orbit is over or near the dawn/dusk line
then the satellite is never in Earth's shadow.
Note that you can be in a continuously sunlit orbit without being in a
sun synchronous orbit.
Alain Fournier
Smit Kamal
That was helpful. I wanted to know about the time spent by the satellite in eclipse per day or per year when it is in a polar sun synchronous orbit with 98 degree inclination
Thankyou
Regards
Smit Kamal