- This topic has 5 replies, 5 voices, and was last updated 5 years, 11 months ago by
.
-
Posts
-
It seems a long time since July 2015 when NASA’s New Horizons spacecraft flew past Pluto and sent back close-up images of the dwarf planet, its relatively large moon Charon and its other companion moons.
Its next target is the 30 km Kuiper Belt object (486958) 2014 MU69, unofficially known as Ultima Thule. At 05:33 UT on New Year’s Day New Horizons will fly past 2014 MU69 and give us an opportunity to study a KBO from a distance of only 3,500 km.
From recent occultations of a star by ‘Ultima Thule’ its shape profile suggests that it is an elongated object, either a contact binary or perhaps two separate bodies gravitationally linked to each other, such as in this detailed outline of asteroid (90) Antiope obtained in 2011 by occultation observers in the USA.
Comprehensive information about the New Horizons mission is available here.
Let’s hope that the flyby is successful, that good data trickle back to Earth, and we don’t have to wait too long before some first results are announced.
Clear skies for 2019.
Alex.
The “Still Alive” signal should hit ground 15:39UT today together with a bunch of status telemetry. First results tomorrow hopefully.
Should be fun. An exciting time to be alive.
By accident I have just deleted my original comment. so I’ve reconstructed it below. It is a bit techy but basically the data rate back from NH is around 1kbps (equivalent to around 5hrs to transmit a mobile phone picture) achieved using a transmitter with the power of a small LED lightbulb.
The data rate from NH is around 1kbps at the moment achieved using a 12W transmitter and a 2.1m high gain antenna on board the spacecraft. This is received by a 70m DSN antenna on the ground. The link is at X-band and the data is encoded using a rate 1/6 Turbo code. This expands every bit of data into 6 symbols which are then transmitted to the ground. The coding allows the ground to extract good data at a very low signal-to-noise ratio (Eb/No = 0dB). The data is partially phase modulated onto a carrier with some power remaining in the carrier. This provides a discrete frequency tone for synchronisation.
For those that are interested the link budget goes something like:
Spacecraft downlink EIRP +52.8dBW Free space path loss -307.2 dB DSN antenna G/T +61.5dB/K Boltzmann constant +228.6 dBW/K/Hz Bit rate -30dBHz Eb/No 5.7 dB The actual margin is much less than this since NH is currently very near solar conjunction (6 deg today, down to 2 deg on Jan 7) and I haven’t included any factors for transmit and receive losses, modulation loss, pointing losses, weather etc.
A fun fact is that NH is currently around 6.6 billion Km away which is around 160000 times further than a geostationary satellite so the path loss is around 104dB larger than from GEO. That’s a lot.
Voyager-1 is currently 3.3 times further away (21.7 billion km vs 6.6 billion km) so the path loss is 10.3dB more. It has an 18W X-band transmitter (NH is 12W) and a larger HGA (3.7 m vs 2.1m). This gives an overall spacecraft EIRP increase of 6.7dB. On the negative side Voyager uses an old FEC code (rate 1/2 concatenated vs rate 1/6 Turbo) which loses around 2.5 dB. Overall the link is around 6 dB worse. I believe that Voyager 1 currently supports TM rates of up to 600 bps to the 70-m DSN stations so this would imply a margin of around 2dB for all losses in that case (it is currently far from solar conjunction).
- You must be logged in to reply to this topic.