A transponder consists of a number of electronic components onboard the satellite. They work together to create an RF communications channel of a standard bandwidth (such as 36 MHz). In the case of Horizons-1 and Horizons-2, 24 transponders comprise the payload. The transponders can be employed by different users who apply them to a variety of purposes.
- Wideband components (like the antenna and wideband receiver) are shared by a group of transponders
- Each transponder has its own input filter to separate out the particular 36 MHz of bandwidth that is allocated to it
- Each transponder has its own power amplifier, which in the case of Horizons-1 is a 108 watt traveling-wave tube amplifier (TWTA). In addition to the 24 TWTAs needed for the 24 transponders, the repeater includes a quantity of spare TWTAs to be used in case of amplifier failure.
This is a repeater section of the payload, showing the elements of six transponders. A transponder includes a share of the uplink and downlink antennas and of the wideband receiver.
It consists of
- A numbered input filter (such as 9-K),
- A pre-amplifier, called a driver/limiter/amplifier (DLA)
- An output amplifier
- A numbered output filter.
Inherent in the repeater design is the frequency plan that delineates the frequencies of the individual transponders.
The frequency plan shown below is for the Horizons-1 satellite; it is very similar to that of Horizons-2 and many other GEO satellites currently in operation. However, the specific numbering plan, such as 1-K through 24-K, is unique to each satellite.
Note that the center frequency of transponder 9-K is 14,180 MHz (14.18 GHz) on the uplink side and 11,880MHz (11.88GHz) on the downlink side.
- Every transponder has a similar pair of frequencies because there must be an uplink and a downlink; the satellite does not modify the bandwidth or modulation of the signal, but merely translates its center frequency and amplifies it to the appropriate power for the downlink
- A single transponder can relay as much as 90 Mbps, if a suitable antenna, power amplifier and modulation equipment is used on the ground. Normally, a transponder is used to carry several VSAT networks or up to 12 digital TV channels.
This is the frequency plan of Horizons-1, showing a total of 24 transponders in the uplink and downlink. The odd-numbered transponders are cross-polarized with the even-numbered transponders, and each has a usable bandwidth of 36 MHz.