diff --git a/Cargo.lock b/Cargo.lock
index b783648..78cdaf9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -567,6 +567,7 @@ dependencies = [
  "paste",
  "postcard",
  "serde",
+ "serde_json",
  "spacepackets",
  "zerocopy",
 ]
diff --git a/satrs-core/Cargo.toml b/satrs-core/Cargo.toml
index 9969718..4918158 100644
--- a/satrs-core/Cargo.toml
+++ b/satrs-core/Cargo.toml
@@ -40,6 +40,7 @@ path = "../spacepackets"
 serde = "1.0"
 zerocopy = "0.6"
 once_cell = "1.13"
+serde_json = "1"
 
 [dev-dependencies.postcard]
 version = "1.0"
diff --git a/satrs-core/tests/hk_helpers.rs b/satrs-core/tests/hk_helpers.rs
new file mode 100644
index 0000000..9a30368
--- /dev/null
+++ b/satrs-core/tests/hk_helpers.rs
@@ -0,0 +1,157 @@
+#![allow(dead_code)]
+use core::mem::size_of;
+use serde::{Deserialize, Serialize};
+use spacepackets::ecss::{Ptc, RealPfc, UnsignedPfc};
+
+enum NumOfParamsInfo {
+    /// The parameter entry is a scalar field
+    Scalar = 0b00,
+    /// The parameter entry is a vector, and its length field is one byte wide (max. 255 entries)
+    VecLenFieldOneByte = 0b01,
+    /// The parameter entry is a vecotr, and its length field is two bytes wide (max. 65565 entries)
+    VecLenFieldTwoBytes = 0b10,
+    /// The parameter entry is a matrix, and its length field contains a one byte row number
+    /// and a one byte column number.
+    MatrixRowsAndColumns = 0b11,
+}
+
+const HAS_VALIDITY_MASK: u8 = 1 << 7;
+
+struct ParamWithValidity<T> {
+    valid: bool,
+    val: T,
+}
+
+struct TestMgmHk {
+    temp: f32,
+    mgm_vals: [u16; 3],
+}
+
+struct TestMgmHkWithIndividualValidity {
+    temp: ParamWithValidity<f32>,
+    mgm_vals: ParamWithValidity<[u16; 3]>,
+}
+
+#[derive(Serialize, Deserialize)]
+struct TestMgmHkWithGroupValidity {
+    last_valid_stamp: [u8; 7],
+    valid: bool,
+    temp: f32,
+    mgm_vals: [u16; 3],
+}
+
+impl TestMgmHk {
+    pub fn write_to_be_bytes(&self, buf: &mut [u8]) -> Result<usize, ()> {
+        let mut curr_idx = 0;
+        buf[curr_idx..curr_idx + size_of::<f32>()].copy_from_slice(&self.temp.to_be_bytes());
+        curr_idx += size_of::<f32>();
+        for val in self.mgm_vals {
+            buf[curr_idx..curr_idx + size_of::<u16>()].copy_from_slice(&val.to_be_bytes());
+            curr_idx += size_of::<u16>();
+        }
+        Ok(curr_idx)
+    }
+}
+
+/// This could in principle be auto-generated.
