PyCG-DTN

Build DTN contact graphs for deep-space networks from real ephemerides.

Give it a set of bodies and a number of days. It works out which SPICE kernels it needs, downloads them from NASA/NAIF, computes when every pair of nodes can actually see each other, and writes a contact plan.

from pycg_dtn import ContactGraph

cg = ContactGraph()
cg.AddCelestial("Earth")
cg.AddCelestial("Mars")
cg.AddSatellite("MRO-LIKE", "Mars", altitude_km=400, inclination_deg=93.0)

plan = cg.GenerateContactGraph(days=780)
plan.Write("out/")

out/contactGraph.csv     ION contact plan
out/contactGraph.json    full plan with metadata
out/summary.json         per-link statistics

Install

pip install pycg-dtn

Requires Python 3.10+.

What it computes

A contact is an interval during which two nodes can exchange data. Two things take a deep-space link away even when both endpoints are nominally in view:

Occultation — a third body sits in the line of sight. Mars hides Phobos; Jupiter hides Io.

Solar conjunction — the signal path passes close to the Sun, whose corona scatters and delays the signal badly enough that operators stand the link down. The threshold is the Sun–Earth–probe angle; below about 3° the link is treated as unusable, following DSN practice (DSN handbook 810-005).

Contact time is the analysed span minus the union of everything that blocks it. Each surviving interval is then cut into sub-contacts wherever the achievable data rate has drifted more than 10%, so no single contact misrepresents how much data fits through it.

Rates come from a Friis free-space path loss → SNR → Shannon capacity chain, with defaults modelling an X-band spacecraft high-gain dish talking to a DSN 70 m antenna (74.18 dBi, Rodemich 1989).

Bodies

AddCelestial accepts anything NAIF names, case-insensitively — or a NAIF integer ID code.

cg.AddCelestial("Titan")
cg.AddCelestial("Europa", eid="ipn:5.2")   # custom ION endpoint identifier
cg.AddCelestial("401")                      # Phobos, by ID

An unrecognised name raises UnknownCelestialBodyError.

Satellites

Satellites here are defined by you, as a classical Keplerian orbit about a central body:

cg.AddSatellite("MRO-LIKE", "Mars", altitude_km=400,
                eccentricity=0.001, inclination_deg=93.0)

Size the orbit with either altitude_km (above the central body's equatorial radius) or semi_major_axis_km, and give exactly one of them. The rest — eccentricity, inclination_deg, raan_deg, arg_periapsis_deg, mean_anomaly_deg, epoch_utc — default to a circular orbit at the start epoch.

A satellite is a node like any other. It is treated as a point target, so it never occults anything, but things certainly occult it, a low orbiter is hidden by its own planet once per revolution, which is what dominates its contact plan:

MERCURY   MARS      inter in contact  95.8%   longest outage   4.27 d
MERCURY   MRO-LIKE  inter in contact  71.5%   longest outage   4.27 d
MARS      MRO-LIKE  intra in contact 100.0%   longest outage   0.00 d

Kernels

A kernel is NAIF's term for a SPICE data file. Only the ones your bodies need are downloaded, which matters — the Jovian satellite ephemeris alone is over a gigabyte, and an Earth–Mars scenario should never pay for it.

Check the cost before committing:

pycg kernels --bodies Earth Mars Phobos

kernel directory: /home/you/kernels
4 kernels, about 96 MB total

  [need] naif0012.tls          0.0 MB   leap seconds, for UTC <-> ET conversion
  [need] pck00011.tpc          0.1 MB   body radii and IAU body-fixed orientation
  [need] de440s.bsp           31.2 MB   planetary ephemeris DE440 (short), 1849-2150
  [need] mar099s.bsp          64.5 MB   Phobos and Deimos

All files come from NAIF's public generic-kernel archive.

Configuring the link budget

Every radio parameter has a getter and a setter. Setters chain and validate.

lb = cg.GetLinkBudget()
lb.SetFrequency(32.0e9)     # Ka-band instead of X-band
lb.SetRxGain(79.0)          # a larger ground antenna
lb.SetBandwidth(5.0e6)
lb.SetMinRate(1000.0)       # drop contacts below 1 kbps

print(lb.GetWavelength(), lb.GetNoisePower())

Configuring the geometry search

Same pattern on cg.GetGeometry():

geo = cg.GetGeometry()
geo.SetSepExclusion(2.0)     # tighter solar exclusion, degrees
geo.SetOccultStep(300.0)     # finer occultation search, seconds
geo.SetRateTolerance(0.05)   # split contacts on 5% rate drift

Choosing a span

Contact plans repeat on the synodic period — the time for two bodies to return to the same relative arrangement, which for planets with orbital periods Pa and Pb is 1 / |1/Pa − 1/Pb|.

Reading the results

plan = cg.GenerateContactGraph(days=780)

len(plan)                          # number of contacts
plan.ForLink("EARTH", "MARS")      # contacts on one link, time-ordered

for s in plan.LongestOutages(5):
    print(s.a, s.b, s.t_maxgap_days)

The CSV is the contact plan format ingested by ION, NASA JPL's reference DTN implementation. Each contact becomes four rows — a contact and a range line in each direction.

Command line

pycg kernels --bodies Earth Mars Phobos
pycg fetch   --bodies Earth Mars Phobos
pycg build   --bodies Earth Mars --days 780 --out out/
pycg build   --bodies Earth Mars --days 780 --frequency 32e9 --rx-gain 79

Satellites take a NAME,CENTRAL,key=value,... spec and the flag repeats:

pycg build --bodies Earth Mars --days 780 \
           --satellite RELAY-1,Mars,alt=400,inc=93 \
           --satellite RELAY-2,Mars,sma=20000,ecc=0.3

Keys are alt or sma (give one), ecc, inc, raan, argp, ma, eid. The central body does not have to be a node — it still occults its own orbiter.

License

GNU General Public License v3.0 or later. See LICENSE.