see also:

• radio basics
• software defined radio (SDR) receivers
• transceivers - software defined radio (SDR)
• radio signal intelligence (SigInt)
• radio antennas

• there are now thousands of satellites orbiting earth, each sending (and some receiving) radio signals to earth, many of which can be decoded by consumer radio receivers such as software defined radio (SDR) receivers - if you have the correct antenna and set up

• ~850km
• these allow high bandwidths of data to be received/transmitted hence are used for internet services (eg. Starlink) and other purposes such as weather image capture and transmission (eg. NOAA, Meteor) and LEOSAR (search and rescue)
• the drag on them results in issues which makes them less suitable for GPS
• LEOSAR Search and Rescue system
  • the first generation of search and rescue satellites in the Cospas-Sarsat program, introduced in the early 1980s
  • 850 kilometers and follow a near-polar orbit, taking approximately 102 minutes to complete one orbit
  • detect 406 MHz distress beacon signals and store the data onboard if they are not in view of a ground station, later re-transmitting when they pass over the ground station
  • can accurately calculate the location of beacons using Doppler processing techniques as they move relative to the beacon
  • their polar orbit also allows good coverage of polar regions, which geostationary satellites cannot cover
  • coverage is dependent on satellite passes and ground station visibility
• OrbComm
  • 31 satellites for machine-to-machine and IoT
  • transmit around 137-148MHz on narrow FM sending ASCII data
  • see https://www.youtube.com/watch?v=vEha9se62NQ
• Starlink

• 19,000-24,000km
• these include:
  • MEOSAR (Medium Earth Orbit Search and Rescue) satellites
    • combines advantages of older satellite systems (LEOSAR and GEOSAR) while overcoming their limitations
    • MEOSAR uses multiple constellations of navigation satellites equipped with Search and Rescue payloads: the US GPS, European Galileo (LHCP downlink 1544.1MHz), Russian GLONASS (LHCP downlink 1544.9MHz), and more recently China’s BeiDou (RHCP downlink 1544.2KHz)
    • detect distress signals on the 406 MHz frequency from EPIRBs etc and relay these signals in near real-time to ground stations called MEOLUTs (MEOSAR Local User Terminals)
    • using signals from multiple satellites, MEOSAR can calculate the location of the distress beacon with high accuracy through techniques like Doppler shift and time difference of arrival - MEOSAR can determine the location of a distress beacon within 10 minutes, 95% of the time
    • commenced in 2016, but fully operational in 2023
    • system is managed internationally by Cospas-Sarsat, with satellites being a secondary mission payload on navigation satellites primarily intended for positioning and timing
  • GPS S-band (LHCP downlink 2226.472MHz)
  • GPS L-band (RHCP downlink 1544.9MHz)

• 36,000km
• this is where the current global positioning system (GPS) / global navigation satellite systems (GNSS) satellites reside as well as geostationary satellites:
  • GEOSAR (Search and Rescue)
    • orbit the Earth at an altitude of about 36,000 kilometers directly above the equator, staying fixed relative to the Earth's surface, providing continuous coverage over roughly one-third of the globe at a time
    • three geostationary satellites spaced evenly around the Earth can provide near-continuous coverage between about 70 degrees north and south latitudes, excluding the polar regions
    • continuously monitor distress beacons transmitting at 406 MHz within their coverage area, offering rapid alert capability
    • as they are geostationary, they cannot use Doppler shift methods to locate beacons, instead, location is either encoded in the distress signal via an internal or external GPS receiver or derived from complementary systems like LEOSAR.
    • work in synergy with LEOSAR, which covers polar regions and can independently locate beacons using Doppler processing
    • GEOSAR satellites include NOAA’s GOES satellites (RHCP downlink 1544.5MHz), ESA’s Meteosat series (horizontal downlink 1544.5MHz), and India’s INSAT satellites (linear downlink 4507MHz)
  • Elektro-L (LHCP downlink 1544.5MHz)

• there are many apps and methods to visualize the orbits of the satellites
  • internet-based services
    • https://www.n2yo.com
  • standalone device apps
    • SatDump for Windows, although designed for decoding satellite data with a SDR device, it includes a satellite tracking function which will function without a SDR device
  • radio receiver apps
    • see software defined radio (SDR) receivers

