satellite footprint w.r.t beam width

Hi All,

Found a couple of docs online which allow the user to calculate the footprint radius of a LEO sat in km^2 based on orbital geometry.

This is fine. However, none of these take into consideration transmitting antenna 'gain'. Surely the footprint is determined by how focused the satellite beam is ? i.e. the 'gain' of the transmit antenna ?

could anyone shed some light on this please.

Thanks,

J

The calculation probably shows the area at the foot of a cone where the transmitting antenna is the peak and the angle is known from the antenna radiation pattern. That doesn't necessarily change with gain although there would be some relationship between them. The gain might detemine the strength within the footprint but not it's size.

Brian.

Hi Brian,

Thanks for the reply,

so would i wrong to think that that an antenna with a higher gain produces and narrower beam width? I can't quite get my head around this,

J



As an example,

I have currently specified (with no consideration to antenna gain), a earth coverage % of around 5% for my low earth orbit sat (at a minimum elevation angle of 10 degrees).

Now to improve my link margin (which at this point has assumed 0dBi antennas), I am going to specify a mico strip patch antenna with 6dBi gain.

Would specifying this antenna void my earth overage claims ?

thanks again,

J

Well it depends how you view it. If the satellite has a high gain antenna focused on the earth below it it will only radiate a limited portion of the earth so tecknet2 is correct in that a footprint is just a theoretically maximum illumination area and not a true representation of what part of earth get illuminated.

To determine the radiated area one has to know the gain of the transmitter AND its orientation. Usually satellites are not pointing their antennas perfectly down so it's hard to know what part is being illuminated or not.
The exception is Geostationary satellites, these usually have very well defined illumination pattern.

Clear as mud?

What are you trying to do ?



what are you trying to do ? really I do not understand.
If you have a beam coming from an antenna installed on a spacecraft its footprint on the Earth is just a series of isolevel contour plots (including or not the Earth curvature it depends on what you want see) in terms of antenna gain or EIRP. If you have a circular foot print and assuming that the antenna boresight is pointing at nadir on the Earth, you have a circular symmetric beam (eg pencil beam, isoflux, etc...) the isolevels shall depend on the antenna gain (eg abolsute or relative peak, -1dB -2dB and so on..). If the antenna is not active one the bigger is the antenna, the higher is the gain, the narrower is the foot print... but usually in space based antenna at stage one the link budget is established for a Earth portion, then the "minimum" gain is specified for the antenna in that area (not necessary circular), and consequently the antenna is dimensioned (strongly constrained by the spacecraft allowable volume), essentially the resulting footprint is showing the antenna patter projected on the Earth
what are you trying to do ?