3 ms·

Expensive, yes, but difficult, not as much as you think. I'm assuming they are operating in low earth orbit. Which means that the satellite could be around 200

Expensive, yes, but difficult, not as much as you think. I'm assuming they are operating in low earth orbit. Which means that the satellite could be around 200-300km above the earth. Apart from the atmosphere its direct line of sight. You have the advantage that you are not really trying to decode data, just figure out where something is, so you can use a large phased array antenna to electronically sweep an area. Because you're in orbit, you can localise a RF source as you fly over. in terms of RF power, GPS is transmitted at ~40 watts from ~20200km I haven't done the maths, but as its inverse square, I'd punt that GPS signal is weaker than a phone signal at 200km

instagib 3y ago
SAR is nice for satellites too. https://www.mathworks.com/help/radar/ug/spaceborne-synthetic-aperture-radar-performance-prediction.html https://www.mathworks.com/help/radar/ug/spaceborne-synthetic... How much POWER do the GPS Satellites output on the 1575mhz L1 frequency? In the frequency allocation filing the L1 C/A power is listed as 25.6 Watts. The Antenna gain is listed at 13 dBi. Thus, based on the frequency allocation filing, the power would be about 500 Watts (27 dBW). Now, the free space path loss from 21000 km is about 182 dB. Take the 500 Watts (27 dBW) and subtract the free space path loss (27 - 182) and you get -155 dBW. The end of life spec is -160 dBW, which leaves a 5 dB margin. And if you really get into it, you'll discover ALL of the following represent the same approximate signal strength for GPS on the face of the earth (m stands for milliwatts and m2 stands for meters squared): -160 dBW, -130 dBm, -135 dBW/m2, -105 dBm/m2, -223 dBW/Hz, -163 dBW/MHz, -193 dBm/Hz, -198 dBW/m2/Hz, -138 dBW/m2/MHz Once you figure out why they're all the same, you're well on your way to understanding power, power density, and power flux density as it relates to GPS. For those that wish to quibble, I am assuming an even distribution of power density over a 2 MHz C/A bandwidth. http://gpsinformation.net/main/gpspower.htm http://gpsinformation.net/main/gpspower.htm
- KaiserPro 3y ago
  thanks for this, my maths is a bit weak and I last did this sort of calculations in 2008. I got around -79dbm for a phone at 200km, now I think thats probably a bit high. Even so even -90dbm is certainly something thats workable with

instagib 3y ago

SAR is nice for satellites too. https://www.mathworks.com/help/radar/ug/spaceborne-synthetic-aperture-radar-performance-prediction.html https://www.mathworks.com/help/radar/ug/spaceborne-synthetic... How much POWER do the GPS Satellites output on the 1575mhz L1 frequency? In the frequency allocation filing the L1 C/A power is listed as 25.6 Watts. The Antenna gain is listed at 13 dBi. Thus, based on the frequency allocation filing, the power would be about 500 Watts (27 dBW). Now, the free space path loss from 21000 km is about 182 dB. Take the 500 Watts (27 dBW) and subtract the free space path loss (27 - 182) and you get -155 dBW. The end of life spec is -160 dBW, which leaves a 5 dB margin. And if you really get into it, you'll discover ALL of the following represent the same approximate signal strength for GPS on the face of the earth (m stands for milliwatts and m2 stands for meters squared): -160 dBW, -130 dBm, -135 dBW/m2, -105 dBm/m2, -223 dBW/Hz, -163 dBW/MHz, -193 dBm/Hz, -198 dBW/m2/Hz, -138 dBW/m2/MHz Once you figure out why they're all the same, you're well on your way to understanding power, power density, and power flux density as it relates to GPS. For those that wish to quibble, I am assuming an even distribution of power density over a 2 MHz C/A bandwidth. http://gpsinformation.net/main/gpspower.htm http://gpsinformation.net/main/gpspower.htm

KaiserPro 3y ago
thanks for this, my maths is a bit weak and I last did this sort of calculations in 2008. I got around -79dbm for a phone at 200km, now I think thats probably a bit high. Even so even -90dbm is certainly something thats workable with