• GPS data
  • GPS satellites broadcast on several specific frequencies within the L band:
    • L1: 1575.42MHz (primary civil signal, used by virtually all GPS receivers)
    • L2: 1227.60MHz (used for military and some advanced civil receivers)
    • L5: 1176.45MHz (newer, high-precision civil signal, used for aviation and advanced applications)
  • see global positioning system (GPS) / global navigation satellite systems (GNSS)
• Emergency txt communications
  • iPhone:
    • uplink (sending signal from iPhone to satellite) operates around 1.6GHz (the L band, including 1610MHz, used by Globalstar)
    • downlink (receiving signal from satellite to iPhone) can use the S band and sometimes higher frequencies such as 2.4GHz (Band 53/n53)
    • the iPhone’s internal antenna supports these frequencies without any external hardware, but the connection requires a clear line of sight to the sky and users must point their phone directly at the satellite as indicated by the device interface. Unlike bulky satellite phones, the iPhone relies on smart software and directional transmission with short compressed text messages optimized for low bandwidth
    • probably uses Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK)
  • Zoleo and Garmin - Iridium
    • uplink and downlink frequency: 1616MHz to 1626.5MHz

• generally send 406MHz signals to LEOSAR, MEOSAR and GEOSAR satellites which re-transmits these on 1544.5MHz
  • Galileo transmits with L handed polarity
  • if people illegally transmit voice on 403MHz, they will be sent back to earth and can be heard on the 1544.5MHz - see https://www.youtube.com/watch?v=U_pCHTeamn8
  • GPS satellites transmit down at 2226.47234MHz

• VHF 137.9MHz satellites:
  • NOAA Weather VHF satellites now de-commissioned!
    • the last NOAA Weather satellites to use VHF, NOAA 15 and 19 were shut down Aug 2025
  • Meteor-M satellites (Russian polar orbiters, M1 137.1, M2 series) still transmit APT: 136-138 MHz generally 137.9MHz
• Elektro-L satellites - Russian geostationary weather satellites 1691-1693 MHz
• new NOAA X-band 8GHz:
  • NOAA-20, NOAA-21, and the JPSS (Joint Polar Satellite System) series utilize X-band frequencies (~8GHz) for data transmission, which are not compatible with the VHF receivers used by hobbyists for APT (Automatic Picture Transmission) signals
• Australian BOM service 18GHz:
  • Himawari-9, a geostationary satellite operated by the Japan Meteorological Agency (JMA) which provides weather images to Australia's BOM on Ka-band (18.1–18.4GHz) - this may be relayed on 402MHz
    • HimawariCast (the rebroadcast of Himawari data for the Asia-Pacific region) is sent via DVB-S2 protocol on the C-band (approximately 3.7–4.2GHz)

• mainly on L band:
  • Inmarsat 3, 4 (“Alphasat”) and 6 satellites
    • Inmarsat Aero provides secure and reliable voice and data communication services for aviation 1530 MHz to 1660 MHz
    • Inmarsat-C STD-C EGC messages are emergency warning broadcasts sent via satellite to deliver critical safety and weather information to aircraft and ships.
      • 1545-1547 MHz in L-band for communications with aircraft
    • Inmarsat Aero and Inmarsat-C STD-C EGC can also use 3.685-3.687 GHz

• Starlink satellites in Australia primarily use the Ku-band (12-18 GHz) and Ka-band (26.5-40 GHz) for transmitting internet data

• these are paid services such as provided by Foxtel
• current protocol uses DVBS1 but this will be replaced by DVBS2 which may require new satellite dishes
• Foxtel downlinks use Ku-band 11.720GHz H and 12.478GHz H from satellites such as Optus D3/10 at 156°E
  • when the satellite dish receives the Ku-band signal, the LNB (Low Noise Block downconverter) on the dish down-converts it to an “Intermediate Frequency” in the 950-2350MHz range. This enables efficient transmission over standard coaxial cables (usually RG6) to indoor satellite STB receivers without excessive signal loss

• Ka-band, Ku-band as for Starlink, etc
• requires a special directional satellite dish type antenna

• whilst you may be able to get by with a V dipole antenna plus a L band filter for the wavelengths you need plus a LNA, the signal to noise ratio may not suffice to allow decoding of the signals
• a better option may be a L band helical antenna +/- aimed into a 70cm or larger parabolic dish
• eg. Discovery Dish with specific feeds

• you will need a 137.9Mz Sawbird filter/amp
• antenna options include:
  • V dipole antenna tuned to 137.9 MHz
    • each arm should be 0.515m (142/freq in MHz gives 1.03m total length) but start slightly longer
    • set up the antenna in its intended position (as close as possible to final operating height and orientation) because the surrounding environment affects tuning
    • measure standing wave ratio (SWR) on the frequency of 137.8 MHz using an antenna analyzer or SWR meter connected at the feed point
    • adjust the length to tune
      • if SWR minimum is at a lower frequency than 137.8 MHz, the antenna is too long, shorten both arms equally, vice versa if frequency is higher
  • Quadrifilar Helix (QFH) antenna
    • provides right-hand circular polarization (RHCP), matching the polarization of NOAA satellite signals, and offers consistent performance without needing constant reorientation
    • ideal for fixed installations or where you want “set and forget” operation
  • Yagi-Uda antenna:
    • more directional and provides higher gain but bulkier and less practical for casual or portable use.