Satellite Broadband Modems


A satellite modem or satmodem is a modem used to establish data transfers using a communications satellite as a relay. The satellite modem's main function is to transform an input bitstream to a radio signal and vice versa.

There are some devices that include only a demodulator (and no modulator, thus only allowing data to be downloaded by satellite) that are also referred to as "satellite modems." These devices are used in satellite Internet access (in this case uploaded data is transferred through a conventional PSTN modem or an ADSL modem).

A satellite modem is not the only device needed to establish a communication channel. Other equipment that are essential for creating a satellite link include satellite antennas and frequency converters.

Data to be transmitted are transferred to a modem from data terminal equipment (e.g. a computer). The modem usually has Intermediate Frequency (IF) output (that is, 50-200 MHz), however, sometimes the signal is modulated directly to L-band. In most cases frequency has to be converted using an upconverter before amplification and transmission.

A modulated signal is a sequence of symbols, pieces of data represented by a corresponding signal state, e.g. a bit or a few bits, depending upon the modulation scheme being used. Recovering a symbol clock (making a local symbol clock generator synchronous with the remote one) is one of the most important tasks of a demodulator.

Similarly, a signal received from a satellite is firstly downconverted (this is done by a low-noise block converter - LNB), then demodulated by a modem, and at last handled by data terminal equipment. The LNB is usually powered by the modem through the signal cable with 13 or 18 V DC.


The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also define error correction codes and framing formats.

Popular modulation types being used for satellite communications:

  • Binary phase shift keying (BPSK);
  • Quadrature phase shift keying (QPSK);
  • Offset quadrature phase shift keying (OQPSK);
  • 8PSK;
  • Quadrature amplitude modulation (QAM), especially 16QAM.

The popular satellite error correction codes include:

  • Convolutional codes:
    • with constraint length less than 10, usually decoded using a Viterbi algorithm
    • with constraint length more than 10, usually decoded using a Fano algorithm
  • Reed-Solomon codes usually concatenated with convolutional codes with an interleaving
  • New modems support superior error correction codes (turbo codes and LDPC codes)

Frame formats that are supported by various satellite modems include:

  • Intelsat business service (IBS) framing
  • Intermediate data rate (IDR) framing
  • MPEG-2 transport framing (used in DVB)
  • E1 and T1 framing

High-end modems also incorporate some additional features:

  • Multiple data interfaces (like RS-232, RS-422, V.35, G.703, LVDS, Ethernet);
  • Embedded Distant-end Monitor and Control (EDMAC), allowing to control the distant-end modem;
  • Automatic Uplink Power Control (AUPC), that is, adjusting the output power to maintain a constant signal to noise ratio at the remote end;
  • Drop and insert feature for a multiplexed stream, allowing to replace some channels in it.


Satellite modems are often used for home internet access.

There are two different types, both employing the Digital Video Broadcasting (DVB) standard as their basis:

  • One-way satmodems (DVB-IP modems) use a return channel not based on communication with the satellite, such as telephone or cable.
  • Two-way satmodems (DVB-RCS modems, also called astromodems) employ a satellite-based return channel as well; they do not need another connection. DVB-RCS is ETSI standard EN 301 790.

There are also industrial satellite modems intended to provide a permanent link. They are used, for example, in the Steel shankar network.

How to choose a satellite modem?

When it comes to satellite modems, there is a broad range to choose from. They range from very inexpensive satellite modems to obtain home Internet access to costly multipurpose modem devices and machinery for commercial use. The word modem means modulator or demodulator. Satellite modems transform and receive an input bit stream to a radio signal. However, some satellite modem devices only have a demodulator that is used to access satellite Internet, whereas data is transferred in other directions via the use of a conventional PSTN model or an ADSL modem.

What is the best modem for satellite internet?

The best modem for satellite internet is the one you get from your provider because it will already be calibrated to the satellite signal you’re receiving. However, you might want to use a faster or more powerful router than the one that comes built in to the modems offered by the ISPs. Currently, the only companies offering satellite internet are HughesNet and Viasat. Both providers lease modems with built-in Wi-Fi routers to their customers. The modems from both ISPs have similar specs, but Viasat gives you more ports and security options. HughesNet uses the HT2000W satellite modem, while Viasat doesn’t list the exact model number you’ll get.

Viasat Wi-Fi Modem Specs

  • Band: 2.4 GHz and 5 GHz
  • Protocol: 802.11 a/b/g/n
  • Security: WPS, WEP, and WPA
  • Ethernet Ports: 4 Includes adapter for Viasat Voice

HughesNet Wi-Fi Modem Specs

  • Band: 2.4 GHz and 5 GHz
  • Protocol: 802.11 a/ac/an/b/g/n
  • Security: WPS
  • Ethernet Ports: 1


WiFi Gateway

The modem takes care of translating IP packets into a format compatible with transmission over a satellite link. Its ouput is a modulated carrier wave in the L band (950-1450 MHz). Earlier DVB (Digital Video Broadcast) modulation was used, the same type of coding used for satellite TV. It had the advantage of already being widely deployed, so the hardware was already relatively cheap. New advances in technology made it economical to use WIMAX (Worldwide Interpretable Microwave Access) , and IPoS (Internet Protocol over Satellite).