+impl TestMgmHkWithIndividualValidity {
+    pub fn write_to_be_bytes_self_describing(&self, buf: &mut [u8]) -> Result<usize, ()> {
+        let mut curr_idx = 0;
+        buf[curr_idx] = 0;
+        buf[curr_idx] |= HAS_VALIDITY_MASK | (self.temp.valid as u8) << 6;
+        curr_idx += 1;
+        buf[curr_idx] = Ptc::Real as u8;
+        curr_idx += 1;
+        buf[curr_idx] = RealPfc::Float as u8;
+        curr_idx += 1;
+        buf[curr_idx..curr_idx + size_of::<f32>()].copy_from_slice(&self.temp.val.to_be_bytes());
+        curr_idx += size_of::<f32>();
+        buf[curr_idx] = 0;
+        buf[curr_idx] |= HAS_VALIDITY_MASK
+            | (self.mgm_vals.valid as u8) << 6
+            | (NumOfParamsInfo::VecLenFieldOneByte as u8) << 4;
+        curr_idx += 1;
+        buf[curr_idx] = Ptc::UnsignedInt as u8;
+        curr_idx += 1;
+        buf[curr_idx] = UnsignedPfc::TwoBytes as u8;
+        curr_idx += 1;
+        buf[curr_idx] = 3;
+        curr_idx += 1;
+        for val in self.mgm_vals.val {
+            buf[curr_idx..curr_idx + size_of::<u16>()].copy_from_slice(&val.to_be_bytes());
+            curr_idx += size_of::<u16>();
+        }
+        Ok(curr_idx)
+    }
+}
+
+impl TestMgmHkWithGroupValidity {
+    pub fn write_to_be_bytes_self_describing(&self, buf: &mut [u8]) -> Result<usize, ()> {
+        let mut curr_idx = 0;
+        buf[curr_idx] = self.valid as u8;
+        curr_idx += 1;
+        buf[curr_idx..curr_idx + self.last_valid_stamp.len()]
+            .copy_from_slice(&self.last_valid_stamp);
+        curr_idx += self.last_valid_stamp.len();
+        buf[curr_idx] = 0;
+        curr_idx += 1;
+        buf[curr_idx] = Ptc::Real as u8;
+        curr_idx += 1;
+        buf[curr_idx] = RealPfc::Float as u8;
+        curr_idx += 1;
+        buf[curr_idx..curr_idx + size_of::<f32>()].copy_from_slice(&self.temp.to_be_bytes());
+        curr_idx += size_of::<f32>();
+        buf[curr_idx] = 0;
+        buf[curr_idx] |= (NumOfParamsInfo::VecLenFieldOneByte as u8) << 4;
+        curr_idx += 1;
+        buf[curr_idx] = Ptc::UnsignedInt as u8;
+        curr_idx += 1;
+        buf[curr_idx] = UnsignedPfc::TwoBytes as u8;
+        curr_idx += 1;
+        buf[curr_idx] = 3;
+        for val in self.mgm_vals {
+            buf[curr_idx..curr_idx + size_of::<u16>()].copy_from_slice(&val.to_be_bytes());
+            curr_idx += size_of::<u16>();
+        }
+        Ok(curr_idx)
+    }
+}
+
+#[test]
+pub fn main() {
+    let mut raw_buf: [u8; 32] = [0; 32];
+    let mgm_hk = TestMgmHk {
+        temp: 20.0,
+        mgm_vals: [0x1f1f, 0x2f2f, 0x3f3f],
+    };
+    // 4 byte float + 3 * 2 bytes MGM values
+    let written = mgm_hk.write_to_be_bytes(&mut raw_buf).unwrap();
+    assert_eq!(written, 10);
+
+    let mgm_hk_individual_validity = TestMgmHkWithIndividualValidity {
+        temp: ParamWithValidity {
+            valid: true,
+            val: 20.0,
+        },
+        mgm_vals: ParamWithValidity {
+            valid: true,
+            val: [0x1f1f, 0x2f2f, 0x3f3f],
+        },
+    };
+    let written = mgm_hk_individual_validity
+        .write_to_be_bytes_self_describing(&mut raw_buf)
+        .unwrap();
+    // 3 byte float description, 4 byte float, 4 byte MGM val description, 3 * 2 bytes MGM values
+    assert_eq!(written, 17);
+
+    // The easiest and probably best approach, trading off big advantages for TM downlink capacity:
+    // Use a JSON format
+    let mgm_hk_group_validity = TestMgmHkWithGroupValidity {
+        last_valid_stamp: [0; 7],
+        valid: false,
+        temp: 20.0,
+        mgm_vals: [0x1f1f, 0x2f2f, 0x3f3f],
+    };
+    let mgm_as_json_str = serde_json::to_string(&mgm_hk_group_validity).unwrap();
+    println!("{}", mgm_as_json_str);
+}
diff --git a/spacepackets b/spacepackets
index fc05eaa..251fcdd 160000
--- a/spacepackets
+++ b/spacepackets
@@ -1 +1 @@
-Subproject commit fc05eaa925059a23ab0c9d2f0be50105ae969d48
+Subproject commit 251fcdd6c8d366484e10fb9480925f0160d569ff