# I can't see other Ultralight traffic,even when they have S-Mode transponder **URL:** \ **Category:** English\ **Created:** [May 26, 2023, 5:32pm UTC](https://forum.safesky.app/t/i-cant-see-other-ultralight-traffic-even-when-they-have-s-mode-transponder/1175 "2023-05-26T17:32:34Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![pedro.custodio](https://avatars.discourse-cdn.com/v4/letter/p/ac91a4/32.png) [@pedro.custodio](https://forum.safesky.app/u/pedro.custodio)\ **Post date:** [May 26, 2023, 5:32pm UTC](https://forum.safesky.app/t/i-cant-see-other-ultralight-traffic-even-when-they-have-s-mode-transponder/1175/1 "2023-05-26T17:32:34Z")
Hi, I am using Safesky and I can see ohter GA planes. Strangely, most Ultralight do not appear. I have tested flying next to an airplane with a Funke TRT800 S-Mode transponder in mode Sierra, and it was not visible in the app. Could this be an issue with the Transponder type? ---
**Author:** ![Mariko](https://dub1.discourse-cdn.com/flex017/user_avatar/forum.safesky.app/mariko/32/474_2.png) [@Mariko](https://forum.safesky.app/u/Mariko)\ **Post date:** [May 27, 2023, 7:48am UTC](https://forum.safesky.app/t/i-cant-see-other-ultralight-traffic-even-when-they-have-s-mode-transponder/1175/2 "2023-05-27T07:48:50Z")
Hi, I have the same trasponder. It as S-ES capability, but you need to feed it with a GPS source in order to trasmit a ADS-B positional message (about 1 per second) Without a GPS source, it only replay to mode A,C and S interrogation from SSR. Using some groung stations, the aircraft position is determinated using MLAT (multilateration) tecnique, but it needs at least 3 or 4 stations receiving mode S replay from same aircraft to calculate its position. So, when an aircraft wit a TRT800-S trasponder not feeded with a GPS source, is at ground or fly at low altitude in an area without enough ground stations, you cannot see it on Safesky, I think.