Satgen 118  Geosat VHF: Faraday: Minisats: GPS: Mars Comms   30th June 91

Questions and Answers :- A1. In the RSGB Space Radio Handbook several
experiments use VHF geosats. Are there any easy modern geosats. Yes there
are . Point your antenna southwest at about 25 degrees elevation and tune
anywhere between 240 and 270 MHz. You will find a geosat with lots of
strong signals. 244.078 is particularly useful for propagation tests.

A2. Why did some stations get poor reception with fading from GB1MIR ?
Because Faraday rotation kept changing the polarisation of the downlink,
which went from horizontal polarisation through vertical and back to
horizontal at least 4 times in a single orbital pass. This caused deep
fades at stations with fixed polarisation. So if you want to hear Mir
clearly and continuously , use switchable polarisation on your antenna.

A3. When can we expect the Russians to launch mini test sats from Mir ?
Soviet controllers have said that the present on board crew are due to
launch 2 minisats for exploration of the upper atmosphere sometime before
October 2nd.

A4. Why are the GPS navsats on 1575.42 inaudible ? Because their signal
sounds just like noise. It is in fact pseudo random noise obtained by
multiplying the 100Hz bandwidth nav data sig by a Gold code binary signal
which is a long string of irregular patterns of zeroes and ones. This
results in a signal 2 MHz wide which cannot be heard , but can be tuned on
FM wideband using a centre frequency tuning meter ( as available on the
ICOM R7000) . All GPS sats use the same frequency . You pick out
individual sats by looking for their individual Gold code. By contrast all
Russian Glosnav sats use the same Gold code but transmit on separate
frequencies in the band 1626.5 to 1646.5 MHz.
The same rules apply to tuning the high speed digital packet signal from
Uosat3 Oscar 14 on 435.07 MHz. At 9600 bps with pseudo random binary
modulation , it cannot be tuned audibly. So like the GPS sats you tune
either on a scope "Eye" diagram or a centre tuning meter in FM wideband.
Uosat F 435.125 MHz due to launch in late July will tune the same way as
Uosat 3.

A5. Last but not least . What sort of station will be needed for an SSB
link to Mars. Answer - each station , at the Earth end and the Mars end
will need, assuming no solar or galactic QRN. :-
When Mars is closest to Earth ... 3m dish, 100 watts, SysTemp 100k,2.4 GHz
   (80 million kms approx)    or. 1m dish,  "   "   ,   "      "   24 GHz
When Mars is furthest away    ... 6.5m dish, "  "   ,   "      "  ,2.4 GHz
  (380 million Kms approx)    or. 2m dish,   "  "   ,   "      "  ,24 GHz
73 de John GM4IHJ @ GB7SAN