MORE MIR
OPERATING HINTS:
Doppler
Shift
by John
Magliacane
The Mir operating hints that have
been carried in SpaceNews over the past several weeks have
received wide acclaim from all corners of the world.
It is hoped that these tips will help make contacts with Mir more
practical and rewarding for all those involved. Many thanks go to
G. Miles Mann, WF1F and Dr. Dave Larsen, N6JLH for contributing
their sage advice to those recent articles. As a follow-up, I
thought I'd throw my hat into the ring and offer some comments on
the effect that Doppler shift plays in communicating with a
simplex communications satellite such as the Amateur Radio
station on Mir.
Doppler shift is nothing new in satellite communications, but it
takes on an interesting "twist" when a simplex
communications link is employed. Doppler shift is the apparent
frequency change observed as the result of the motion of either
the transmitter or the receiver in a communications link.
Both the satellite's motion and the Earth's rotation contribute
to Doppler shift, with the satellite's motion playing the larger
role.
Doppler shift's magnitude is determined by the rate of motion,
while its polarity is determined by the direction of motion. The
polarity is also determined by whether the transmitter or the
receiver is in motion.
When Mir is approching your ground station, the spacecraft
appears to be moving towards you at a high rate of speed. This
motion causes the signal received from Mir to appear about 3.5
kHz ABOVE Mir's actual transmitting frequency of 145.985 MHz. At
the same time, while Mir races toward your ground station, it
receives signals from your uplink transmitter about 3.5 kHz ABOVE
your actual transmitting frequency.
No Doppler shift compensation is performed on the Mir spacecraft.
All compensation must all be handled by individual ground
stations.
In order to compensate for the effects of Doppler shift, ground
stations wishing to communicate with Mir must tune their
receivers several kilohertz ABOVE 145.985 MHz at the time of
acquisition of signal (AOS), AND their transmitters several
kilohertz BELOW 145.985 MHz. This involves transmitting and
receiving on separate (split) frequencies approximately 7 kHz
apart.
Most ground stations use transceivers that transmit on the same
frequency on which they receive (simplex), and if they tune their
transceivers 3.5 kHz (or 5 kHz if their equipment tunes in 5 kHz
steps) above 145.985 MHz at the start of a pass, their uplink
signals are received 7 kHz (or 8.5 kHz) ABOVE what the Mir space
station receiver is tuned to. The chances of establishing a radio
contact under these conditions are extremely remote, even if
there are no other ground stations competing for Mir's uplink
receiver.
Around the time of closest approach (TCA), which also occurs
around the time of maximum elevation for satellites such as Mir
in circular orbits, the Doppler shift between Mir and the ground
station approaches zero. It then quickly reverses polarity as Mir
recedes at an ever increasing rate away from the ground station.
At TCA, it is safe for ground stations to set their transceivers
to 145.985 MHz and not worry about Doppler shift.
As Mir recedes however, stations should tune their receivers
several kilohertz BELOW 145.985 MHz, and their uplink
transmitters several kilohertz ABOVE 145.985 MHz to compensate
for the increasing Doppler shift. Failure to properly compensate
for the Doppler shift on the uplink would, again, result in the
ground station's signal falling outside the passband of the
narrowband FM receiver on Mir shortly after TCA.
As Doppler shift increases, so must the split between the ground
station's transmitting and receiving frequencies. As DOWNLINK
signals drift DOWN in frequency during a pass, UPLINK signals
must drift UP.
T/R offset is zero at TCA, and maximum at AOS and LOS.
The effects of Doppler shift play a greater role in packet radio
communications than they do in FM voice communications. 1200 baud
packet radio communications (1 kHz shift AFSK on a narrowband FM
carrier) occupy a much greater signal bandwidth than does FM
voice, 1200 baud Manchester encoded FSK, or even 9600 baud FSK
packet.
If an uplink signal isn't properly centered within the passband
of the FM receiver on Mir, it will be received with severe
distortion if it is received at all. Tests have shown that a 1200
baud FM packet signal must be strong enough to produce at least
25 dB of receiver quieting for an acceptable bit error rate
performance.
Along the same lines, if other stations are competing with one
another on the uplink frequency, it would take a signal strength
at least 25 dB above the SUM TOTAL OF ALL OTHER SIGNALS on
frequency to be received reliably on Mir because of the capture
effect of FM. If just one bit of a packet frame is received in
error, then the entire packet frame is damaged and must be
re-sent by the transmitting station until it is received cleanly
without error. Incidentially, a signal 25 dB (300 times) stronger
than the sum total of all others is an ASTRONOMICAL signal level
even if only a few other ground stations are colliding with one
another on Mir's uplink frequency.
That's why QRM is so damaging and following proper operating
procedures is so important when communicating with Mir.
While it may be possible to successfully pass a short connect
request packet to Mir and receive a connection acknowledgement in
between interference bursts, it's much more difficult to go much
beyond that and transmit longer information <I> frames to
issue PMS commands and send a message to Mir in the midst of
heavy interference.
In closing, please keep in mind that the Personal Message System
on Mir is a single-user BBS with very limited storage
capabilities. Only ONE station may connect to Mir at a time. ALL
OTHERS MUST WAIT. PLEASE DO NOT connect to Mir unless the PMS is
open and transmitting CQ frames.
PLEASE DO NOT connect to Mir unless you have a reason for doing
so.
PLEASE DO NOT use Mir for passing terrestrial packet radio
traffic.
PLEASE DO NOT use Mir as a digipeater, and PLEASE DO NOT connect
to the keyboard port (R0MIR) unless it is in active use by a
Cosmonaut wishing to make a contact. PLEASE LISTEN carefully to
Mir's downlink frequency to avoid causing interference to other
stations who may already be in contact with Mir and to determine
what mode of communications the Cosmonauts are using on the Mir
spacestation.
John Magliacane, KD2BD - SpaceNews