Tutorial Globalstar

Tutorial: Globalstar

What it is. Globalstar is a satellite phone and IoT network. Its signals are spread-spectrum (CDMA) and encrypted, so there is nothing to read. OnAir is therefore an activity monitor: it shows, per channel, power, signal-to-noise, how often the channel is occupied and the Doppler shift, and plots the Doppler track of the satellites the pass planner predicts. Think of it as a way to see that terminals and satellites are active, not what they carry.

Which band the mode tunes. The receiver covers 1610 to 1626.5 MHz. That is the Globalstar L-band, which carries the terminal uplink (handsets and trackers to the satellite). Globalstar's satellite downlink is in the S band, 2483.5 to 2500 MHz, which this mode does not tune. In the frequency database the mode default is 1618.25 MHz (the L-band centre; the S-band centre 2491.75 MHz is listed for reference). The Mode-Globalstar page and some labels call the L-band signal the "downlink"; in Globalstar's own plan it is the terminal uplink, so what you hear in activity is ground terminals, not the satellites.

What you need

• Radio. A radio that tunes 1.6 GHz and runs at about 10 Msps (the lowest rate that works is 3 Msps; the channel width is 10 MHz). A HackRF, Airspy, bladeRF or USRP. An RTL-SDR does not tune this high. See Supported-radios.
• Antenna. An L-band patch or helical antenna with an LNA and a band-pass filter, outdoors with a clear sky. Terminal transmissions are weak and short, so expect low activity unless you are near terminals. Antenna power: bias-tee or injector.
• Frequencies. 13 CDMA channels of 1.23 MHz inside 1610 to 1626.5 MHz; start at 1618.25 MHz.

Before you start

• Gain: raise it to see the noise floor move; stop before overload.
• At 3 Msps only a part of the 13 channels is sampled: the panel says "n of 13 in the band sampled (x Msps), n active now". Use 10 Msps for the whole band.
• When the input is a file with unknown centre, the band centre 1618.25 MHz is assumed.

Step by step

1. Set your position in the map, then View > Passes: download TLEs. The Globalstar satellites in the planner are drawn on the Doppler view.
2. Mode list, group Satellite & navigation, entry Satellites, 1.6 GHz.
3. Press Start.
4. Look at Channels, Doppler and Events.

What you should see

On Channels: for each of the channels MHz, Power, SNR, Occupancy and Doppler; "no channel active" when quiet. On Doppler, the Doppler of the channel centroid over time with the predicted tracks of satellites above the horizon; the Pass planner overlay can be cleared. On Events: Time, Channel, Duration, Peak SNR, Doppler start to end, Clear.

Reading the screen

• Doppler. The centroid of a channel's spectrum against its nominal centre. Two satellites on one channel give a mixture. The predicted Doppler of every Globalstar satellite above the horizon comes from the TLEs of the pass planner.
• Events. Each burst of activity as a row.
• State: "simulated" marks the built-in test signal.

Options and settings

None that change decoding; the panel notes "Nothing is decoded, and nothing can be: the content is not for listening."

Troubleshooting

• No channel active. Terminals transmit rarely; wait or choose a busier location.
• Doppler plot empty. Download TLEs and set your position.
• Noise. Add a 1.6 GHz filter.
• Rate too low. Under about 3 Msps the mode does not run.
• No signals on S-band. Not tuned by this mode.

Tips

Use the same antenna and LNA as for Tutorial-Iridium; the two bands are close.

Legal and privacy notes

Receive only: no decoding, no decryption. Check your local rules before listening to satellite phone and data links. See Legal.

Under the hood

CDMA with 1.23 MHz channels (13 in the L-band uplink), spread and encrypted; activity detected from power and occupancy; Doppler from the spectral centroid.

Related

Mode-Globalstar, Tutorial-Iridium, Tutorial-Orbcomm, Legal, Troubleshooting